This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 635

Cite this: Chem. Commun., 2012, 48, 635–653

Interaction of niobium and tantalum pentahalides with O-donors:

coordination chemistry and activation reactions

Fabio Marchetti and Guido Pampaloni*

Received 27th July 2011, Accepted 17th October 2011

DOI: 10.1039/c1cc14592d

The chemistry of niobium and tantalum pentahalides, MX5, with oxygen compounds is reviewed

herein. The polynuclear structure of MX5 is readily broken by addition of oxygen-containing

organic molecules, L, to give either mononuclear or ionic dinuclear coordination adducts. Then

activation of the organic ligand may take place favoured by several factors, i.e. low M–X bond

energy, high temperature, presence of more than one oxygen function within L, L/M molar

ratio Z 2. The activation reactions are often uncommon in the context of metal halides; they

include the cleavage of Csp3–O, Csp2–O, C–H and C–C bonds, and eventual successive

rearrangements proceeding with C–O or C–C couplings. The recently elucidated reactivity of MX5

with limited amounts of oxygen compounds will be presented, and possible connections with the

relevant MX5-directed syntheses reported in the literature will be outlined.

1. Introduction

Niobium and tantalum pentahalides, MX5, are known for all

the halides: they can be prepared by direct combination of the

elements at high temperature. The compounds MX5 are relatively

volatile solids (that is indicative of the covalency of the M–X

bonds), in which the metal centre attains the hexacoordination by

means of halide bridges. The pentafluorides are tetramers, while

the heavier halides are dimers. The colours vary from colourless

(NbF5, TaF5, TaCl5) to yellow (NbCl5, TaBr5), red (NbBr5, TaI5)

and brown-black (NbI5).1

Niobium and tantalum pentahalides are scarcely soluble in

non-coordinating solvents, and easily susceptible to hydrolysis

by traces of moisture. This feature makes such materials

particularly difficult to handle and store. This is probably

the main reason why the coordination chemistry of MX5 (M =

Nb, Ta) could be significantly less developed than that of other

Universita di Pisa, Dipartimento di Chimica e Chimica Industriale,Via Risorgimento 35, I-56126 Pisa, Italy.E-mail: pampa@dcci.unipi.it; Fax: +39 050 2219246;Tel: +39 050 2219219

Fabio Marchetti

Dr Fabio Marchetti received thedegree in Industrial Chemistryfrom the University of Bolognain 1999 and PhD in 2003. Afterpost-doctoral fellowship inBologna, he moved to Pisawhere he has been a permanentresearcher since 2006. Hespent research periods at theUniversities of Bristol (1999),Zaragoza (2001–02) andHelsinki (2006). F. Marchettiis a co-authour of about 70papers on international journals,mainly concerning the synthesisand the reactivity of transitionmetal compounds.

Guido Pampaloni

Prof. Guido Pampaloni receivedthe degree in Chemistry fromthe University of Pisa in 1979.After a post-doctoral fellowshipin Aachen (Germany) withProf. G. E. Herberich (1979–1980), he came back to Pisawhere he received PhD in chem-istry from the Scuola NormaleSuperiore in 1983. Since 1992,G. Pampaloni has been anassociate professor at the Dipar-timento di Chimica e ChimicaIndustriale of the University ofPisa. Prof. Pampaloni is aco-author of ca. 150 papers on

international journals and 10 patents. His main interests concern thesynthesis, the reactivity and the catalytic properties of inorganic(metal halides and their coordination adducts) and organometallic(carbonyl-, cyclopentadienyl- and arene metal derivatives) com-pounds of early transition elements.

ChemComm Dynamic Article Links

www.rsc.org/chemcomm FEATURE ARTICLE

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2DView Online / Journal Homepage / Table of Contents for this issue

636 Chem. Commun., 2012, 48, 635–653 This journal is c The Royal Society of Chemistry 2012

transition metal halides (e.g. group 4 metal tetrahalides), and

the data reported in the literature up to 20072 were sparse and

incomplete.

Likewise, the use of MX5 in metal-mediated synthesis has

been rather circumscribed; in this context, precursors based on

group 5 metals have been much less studied than the group 4

congeners (according to Kempe and coworkers, ‘‘Niobium and

tantalum compounds live in the shadow of metal complexes of

group 4’’3) and, within group 5 itself, the prevalent attention

has been devoted to vanadium compounds which have found

application mainly in industrial processes.4

Nevertheless, the employment of MX5 (M = Nb, Ta) in

catalytic reactions has seen a significant growth in the recent

past, as witnessed by two reviews appeared in 2004.5,6 Niobium

and tantalum pentahalides have shown in some cases unusual

and striking behaviour as catalysts, in comparison with the

reactivity exhibited by more traditional transition metal halides

(e.g. group 4 metal tetrachlorides) in the analogous reactions.7

In the conviction that increased knowledge of coordination

chemistry of MX5 may help understanding and advancing

of the related catalytic processes, we have spent the very last

years on investigating the reactivity of MX5 with limited and

controlled amounts of potential oxygen donor ligands.

In this feature article, we intend to describe the nature of the

products obtained, which include both coordination adducts

and derivatives formed as a result of activation reactions. Remark-

ably some of the activations, occurring selectively and under

mild conditions, are unprecedented in the literature. The results

will be discussed also with reference to the relevant MX5-directed

syntheses reported in the literature.

2. Synthesis of coordination compounds

2.1 Mononuclear complexes MX5L

The addition of a variety of oxygen, sulfur and nitrogen

molecules to [MX5]n results in the rupture of the polynuclear

structure and, in a number of cases, affords mononuclear,

hexacoordinated derivatives, normally soluble in chlorinated

solvents. Thus compounds of general formula MX5L have been

known since about 50 years ago, especially for X = Cl.

However exhaustive characterization of these products was

not reported, and only a few chloro-complexes were structurally

characterized by X-ray diffraction before 2007,8 examples being

MCl5(OPCl3) (M = Nb,9 Ta10), TaCl5(OSCl2),11 TaCl5(OEt2),

12

TaCl5[OC(tBu)(p-tolyl)],13 NbCl5(SPPh3),14 (NbCl5)2(m-S4tio-

ether),15 and NbCl5(NCMe).16 For what concerns the oxygen

derivatives, several adducts with ethers,17 ketones,18 oximes,19

ureas,20 PQO21 and SQO containing species22 were isolated in

the solid state, while others were identified in solution only.23

Helm and Merbach performed detailed kinetic studies on

the ligand exchange process MX5L+L*-MX5L*+ L (M=

Nb, Ta; X=F, Cl, Br; L= ether, phosphoryl, sulfide, selenide,

telluride) in chlorinated solvents. The exchange is normally very

fast, with rate constants varying between 19 (TaCl5OEt) and

960 s�1 (TaBr5OEt).24 It proceeds either via a dissociative

mechanism when L = ether or phosphoryl, or with an asso-

ciative mechanism when L = EMe2 (E = S, Se, Te); in both

cases, the substitution may be slowed by steric factors.25

It has been reported that the stability of MX5L complexes

depends on the nature of both X and L. In the case of MX5(ER2)

(X= Cl, Br; E =O, S, Se, Te), the stability increases on passing

from X= Br to X= Cl, and on increasing the atomic weight of

the chalcogen.23c,e,26 Since soft donor atoms, rather than oxygen,

confer higher stability to MX5(ER2), a relatively soft character of

the strong acids MX5 might be envisaged, in apparent contrast

with the HSAB principle.27

The reactions of MX5 with oxygen compounds were occa-

sionally reported to take place with the formation of the oxo

unit MQO,17b,18a,20,21a,d,28 and this feature was attributed to

oxygen abstraction from the organic reactant by the highly

oxophilic metal centre. The formation of the MQO bond via

oxygen abstraction must be accompanied by deoxygenation

of the organic substrate. Actually the detailed study of the

systems NbCl5/2-butanone and NbCl5/9-heptadecanone clearly

pointed out the generation of oxygen-depleted organic products,

whose nature depends on the solvent.29 Otherwise, products of

formula NbOCl3L2 were claimed to be formed by the reactions

of NbX5 (X = Cl, Br) with excess of oxygen molecules (L =

Ph3PO,28,30 Me2SO,31 ketones,18a ureas20,32), but evidence for the

contextual production of deoxygenated organics was given in few

cases only.28,31,32

In the course of our studies on the chemistry of MX5 with

limited amounts of oxygen compounds, we could isolate and

fully characterize, by spectroscopic (IR, NMR) and analytical

(elemental analysis, solution conductivity) techniques, a series

of stable MX5L (M = Nb, Ta; X = F, Cl, Br) complexes.

These were prepared by the addition of the appropriate

oxygen reactant (L) to a suspension of MX5 in a chlorinated

solvent (chloroform, dichloromethane), by using strictly con-

trolled stoichiometry (L/M = 1). In fact an excess of oxygen

reactant may induce the formation of ionic products and/or

activation processes (Sections 2.2 and 4.2). The list of MX5L

compounds by ourselves includes adducts with aryl-ethers,33

halo-ethers (e.g. MeOCH2Cl and MeOCH2CH2Br),34 cyclic

ethers (tetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetra-

hydropyran, 1,4-dioxane,35,36 1,3-dioxolane37), esters (e.g.

HCO2Me),36 aldehydes/ketones (e.g. PhHCO and Me2CO),2,36

amides/ureas,36,38 Ph3PO,38 Et2NCO2Me and N2CHCO2Et,34

and MF5(EOH) [M = Nb, Ta; E = alkyl, alkyl-C(QO)].36

In the course of our research, it has been demonstrated that

the reaction between MCl5 and simple tertiary amides could

proceed through the formation of a kinetic product,38 in which

the amide is N-coordinated to the metal centre, evolving to the

final stableO-donor containing complex. We have not observed

any CQO activation by treating the complexes MX5L [X = F

or Cl; L = OC(NEt2)2, MeC(O)NHPh, OCMe2] with excess L

at room temperature or higher,34–38 when working under rigorously

anhydrous conditions. This evidence suggests that, in contrast with

what was reported in the past by other authors (see above), the

possible formation of metal-oxo derivatives is ascribable more

properly to adventitious water. Notwithstanding, the recent find-

ings that NbCl5 is able to dehydrate, respectively, cyclohexylurea

to the relevant carbodiimide32 and benzamide to benzonitrile39

should be mentioned (see Section 7).

The solid state structures of some representative MX5L com-

pounds have been determined by X-ray diffraction and are

depicted in Scheme 1, i.e. TaCl5(thf),35 NbCl5(1,4-dioxane),

35

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 637

TaCl5(OCPh2),2 NbCl5(dmf),38 NbCl5(teu),

38 TaCl5(OPPh3)

(thf = tetrahydrofuran; dmf = N,N-dimethylformamide;

teu = N,N,N0,N0-tetraethylurea).38

Dinuclear compounds with a bifunctional bridging ligand of

general formula (MX5)2[m-k2-(O–O)] [O–O = 1,4-(OMe)2C6H4,

1,4-F2-2,5-(OMe)2C6H2, 1,3-(OMe)2C6H4, PhO(CH2)2OPh, trans-

(EtO2C)CHQCH(CO2Et)] have been obtained upon 2 : 1 molar

reactions of MX5 (X= F or Cl) with, respectively, para and meta

dialkoxybenzenes,33 1,2-diphenoxyethane33 or diethylfumarate,

see Scheme 2.34 Some of these adducts are inert towards the

addition of further oxygen substrate, i.e. the formation of mono-

nuclear MX5[k1-(O–O)] is inhibited in some cases.

DFT calculations have suggested that the gas-phase formation

of the mononuclear derivative TaCl5[k1-(1,4-dimethoxybenzene)],

from equimolar amounts of TaCl5 and 1,4-dimethoxybenzene, is

favoured by 0.5 kcal mol�1 with respect to the formation of the

dinuclear (TaCl5)2[m-k2-(1,4-dimethoxybenzene)]. This implies that

effects due to the solvent (usually CH2Cl2) might be responsible for

addressing the reaction selectively to the synthesis of dinuclear

adducts rather than mononuclear ones.33

In summary, recent results clearly indicate that the penta-

halides of the group 5 heavier metals may behave as typical

Lewis acids, i.e. they are able to coordinate a large variety of

oxygen species affording simple, mononuclear adducts. How-

ever 1 : 1 (or 1 : 2) L/M stoichiometry must be strictly respec-

ted; else the use of excess of organic reactant may result in the

formation of ionic adducts and/or in activation processes, as

will be shown later on.

The mononuclear derivatives resulting from O-donor addi-

tion to MX5 possess properties (e.g. solubility, reactivity)

significantly changed with respect to those of the precursors.

