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Hypervalent Silicates:Properties and Synthetic Utility
A Scheidt Group Literature Presentation
Robert B. Lettan IISeptember 21, 2004
Leading Reference: Chult, C.; Corriu, R. J. P.; Reye. C.; Young, J. C. Chem. Rev. 1993, 93, 1371-1448.
Si Si Si
Outline
I. Introduction
II. Preparation of Hypercoordinate Silicon Compounds
A. Pentacoordinate Compounds B. Hexacoordinate Compounds
III. Structures of Hypervalent Silicon Compounds
IV. Stereochemical Nonrigidity of Hypervalent Silicon Compounds
V. Reactivity of Silicon Compounds
A. Pentacoordinate Compounds B. Hexacoordinate Compounds
VI. Synthetic Methods Involving Hypervalent Silicon Intermediates
VII. Conclusion
Introduction
Silicon compounds with a coordination number higher than four have been know since the beginning of the 19th century.
SiFF F
F
F
F
2-
SiFF F
F
NH2
NH2
2-
1. Gay-Lussac, J. L.; Thenard, L. J. Mémoires de Physique et de Chimie de la Société d' Arcueil 1809, 2, 317.2. Davy, J. Phil. Trans. Roy. Soc. London 1812, 102, 352.
3. Colvin, E. W. Silicon in Organic Synthesis; Butterworths: London, 1981.4. Wbere, W. P. Silicon Reagents for Organic Synthesis; Springer-Verlag: Berlin, 1983.5. Fleming, I. Comprehensive Organic Chemistry; Jones, N., Ed; Pergamon Press: Oxford, 1979, Vol. 3, p. 554.6. Corriu, R. J. P.; Perz, R.; Reyé, C. Tetrahedron 1983, 39, 999.7. Müller, R. Organometal. Chem. Rev. 1966, 1, 359.8. Müller, R. Z. Chem. 1984, 24, 41.9. Kumada, M.; Tamao, K. Yoshida, J. J. Organomet. Chem. 1982, 239, 115.
In the past thirty years considerable interest has been paid to the distinctive reactivity of hypervalent silicon compounds
1) Nucleophilic activation and catalysis in the application of of organosilicon compounds as intermediates in organic synthesis.3-6
2) Formation and reactivity of organofluorosilicates [RSiF5]2-.7-9
Preparation of Pentacoordinate Silicon Compounds
There are three general methods:
1) Anion Addition to Tetracoordinate Silicon Compounds
RnSiX4-n + X- [RnSiX5-n]-
2) Inter- or Intramolecular Coordination of a Neutal Donor to Silicon
SiR4
N
3) Substitution of a Trifunctional Organosilane
a) By use of a Bidentate Ligand (i.e. catechol)
b) By use of trialkanolamines, N[(CH2)nOH]3 or tris-(2-aminoethyl)amine N[(CH2)nNHR]3 to give silatranes or triazasilatranes respectively.
Anion CoordinationFluoride Donation
SiF2R1R2
R1=F,Me,PhR2=F,Ph
FSiF2R1R2R4N+F-
NMR1and vibrational spectroscopic data2 suggest a pentacoordinate silicon.
18-crown-6 allowed isolation of the salt/crystal structure3
Hydride Donation4
HSi(OR)3 [H2Si(OR)3] + [HSi(OR)4]
R = Et, Pri, Bun, Bus, c-C6H11
4[H2Si(OR)3]-K+ 3[HSi(OR)4]-K+ + SiH4 + KH
R = Me, Et, Bun
KH
KH
a b
a:b increases as steric bulk of R increases
Alkoxide Donation5
PhnSi(OMe)4-n + KOMe/ 18-c-6 [PhnSi(OMe)5-n]-[K,18-c-6]+
MePhSi(OEt)2 + KOEt/ 18-c-6 [MeSi(OEt)4]-[K,18-c-6]+
THF or
DME29Si = 80-100 120-135HSi(OR)3 + KOR [HSi(OR)4]-K+
1. Klanberg, F. Muetterties, E. L. Inorg. Chem. 1968, 7, 155.2. Ault, B. S. Inorg. Chem., 1979, 18, 3339.3. Harland, J. J.; Payne, J. S.; Day, R. O.; Holmes, R. R.; Inorg. Chem. 1987, 26, 760. 4.Corriu, R. J. P.; Guérin, C.; Henner, B. J. L.; Wang, Q. Organometallics, 1991, 10, 3574.5.Damrauer, R.; Danahey, S. E. Organometallics 1986, 5, 1490.
