HIGHLIGHT

Nitroxide Decomposition: Implications toward NitroxideDesign for Applications in Living Free-RadicalPolymerization

AARON NILSEN, REBECCA BRASLAUDepartment of Chemistry and Biochemistry, University of California at Santa Cruz,Santa Cruz, California 95064

Received 22 September 2005; accepted 17 October 2005DOI: 10.1002/pola.21207Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Nitroxides bearing

an a-hydrogen decompose upon

heating in a bimolecular reaction. A

new mechanism is proposed for the

decomposition of t-butylisopropyl-

phenyl nitroxide (TIPNO) involving

the formation of a head-to-tail dimer,

single electron transfer to form an

oxammonium salt, epimerization to

the corresponding nitrone, and elimi-

nation to form a conjugated oxime.

This mechanism may provide in-

sights into designing new nitroxides

for use in controlled polymerization.

VVC 2005 Wiley Periodicals, Inc. J Polym Sci

Part A: Polym Chem 44: 697–717, 2006

Keywords: decomposition; elec-

tron transfer; initiators; living poly-

merization; nitroxide; radical poly-

merization

Aaron Nilsen is currently a post-doctoral fellow in the department of

Chemistry and Chemical Biology at the University of California, San

Francisco. He received a Ph.D. in Chemistry, focusing on nitroxide-

mediated polymerization, from the University of California, Santa

Cruz in 2005. Before pursuing his Ph.D., he spent several years in the

pharmaceutical industry. He holds a B.S. in biochemistry from the

University of California, San Diego.

AARON NILSEN

This article is dedicated to the memory of Andre Rassat.Correspondence to: R. Braslau (E-mail: braslau@chemistry.

ucsc.edu)

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 697–717 (2006)VVC 2005 Wiley Periodicals, Inc.

697

INTRODUCTION

Nitroxides are kinetically persistent free radicals that

are easily oxidized or reduced and act as reversible

traps for other free-radical species. These unusual

properties have led to a wide range of applications

spanning their use in biology as spin labels1–6 to the

development of organomagnetic materials.7 In organic

chemistry, nitroxides have been used in kinetic stud-

ies,8–11 as oxidizing species in the form of the corre-

sponding N-oxo ammonium salts,12–23 as temporary

caps for transient carbon radicals,24–31 and as probes

for stereocontrol with prochiral carbon radicals.32–36 In

polymer chemistry, nitroxide-mediated polymerization

(NMP)37–47 has become popular as a method for pre-

paring living polymers48–51 under mild, chemoselective

conditions with good control over both the polydisper-

sity and molecular weight. In designing nitroxides for

applications in NMP, much attention has been directed

toward fine-tuning the nitroxide structure to lower the

bond dissociation energy (BDE) of the transient N-alkoxyamines formed during the polymerization pro-

cess, with the goal of running NMP at temperatures

above room temperature but low enough to be compat-

ible with complex functionality and to allow conven-

ient industrial preparation. A number of elegant stud-

ies52–59 have investigated the details of the kinetics of

N-alkoxyamine thermal homolysis (kdissociation) and

recombination (kcombination) as it applies to NMP. In

addition to kinetic considerations, the stability of the

free nitroxide under the polymerization conditions is

also critical. This highlight is focused on decomposi-

tion studies of the a-hydrogen nitroxide t-butylisopro-

pylphenyl nitroxide (TIPNO) developed in this labora-

tory in conjunction with the laboratory of Hawker,45

with implications toward nitroxide design for advances

in NMP.

The persistent radical effect60–63 is the key under-

lying principle of living free-radical polymerization:

highly selective cross-coupling of a persistent radical

and a transient radical during the polymerization proc-

ess provides a protected form of the growing polymer

chain. Transient and persistent radicals are formed at

the same rate. Although in the first moments of the

polymerization transient radicals undergo rapid self-ter-

mination reactions, persistent radicals do not. The

resulting increase in the concentration of persistent rad-

icals makes the subsequent trapping of all new transi-

ent radicals selective for the cross-coupled product. In

addition to nitroxides used in NMP, other persistent

radical species commonly used in living radical poly-

merization include metal complexes in atom transfer

radical polymerization64–76 and tertiary thiocarbonyl-

derived radicals in reversible addition–fragmentation

transfer77–80 polymerization. In the course of polymer-

ization, the concentration of transient polymeric carbon

radicals remains very low, whereas the concentration

of the persistent species remains higher. For successful

control in polymerization, the trapping of transient car-

bon polymer radicals with the persistent species must

occur faster than the addition to the olefin monomer.