Thus a large increase of catalytic activity has been observed on

going from NbCl5 to NbCl5L (L= oxygen molecule), as far as

the ethylene polymerization reaction is concerned.40 On the

other hand, NbX5 (X= F, Cl) works much better than several

NbX5L compounds as b-pinene polymerization catalysts.41

2.2 Halide-transfer and formation of ionic species

The pentahalides of niobium and tantalum have a tendency to

add one halide ion to give the hexahalometalates [MX6]�.1

MCl5 and MBr5 show significantly less affinity for the homo-

logous halide than does MF5 for F�. In agreement with the

increase of bond strength on descending a vertical sequence

of transition elements,1 the [TaX6]� ions are thermally more

stable than the niobium analogues.42 A variety of salts con-

taining the [MX6]� anion (X = F, Cl, Br)36,43 or the dinuclear

[M2F11]� 44 are found in the literature; the [M2F11]

� unit is the

formal result of the exothermic combination of MF5 with

[MF6]�.44a

As far as oxygen species are considered, formal self-ionization

of NbF5 was observed in the 1 : 1 reaction with guanine in

acetonitrile solution, yielding the ionic salt [NbF4(guanine)2]-

[NbF6].45 Moreover the formation of [NbF6]

� and/or [Nb2F11]�

was detected by NMR in the 1 : 1 reactions of NbF5 with

monooxygen donors like N,N-dimethylformamide,46 N,N,N0,N0-

tetramethylurea and mesityl oxide.36

A crucial dependence of the product structure on the halide

has been ascertained in the 1 : 1 reactions of TaX5 withN,N,N0,N0-

tetramethylurea (tmu). The mononuclear TaX5(k-tmu) (X =

F, Cl) are formed selectively from the respective parent

compounds, while TaBr5 adds tmu affording the crystallo-

graphically-characterized complex [TaBr4(k-tmu)2][TaBr6] in

high yield, see Scheme 3.2,36

In the very recent years, we have synthesized new

compounds of general formula [MX4(O–O)2][MX6] [O–O =

1,2-dimethoxyethane (dme), MeOCH2CO2Me (mma), cis-

(MeO2C)CHQCH(CO2Me), cis-(EtO2C)CHQCH(CO2Et),

CH2(CO2Me)2],34,47 by reaction of MX5 (M = Nb or Ta;

X = F, Cl or Br) with equimolar amounts of the

appropriate bidentate ligand. Interestingly, the complexes

Scheme 1 X-ray structures of MX5L adducts.

Scheme 2 Reactions of MX5 with bifunctional oxygen molecules.

Scheme 3 Addition of N,N,N0,N0-tetramethylurea to tantalum

pentahalides.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

638 Chem. Commun., 2012, 48, 635–653 This journal is c The Royal Society of Chemistry 2012

[NbX4(dme)2][NbX6] (X = F, Cl, Br) exhibit different structures

depending on X. The X-ray structure of [NbF4(dme)2][NbF6]

shows two bidentate dme ligands binding the octa-coordinated

metal. Instead DFT calculations and low-temperature

NMR investigations have outlined that one bidentate and one

monodentate dme bind the heptacoordinated niobium in the

chloro- and bromo-species (see Fig. 1). The difference is probably

the consequence of the higher steric hindrance exerted by the

heavier halide (Cl� or Br�) with respect to the fluoride.48

The two possible outcomes (i.e. formation of neutral,

MX5L, or ionic, [MX4L2][MX6], adducts) of the 1 : 1 molar

reactions ofMX5 withO-donors (L) are the result of two distinct

modes of breaking of the polynuclear structure of MX5 by

addition of L. Thus the neutral MX5L appear to be formed by

symmetrical breaking of [MX5]n, while [MX4L2][MX6] derive

from asymmetrical breaking of [MX5]n with heterolytic cleavage

of one M–(m-X) bond per metal centre. Probably several factors

(e.g. steric and electronic properties of L, M–X bond polariza-

tion, stability of the [MX6]� ion, chelating effect, solvent) concur

in determining the nature of the coordination product, and the

overall situation is probably more complicated than what was

tentatively theorised in the past.49,50

The species MX5L and [MX4L2][MX6] can be discriminated

on the basis of electrical molar conductivity (LM) measured

in a chlorinated solvent.51 In fact, the former compounds

generally show LM in the range 0.05–0.50 S cm2 mol�1, while

the latter show LM > 1.5 S cm2 mol�1.

Compounds of formula MX5L2 (M = Nb, Ta; X = halide;

L = O-donor), containing a heptacoordinated metal centre

and obtained by addition of two molar equivalents of L to

MX5, have been rarely reported.17b,22,45 Instead it was demon-

strated that the addition of an excess of L (two equivalents or

more) to the pentafluoride MF5 could result in selective

formation of the ionic products [MF4L4][MF6], comprising

an octacoordinated metal center.22a,23c,e,49 Actually a large

number of O-species (L) form selectively mononuclear com-

plexes (MF5L) on reacting with MF5 in a 1 : 1 molar ratio (see

Section 2.1). Nevertheless the use of excess of L determines the

generation of [MF4L4][MF6] salts. This is the case, for instance,

of alcohols52 and tetrahydrofuran.35

The formation of the [MX6]� ion (X = Cl, Br) by combi-

nation of MX5 with simple O-donors (L) may be favoured on

increasing the L/M molar ratio. For instance, the neutral

mononuclear MCl5L (L = thf,35 e-caprolactam45) can be

isolated as crystalline compounds after reaction of MCl5 with

L in a 1 : 1 ratio. However the use of excess of L results in the

formation of [MX6]� salts of cations containing organic

fragments derived from ring opening of L and C–O coupling

(vide infra).35,53

19F NMR spectra (in CD2Cl2 or CDCl3) are diagnostic in

the distinction of MF5 derivatives. More in detail, octahedral

MF5L complexes normally show one broad resonance at

room temperature, which comes split into two resonances at

183–213 K. These two resonances integrate 1 : 4 and account

for fluorine nuclei, respectively, in trans and cis positions with

respect to L [e.g. in the case of NbF5(CH3COOH):36 d= 206.4

(1 F), 141.6 (4 F) ppm]. Otherwise [MF4Lx][MF6] (x = 2, 4)

complexes display at room temperature the 19F resonance charac-

teristic of the [MF6]� anion (at ca. 40 and 103 ppm for M = Ta

and Nb, respectively). [NbF6]� resonates as a decet, due to

coupling with niobium (I = 9/2); instead [TaF6]� appears as a

singlet even at low temperature, probably due to the fast

quadrupole relaxation of tantalum (I = 7/2).23g The fluorine

atoms in the cations [MF4(O–O)2]+ give sharp singlets [e.g. at

182.0 ppm for NbF4(k2-dme)2

+ 47] whose shape does not vary

significantly with temperature; conversely [MF4L4]+ cations

(L = monodonor ligand) might be non-detectable by 19F

NMR, probably for effects ascribable to short relaxation times

or fast fluorine exchange.23g,36,49 The [M2F11]� ions exhibit

typical low temperature 19F NMR patterns, each consisting of

three resonances distributed on a large range of chemical shifts

(at ca. 190, 150, �50 ppm for M= Nb).

3. Stabilization of uncommon cations

Traditional halo anions based on group 15 elements, e.g.

[AsF6]�, [SbF6]

�, and [Sb2F11]�, have been successfully asso-

ciated with reactive organic or organometallic cations for the

isolation of the respective salts. The list of cations ‘‘stabilized’’

by this approach includes metal carbonyls,54 dinitrogen fluoride,55

oxonium and sulfonium species,56 gold(I)dinitrile,57 ferrocenyl-

methanol,58 arene radicals,59 and protonated arenes (arenium

cations).60 The anion [Sb2F11]� has been recently partnered with

the hexafluorobenzene radical cation,61 and the first example of

the coordination compound containing XeF2 as ligand, namely

[Ba(XeF2)5]2+, has been isolated with [MF6]

� (M=As, Sb, Nb)

counterions.62

Although perfluorinated anions based on group 15 elements

may fulfil some of the characteristics typical of the so-called

non-coordinating anions (presence of very weakly basic sites in

the periphery; high degree of charge delocalization; high

thermodynamic stability),63 they may give fluorine transfer

reactions at room temperature or above.59

Niobium(V) and tantalum(V) halo anions (or mixed alkoxo-

halo anions), whose structures resemble those of the group

15 analogues, have revealed to be effective for the stabilization

of reactive cationic species. Thus hexahalometalates [MX6]�

have permitted the solid-state isolation of protonated

ketones,2 protonated [MeO(CH2)2]2O (diglyme),64 and proto-

nated thioethers.44b

Although protonation of diglyme and thioethers probably

occurs via adventitious hydrolysis,44b,64 the formation of proto-

nated ketones is a side-reaction resulting from ketone C–H

activation, see Scheme 4 and Section 2.1.

Fig. 1 (A) View of the [NbF4(dme)2]+ cation in [NbF4(dme)2][NbF6]

(X-ray structure); (B) View of the [NbCl4(dme)2]+ cation in

[NbCl4(dme)2][NbCl6] (DFT-calculated structure for the gas phase).

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 639

The reaction of CMe2(OMe)2 with NbCl5 proceeds with C–O

activation (Section 4.2.3) and affords selectively the stable

methylated-mesityl oxide species [Me2CQCHC(QOMe)Me]

[NbCl5(OMe)], see Scheme 5.37

Very few protonated ketones have been isolated in the

solid state up to now, the unique additional example being

the salt [(C12H8D2O)2H][SbCl6].65 Moreover, O-alkylated

ketones (or aldehydes) are rare in the literature, and they

usually act as reactive intermediates in organic syntheses;66 the

[Me2CQCHC(QOMe)Me]+ cation reported by ourselves

represents the first example of a structurally-characterised,

non-coordinated alkylated ketone.

It should be mentioned that stable [TaCl6]� salts containing

indanyl- and anthryl cations have been prepared by reactions

of TaCl5 with poly-alkylated arenes (see Scheme 6).67

A striking result in the field of stabilization of reactive cations

by M(V)-based anions has been recently reported: deeply-coloured

solutions of [Arene][Nb2F11] (Arene = benzene, 1,4-difluoro-2,5-

dimethoxybenzene, 1,4-dimethoxybenzene, 2,5-diethoxytoluene)

have been obtained44a by treating the appropriate monocyclic

aromatic compound with NbF5, according to eqn (1).68

3NbF5 + Arene - [Arene][Nb2F11] + NbF4 (1)

The reactions involving O-species presumably proceed with

intermediate formation of dinuclear coordination adducts,

(MX5)2[m-k2-(O–O)] (see Section 2.1). The unprecedented

association of the [Arene]�+ cation with the [Nb2F11]� anion

provides outstanding inertness to the former, so that its complete

degradation in chloroform solution at room-temperature requires

several days. It must be remarked that the clear detection of the

benzene radical cation as a non-transient species in solution at

room temperature was unprecedented. The solid state structure of

[1,4-F2-2,5-(OMe)2C6H2][Nb2F11] is reported in Fig. 2.

Spectroscopic analyses on compounds [Arene][Nb2F11]

(Arene = monocyclic arene) have outlined the presence of

cation–anion interactions in solution, giving rise to the sur-

prisingly long life of the radical species. In particular, the EPR

spectra show coupling of the unpaired electron in the cation

with some terminal fluorides in the [Nb2F11]� anion and, in the

case of benzene, the coupling is extended to the niobium

nuclei. According to computational results, low steric hindrance

at the cation favours interaction with the anion, see Table 1,

enhancing the kinetic inertness of the salt. The simple benzene

derivative [C6H6][Nb2F11] is obtained at 70 1C and resists in

chloroform solution at room temperature for much longer periods,

with respect to the substituted [1,3-(OMe)2C6H4][Nb2F11] and

[2,5-(OEt)2C6H3(Me)][Nb2F11]. On the other hand, the benzene

radical species forms in significantly lower yields compared to

the alkoxy-substituted analogues: this fact is driven by thermo-

dynamic features and is reasonably due to the absence of stabiliz-

ing electron-donor substituents on the [C6H6]+ ring.68

The reactions of 1,3-dimethoxybenzene with MF5, in the

chlorinated solvent, quickly afford the coordination compounds

(MF5)2[k2-1,3-(OMe)2C6H4] (M = Nb,44a Ta,33) which slowly

convert into [2,4-(OMe)2C6H5][M2F11], see Scheme 7.

Scheme 4 Synthesis of the hexachlorotantalate salt of protonated

acetophenone.

Scheme 5 Synthesis of the carboxonium salt [Me2CQCHC(QOMe)Me]

[NbCl5(OMe)].

Scheme 6 Synthesis of stable indanyl- and anthryl [TaCl6]� salts.