Neutral Donors
Intermolecular Donation
Me2SiHCl NMI SiNMIMe
H
Me
NMI
Most cases of intermolecular donation areeither to weak to interact, or results in completeanionic dissociation (i.e. a new tetravalantcomplex). For complexation to work, morethan one electronegative ligand must beattached, or hydrogen must be a ligand as well,as in the example above, which wasconfirmed by X-ray structural analysis.1
Intramolecular Donation
Me2N Si FF F
NNSi F
F F
R
Me2N SiXYZ
OAr
O Si FF F
Me
O
SiClMe2
Si ClMe Me
N
O Si FMe Me
Although rigid and favorable geometries help allowinteractions, intramolecular coordination is moredependent on the remaining substituents on silicon.Bond distances and hypervalency confirmed by X-rayanalysis.2
1. Hensen, K.; Zengerly, T.; Pickel, P.; Z. Anorg. Allg. Chem. 1988, 558, 21.2. Onan, K. D.; McPhall, A. T.; Yoder, C. H.; Hillyard, R. W. J. Chem. Soc., Chem. Commun. 1978, 209.
Substitution in a Trifunctional OrganosilaneBy a Bidentate Ligand1-3
OH
OH2
R-Si(OMe)3
Et3N
( R = Ph)
Me4N+OH-
MeOM
MeOH
OSi
OPh
2
OSi
OR
2
OSi
OR
2
Et3NH+
Me4N+
M+
OHOH
2O
SiO
RO
2
BunNH3+
Si(OR)4
BunNH2
Li
F3C OLiF3C
RSi
O
O
F3C CF3
F3C CF3
RSiCl3Li+
Trialkanolamines and Tris(2-aminoethyl)amines4,5
RSi(NMe2)3 N NH2 3HN Si
NH
NH
N
R
RSi(OR')3 N OH3
O SiOO
N
R
Silatrane Azasilatrane
1. Frye, C. L. J. Am. Chem. Soc. 1964, 86, 3170.2. Frye, C. L. J. Am. Chem. Soc. 1970, 92, 1205.3. Perozzi, E. F.; Martin, J. C. J. Am. Chem. Soc. 1979, 101, 1591.4. Frye, C. L.; Vogel, G. E.; Hall, J. A. J. Am. Chem. Soc. 1961, 83, 996.5. Lukevics, E.; Zelchans, G.; Solomennikova, I. I.; Liepins, E. E.; Jankovska, I.; Mazeika, I. Chem. Abstr. 1977, 86, 171536j.
Hexacoordinate Silicon CompoundsFluoride Donation1 Intermolecular Coordination2
Intramolecular Coordination3 Bidentate Ligand Substitution4,5
RSiX3
X = Cl, Br, I, OR
SiRF F
F
F
FK
KF (excess)
H2O, 0 °C N
N
SiCl4
SiXYI2
Si2Br6
N
NSiCl4
N
NSi
N
NSi
X
Y2
3
2+
4+
2I-
4Br-
N
N= 2,2' bypyridine or 1,10-phenanthroline
Si XX
NMe2
Me2NX = H, F
SiX4
X = Cl, ORY = O, NR
O
Y
O
YSi
3
R2SiX2
X = Cl, ORY = O, NR
O
Y
O
YR2Si
2
Ligand Examples:
OOSi
Z
Z
Z = O, NMe
N
HO OH
OH
1. Tamao, K.; Yoshida, J.; Yamamato, H.; Kakui, T.; Matsumoto, H.; Takahashi, M.; Kuritam A.; Murata, M.; Kumada, M. Organometallics, 1982, 1, 355.2. Hensen, K.; Busch, R. Z. Naturforsch. 1982, 37B, 1174.3. Brelière, C.; Carré, F.; Corriu, R. J. P.; Poirier, M.; Royo, G.; Zwecker, J. Organometallics 1989, 8, 1831.4. Dilthey, W. Chem. Ber. 1903, 36, 923.
Structures of Hypervalent Silicon CompoundsPentaflurosilicates
F SiRR
F
F
R = F, organic groups
Organic groups occupy theequatorial positions.
Axial Si-F bond lengths are alwaysgreater than equatorial Si-F bonds.
Si-F bond lengths increase with increasing steric bulk of organic groups as well as with thediminishing number of electronegative atoms.