This ensures that all polymer chains grow at the same

rate. However, if the concentration of persistent radi-

cals becomes too high, virtually every transient radical

that is formed is trapped before any addition to the

monomer can take place. In this situation, polymeriza-

tion is inhibited, and incomplete consumption of the

olefin monomer is observed. Thus, the concentration of

the persistent species must be high enough to ensure

selective cross-coupling but not so high that addition

to the olefin monomer (chain elongation) is prohibited.

Thus, the chemical stability of the persistent species

under polymerization conditions is critical.

Early work in NMP was conducted with robust

nitroxides such as 2,2,6,6-tetramethylpiperidinyl-1-oxy

(TEMPO), di-t-butyl nitroxide (DTBN), and tetraethyli-

soindoline-2-oxyl nitroxide.41 These polymerizations

demonstrated good control with styrene monomers, but

Rebecca Braslau is an Associate Professor at the University of California,

Santa Cruz. Her group focuses on reactions involving free radical

intermediates: both in the development of novel synthetic methodol-

ogy, and in the use of nitroxide mediated polymerization to design

new materials. She joined the faculty at UCSC in 1991, following

postdoctoral studies in organic free radical chemistry with Bernd Giese

at the Universitat Basel. She received her Ph.D. under Barry Trost in

organopalladium catalyzed transformations and organosulfur chemistry

from the University of Wisconsin, Madison in 1989. Prior to graduate

studies, Rebecca studied marine natural products chemistry as a Wat-

son Fellow with John Coll in Australia. Her undergraduate degree in

chemistry is from Reed College. Rebecca grew up in California.REBECCA BRASLAU

698 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 44 (2006)

not with acrylates, acrylamides, acrylonitrile, and dienes.

With styrene, the autopolymerization mechanism, as de-

scribed by Mayo,81 generates small amounts of carbon

radicals that effectively scavenge excess persistent radi-

cals, keeping the concentration of persistent radicals in

balance. However, nonstyrenic monomers do not undergo

analogous reactions. Thus, additives [e.g., acetic anhy-

dride and camphorsulfonic acid (CSA)] have been em-

ployed to keep the concentration of nitroxide from climb-

ing too high. The advent of the use of somewhat robust

a-hydrogen-bearing nitroxides such as TIPNO45 and

diethylphosphono nitroxide (DEPN) (SG1)44,46,47 among

others45,56,82–87 has allowed the extension of NMP to

a much wider range of nonstyrenic olefin monomers.

These a-hydrogen-bearing nitroxides are stable and

easily handled at room temperature but do undergo

bimolecular disproportionation under the thermal con-

ditions of polymerization. Thus, a fine balance is

achieved between keeping the concentration of nitro-

xide high enough to ensure trapping of the growing

polymer chain and preventing excessive nitroxide

buildup that inhibits chain elongation.

There has been much discussion concerning the mech-

anism of disproportionation of a-hydrogen nitroxides.

Thus, we have studied the decomposition of TIPNO as a

representative member of the class of a-hydrogen nitro-

xides. The following is an examination of the pertinent

literature, in conjunction with these decomposition stud-

ies, with the hope of gaining a clearer understanding of

the structural features that will allow the development of

even better nitroxides for use in NMP.

NITROXIDE BACKGROUND

Nitroxides are N,N-disubstituted N��O compounds

with an unpaired electron delocalized in the N��O psystem. This three-electron p system effectively forms

a bond order of one-and-a-half as indicated by the

bond energy of 100 kcal/mol, midway between the

energy of a N��O single bond (53 kcal/mol) and a

N¼¼O double bond (145 kcal/mol).88 The gain in

energy from the delocalization of the unpaired electron

has been calculated to be approximately 30 kcal/mol.89

Nitroxides are commonly formed by the treatment

of corresponding hydroxylamines such as 1 with mild

oxidants such as oxygen, with or without a catalyst

(Scheme 1). Further oxidation of the nitroxide with a

slightly stronger oxidant such as bleach or bromine

produces oxammonium salt 2. The reduction of nitrox-

ides with mild reducing agents such as phenyl hydra-

zine90 and ascorbic acid91 gives the corresponding

hydroxylamine 1. The further reduction of the hydroxyl-

amine with a stronger reducing agent can result in cleav-

age of the N��O bond, providing the corresponding sec-

ondary amine.92

The use of catalytic nitroxides in the form of the

corresponding oxammonium salts (2) for the oxida-

tion of primary and secondary alcohols to aldehydes

and ketones was first described by Golubev et al.93

(Scheme 2). Amines, phosphines, phenols, and ani-

lines can also be oxidized with nitroxides. Active

oxammonium salt 2 can be isolated and used as a

stoichiometric oxidant with the judicious choice of a

counterion,17 or more commonly it is formed in situfrom catalytic nitroxide with the aid of a primary