Fig. 2 View of the molecular structure of [1,4-F2-2,5-(OMe)2C6H2]-

[Nb2F11].

Table 1 Calculated Nb-centroid distance in [Arene][Nb2F11]compounds

Arene

Nb–centroid distance/A 4.379 4.606 4.573 4.698 4.879

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

640 Chem. Commun., 2012, 48, 635–653 This journal is c The Royal Society of Chemistry 2012

The arenium salts [2,4-(OMe)2C6H5][M2F11] (M = Nb, Ta)

have been isolated in the solid state at room temperature

and have shown to resist thermal treatment in CHCl3.

Contrastingly, dialkoxybenzenium ions, as obtained with

conventional superacidic systems (e.g. SbF5/HF), could be

detected previously at low temperature only.69 The formation

of [2,4-(OMe)2C6H5][M2F11] may proceed via the redox

mechanism proposed in eqn (1) for M = Nb.

The synthesis of long-lived arene radical salts (and stable

dimethoxybenzenium species) is based on two important

features: (i) the [M2F11]� counterions are highly innocent;

(ii) niobium and tantalum have two close and stable oxidation

states (+5 and +4), allowing MF5 to act both as mono-

electron oxidants towards the arene and precursors of the

counterion. The oxidizing behaviour of MF5 is not common,

for instance the published procedures for the synthesis of

NbF4 from NbF5 require drastic conditions.70 Moreover it

was reported that NbF5 was not reduced by pyridine,71 while

NbX5 (X = Cl, Br, I) underwent reduction72 with the formation

of NbX4(py)2 and halide transfer products (Scheme 8).72a,73

In other terms, reaction (1) is the result of the combination

of a bad oxidant (NbF5) with a bad reductant (arene). The

driving force for such an unexpected redox process probably

resides in the favourable generation of the anion74 and in the

cation/anion interactions in the products (see above).

4. Activation reactions

4.1. Activation of O–H bonds

The chemistry exhibited by alcohols, ROH, or carboxylic

acids, RCOOH, with MX5 (M = Nb, Ta; X = F, Cl, Br, I)

is significative of the key importance of the M–X bond energy

in determining the nature of the products. As stated above,

MF5(ROH) or [MF4(ROH)4][MF6] complexes [M = Nb, Ta;

R = Me, Et] are obtained selectively by treatment of MF5

with, respectively, one or two equivalents of alcohols

(see Sections 2.1 and 2.2).36 These reactions preserve altogether

the highly energetic M–F bonds, thus preventing the activation

of O–H bonds. On the other hand, it has been known since many

years that the heavier pentahalides MX5 (M=Nb, Ta; X = Cl,

Br, I) react with alcohols/phenols1,34,75 or carboxylic acids,76,77 in

variable molar ratios, giving alcoholato/carboxylato derivatives

via O–H bond cleavage and HX release. The latter derivatives

generally maintain the dinuclear structure typical of the

starting materials, unless additional donor atom is present in

the organic moiety. Two pertinent molecular structures are

shown in Fig. 3.

A series of mixed chloro-alcoholato complexes NbCln(OR)5�n(n = 2–4) have been recently individuated as convenient

Nb-based precursors for the ethylene polymerization reaction,

in association with Al-cocatalysts.40a

Niobium and tantalum pentachlorides have found applica-

tion in organic syntheses which make use of alcohols. Thus

NbCl5 is able to promote room-temperature chlorination of

tetrahydrofurfuryl alcohol, cyclohexylmethanol, cyclohexanol

and similar compounds, Scheme 9.78 Also b-hydroxy-a,b-unsaturated ketones can be transformed into the corresponding

b-chloro-a,b-unsaturated ketones by means of NbCl5.79

Furthermore, efficient C-, N- and S-centred nucleophilic

substitution reactions on diaryl carbinols and aryl–alkyl carbi-

nols catalyzed by NbCl5 have been developed.80 These reactions

are fast, high yielding and proceed at room temperature.

Niobium and tantalum pentachlorides as such or supported

on silica have been used for the tetrahydropyranylation or acetyl-

ation of alcohols and phenols.81 The advantages of employing

these metal systems over previously reported methods are

numerous: lower reaction temperatures, shorter reaction

times, higher substrate/catalyst ratios, higher yields, easier

work-up conditions and better tolerance of functional groups

present in the substrates.82

Scheme 7 Synthesis of thermally stable 1,3-dimethoxybenzenium salts.

Scheme 8 Redox reactions involving NbX5.

Fig. 3 X-ray structures of [NbCl4(m-O2CCl3)]2 and NbCl4[k2-OCH2-

CH2OMe].

Scheme 9 Chlorination of alcohols promoted by NbCl5.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 641

Suspensions of NbCl5/Al2O3 in carboxylic acid/alcohol

mixtures afford the corresponding carboxylic esters very

efficiently under microwave irradiation.83 This catalytic system

is almost as valid as the powerful microwave-driven catalyst

ZnCl2/SiO2 reported previously,84 and even more efficient if

the alcohol partner bears an aromatic ring.

Fleming and coworkers developed an innovative, direct

method for the synthesis of allylic and propargylic halides,

making use of stoichiometric quantities of NbX5 (X= Cl, Br).

Allylic halides are generally prepared by halogenation of

allylic alcohols; in turn these latter are obtained by olefination–

reduction sequences starting from aldehydes or ketones,85

because the direct halo-olefination of carbonyls suffers facile

halide elimination.

Fleming’s procedure consists of ‘‘halo-olefination’’ of aromatic

and aliphatic aldehydes by sequential addition of vinyl-

magnesium bromide and NbX5 (X = Cl, Br). The reaction

proceeds with intermediate formation of a coordinated allyl

alkoxide, which rearranges to give the uncoordinated allyl

halide through oxygen abstraction by the metal centre, see

Scheme 10.86 Propargylic alcohols react similarly with NbBr5to afford allenyl bromides. The organic products are efficiently

isolated by simple extraction and can be used in subsequent

reactions without further purification.

As far as carboxylic acids are concerned, it should be

mentioned that TaCl5 promotes the efficient synthesis of

amides from RCOOH and amines, via the probable formation

of chloro-carboxylato intermediates.87,88 This procedure has

been successfully applied to the reaction of secondary amines

with encumbered carboxylic acids, and for the preparation of

N–Me peptides, see Scheme 11.88

4.2. Activation of Csp3–E bonds (E = O, Si).

4.2.1. Monoethers and silyl-ethers. The coordination com-

pounds of group 5 metal pentahalides with monofunctional alkyl

ethers (see Section 2.1) are usually stable at room temperature.

However, thermal treatment can induce C–O cleavage in corres-

pondence of the heaviest halides. Thus, compounds of general

formula MX5(ORR0) [M = Nb, Ta; X = Cl, Br; ORR0 =

OMe2, OEt2, OMeCH2Cl, OMeCH2CH2Cl, O(CH2CH2Cl)2,

1,4-dioxane] convert into MOX3 and alkyl halides upon heating

at 90–100 1C, eqn (2).34,89

MX5(ORR0) - MOX3 + RX + R0X (2)

Alkyl–aryl ether adducts may undergo both Csp3–O and Csp2–O

rupture at high temperature, as evidenced by the detection of

C6H5OH andMeCl in the mixture obtained by thermal treatment

of NbCl5[O(Me)C6H5], followed by hydrolysis.33

The reactions of MCl5 (M = Nb, Ta) with silyl ethers

proceed readily with Si–O cleavage at room temperature, and

represent a clean and selective way for the preparation of

chloro-alcoholates (Scheme 12).34,90

We have recently found that mixed fluoro-alcoholato com-

plexes, MF5�n(OR)n (M = Nb, Ta; n = 1–3; R = Me, Et,

Ph), can be efficiently prepared by allowing MF5 to react with

trimethylsilyl-ethers;91 reasonably the formation of strong

Si–F bonds constitutes the driving force for the activation of

the strong M–F bonds.

The cleavage of the ethereal C–O bond is a key-step in

several organic syntheses, which takes advantage of the use of

oxophilic metal-based catalysts.92 Therefore, MCl5 have found

application in this field, and particularly in the acylative

cleavage of ethers, i.e. the synthesis of esters by combination

of ethers with acyl chlorides.93 It was proposed that the

intermediate metal alkoxo-chloride (RO-MCl4), formed at

high temperature from the starting halide and the ether,

reacted with the acyl chloride to afford the ester product and

to regenerate the catalyst, Scheme 13.

4.2.2. Cyclic ethers. In general, the activation of cyclic

ethers by MX5 comes favourable on decreasing the ring size

Scheme 10 NbX5-mediated synthesis of allylic and propargylic

halides. Scheme 11 Synthesis of stereohindered amides and peptides

mediated by TaCl5.

Scheme 12 Preparation of chloro-alcoholates from MCl5 and

trimethylsilyl ethers.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

642 Chem. Commun., 2012, 48, 635–653 This journal is c The Royal Society of Chemistry 2012

and the degree of substitution. Therefore, the five-membered

2,5-dimethyl-tetrahydrofuran and the six-membered tetra-

hydropyran and 1,4-dioxane add toMX5 giving room temperature

stable adducts MX5L.35 Conversely, five-membered unsubsti-

tuted strained cycles such as 1,3-dioxolane and thf undergo

ring opening. This may be promoted either by excess of ligand

(thf, see below)35 or by thermal treatment (1,3-dioxolane, see

Scheme 14).37

MX5 have revealed to be efficient promoters of the ring-opening

polymerization of tetrahydrofuran (thf). The activities increase

along the series TaBr5 o NbCl5 o TaCl5 o TaF5; TaF5 supplies

high activity and produces relatively high molar mass poly-tetra-

hydrofuran (67100 g/mol).53 Mononuclear complexes MX5(thf)

can be obtained as stable compounds by allowing MX5 to react

with thf in 1 : 1 ratio (see Section 2.1); otherwise, when thf is used in

50% excess, the prevalent product is the ionic species [MX4(thf)-

{O(CH2)4 O(CH2)3CH2}][MX6] (M=Nb, Ta; X=F, Cl, Br).35,53

Both MX5(thf) and [MX4(thf){O(CH2)4 O(CH2)3C H2}][MX6]

were proved to act as early-stage intermediates in the course of

the polymerization reaction of tetrahydrofuran promoted by

MX5, see Scheme 15.53,94

The structure of [MX4(thf){O(CH2)4 O(CH2)3C H2}][MX6]

(see Fig. 4 for M = Ta and X = Cl) includes the zwitterionic

4-(tetrahydrofuran-1-ium)-butan-1-oxo ligand, and represents

a noticeable corroboration of the generally accepted mechanism

for the cationic polymerization of cyclic ethers, i.e. the propagating

species is a tertiary oxonium ion undergoing nucleophilic

attack by the monomer at the a-carbon.95

Epoxides are highly reactive three-membered cyclic ethers

which have been employed as feasible materials for organic

syntheses, and recent examples refer to the preparations of

natural products,96 lactones,97 and amino-alcohols. All these

compounds are in turn versatile intermediates for the synthesis

of biologically and pharmaceutically-active substances.98 The

former reactions proceed preferentially via epoxide ring-opening

and, in a number of cases, they require the mediation of metal

species.99

As an example, NbCl5100 or silica-supported TaCl5 promotes

the formation of b-amino alcohols from epoxides and aromatic

amines, see Scheme 16.101 The reactions are complete in 1 h;

the ring opening is influenced by the nature of the epoxide

substituents and takes place with good-to-high regioselectivity.

Furthermore, carboxylation of epoxides to cyclic organic

carbonates has been realized by using either NbCl5 or Nb2O5

as a catalyst (Scheme 17),102 with the former showing lower

activity than the latter.

Constantino and coworkers reported detailed studies on the

ring opening of epoxides by NbCl5, showing the better

performance provided by this halide with respect to the more

commonly used BF3.103 The reactions, performed under

variable conditions, followed different pathways according to

the structure of the epoxide and the reaction medium; in most

Scheme 13 Acylative cleavage of ethers catalyzed by MCl5 (M =

Nb, Ta).

Scheme 14 Thermal activation of 1,3-dioxolane by NbCl5.

Scheme 15 Possible mechanism for the formation of [MX4(thf)-

{O(CH2)4 O(CH2)3C H2}][MX6] from MX5 and thf.

Fig. 4 X-ray structure of [TaCl4(thf){O(CH2)4 O(CH2)3C H2}][TaCl6].