Pentacoordinate Bicyclic Silicates
Si
Si
vs.
trigonal bypyramid(TBP)
rectangular pyramid(RP)
RO SiOO
O
O
RP TBPInc. Steric Bulk of R
SiO
O O
O
Ph
97.6% RP
Intramolecular Coordination
R SiOO
HN
X
SiMeMe
Y
N
O
LessEWX
MoreEWX
Inc. Bond
LengthΣ rvw(N-Si) = 3.5AN-Si above = 2.0-2.2 A
o
o
Y = Cl, Br, I
Si-Y B.L.
Si-O Dist.Cl I
The heavier the halogen, thelonger the Si-Y bond, and the closer the Si-O distance.
Hexacoordinate Silicon
SiClCl N
N
R
R
N-Si interactions are always opposite to chlorine atoms.
BondN-SiCl-SiMe-Si
Oct. length (A)1.95-2.202.15-2.40
1.90
Tet. length (A)-
1.90-2.101.90
Stereochemical Nonrigidity
SiMeF
F
N Me
MeMe H
4 NMe2 peaks in 1H NMR spectra2 peaks in 19F DNMR spectra
Intramolecular F ligand exchange 2 NMe2 peaks in 1H NMR (ΔG‡ = 9.4kcal/mol)1 peak in 19F DNMR spectra (ΔG‡ = 9.3kcal/mol)
Si-N opening/ rotation/ inversion1 NMe2 peak in 1H NMR (ΔG‡ = 11.8kcal/mol)
[SiF5]- and [RSiF4]-
[R2SiF3]-
Exchange Extremely FastSingle 19F Resonance at LT
Axial and equatorial fluorine atomsdistinguished at LT 19F NMR (ΔG‡ = 9 -12 kcal/mol)
Inc. ΔG‡StrongerEWG
WeakerEWG
Pentavalent Silicon1,2
Hexacoordinate Silicon3
SiR
RR
NR2
R
Si-N coordinate bond < 6 kcal/molΔG‡ < 7 kcal/molVery Fluxional
1. Damrauer, R.; O'Connell, B.; Danahey, S. E.; Simon, R. Organometllics 1989, 8, 1167.2. Corriu, R. J. P.; Kpoton, A.; Poirier, M.; Royo, G.; Corey, J. Y. J. Organomet. Chem. 1984, 277, C25.3. Kessler, H. Angew. Chem., Int. Ed. Engl. 1970, 9, 219.
Incr
easin
g Te
mpe
ratu
re
Reactivity of Pentacoordinate Silicon Compounds
R1R2R3Si-Cl Nu R3SiR2R1
Cl
Nu
R3SiR2R1
Nu
Nu
Cl-
+
SiR2R1 Cl
R3
O
Nu
SiR2R1 Cl
R3
Nu
Nu
H H
Racemisation
R1R2R3Si-OH
Nu
H2O
v = krac [R1R2R3Si-Cl] [Nu]2
vrac = k [R1R2R3Si-Cl] [H2O] [Nu]
H2O
H2O
ΔS‡ = -40 to -60 e.u.ΔH‡ < 3 kcal/mol
Indicate a MechanismControlled by ΔS‡
Step 1: Initial and reversible attack of activating nucleophilic catalyst on substrate.
Step 2: Rate-determining step. A second Nu must attack. Pentacoordinate species must be more reactivethan tetracoordinate silane. RDS involves attack on pentacoordinate Si, so must result in hexacoordinateintermediate (or Transition State)Large negative values of ΔS‡ are consistent
with a highly organized transition state
1. Corriu, R. J. P.; Dabosi, G.; Martineau, M. J. Organomet. Chem. 1978, 150, 27.2. Corriu, R. J. P.; Dabosi, G.; Martineau, M. J. Organomet. Chem. 1980, 186, 25.
Spillover Effect
1. Voronkov, M. Top. Curr. Chem. 1986, 131, 99.2. Michael, F. Evans Group Seminar 1998-99.
XSi
X XX X Si
XX
X
X
SiXX X
X
X
X
2+ X- + X-
All B.O. = 1
Formal Charges:X = 0Si = 0
B.O. = 0.83
B.O. = 0.75
All B.O. = 2/3
Formal Charges:X = -0.33Si = 0
Formal Charges:Xax = -0.25Xeq = -0.17Si = 0
Ab initio study:
Species
SiH4SiH5
-
SiH3FSiH3F2
-
SiF4SiF4 NH3
SiF4 2NH3
Si Charge
+0.63+0.84
+1.10+1.26
+1.434+1.470+1.463
Ligand Charge
-0.16-0.29(eq), -0.49(ax)
-0.15(H), -0.67(F)-0.26(H), -0.74(F)
-0.358-0.397(F,eq), -0.385(F,ax), -0.084(NH3)-0.463(F), +0.196(NH3)
Formal charge does not change, but the number of electronegative substituentsincreases with increasing coordination.
Pentacoordinate Alkyl- and ArylsilicatesFluoro- and Methoxyorganosilicates
[R2SiX3]- [K, 18 Crown 6]+
R2SiH2
R2Si(OMe)2
R2SiMe2
R2Si
R2SiR'2
FBut
LiAlH4
NaOMe
R'Li
ButMgX
MeMgX
Rel. React.(penta/ tetra)
> 100:1
Ar-X + [(CH3)3SiF2]- Ar-CH3(η3-C3H5PdCl)2
Cross-Coupling Reactions( X = F, OMe)
Bis(1,2-benzenediolato)organosilicates
OSi
OR
2
-
K+O
O
R'2RSi
Ph CMgBr
R'2RSiC CPh
LAHR'
2RSiH
R'2RSiSiPh3
R'2RSiCl
R'2RSiOMe
2MeOLiMeOH
R'2RSi
Ph3SiLi
HClMgBr2
2 eq. R'Li
OSi
O 2
-
LAHEt2O
excessR'Li/ R'MgBr
BF3
Et2O
RSiH3
RSiR'3
R3MgBr
Cp2TiCl2
R2MgBr
Cp2TiCl2
OSi
OB
2
-
K+
R3RSiH2
R2RSiH2
R2 = primary grignard reagentR3 = secondary grignard reagent
Et3NH+ + Ar-X Ar[Pd]
Pentacoordinate HydridosilicatesAlkoxyhydridosilicates
[HSi(OEt)4]- K+ Cp(CO)2FeI
Ag + KBF4 + 0.5H2 + Si(OEt)4
0.5 [Cp(CO)2Fe]2 + KI + 0.5H2 + Si(OEt)4
R3SiH
Si(OR)4 + H2
3 RMgBr
RYCORCH3OH
RCO2EtRCHO
RCONMe2RCHO
ROH
AgBF4
single electron transfer
reductions
RXRH alkyl halide reductions
alcoholysis
Grignard Additions
Bis(diolato)hydridosilicates
Y = H, R
OSi
OH
2
-
Li+
Can also reduce aldehydes andketones, but not esters or amides.
H3C OLi
OLiH3C
OLi
OLi
PhH2CO
PhH2CO
OLi
OLi
H3CO
H3CO PhH2C NHLi
OLiOLiOLi
Ligands for Enantioselective Reductions (20-90 %ee)1,2
1. Kohra, S.; Hayashida, H.; Tominga, Y.; Hosomi, A. Tetrahedron Lett. 1988, 29, 89.2. Schiffers, R.; Kagan, H. B. Synlett 1997, 1175.
Pentacoordinate AllylsilicatesBis(1,2-benzenediolato)allylsilicates
SiOO
2
-
Me4N+
R R'(H)
O
KF, NaOMe,
or KOMe
ketones or aldehydes
(H)R'R OH
Ketones are less reactive, need alkoxide base to promote. Selectivealdehyde addition possible in presence of ketones.
(MeO)3Si
In presence of KF, cleavage of Si-O bond occurs, and crotonization or a Cannizzaroreaction is favored.
R2
R1
Fluoroallylsilicates Mechanism
SiF3
R2
R1
hydroxy compound
Et3N, CH2Cl2
R H
O
R
OH
R1 R2
R2R1
O
O
O
OOH
RR2
R1
High regio- and stereo-selectivity
Silatranes and Azasilatranes
Silatranes
O SiOO
N
R
R = H: Can reduce alkyl halides, acid chlorides and ketones, but with a much lower reacivity than hydridosilicates. Need long reaction times and heat.
R = I:
R = aryl: Can reduce with LAH. Can alkylate with R'Li.
R = vinyl: R'Li yields If R' is sterically bulky (i.e. t-Bu) substitution to the silane doesn't occur.