Scheme 16 MCl5-catalyzed synthesis of b-amino alcohols.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 643

cases, more than one product was observed. A tentativemechanism

was proposed for the ring opening of a-pinene epoxide. Accordingto low-temperature investigations, the first step was supposed to

be the coordination of the Lewis acid to the oxygen atom, see

Scheme 18.103

An easy access to the acetate MeC(O)OCH2C(CF3)2OH

has been gained by the TaF5-promoted reaction of 2,2-

bis(trifluoromethyl)oxirane with acetic acid, see Scheme 19.104

Our contribution to the understanding of the direct inter-

action of niobium and tantalum pentahalides with epoxides

has appeared in a recent paper.105 The reactions of MX5 (X =

Cl, Br) with epoxides in variable molar ratios proceed rapidly,

exothermically and regioselectively with epoxide ring opening

and insertion into M–X bonds. In particular, the 1 : 3 reactions

afford selectively stable dinuclear species containing both halide

and 2-haloalcoholato ligands, see Scheme 20.

A NMR study of the product obtained by 3 : 1 reaction of

1,2-epoxybutane with NbCl5 (Scheme 21) has pointed out the

presence of the 2-chloroalkoxide ligands –OCH(Et)CH2Cl,

instead of –OCH2CH(Et)(Cl). This evidence suggests that the

multiple insertion of 1,2-epoxybutane into the metal–chloride

bonds of MCl5 (M = Nb, Ta) occurs in a regioselective mode,

with one halide attacking the less hindered carbon atom.105

Respecting the general rule that M–F bonds are preserved in

the reactions of MF5 with organic compounds, NbF5 interacts

with one equivalent of 2,3-dimethyl-2,3-epoxybutane giving

the mononuclear NbF5[OQC(Me)(But)] in high yield,105

Scheme 22, as a result of Lewis acid-promoted epoxide-to-

ketone isomerization (Meinwald rearrangement).106

4.2.3. Polyethers. The activation of ethereal C–O bonds by

MX5 becomes favourable when the organic substrate presents

more than one ether function. Ionic complexes [MX4(O–O)2][MX6]

(X = Cl, Br; O–O = 1,2-dialkoxyalkane) can be generated

easily by the addition of 1,2-dialkoxyalkanes (including dme,

see Section 2.2) to MX5. These complexes may be detected at

low temperature only, due to facile C–O bond cleavage

occurring at room temperature. Hence the room temperature

reactions of MX5 (M = Nb, Ta; X = Cl, Br, I) with a variety

of 1,2-dialkoxyalkanes in the 1 : 1 molar ratio yield equimolar

amounts of stable alkoxy-derivatives and alkyl halides, via

intermediate formation of [MX4(O–O)2][MX6].35,48,107 Alter-

natively, when MCl5 (M = Nb, Ta) are allowed to react at

room temperature with a two-fold excess of 1,2-dimethoxy-

ethane (dme), MOCl3(dme), CH3Cl and 1,4-dioxane (eqn (3):

C4H10O2 = dme; C4H8O2 = 1,4-dioxane) are produced

selectively via unusual multiple C–O activation of the organic

substrate.64,108 The formation of dioxane is the result of the

establishment of new C–O bonds and, therefore, implies

reorganization of some units derived from the fragmentation

(see Scheme 23). This outcome contrasts with the general trend

observed in the activation reactions of diethers by means of

oxophilic metal compounds, because such reactions are exclusively

Scheme 17 Carbonylation of epoxides catalyzed by niobium(V)

compounds.

Scheme 18 NbCl5-promoted reactions of a-pinene epoxide.

Scheme 19 TaF5-mediated reaction of 2,2-bis(trifluoromethyl)oxirane

with acetic acid.

Scheme 20 Reactions of MX5 (M = Nb or Ta; X = Cl or Br) with

epoxides.

Scheme 21 Possible insertion modes of epoxides into the Nb–Cl

bond.

Scheme 22 Epoxide to ketone isomerization promoted by NbF5.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

644 Chem. Commun., 2012, 48, 635–653 This journal is c The Royal Society of Chemistry 2012

fragmentations,109,110 i.e. they do not proceed with the formation

of new C–O bonds by coupling of the fragments produced.

MCl5 + 2C4H10O2 - MOCl3(C4H10O2) + 2CH3Cl

+ 12C4H8O2 (3)

On account of the fact that MOCl3(dme) can be easily

reduced to MCl3(dme) by oxygen abstraction,64 the reaction

of MCl5 (M = Nb, Ta) with excess dme may be considered as

a convenient entry into the chemistry of M(III). Indeed dme

was used as a solvent for a number of organic syntheses,

employing MCl5 as precatalyst in the presence of a reducing

agent, which were claimed to proceed by the mediation of

M(III) species5,111 (an example is given in Scheme 24111a,b).

Our attempts to generalize the outstanding 1 : 2 reaction of

MCl5 with dme (eqn (3)) led us to the conclusion that the

formation of dioxane was regulated by kinetic factors, being

inhibited by the use of either heavy MX5 (M = Nb, Ta; X =

Br, I) or 1,2-dialkoxyalkanes larger than dme. In these cases,

monoalkoxo-tetrahalo compounds were obtained prevalently,

with nearly one equivalent of organic material remaining

unreacted. For instance, the generation of NbOBr3(dme)/

1,4-dioxane has not been observed from NbBr5/excess dme,

although computer calculations suggest that this reaction is

highly exoergonic.48

The ionic derivatives [MF4(k2-ROCH2CH2OR)2][MF6],

prepared by addition of 1,2-dialkoxyalkanes (R = Me, Et)

to MF5 (see Section 2.2), undergo activation of the organic

part at high temperature. The activation is driven by the

strength of the M–F bond, preserving the [MF5] frame, and

gives selectively OR2 (R = Me, Et) and 1,4-dioxane, see

Scheme 25.47 It should be remarked that this transformation takes

place efficiently even by employing catalytic amounts of MF5.

Theoretical calculations for the gas phase have confirmed that

the formation of Me2O and dioxane is the most favoured reaction

between NbF5 and dme (DGor = �15.16 kcal mol�1Nb), while the

formation of MeCl and dioxane is the most favoured reaction in

the case of NbCl5/dme (DGor = �53.13 kcal mol�1Nb).

47

The reactions of MX5 (M = Nb, Ta; X = F, Cl, Br) with

equimolar amounts of acetals/ketals (1,1-dialkoxyalkanes) or

trimethylformate proceed under mild conditions according to

various pathways which all include C–O bond cleavage. Quite

unusually for the chemistry of 1,1-dialkoxyalkanes with acidic

species,92c,112 C–O bond activation may be accompanied by

less common C–H and C–C cleavage (see also Scheme 14); in

addition, C–O and C–C couplings may follow.

In accordance with the general trend, the reactions are

driven by the nature of the halide, X. Thus, the relatively high

polarity of the M–F bonds generally confers major reactivity

to MF5 with respect to MX5 (X = Cl, Br). Furthermore, the

strength of the M–F bonds disfavours the formation of alkyl

fluorides, whereas alkyl halides are usually generated in the

course of the fragmentation processes involving niobium or

tantalum chlorides or bromides [see Scheme 26 for NbX5 +

CH(OMe)3].

In general, we may state that ethereal bonds can be broken

byMX5 and the rate of the process is enhanced by the presence

of further vicinal ether function. This feature was exploited for

attaining the room-temperature NbCl5-mediated cleavage of

methoxymethyl ethers (acetals) or esters to the corresponding

alcohols/carboxylic acids, Scheme 27a.113 The use of NbCl5allows milder reaction conditions and higher yields with

respect to the commonly employed deprotecting agents, such

as HCl, TiCl4, ZrCl4 and BMe2Br.113

A simple and facile method for the dealkylation of alkyl–

arylethers mediated by NbCl5 was also reported, Scheme 27b.114

The Sakurai–Hosomi reaction is another example of NbCl5-

directed synthesis involving acetals. It proceeds in the presence

Scheme 23 C–O bonds cleavage/formation in the reaction between

MCl5 and dme.

Scheme 24 Low-valent NbCl5-mediated activation of aryl trifluoro-

methyl groups.

Scheme 25 The reactions of NbX5 (X = F, Cl) with 1,2-

dialkoxyalkanes.

Scheme 26 Different activation routes in the reactions of NbX5 (X=

F, Cl, Br) with trimethylformate.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 645

of a silver salt and consists of the coupling of acetal/allyltri-

methylsilane to give homoallylic ethers, Scheme 28.115

5. Activation of Csp2–O bonds

The stability of the ionic adducts [NbCl4(O–O)2][NbCl6]

[O–O=MeOCH2CO2Me (mma), cis-(MeO2C)CHQCH(CO2Me),

cis-(EtO2C)CHQCH(CO2Et), CH2(CO2Me)2], obtained by

combination of NbCl5 with ester-containing molecules

(see Section 2.2), has been investigated. All these compounds

may be effectively converted into more stable neutral m-oxospecies, via selective fragmentation (consisting of C–O bonds

cleavage) of half organic material (Scheme 29).34

A comparative view of the degradation pathways originating

the oxo-unit from different bidentate oxygen donors is given in

Scheme 30. In the case of MeOCH2CO2Me, the oxo-unit derives

from the room-temperature cleavage of two Csp3–O bonds per

fragmented molecule. Otherwise the formation of the oxo-unit

from cis-(MeO2C)CHQCH(CO2Me) [or CH2(CO2Me)2] must

involve the cleavage of at least one Csp2–O bond, thus requiring

high temperature conditions.

By contrast with the Cl-analogues, the fluorine compounds

[NbF4(O–O)2][NbF6] [O–O = cis-(MeO2C)CHQCH(CO2Me),

CH2(CO2Me)2] survive thermal treatment in chloroform

solution. Further demonstration of the influence of the M–F

bond strength on the chemistry of MX5 (M = Nb, Ta) is

carried out by the reactions of NbX5 (X = F, Cl) with

ethyldiazoacetate. A simple coordination adduct was obtained

from NbF5, while the reaction with NbCl5 yielded a complicated

mixture of organic products, which could be identified after

hydrolysis of the reaction mixture (see Scheme 31).34

6. Reactions involving aldehydes and ketones

6.1. Synthesis of metal acetylacetonates

2,4-Pentanediones, MeC(O)CH(R)C(O)Me (R = H, Me),

react with TaX5 (X = Cl, Br) in a 1 : 1 molar ratio to afford

hexacoordinated derivatives (see Scheme 32),2 coherently with

the reactivity normally observed for acidic metal halides with

acetylacetone, MeC(O)CH2C(O)Me.116

More than one equivalent of 2,4-pentanedione can react per mole

of TaX5: for instance the ionic [TaF{OC(Me)C(Me)C(Me)O}3]

[TaF6] (Fig. 5) has been obtained by treatment of TaF5 with

a three-fold molar excess of MeC(O)CH(Me)C(O)Me. In this

case, the exceptional stability provided by the newly formed

six-membered rings overcomes the strength of the highly-

energetic Ta–F bond.2

6.2. C–H bond activation

The direct reaction ofMCl5 (M=Nb, Ta) with two equivalents

of a suitable ketone proceeds with activation of the C–H bond

in the a-position with respect to the carbonyl function. For

instance the initial formation of the complex TaCl5(acetone)

(see Section 2.1) is followed by attack of a second molecule of

acetone at room temperature. This determines HCl release and

Scheme 27 Dealkylation of ethers mediated by NbCl5.

Scheme 28 Sakurai–Hosomi reaction.

Scheme 29 NbCl5-mediated activation of ester compounds.

Scheme 30 Comparative view of the fragmentations of bifunctional

oxygen donors in the presence of NbCl5.

Scheme 31 Reactions of NbX5 with diethyldiazoacetate.

Scheme 32 Formation of b-diketonato derivatives of tantalum(V).

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

646 Chem. Commun., 2012, 48, 635–653 This journal is c The Royal Society of Chemistry 2012

intermolecular C–C bond coupling, to afford an aldolate

derivative (Scheme 33).2

The general mechanism of formation of b-ketoalcoholsfrom the corresponding ketones catalyzed by Lewis acids is

probably operative in this reaction.117 A condensation similar

to that reported in Scheme 33 has been observed by Bazan and

coworkers in the reaction of TaCl3[C4H4B–N(CHMe2)2] with

two equivalents of acetone;118 the possible formation of an

intermediate coordination adduct containing acetone could

not be demonstrated.

It should be noted that the aldol condensation reactions

cited above are mediated by rather complex systems (e.g.

titanium and zirconium enolates117a,b and sterically crowded

aluminium aryloxides117c or tantalum borollide118), thus the

result obtained with MCl5 (M = Nb, Ta) provides consider-

able improvement due to the easy availability of

pentachlorides.

The capability of MCl5 to promote carbon–carbon coupling

via activation of the C–H bond adjacent to carbonylic function

has been exploited for synthetic purposes.119,120 A pertinent

example of fast, room temperature reaction is illustrated in

Scheme 34.