R = allyl: Need L.A. TiCl4 seems to work best. Allows addition to aldehydes. Will also add to allyl group to vinyl esters, yielding
Reactant(Me3Si)2OR'C CHR'R"HgR'OR"
MeCO2EtMeCHO
ProductR = Me3SiO R = C CR'R = R' or R"
R= OR' or OR"R = MeCO2
R = MeCHIO
SiR'3R'
CO2Et
Azasilatranes
HN SiNH
NH
N
R
1. R'Me2SiCl as a reactant: If R = H , OEt, or Me silylation of 2 of the equatorial NH groups occurs with NEt3. Silylation of all 3 amines proceeds readily with NEt3 when R = OEt. Silylation of the third amine when R = Me requires nBuLi. Following this silylation, it is possible to methylate the Nax with CF3SO3Me, yielding
2. MeOH as a reactant: If R = H , OEt, or Me, solvolysis gives RSi(OMe)3
N SiNN
N
MeSiMe2R
SiMe2RRMe2Si
Me
Neutral Pentacoordinate Silicon DerivativesSilicon Hydrides
Carboxylic Acids and Acid Chloridesto Aldehydes.
Isocyanates to formamides.
Thioisocyanates to N-acylthioformamides
PhN=C=X
(X = O,S)Si
NH
Ph
NMe2
X
HPh
R'COClN HR'
O X
Ph
alkylcarbodiimides (R-N=C=N-R)Reductions:
SiNH
Ph
NMe2
p-tolN
p-tol
Si HPh
NMe2
R' = Pri, c-C6H11
HCO2H
R"COCl
R"COCl
N"R
O
p-tolN
p-tol
HN N
RR
"RNH
NR'R'
O
Diaminosilanes
SiNMe2
NMe2
R
NMe2
CS2 (10 equiv.)
CS2 (1 equiv.)
Me2N NMe2
S
NMe2S
NMe2
S
S
Ph-N=C=X
(X = O,S)
CO2, Δ
Me2N NMe2
O
SiHH
R
N
SiNNMe2
R
N
NHR'
NR'
Ph
X
NMe2
SiXNMe2
R
NNPh
NMe2
1,3-migrationΔ
Me2N NMe2
NPh
Reactivity of Anionic Hexacoordinate Silicon Compounds
Organopentafluorosilicates
RR
SiF5
2M+AgI/ CuI
homocoupling
RR
RR
2
2CuX2
oxidation
RR
XE+ + CuII
Si-C bond cleavage
RR
E
C-C bond formation
EWG
RR
EWG
CO + ROH
carbonylation
RR
CO2R
NBS or X2RR
X
MCPBA
oxidative cleavage
RR
OH+ Pd(OAc)2
Heptacoordination at Silicon
Si
Me2N
Me2N NMe2
F
OSi
O2
NMe2
NMe2
PPN+
NMR Evidence:
29Si NMR spectrum has the chemical shift d = -129.8ppm, which is upfield in comparison to the monoaminehexacoordinate variant (d = 121.2ppm)
1H NMR spectrum shows one NMe2 peak at RT. Since it is assumed that the silicon atom is at least hexacoordinated,the equivalence must be due to reversible attack of the freeNMe2 group on the the silicon atom, with displacement of the chelated one. OR, nucleophilic attack on a hexacoordinate silicon species, which could occur via a heptacoordinate transition state.
Crystal Structure Evidence:
Basic tetrahedral geometry (tricapped tetrahedral) ofthe fluorosilane is retained.
Lone pairs of three NMe2 groups are oriented toward the silicon atom, even though there is no geometric constraint to force this, since the NMe2 groups in the benzylic positions are free to rotate.
N-Si bond distances vary from 3.00-3.49A,corresponding to weak intramolecular interactions. System prefers three weak interactions, as comparedto a single one (pentacoordination at silicon), or two(hexacoordination).
F Si
NMe2 3
Synthetic Methods Involving Hypervalent Silicon Intermediates
Activation of the Si-H Bond
ROH ROSiR1R2R3HSiR1R2R3
CsF/Im/DMF
RR'OSiR1R2R3HSiR1R2R3
TBAF/HMPA/RTR R'
O H3O+
RR'OH
primary>secondary>tertiary
mild reduction
R Z
O (EtO)3SiH, KF/CsF
Z = H, R, OR R
OH
Z
Can reduce in presence of olefins, bromo, nitro,or amido groups.
Can also selectively reduce aldehydes over ketones, and ketones over esters.
Mechanism
R1R2R3SiH R3SiR2R1
H
F
SiR2R1 O
R3
H
F
SiR2R1 O
R3
H
F
HR4
R4
R5
F-
R4OH
R4R5CO
F-
F-
R4OSiR1R2R3
R4R5CHOSiR1R2R3
Silicon-Oxygen Activation
OSiMe3
RX, F-
OR
OSiMe3
1)PhCHO, TBAF
2)H2O
O
Ph
OH
O
2Si(OR)4, KF/CsF
O
OSiMe3
PhO
CsF
silyl enol ethersO
Ph
O
RCOCH2R'
Me
CONH2
Si(OMe)4
CsF R
O
R' Me
CONH2
HN
OMe
RR'
OOSiMe3
OMeH3C
O
CO2Me
CH3
F-
Silyl Enol Ethers
Aldol Rxns
MIchael Rxns
Silyl Ketene AcetalsMechanistic Example
SiH3CH3C
CH3
O
F
R1
R3R2
OR
H
Silicon-Carbon Bond Activation
H3C
H3C SiMe3
X
TBAF or
KF, 18-crown-6 H3C
H3Cα-Elimination
Cl Cl
SiMe3
CsF ClO
Ph
Ph
O
Cl
Ph
Ph
β-Elimination
Elimination Rxns
NMe2
SiMe3
O
H3C
H
MeOCH3
H
CH3
H H
OCH3
CH3
H1) MeI
2)CsF MeCN
1,4-Elimination
Allylsilanes
Z3Si 1) F-,
2) H3O+
(Z = R, F)
R R'
Y
OH
R R'
Addition to Carbonyls and Imines
Also works with Michael Acceptors
SiMe3
CO2CH3
TBAF
CO2CH3
Alkynyl-, Propargyl-, and Benzyl-, Aryl-, and Alkenylsilanes
Activated with fluoride or alkoxide promoters,and applications in additions to carbonylcompounds.
Oxiranyl- Cyclopropylsilanes
X
(X = O, CH2)
EWGSiMe3
F- + E+ X EWGE
H3C
H3C
Silicon-Carbon Bond Activation: TMS derivatives with Anion-Stabilizing Groups in the α-Position
Anion Equivalents
SiMe3
H ClCl
1) F-, RCHO
2) H3O+ R CHCl2
O
Ph CH2SiMe3
OCsF
PhCHO
PhCH2Br
OPhCOCH=CHPh
PhCOCH2CH2Ph
O
CH2COPh
Ylide Equivalents
Ph3P CH2SiMe3 TfO-CsF
CH3CN
Wittig reaction
CH2
Ph
Me3SiH2C P(OMe)2
O
Ph
CsF, R1R2OCHPh
R2
R1
Horner-Emmonstype reaction
O
SPh
O
SPhCH2SiMe3
TMSCh2OTf
O
SPh
CsF
CH3CN
TfO-
Sulfur Ylides
NSR
SiMe3N
SR
SiMe3H3C
I-
MeI CsF
EWG EWG
NCH3
CO2CH3
H3CO2C
Azomethine Ylides
Acylsilanes
XSiMe3
O CsF or KF/18c6
PhCHO(X = O, S, NMe)
XO
Ph
OH
O
Ph
Palladium-Catalyzed Cross-Coupling Reactions
RX
R1SiZ3
F-
X = Br, I, OTfSiZ3 = SiMe2(OEt) SiMe(OEt)2 SiMe(OEt)3
RR1
E,Z retained
Vinylsilanes Arylsilanes
R1SiRF2
R2I
R2R1
KF/DMF
(η3-C3H5PdCl)2
CO insertions also possible
Alkynylsilanes
R SiMe3
BrPh
TAS TMSF2
(η3-C3H5PdCl)2
Ph
R
Alkyltrifluorsilanes
R1TfO
TBAF
(PPh3)4PdRSiF3
R1R
Allylsilanes
SiMe3
TAS TMSF2/ (η3-C3H5PdCl)2
TAS TMSF2/ (η3-C3H5PdCl)2
BrPh
PhBr
H3CPh
Ph
RI
SiMe3H3C
CH3 TBAF
(PPh3)4Pd R
CH3H3C
More Recent Advances in Silicon ChemistryRing Opening Halosilyations1
O
n
n = 1,2,3Also Applicable
to Lactones
XOSiR3
X = Br, IR = Me, Et
EtSiH/ MeI/ PdCl2 or
Me3SiNEt2/ MeI
1. Kunai, A.; Oshita, J. J. Organomet. Chem. 2003, 686, 3.2. Bassindale, A. R.; Parker, D. J.; Patel, P.; Taylor, P. G. Tetrahedron Lett. 2000, 41, 4933.3. Kirpichenko, S. V.; Suslova, E. N.; Albanov, A. I.; Shainyan, B. A. Tetrahedron Lett. 1999, 40, 185.