The same approach has been applied to Mannich-type

reactions for the synthesis of b-amino carbonyl compounds,121

see Scheme 35.

Further MX5-directed syntheses employing ketones/

aldehydes and involving the possible initial carbonyl-to-metal

coordination have been worked out. The most significant

examples will be presented in the following.

6.3. Stereoselective synthesis of 1,2-ethandiols

A highly stereoselective C–C bond forming self-dimerization

of aryl aldehydes to give D,L-1,2-diaryl-1,2-ethanediol is

mediated by MCln/NBu4I (MCln = TiCl4, NbCl5), see

Scheme 36.122 A mechanistic hypothesis has been proposed

for the titanium system which considers the monoelectron

reduction of the aldehyde, followed by dimerization of the

resulting ketyl radical. In view of the viable Nb(V) - Nb(IV)

reduction,68 a similar mechanism could be invoked when

NbCl5 is used as a promoter.

6.4. Ferrier reaction123

Per-O-acetylated glycals react with primary and secondary

alcohols, in thf in the presence of catalytic amounts of NbCl5,

with exclusive formation of the a-anomers of 2,3-unsaturated

glycosides (Scheme 37).124,125 A four-step mechanism has been

suggested: (i) coordination of NbCl5 to the carbonyl oxygen of

the C5-bound side chain acetyl group; (ii) transfer of a

chloride from niobium to C1 via an eight-membered transition

state; (iii) release of ‘‘NbCl4(O2CMe)’’ (see Section 4.1); (iv)

addition of alcohol to the resulting b-anomer, giving the final

a-anomer. However, on account of knowledge accomplished

about the behaviour of NbCl5 in the presence of excess thf

(see Section 4.2.2), ionic metal species may be involved in the

process.

6.5. Synthesis of a-aminophosphonates

A procedure for the synthesis of a-aminophosphonates based

on the MCl5-mediated, one pot, three-component reaction of a

carbonyl compound, amine and diethylphosphite, has been

Fig. 5 X-ray structure of the heptacoordinated cation in

[TaF{OC(Me)C(Me)C(Me)O}3][TaF6].

Scheme 33 Synthesis of tantalum(V) chloro-aldolate complex.

Scheme 34 C–C coupling reaction involving carbonylic compounds

promoted by NbCl5.

Scheme 35 Synthesis of b-amino carbonyl compounds.

Scheme 36 Stereoselective synthesis of 1,2-ethanediols from

aldehydes.

Scheme 37 Proposed mechanism for the formation of a-anomers in

the NbCl5-promoted reaction of acetylated glycals with alcohols.124,125

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 647

reported, Scheme 38. The synthesis was initially performed in

the presence of TaCl5 supported on silica,126 but very recently

it has been observed that the use of NbCl5, at ca. 50 1C in the

absence of a solvent, is very effective in terms of yield and

reaction rate.127

6.6. Homologation reactions

Homologation reactions on a-trialkylstannylmethyl-b-keto-estersin the presence of NbCl5 have been reported.128 The authors

proposed the intermediate formation of a compound resulting

from coordination of the two carbonyl groups to NbCl5, followed

by SnMe3Cl elimination and formation of a niobium-stabilized

cyclopropanolato fragment (see Scheme 39).

6.7. Cyclization reactions

Lacerda Junior and coworkers129 found that usually poor

dienophiles (e.g. 2-cycloenones) and cyclopentadiene underwent

Diels-Alder reaction at �78 1C with outstanding stereoselectivity

in the presence of NbCl5 (Scheme 40). In comparison with other

Lewis acids (AlCl3, SnCl4), niobium pentachloride is preferable

not only in terms of stereoselectivity, but also yield, reaction time

and temperature.

The NbCl5-catalyzed Diels-Alder reaction has served in the

synthesis of eremophilanes and bakkanes, two biologically

active compounds (Scheme 41). The coordination adduct A

has been proposed as a transition state to account for the high

regio- and stereoselective character of the reaction.130

Catalytic amounts of NbCl5 (TaCl5 is less active) promote

the cyclization of citronellal to a mixture of isopulegol and

neoisopulegol, Scheme 42, under mild conditions.131

6.8. Synthesis of alcohols

Protected acetylated homoallylic alcohols have been prepared

via TaCl5-mediated Sakurai reaction and subsequent in situ

acetylation with acetic anhydride (Scheme 43).132

6.9. Synthesis of heterocycles

Much effort in organic chemistry has been devoted to the

development of new methods for the preparation of hetero-

cyclic rings. The use of MX5 derivatives as promoters in this

kind of reactions has seen a progress in the last decade.

A selection of products, which in turn are precursors of

natural molecules possessing biological or medicinal activity,

prepared by mediation of MX5 under variable experimental

conditions, is shown in Scheme 44.133

Tantalum pentahalides supported on silica gel have been used

for the conversion of carbonyl compounds into the corresponding

1,3-oxathiolanes,134 for the cyclization of 20-aminochalcones

to the corresponding dihydroquinoline derivatives135 and for

the condensation of olefins with aldehydes (Prins reaction),136

see Scheme 45. The use of silica-supported TaX5 is justified by

Scheme 38 One-pot synthesis of a-aminophosphonates.

Scheme 39 NbCl5-mediated homologation reaction.

Scheme 40 Diels-Alder NbCl5-mediated reactions of 2-cycloenones.

Scheme 41 Diels-Alder reactions in the NbCl5-mediated synthesis of

biologically-active compounds.

Scheme 42 Cyclization of citronellal.

Scheme 43 One pot synthesis of protected acetylated homoallyl

alcohols.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

648 Chem. Commun., 2012, 48, 635–653 This journal is c The Royal Society of Chemistry 2012

the increase of the tantalum oxophilicity on supportation,

resulting in fast reactions, absence of by-products and easy

work-up.134

The synthesis of chlorotetrahydropyrans137 and the sulfur

transfer in a,b-unsaturated N-acylimides138 have been performed

in high yields by using NbCl5 as a promoter, Scheme 46. In the

case of the synthesis of chlorotetrahydropyrans, NbCl5 is a

convenient choice. On the other hand most of the classical

methods demand long reaction times and the use of expensive

reagents, and generate a mixture of products. A mechanism is

proposed which considers the formation of a hemi-acetal (see

Section 4.2.3), O-coordinated to NbCl5, which undergoes

facile37 O-abstraction and Prins-type cyclization. As far as

the sulfur transfer reaction is concerned, the authors stated

that ‘‘. . .an additional improvement of the methodology comes

from the fact that the reaction course is readily followed by the

colour change, from red to yellow. . .’’.138

Finally, NbCl5-catalyzed, solvent-free, one pot synthesis of

substituted coumarins has been recently proposed, see

Scheme 47. Low catalyst loading and short reaction times

are the most convenient features of the process.139

7. CQQQO bond activation of amides and ureas

We have stated that N-fully substituted amides and ureas form

thermally-stable coordination compounds with MX5 (M =

Nb, Ta; X= F, Cl, Br), see Section 2.1. On the other hand, the

presence of nitrogen-bound hydrogen atoms may determine

interesting activation processes. It has been found that the

N,N0-dicyclohexylurea adduct NbCl5[k-CyHNC(O)NHCy] is

efficiently deprotonated by triethylamine at room temperature.

Subsequent rearrangement occurs with final formation of the

carbodiimide CyNQCQNCy, as a result of dicyclohexylurea

dehydration (Scheme 48).32

Further report deals with the dehydration of benzamide by

means of NbCl5 in refluxing benzene, see eqn (4).39

NbCl5 + PhCONH2 - NbOCl3(NCPh) + 2HCl (4)

These very recent findings may give an impulse to the

development of synthetic protocols making use of MX5 and

involving the dehydration of amides/ureas.

8. Acylation reactions

We have recently outlined that the direct interaction of MX5

(X = Cl, Br) with acetic anhydride (or halo-substituted acetic

anhydrides) results in single C–O bond cleavage and formation

Scheme 44 Heterocycles obtained by MX5-mediated reactions.

Scheme 45 Reaction catalyzed by silica-supported TaX5.

Scheme 46 Synthesis of chlorotetrahydropyrans and sulfur transfer

in a,b-unsaturated N-acylimides.

Scheme 47 Synthesis of coumarins.

Scheme 48 Conversion of N,N0-dicyclohexylurea into dicyclohexyl-

carbodiimide promoted by NbCl5.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 649

of the corresponding acyl-halide and the dinuclear bridging

carboxylato-derivative (see eqn (5) and Section 4.1).76

2MX5+2(RCO)2O-[MX4(m-OOCR)]2+2RCOX

M=Nb, Ta; X=Cl, Br; R=CH3, CHCl2, CF3 (5)

The reaction illustrated in eqn (5) is probably involved in

those MCl5-catalyzed acetylations in which acetic anhydride is

employed as a reagent. A pertinent example is the NbCl5-

catalyzed acetylation of 1,2-dimethoxybenzene, developed by

Arai, Nishida and coworkers, see Scheme 49.115 A dramatic

yield increase (from 19 to 93%) was achieved by addition of

AgClO4, generating the highly reactive species [NbCl4]+.

9. Activation of S-containing compounds

Carbonyl compounds have been obtained from dithioacetals

by oxidation with hydrogen peroxide in the presence of

catalytic amounts of TaCl5/NaI.140 The iodonium ion I+,

formed via I�- I+ oxidation, attacks the dithioacetal moiety

giving a iodosulfonium cation, which generates the carbonyl

compound upon hydrolysis, Scheme 50.140

Sulfides are oxidized to sulfones and sulfoxides by MCl5/

H2O2 (M = Nb, Ta).141 A detailed investigation of the

mechanism has shown that the oxidant is a tantalum peroxide,

formed by reaction of TaCl5 with aqueous H2O2, Scheme 51.

It has been found that the addition of I� inhibits the formation

of the oxidation products. In fact iodosulfones, which are

unreactive towards the metal peroxide, form under these

conditions.141

It should be mentioned here that bromination reactions of

C–C multiple bonds, aromatic compounds and diketones have

been performed with the bromonium ion as obtained by

selective oxidation of bromide with H2O2 in the presence of

tantalum chloride.142

Conclusions

The chemistry of niobium and tantalum pentahalides, MX5,

with oxygen compounds has been elucidated in the very last

years. Different types of coordination adducts (neutral or

ionic) can be isolated upon reaction of controlled amounts

of oxygen donors with MX5. The formation of coordination

compounds may be the first step of successive activation

reaction involving the organic substrate, sometimes unusual

in the context of transition metal derivatives.

The progress in the knowledge of the direct interaction of MX5

with oxygen compounds may serve in the full comprehension of

the existing, relatedMX5-mediated processes, and it may hopefully

promote the development of new MX5-mediated syntheses. This

development is desirable also on considering the easy availability

of MX5 and the substantial bioinertness/biocompatibility of the

elements niobium and tantalum.

Although MF5 have been significantly less employed in

catalysis than the heavier halide congeners, they have recently

shown interesting, novel features in the direct reactions with

oxygen species that could encourage their use in organic

synthesis. In fact, MF5 may work as a monoelectronic transfer

agent with respect to non-easily oxidizable systems, and

stabilize reactive cations by generating [MF6]� and [M2F11]

�

anions. The stability/inertness achieved with these anions

seems to overcome that supplied by the group 15 homologues.

In general, organic halides are usually generated in the course

of the activation processes involving MX5 (X=Cl, Br, I), while

MF5 are able to activate several oxygen-containing molecules

without the formation of organic fluorides (i.e. MF5 do not

contribute in terms of atoms to the organic products of the

activation reaction). The different behaviours exhibited by MF5

andMX5 (X=Cl, Br, I) must be essentially related to theM–X

bond energy scale.

Acknowledgements

This article is, in part, an account of the research activity carried

out by the authors in Pisa, which could not have been carried out

without the important contribution of the co-workers whose

names appear in the references. The Ministero dell’Istruzione,

dell’Universita e della Ricerca (MIUR, Roma) is acknowledged

for financial support. Thanks are due to Dr Simona Samaritani

(University of Pisa) for helpful discussion.

Notes and references

1 (a) L. G. Hubert-Pfalzgraf, M. Postel and J. G. Riess, Niobiumand Tantalum, in Comprehensive Coordination Chemistry, ed.G. Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon,Oxford, 1987, vol. 3, pp. 586–697; (b) T. Waters, A. G. Wedd,M. Ziolek and I. Nowak,Niobium and Tantalum, in ComprehensiveCoordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer,Elsevier, Oxford, 2003, vol. 4, pp. 242–312.

Scheme 49 Catalytic Friedel-Crafts acetylation using NbCl5.

Scheme 50 Deprotection of dithioacetals with TaCl5/NaI/H2O2.

Scheme 51 H2O2-oxidation of sulfides to sulfones and sulfoxides.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

650 Chem. Commun., 2012, 48, 635–653 This journal is c The Royal Society of Chemistry 2012

2 We published the first paper on the reactivity of group 5 pentahalideswith O-donor ligands in 2007: F. Marchetti, G. Pampaloni andS. Zacchini, Dalton Trans., 2007, 4343–4351.

3 A. Spannenberg, H. Fuhrmann, P. Arndt, W. Baumann andR. Kempe, Angew. Chem., Int. Ed., 1998, 37, 3363–3365.

4 (a) C. Redshaw, Dalton Trans., 2010, 39, 5595–5604;(b) V. C. Gibson and S. K. Spitzmesser, Chem. Rev., 2003, 103,283–315; (c) H. Hagen, J. Boersma and G. van Koten, Chem. Soc.Rev., 2002, 31, 357–364.

5 S. Chandrasekhar, T. Ramachandar and T. Shyamsunder, IndianJ. Chem., 2004, 43B, 813–838.

6 C. K. Z. Andrade, Curr. Org. Synth., 2004, 1, 333–353.7 (a) G. Smitha, S. Chandrasekhar and C. S. Reddy, Synthesis,2008, 829–855, and references therein; (b) V. Rodrıguez-Cisterna,C. Villar, P. Romea and F. Urpı, J. Org. Chem., 2007, 72,6631–6633; (c) C. Mamat, S. Buttner, T. Trabhardt, C. Fischerand P. Langer, J. Org. Chem., 2007, 72, 6273–6275;(d) D. Basavaiah and K. R. Reddy, Org. Lett., 2007, 9, 57–60;(e) V. T. H. Nguyen, E. Bellur, B. Appel and P. Langer, Synthesis,2006, 1103–1110; (f) L. Anastasia, E. Giannini, G. Zanoni andG. Vidari, Tetrahedron Lett., 2005, 46, 5803–5806; (g) M. Shoji,J. Yamaguchi, H. Kakeya, H. Osada and Y. Hayashi, Angew.Chem., Int. Ed., 2002, 41, 3192–3194; (h) Y. Hayashi,M. Nakamura, S. Nakao, T. Inoue and M. Shoji, Angew. Chem.,Int. Ed., 2002, 41, 4079–4082.

8 Structurally characterized MX5L adducts after 2007 include:NbX5(SMe2), X = Cl, Br [M. Jura, W. Levason, R. Ratnani,G. Reid and M. Webster, Dalton Trans., 2010, 39, 883–891] andMCl5(P4Sn), n = 4, 5, 6 [D. Hoppe, D. Schemmel, M. Schutz andA. Pfitzner, Chem.–Eur. J., 2009, 15, 7129–7138].

9 C.-I. Branden and I. Lindqvist, Acta Chem., Scand., 1963, 17,353–361.

10 C.-I. Branden, Acta Chem., Scand., 1962, 16, 1806.11 F. Calderazzo, M. D’Attoma, G. Pampaloni and S. Troyanov

, Z. Anorg. Allg. Chem., 2001, 627, 180–185.12 M. J. Heeg, D. S. Williams and R. Elgammal, quoted as Private

Communication, 2005 on the Cambridge Structural Database(CSD), 2009. Refcodes KARCEM.

13 M. J. Heeg, D. S. Williams and A. Korolev, quoted as PrivateCommunication, 2005 on the Cambridge Structural Database(CSD), 2009. Refcode RATPEI.

14 K. Stumpf, R. Blachnik, G. Roth and G. Kastner, Z. Kristallogr. -New Cryst. Struct., 2000, 215, 589–590.

15 R. E. DeSimone and T. M. Tighe, J. Inorg. Nucl. Chem., 1975, 38,1623–1625.

16 G. R. Willey, T. J. Woodman and M. G. B. Drew, Polyhedron,1997, 16, 351–353.

17 (a) D. B. Copley, F. Fairbrother and A. Thompson, J. Chem. Soc.A, 1964, 315–318; (b) F. Fairbrother, K. H. Grundy andA. Thompson, J. Chem. Soc. A, 1965, 765–770; (c) K. Feenanand G. W. A. Fowles, J. Chem. Soc. A, 1965, 2449–2451;(d) B. M. Bulychev and V. K. Bel’skii, Russ. J. Inorg. Chem.(Transl. of Zh. Neorg. Khim.), 1995, 40, 1765–1776.

18 (a) M. S. Gill, H. S. Ahuja and G. S. Rao, J. Indian Chem. Soc.,1978, 551, 875–878; (b) F. Filippini and B.-P. Susz, Helv. Chim.Acta, 1971, 54, 835–845.

19 A. M. Bol’shakov, M. M. Ershova, M. A. Glushkova andYu. A. Buslaev, Russ. J. Coord. Chem., 1978, 4, 1358.

20 A. O. Baghlaf, K. Behzadi and A. Thompson, J. Less-CommonMet., 1978, 61, 31–37.

21 (a) D. Brown, J. F. Easey and J. G. H. du Preez, J. Chem. Soc. A,1966, 258–261; (b) D. Brown, J. Hill and C. E. F. Rickard,J. Less-Common Met., 1970, 20, 57–65; (c) D. Brown, J. Hilland C. E. F. Rickard, J. Chem. Soc. A, 1970, 476–480;(d) R. J. Dorschner, J. Inorg. Nucl. Chem., 1972, 34, 2665–2668;(e) J. R. Masaguer and J. Sordo, An. Quim., 1973, 69, 1263–1268.

22 (a) J. C. Fuggle, D. W. A. Sharp and J. M. Winfield, J. FluorineChem., 1971/72, 1, 427–431; (b) F. Fairbrother, K. H. Grundyand A. Thompson, J. Less-Common Met., 1966, 10, 38–41.

23 (a) L. V. Kucheruk, I. E. Paleeva, E. P. Buchikhin,O. V. Braverman, I. P. Gol’dshtein and E. N. Gur’yanova, Russ.J. Gen. Chem., 1984, 54, 1500–1504; (b) E. G. Il’in, M. E. Ignatovand Yu. A. Buslaev, Russ. J. Coord. Chem., 1977, 3, 35–38;(c) C. M. P. Favez, H. Rollier and A. E. Merbach, Helv. Chim.Acta, 1976, 59, 2383–2392; (d) S. Brownstein and M. J. Farrall,

Can. J. Chem., 1974, 52, 1958–1965; (e) A. Merbach andJ. C. Bunzli, Helv. Chim. Acta, 1972, 55, 580–593; (f) J. C. Bunzliand A. Merbach,Helv. Chim. Acta, 1972, 55, 2867–2871; (g) J. A. S.Howell and K. C. Moss, J. Chem. Soc. A, 1971, 2483–2487.

24 L. Helm and A. E. Merbach, Chem. Rev., 2005, 105, 1923–1959.25 (a) R. Good and A. E. Merbach, Inorg. Chem., 1975, 14,

1030–1034; (b) C. M. P. Favez and A. E. Merbach, Helv. Chim.Acta, 1977, 60, 2695–2702; (c) H. Vanni and A. E. Merbach,Inorg. Chem., 1979, 18, 2758–2762.

26 (a) R. Good and A. E. Merbach, J. Chem. Soc., Chem. Commun.,1974, 163–164; (b) R. Good and A. E. Merbach, Helv. Chim.Acta, 1974, 57, 1192–1198.

27 R. G. Pearson, J. Am. Chem. Soc., 1963, 85, 3533–3539.28 (a) D. B. Copley, F. Fairbrother and A. Thompson, J. Less-

Common Met., 1965, 8, 256–261; (b) W. van der Veer andF. Jellinek, Recl. Trav. Chim. Pays-Bas, 1966, 85, 842–856.

29 A. P. Bell and A. G. Wedd, J. Organomet. Chem., 1979, 181,81–98.

30 K. Behzadi, A. I. T. Ahwaz Iran and A. Thompson, J. Less-Common Met., 1986, 124, 135–139.

31 D. B. Copley, F. Fairbrother, K. H. Grundy and A. Thompson,J. Less-Common Met., 1964, 6, 407–412.

32 M. Aresta, A. Dibenedetto, P. Stufano, B. M. Aresta, S. Maggi,I. Papai, T. A. Rokob and B. Gabriele, Dalton Trans., 2010, 39,6985–6992.

33 F. Marchetti, G. Pampaloni and S. Zacchini, Eur. J. Inorg.Chem., 2010, 767–774.

34 F. Marchetti, G. Pampaloni and S. Zacchini,Dalton Trans., 2009,6759–6772.

35 F. Marchetti, G. Pampaloni and S. Zacchini, Inorg. Chem., 2008,47, 365–372.

36 F. Marchetti, G. Pampaloni and S. Zacchini, J. Fluorine Chem.,2010, 131, 21–28.

37 F. Marchetti, G. Pampaloni and S. Zacchini,Dalton Trans., 2009,8096–8106.

38 F. Marchetti, G. Pampaloni and S. Zacchini, Eur. J. Inorg.Chem., 2008, 453–462.

39 P. D. W. Boyd, M. G. Glenny, C. E. F. Rickard andA. J. Nielson, Polyhedron, 2011, 30, 632–637.

40 (a) F. Marchetti, G. Pampaloni, Y. Patil, A. M. Raspolli Gallettiand S. Zacchini, J. Polym. Sci., Part A: Polym. Chem., 2011, 49,1664–1670; (b) F. Marchetti, G. Pampaloni, Y. Patil,A. M. Raspolli Galletti and M. Hayatifar, Polym. Int., DOI:10.1002/pi.3139.

41 M. Hayatifar, F. Marchetti, G. Pampaloni, Y. Patil andA. M. Raspolli Galletti, Catal Today, in press.

42 X = Cl: D. R. Sadoway and S. N. Flengas, Can. J. Chem., 1978,56, 2538–2545.

43 Examples of structurally characterized [MX6]� (M = Nb, Ta,

X = F, Cl, Br) anions: (a) J. Beck and G. Bock, Z. Naturforsch.,1996, 51, 119–126; (b) M. Simon and G. Meyer, Eur. J. Solid StateInorg. Chem., 1997, 34, 73–84; (c) J. Beck and A. Fischer,Z. Anorg. Allg. Chem., 1997, 623, 780–784; (d) J. Beck andT. Schlorb, Z. Kristallogr., 1999, 214, 780–785; (e) H. O. Davies,A. C. Jones, M. A. Motevalli, E. A. McKinnell and P. O’Brien,Inorg. Chem. Commun., 2005, 8, 585–587; (f) K. Matsumoto andR. Hagiwara, J. Fluorine Chem., 2007, 128, 317–331;(g) M. S. Fonari, Yu. A. Simonov, W.-J. Wang, S.-W. Tang,E. V. Ganin, V. O. Gelmboldt, T. S. Chernaya, O. A. Alekseevaand N. G. Furmanova, Polyhedron, 2007, 26, 5193–5202;(h) M. S. Fonari, N. G. Furmanova and Yu. A. Simonov,J. Struct. Chem., 2009, 50, S124–S135; (i) J. Burger andH. Henke, Z. Kristallogr., 2009, 224, 358–367; (j) H. Henke,Z. Kristallogr., 2010, 225, 344–348; (k) J. L. Manson,J. A. Schlueter, R. D. McDonald and J. Singleton, J. Low Temp.Phys., 2010, 159, 15–19; (l) T. Xie, W. Brockner and M. Gjikaj,Z. Anorg. Allg. Chem., 2010, 636, 2633–2640.

44 Examples of structurally characterized [M2F11]� (M = Nb, Ta)

anions: (a) F. Marchetti, G. Pampaloni, C. Pinzino andS. Zacchini, Angew. Chem., Int. Ed., 2010, 49, 5268–5272;(b) M. Jura, W. Levason, G. Reid and M. Webster, Dalton Trans.,2009, 7610–7612; (c) I. D. Brown, R. J. Gillespie, K. R. Morgan,J. F. Sawyer, K. J. Schmidt, Z. Tun, P. K. Ummat and J. E. Vekris,Inorg. Chem., 1987, 26, 689–693; (d) A. J. Edwards andG. R. Jones, J. Chem. Soc. A, 1970, 1491–1497.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 651

45 F. Marchetti and G. Pampaloni, Inorg. Chim. Acta, 2011, 376,123–128.

46 K. J. Packer and E. L. Muetterties, J. Am. Chem. Soc., 1963, 85,3035–3036.

47 R. Bini, C. Chiappe, F. Marchetti, G. Pampaloni and S. Zacchini,Inorg. Chem., 2010, 49, 339–351.

48 R. Bini, F. Marchetti, G. Pampaloni and S. Zacchini, Polyhedron,2011, 30, 1412–1419.

49 K. C. Moss, J. Chem. Soc. A, 1970, 1224–1226.50 (a) V. Gutmann, Coord. Chem. Rev., 1976, 18, 225–255;

(b) V. Gutmann, The Donor–Acceptor Approach to MolecularInteractions, Plenum Press, New York, 1978; (c) P.-C. Maria andJ.-F. Gal, J. Phys. Chem., 1985, 89, 1296–1304.