O
Y Me3SiNEt2/ MeI
Y = O, NMe
YOSiMe3
n
Chemoselective Methylation2
R NR'
O (Me3Si)2NH
ClCH2SiMe2Cl SiCH3
CH3
O
Cl
R'N
R
R NR'
O
CH3
CsF
Works with a variety of straight chain, cyclic, conjugated,and aromatic R groups.
Works with both primary and secondary amides.
72-85%Y
Sila-Pummerer Rearrangement3
Me2Si SO
Δ
THF
Me2SiS
O
Si S
O
H3C
CH3Si S
OH3C
CH3
SiH2C
SO
H3C
CH3
Me2SiS
O
Aldol Reaction
Kobayashi, J.; Nakamura, M.; Mori, Y.; Yamashita, Y.; Kobayashi, S. J. Am. Chem. Soc. 2004, 126, 9192.
NF3C
OSiMe3
OMe
OSiMe3
R H
O chiral Zr cat., tBuOMe
[(R)-binol]
78/22 - 90/10 ani/syn85-95 % ee (anti)
R OMe
O
NHCOCF3
OH
TBDPSOH
O
A
A, chiral Zr (10 mol%)
tBuOMe, toluene-20 °C, 95% Y
80/20 anti/syn, 97%ee
TBDPSOOH
OMe
O
NHCOCF3
O O
NHCOCF3
PMP
OTBDPS
1) NaBH4, MeOH
2) PMPCH(OMe)2, TsOH, DMF
OHNH2
OHTBSO 1) TBAF, THF
2) Ac2O, DMAP, pyr3) C12H25MgBr, Li2CuCl4
1) 2N NaOH-EtOH
2) 1N HCl-THF
62 %Y
51 %Y
O O
NHCOCF3
C13H27
PMP
74 %YL-erythro -sphingosine
Allylations
Ar
O
HSiCl3
N N
H3CH3C
CH3CH3
O
PINDOX
-60 °C
40-85%Y77-98 %ee
Ar
OH
*
SiO
NO
Cl
ClH
Ar
CH3
CH3
NCH3
H3C
1. Malkov, A. V.; Orsini, M.; Pernazza, D.; Muir, K. W.; Langer, V.; Meghani, P.; Kocovsky, P. Org. Lett. 2002, 4, 1047.2. (a) Chemler, S. R.; Roush, W. R. J. Org. Chem. 2003, 68, 1319. (b) Chemler, S. R.; Roush, W. R. Tetrahedron Lett. 1999, 40, 4643.
Chiral Ligands1
Chiral Aldehydes2
RMe
O
H
OH MeSiF3
i -Pr2NEtCH2Cl2, 0 °C
SiO
O F
FF
R
Me
H
Me
R3NHR
OH
CH3
OH
CH3
Other reactions
ROSi(H)R'2
"Pt"/ "Ru"H
R SiO
R' R'
ArXPd(dba)2
TBAF
THF, rt
H
R Ar
OH
Silicon Assisted/ Directed Cross-Coupling Reactions1
1. (a) Denmark, S. E.; Pan, W. Org. Lett. 2001, 3 , 61. (b) Denmark, S. E.; Pan, W. Org. Lett. 2002, 4, 4163.2. Lambert, J. B.; Singer, S. R. J. Organomet. Chem. 2004, 689, 2293.
Self-Assembled Macrocycles2
SiOO
O
ONHSi
OO
O
O NH
SiO O
OO
NH
New Materials with possibilities for usein ion exchange, sensor applications, molecular or chiral recognition, catalysis,environmental remediation, and drug discovery.
Conclusion
Silicon has the ability to go pentavalent and hypercoordinate, as demonstrated bynumerous NMR and X-ray experiments.
The "Spill Over" effect lends reasoning to the increasing electrophilicity of silicon, even as more ligands are appended to it.
Numerous synthetic reactions are made possible by the hypervalent transition statesof silicon compounds.
There are many areas left to be understood, explored, and improved upon in the area of hypervalent silicon chemistry.