51 (a) A. Jutand, Eur. J. Inorg. Chem., 2003, 2017–2040.52 J. V. Hatton, Y. Saito and W. G. Schneider, Can. J. Chem., 1965,

43, 47–56.53 F. Marchetti, G. Pampaloni and T. Repo, Eur. J. Inorg. Chem.,

2008, 2107–2112.54 (a) E. Bernhardt, C. Bach, B. Bley, R. Wartchow, U. Westphal,

I. H. T. Sham, B. von Ahsen, C. Wang, H. Willner,R. C. Thompson and F. Aubke, Inorg. Chem., 2005, 44,4189–4205; (b) H. Willner and F. Aubke, Organometallics, 2003,22, 3612–3633; (c) H. Willner and F. Aubke, Angew. Chem., Int.Ed. Engl., 1997, 36, 2402–2425.

55 K. O. Christe, D. A. Dixon, D. J. Grant, R. Haiges, F. S. Tham,A. Vij, V. Vij, T.-H. Wang and W. W. Wilson, Inorg. Chem.,2010, 49, 6823–6833.

56 (a) K. O. Christe, C. J. Schack and R. D. Wilson, Inorg. Chem.,1975, 14, 2224–2230; (b) K. O. Christe, Inorg. Chem., 1975, 14,2230–2233.

57 M. Raducan, C. Rodrıguez-Escrich, X. C. Cambeiro,E. C. Escudero-Adan, M. A. Pericas and A. M. Echavarren,Chem. Commun., 2011, 47, 4893–4895.

58 H. Butenschon, J. Ma, C. G. Daniliuc, I. Nowik andR. H. Herber, Dalton Trans., 2011, 40, 3671–3676.

59 T. J. Richardson, F. L. Tanzella and N. Bartlett, J. Am. Chem.Soc., 1986, 108, 4937–4943.

60 R. Rathore, J. Hecht and J. H. Kochi, J. Am. Chem. Soc., 1998,120, 13278–13279.

61 H. Shorafa, D. Mollenhauer, B. Paulus and K. Seppelt, Angew.Chem., Int. Ed., 2009, 48, 5845–5847.

62 T. Bunic, M. Tramsek, E. Goreshnik and B. Zemva, Collect.Czech. Chem. Commun., 2008, 73, 1645–1654.

63 (a) S. H. Strauss, Chem. Rev., 1993, 93, 927–942; (b) C. A. Reed,Acc. Chem. Res., 1998, 31, 133–139; (c) I. Krossing andA. Reisinger, Coord. Chem. Rev., 2006, 250, 2721–2744, andreferences therein.

64 F. Marchetti, G. Pampaloni and S. Zacchini, Dalton Trans., 2008,7026–7035.

65 R. F. Childs, R. Faggiani, C. J. L. Lock and A. Varadarajan, ActaCrystallogr., Sect. C: Cryst. Struct. Commun., 1984, 40,1291–1294.

66 (a) R. D. Theys and M. M. Hossain, Tetrahedron Lett., 1992, 33,3447–3448; (b) V. Guieu, A. Izquierdo, S. Garcia-Alonzo,C. Andre, Y. Madaule and C. Payrastre, Eur. J. Org. Chem.,2007, 804–810; (c) E. C. Lis, Jr., D. A. Delafuente, Y. Lin,C. J. Mocella, M. A. Todd, W. Liu, M. Sabat, W. H. Myersand W. D. Harman, Organometallics, 2006, 25, 5051–5058.

67 E. Solari, C. Floriani, A. Chiesi-Villa and C. Rizzoli, J. Chem.Soc., Chem. Commun., 1991, 841–843.

68 F. Marchetti, G. Pampaloni and C. Pinzino, J. Organomet.Chem., 2011, 696, 1294–1300.

69 G. A. Olah and Y. K. Mo, J. Org. Chem., 1973, 38, 353–366.70 (a) F. P. Gortsema and R. Didchenko, Inorg. Chem., 1965, 4,

182–186; (b) F. P. Gortsema, Inorg. Synth., 1973, 14, 105–109;(c) J. Chassaing and D. Bizot, J. Fluorine Chem., 1980, 16,451–459.

71 H. C. Clark and H. J. Emeleus, J. Chem. Soc., 1958, 190–195.72 (a) R. E. McCarley, B. G. Hughes, J. C. Boatman and B. A. Torp,

Adv. Chem. Ser., 1963, 37, 243–255; (b) M. Allbutt, K. Feenanand G. W. A. Fowles, J. Less-Common Met., 1964, 6, 299–306;(c) R. E. McCarley and B. A. Torp, Inorg. Chem., 1963, 2,540–546.

73 The pentachlorides MCl5 reduce to the +4 oxidation state whenallowed to react with crown ethers (L. G. Hubert-Pfalzgraf and

M. Tsunoda, Inorg. Chim. Acta, 1980, 38, 43–48), but the oxidationproduct was not identified.

74 The enthalpy variation calculated for the reaction NbF5 +[NbF6]

�- [Nb2F11]� in the gas phase is �33.8 kcal mol�1 44a,68.

75 (a) A. M. Raspolli Galletti and G. Pampaloni, Coord. Chem. Rev.,2010, 254, 525–536, and references therein; (b) T. Matsuo andH. Kawaguchi, Inorg. Chem., 2002, 41, 6090–6098;(c) S. W. Schweiger, M. M. Salberg, A. L. Pulvirenti,E. E. Freeman, P. E. Fanwick and I. P. Rotwell, J. Chem. Soc.,Dalton Trans., 2001, 2020–2031; (d) D. A. Brown, M. G. H.Wallbridge, W.-S. Li, M. McPartlin and I. J. Scowen, Inorg.Chim. Acta, 1994, 227, 99–104; (e) K. C. Malhotra,U. K. Banerjee and S. C. Chaudry, Transition Met. Chem.,1982, 7, 14–16; (f) K. Behzadi and A. Thompson, J. Less-CommonMet., 1977, 56, 9–18; (g) A. A. Jones and J. D. Wilkins, J. Inorg.Nucl. Chem., 1976, 38, 95–97; (h) M. G. B. Drew andJ. D. Wilkins, Inorg. Nucl. Chem. Lett., 1974, 10, 549–552.

76 F. Marchetti, G. Pampaloni and S. Zacchini, Polyhedron, 2008,27, 1969–1976.

77 D. A. Brown, M. G. H. Wallbridge, W.-S. Li and M. McPartlin,Polyhedron, 1994, 13, 2265–2270.

78 E. M. Coe and C. J. Jones, Polyhedron, 1992, 11, 3123–3128.79 M. G. Constantino, V. J. Lacerda, L. C. da Silva Filho and

G. V. Jose da Silva, Lett. Org. Chem., 2004, 1, 360–364.80 J. S. Yadav, D. C. Bhunia, K. V. Krishna and P. Srihari,

Tetrahedron Lett., 2007, 48, 8306–8310.81 Amine and thiols have been acetylated under mild conditions in

the presence of catalytic amounts of NbCl5: J. S. Yadav,A. V. Narsaiah, A. K. Basak, P. R. Goud, D. Sreenu andK. Nagaiah, J. Mol. Catal. A: Chem., 2006, 255, 78–80.

82 (a) K. Nagaiah, B. V. S. Reddy, D. Sreenu and A. V. Narsaiah,ARKIVOC, 2005, 3, 192–199; (b) J. S. Yadav, A. V. Narsaiah,B. V. S. Reddy, A. K. Basak and K. Nagaiah, J. Mol. Catal. A:Chem., 2005, 230, 107–111; (c) S. Chandrasekhar,T. Ramachander and M. Takhi, Tetrahedron Lett., 1998, 39,3263–3266; (d) S. Chandrasekhar, M. Takhi, Y. R. Reddy,S. Mohapatra, C. R. Rao and K. V. Reddy, Tetrahedron, 1997,53, 14997–15004.

83 S. L. Barbosa, G. R. Hurtado, S. I. Klein, V. Lacerda Junior,M. J. Dabdoub and C. F. Guimaraes, Appl. Catal., A, 2008, 338,9–13.

84 S. L. Barbosa, M. J. Dabdoub, G. R. Hurtado, S. I. Klein, A. C. M.Baroni and C. Cunha, Appl. Catal., A, 2006, 313, 146–150.

85 J. F. Arteaga, V. Domingo, J. F. Quılez del Moral andA. F. Barrero, Org. Lett., 2008, 10, 1723–1726.

86 (a) F. F. Fleming, P. C. Ravikumar and L. Yao, Synlett, 2009,1077–1080; (b) P. C. Ravikumar, L. Yao and F. F. Fleming,J. Org. Chem., 2009, 74, 7294–7299.

87 (a) J. Recht, B. I. Cohen, A. S. Goldman and J. Kohn, TetrahedronLett., 1990, 31, 7281–7284; (b) K. Joshi, J. Bao, A. S. Goldman andJ. Kohn, J. Am. Chem. Soc., 1992, 114, 6649–6652.

88 J. B. Fang, R. Sanghi, J. Kohn and A. S. Goldman, Inorg. Chim.Acta, 2004, 357, 2415–2426.

89 A. Cowley, F. Fairbrother and N. Scott, J. Chem. Soc., 1958,3133–3137.

90 (a) V. C. Gibson, T. P. Kee and A. Shaw, Polyhedron, 1988, 7,2217–2219; (b) A. Antinolo, A. Otero, F. Urbanos, S. Garcia-Blanco, S. Martinez-Carrera and J. Sanz-Aparicio, J. Organomet.Chem., 1988, 350, 25–34; (c) H. Yasuda, Y. Nakayama, K. Takei,A. Nakamura, Y. Kai and N. Kanehisa, J. Organomet. Chem.,1994, 473, 105–116.

91 N. Guazzelli, Tesi di Laurea in Chimica, University of Pisa, 2009.92 (a) R. L. Burwell, Jr., Chem. Rev., 1954, 54, 615–685;

(b) M. V. Bhatt and S. U. Kulkarni, Synthesis, 1983, 249–281;(c) R. C. Larock, Ether Cleavage Comprehensive Organic Trans-formations, Wiley-VCH, Weinheim, 1999, 2nd edn, p. 1013.

93 (a) Q. Guo, T. Miyaji, R. Hara, B. Shen and T. Takahashi,Tetrahedron, 2002, 58, 7327–7334, and references therein;(b) Q. Guo, T. Miyaji, G. Gao, R. Hara and T. Takahashi, Chem.Commun., 2001, 1018–1019.

94 Y. Takegami, T. Ueno and R. Hirai, J. Polym. Sci., Polym. Chem.Ed., 1966, 4, 973–974.

95 (a) P. Dreyfuss and M. P. Dreyfuss, Adv. Polym. Sci., 1967, 4,528–590; (b) S. Panczek, P. Kubisa and K. Matyjaszewski, Adv.Polym. Sci., 1980, 37, 1–141.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

652 Chem. Commun., 2012, 48, 635–653 This journal is c The Royal Society of Chemistry 2012

96 (a) Y. Shichijo, A. Migita, H. Oguri, M. Watanabe, T. Tokiwano,K. Watanabe and H. Oikawa, J. Am. Chem. Soc., 2008, 130,12230–12231; (b) J. Justicia, A. G. Campana, B. Bazdi, R. Robles,J. M. Cuerva and J. E. Oltra, Adv. Synth. Catal., 2008, 350,571–576.

97 H. S. Park, D. W. Kwon, K. Lee and Y. H. Kim, TetrahedronLett., 2008, 49, 1616–1618.

98 (a) T. Ollevier and E. Nadeau, Tetrahedron Lett., 2008, 49,1546–1550; (b) M. Curini, F. Epifano, M. C. Marcotullio andO. Rosati, Eur. J. Org. Chem., 2001, 4149–4152.

99 (a) A. Gansauer, A. Barchuk, F. Keller, M. Schmitt, S. Grimme,M. Gerenkamp, C. Muck-Lichtenfeld, K. Daasbjerg and H. Svith,J. Am. Chem. Soc., 2007, 129, 1359–1371; (b) A. M. Anderson,J. M. Blazek, P. Garg, B. J. Payne and R. S. Mohan, TetrahedronLett., 2000, 41, 1527–1530; (c) A. W. Eppley and N. I. Totah,Tetrahedron, 1997, 53, 16545–16552; (d) K. Fujiwara, T. Tokiwanoand A. Murai, Tetrahedron Lett., 1995, 36, 8063–8066;(e) A. E. Vougioukas and H. B. Kagan, Tetrahedron Lett., 1987,28, 6065–6068.

100 A. V. Narsaiah, D. Sreenu and K. Nagaiah, Synth. Commun.,2006, 36, 3183–3189.

101 S. Chandrasekhar, T. Ramachandar and S. Jaya Prakash, Synthesis,2000, 1817–1818.

102 M. Aresta, A. Dibenedetto, L. Gianfrate and C. Pastore, J. Mol.Catal. A: Chem., 2003, 204–205, 245–252.

103 M. G. Constantino, V. Lacerda Junior, P. R. Invernize, L. C.da Silva Filho and G. V. Jose da Silva, Synth. Commun., 2007, 37,3529–3539, and references therein.

104 V. A. Petrov, Synthesis, 2002, 2225–2231.105 F. Marchetti, G. Pampaloni and S. Zacchini, Polyhedron, 2009,

28, 1235–1240.106 (a) J. Meinwald, S. S. Labana and M. S. Chadha, J. Am. Chem. Soc.,

1963, 85, 582–585; (b) M. W. C. Robinson, K. S. Pillinger andA. E. Graham, Tetrahedron Lett., 2006, 47, 5919–5921; (c) G. K.Surya Prakash, T. Mathew, S. Krishnaraj, E. R. Marinez andG. A. Olah, Appl. Catal., A, 1999, 181, 283–288; (d) B. Rickborn, inComprehensive Organic Synthesis, ed. B. M. Trost and I. Fleming,Pergamon, Oxford, 1991, vol. 3, ch. 3.3, pp. 733–775.

107 G. A. W. Fowles, D. A. Rice and K. J. Shanton, J. Chem. Soc.,Dalton Trans., 1978, 1658–1661. The formation of the metalderivative is accompanied by the production of MeCl.

108 F. Marchetti, G. Pampaloni and S. Zacchini, Chem. Commun.,2008, 3651–3653.

109 (a) I. L. Fedushkin, A. N. Lukoyanov, M. Hummert andH. Schumann, Z. Anorg. Allg. Chem., 2008, 634, 357–361;(b) C. P. Larch, F. G. N. Cloke and P. B. Hitchcock, Chem.Commun., 2008, 82–84; (c) C. A. Bradley, L. F. Veiros, D. Pun,E. Lobkovsky, I. Keresztes and P. J. Chirik, J. Am. Chem. Soc.,2006, 128, 16600–16612; (d) I. L. Fedushkin, F. Girgsdies,H. Schumann and M. N. Bochkarev, Eur. J. Inorg. Chem.,2001, 2405–2410; (e) M. C. Cassani, M. F. Lappert andF. Laschi, Chem. Commun., 1997, 1563–1564; (f) D. J. Duncalf,P. B. Hitchcock and G. A. Lawless, Chem. Commun., 1996,269–271; (g) K. Takaki, M. Maruo, T. Kamata, Y. Makiokaand Y. Fujiwara, J. Org. Chem., 1996, 61, 8332–8334;(h) B.-J. Deelman, M. Booij, A. Meetsma, J. H. Teuben,H. Kooijman and A. L. Spek, Organometallics, 1995, 14,2306–2317; (i) C. Eaborn, P. B. Hitchcock, K. Izod andJ. D. Smith, J. Am. Chem. Soc., 1994, 116, 12071–12072;(l) W. J. Evans, T. A. Ulibarri and J. W. Ziller, Organometallics,1991, 10, 134–142.

110 (a) C. A. Bradley, L. F. Veiros and P. J. Chirik, Organometallics,2007, 26, 3191–3200; (b) S. La Caer, M. Heninger, P. Pernot andH.Mestdagh, J. Phys. Chem. A, 2006, 110, 9654–9664; (c) S. Le Caer,M. Heninger, J. Lemaire, P. Boissel, P. Maıtre and H. Mestdagh,Chem. Phys. Lett., 2004, 385, 273–279; (d) M. C. Cassani,Y. K. Gun’ko, P. B. Hitchcock, A. G. Hulkes, A. V. Khvostov,M. F. Lappert and A. V. Protchenko, J. Organomet. Chem., 2002,647, 71–83; (e) Y. K. Gun’ko, P. B. Hitchcock and M. F. Lappert,J. Organomet. Chem., 1995, 499, 213–219.

111 (a) T. G. Driver, Angew. Chem., Int. Ed., 2009, 48, 2–5;(b) K. Fuchibe and T. Akiyama, J. Am. Chem. Soc., 2006, 128,1434–1435; (c) Y. Obora, M. Kimura, T. Ohtake, M. Tokunagaand Y. Tsuji, Organometallics, 2006, 25, 2097–2100; (d) T. Oshiki,K. Tanaka, J. Jamada, T. Ishiyama, Y. Kataoka, K. Mashima,

K. Tani and K. Takai, Organometallics, 2003, 22, 464–472;(e) D. E. Wigley and S. D. Gray, Niobium and Tantalum, inComprehensive Organometallic Chemistry II, ed. E. W. Abel,F. G. A. Stone and G. Wilkinson, Pergamon Elsevier, Oxford,2003, vol. 5, pp. 57–153; (f) R. G. Dushin, in ComprehensiveOrganometallic Chemistry II, ed. E. W. Abel, F. G. A. Stone andG. Wilkinson, Pergamon Elsevier, Oxford, 2003, vol. 12,pp. 335–338, 1087–1089, and the references cited therein.

112 (a) T. W. Greene and P. G. M. Wuts, in Protecting Groups inOrganic Synthesis, ed. J. Wiley, Oxford, III edn, 1999, ch. 4,p. 293; (b) M. B. Smith and J. March, in March’s AdvancedOrganic Chemistry, ed. J. Wiley, Oxford, IV edn, 2007.

113 J. S. Yadav, B. Ganganna, D. C. Bhunia and P. Srihari,Tetrahedron Lett., 2009, 50, 4318–4320.

114 (a) Y. Sudo, S. Arai and A. Nishida, Eur. J. Org. Chem., 2006,752–758; (b) S. Arai, Y. Sudo and A. Nishida, Synlett, 2004,1104–1106.

115 S. Arai, Y. Sudo and A. Nishida, Tetrahedron, 2005, 61,4639–4642.

116 A. R. Siedle, Diketones and Related Ligands, in ComprehensiveCoordination Chemistry, ed. G. Wilkinson, R. D. Gillard andJ. A. McCleverty, Pergamon, Oxford, 1987, vol. 3, pp. 365–412.

117 For general reviews, see for instance: I. Paterson, in Comprehen-sive Organic Synthesis, ed. C. H. Heathcock, Pergamon Press,Oxford, 1991, vol. 2, p. 301; R. O. Duthaler and A. Hafner, Chem.Rev., 1992, 92, 807–832. For references dealing with aldolateformation via organometallic compounds, see for instance:(a) P. Veja, P. G. Cozzi, C. Floriani, F. P. Rotzinger, A. Chiesi-Villa and C. Rizzoli, Organometallics, 1995, 14, 4101–4108;(b) P. G. Cozzi, P. Veja, C. Floriani, F. P. Rotzinger, A. Chiesi-Villa and C. Rizzoli, Organometallics, 1995, 14, 4092–4100;(c) M. B. Power, A. W. Apblett, S. G. Bott, J. L. Atwood andA. R. Barron, Organometallics, 1990, 9, 2529–2534;(d) H. J. Heeres, M. Maters, J. H. Teuben, G. Helgesson andS. Jagner, Organometallics, 1992, 11, 350–356.

118 C. K. Sperry, W. D. Cotter, R. A. Lee, R. J. Lachicotte andG. C. Bazan, J. Am. Chem. Soc., 1998, 120, 7791–7805.

119 J. S. Yadav, D. C. Bhunia, V. K. Singh and P. Srihari, TetrahedronLett., 2009, 50, 2470–2473.

120 J. S. Yadav, B. V. S. Reddy, B. Eeshwaraiah and P. N. Reddy,Tetrahedron, 2005, 61, 875–878.

121 R. Wang, B.-G. Li, T.-K. Huang, L. Shi and X.-X. Lu, TetrahedronLett., 2007, 48, 2071–2073.

122 T. Tsuritani, S. Ito, H. Shinokubo and K. Oshima, J. Org. Chem.,2000, 65, 5066–5068.

123 R. J. Ferrier and O. A. Zukov, Org. React., 2004, 62, 569–736.124 R. N. de Oliveira, A. C. N. de Melo, R. M. Srivastava and

D. Sinou, Heterocycles, 2006, 68, 2607–2613.125 S. Hotha and A. Tripathi, Tetrahedron Lett., 2005, 46, 4555–4558.126 S. Chandrasekhar, S. Jaya Prakash, V. Jagadeshwar and

Ch. Narsihmulu, Tetrahedron Lett., 2001, 42, 5561–5563.127 J.-T. Hou, J.-W. Gao and Z.-H. Zhang, Adv. Organomet. Chem.,

2011, 25, 47–53.128 M. Yamamoto, M. Nakazawa, K. Kishikawa and S. Kohmoto,

Chem. Commun., 1996, 2353–2354.129 L. C. da Silva Filho, V. Lacerda Junior, M. G. Constantino,

G. V. Jose da Silva and P. R. Invernize, Beilstein J. Org. Chem.,2005, 1, 14–19.

130 M. G. Constantino, K. T. Oliveira, E. C. Polo, G. V. J. da Silvaand T. J. Brocksom, J. Org. Chem., 2006, 71, 9880–9883.

131 C. K. Z. Andrade, O. E. Vercillo, J. P. Rodriguez andD. P. Silveira, J. Braz. Chem. Soc., 2004, 15, 813–817.

132 S. Chandrasekhar, P. K. Mohanty and A. Raza, Synth. Commun.,1999, 29, 257–262.

133 (a) M. M. Heravi, F. Nahavandi, S. Sadjadi, H. A. Oskooie andM. Tajbakhsh, Synth. Commun., 2009, 39, 3285–3292;(b) H. Hernandez, S. Bernes, L. Quintero, E. Sansinenea andA. Ortiz, Tetrahedron Lett., 2006, 47, 1153–1156; (c) N. Ahmedand J. E. van Lier, Tetrahedron Lett., 2007, 48, 5407–5409;(d) J. S. Yadav, B. V. S. Reddy, J. J. Naidu and K. Sadashiv, Chem.Lett., 2004, 33, 926–928; (e) L. C. da Silva Filho, V. Lacerda Junior,M. G. Constantino and G. V. Jose da Silva, Synthesis, 2008,2527–2536; (f) J.-T. Hou, Y.-H. Liu and Z.-H. Zhang,J. Heterocycl. Chem., 2010, 47, 703–706; (g) S.-T. Gao, W.-H. Liu,

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online

This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 635–653 653

J.-J. Ma, C. Wang and Q. Liang, Synth. Commun., 2009, 39,3278–3284.

134 S. Chandrasekhar, S. J. Prakash, T. Shyamsunder andT. Ramachandar, Synth. Commun., 2005, 35, 3127–3131.

135 N. Ahmed and J. E. van Lier, Tetrahedron Lett., 2006, 47, 2725–2729.136 S. Chandrasekhar and B. V. S. Reddy, Synlett, 1998, 851–852.137 J. S. Yadav, B. V. S. Reddy, M. K. Gupta and S. K. Biswas,

Synthesis, 2004, 2711–2715.138 A. Ortiz, L. Quintero, H. Hernandez, S. Maldonado, G. Mendoza

and S. Bernes, Tetrahedron Lett., 2003, 44, 1129–1132.

139 S.-T. Gao, C. Li, Y. Wang, J.-J. Ma, C. Wang and J.-W. Zhang,Synth. Commun., 2011, 41, 1486–1491.

140 M. Kirihara, A. Harano, H. Tsukiji, R. Takizawa, T. Uchiyamaand A. Hatano, Tetrahedron Lett., 2005, 46, 6377–6380.

141 (a) M.Kirihara, J. Yamamoto, T. Noguchi andY. Hirai,TetrahedronLett., 2009, 50, 1180–1183; (b) M. Kirihara, J. Yamamoto,T. Noguchi, A. Itou, S. Naito and Y. Hirai, Tetrahedron, 2009, 65,10477–10484.

142 M. Kirihara, K. Okubo, T. Koshiyama, Y. Kato and A. Hatano,ITE Lett. Batteries, New Technol. Med., 2004, 5, 279–281.

Dow

nloa

ded

by U

nive

rsity

of

Mem

phis

on

23 A

ugus

t 201

2Pu

blis

hed

on 0

9 N

ovem

ber

2011

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

1CC

1459

2D

View Online