oxidant.12,14–16,23,94–101

Ma and Bobbitt102 demonstrated that alcohols can

be oxidized by oxammonium salts (3) generated by the

acid-promoted disproportionation of excess TEMPO

(Scheme 3). Essentially, quantitative yields of alde-

hydes and ketones are obtained with both primary and

secondary alcohols by a reaction with a mixture of

TEMPO and p-toluenesulfonic acid. The acid promotes

disproportionation of the nitroxide to the corresponding

hydroxylamine 1 and oxammonium salt 3, driving the

Scheme 2

Scheme 1

HIGHLIGHT 699

reaction to completion by the formation of the hydroxyl-

ammonium tosylate salt 4.Oxammonium-mediated oxidations are not limited

to TEMPO derivatives. There are examples using an

assortment of cyclic21,103–107 and acyclic103,108 nitrox-

ides. However, none of these nitroxides outperforms

TEMPO and its derivatives. Rychnovsky et al.18 meas-

ured the redox potentials of oxammonium salts formed

from cyclic nitroxides, using cyclic voltammetry,

whereas Suemmermann and Deffner109 measured those

of acyclic nitroxides. There are also examples of oxi-

dations carried out with oxammonium salts formed

electrochemically.20,110–112

The photoexcitation of nitroxides leads to additionalpatterns of reactivity. Keana and Baitis113 observed

that the photocycloelimination of nitric oxide fromnitroxide 5 gives diene 6 via a proposed initial a-scis-sion reaction (Scheme 4). Koch and coworkers114,115

found that a scission of DTBN provides a t-butyl radi-cal that is trapped by another equivalent of DTBN to

form N-alkoxyamine 7.Photochemically excited nitroxides also abstract

hydrogen atoms. Call and Ullman116 found that the

preparative photolysis of nitronyl nitroxide 8 yielded

nitronyl nitroxide 9, formed via intramolecular hydro-

gen abstraction by the photochemically excited nitrox-

ide in a 6-membered transition state (Scheme 5). The

primary carbon radical 10 undergoes a rearrangement

to tertiary radical 12, which is trapped by oxygen to

form the observed product 9. The photolysis of

nitronyl nitroxide 13 provided the identical product.

Keana and coworkers117,118 and later Scaiano et al.119

demonstrated that photochemically excited 1,1,3,3-tet-

ramethylisoindolinyl-2-oxy radical (TEMPO) abstracts

hydrogen atoms, whereas Bottle et al.120 also observed

that unactivated hydrogen atoms are abstracted by the

photochemical excitation of the nitroxide TEMIO.

In works by several groups, ground-state nitroxideshave been shown to abstract hydrogen atoms as a slow

but detectable process. In works by both Connolly andScaiano121 and Opeida et al.,122 TEMPO abstracted

activated hydrogen atoms at elevated temperatures.Electron-poor nitroxides such as bis(trifluoromethyl)

nitroxide123–125 and acyl nitroxide phthalimide N-oxyl(PINO)126 readily abstract unactivated hydrogen atomsunder ambient conditions. In an elegant set of experi-

ments, Coseri, Mendenhall, and Ingold studied the pro-pensity of nitroxides to abstract a secondary allylic

hydrogen rather than add to a secondary olefin. Spe-cifically, 4-hydroxy-2,2,6,6-tetramethylpiperidnyl-1-oxy

(TEMPOL), PINO, and di-t-butyliminoxyl radical wereallowed to react with 1,2-dideuteriocyclohexene (14) at64, 70, and 88 8C for 72, 72, and 48 h, respectively.

They conclusively demonstrated that both the nitro-xides and the iminoxyl radical are approximately four

times more likely to follow an abstraction–additionpathway than an addition–abstraction pathway (Scheme

6).127 The abstraction of an allylic hydrogen atom from14 by a nitroxide gives allylic radical intermediate 17,which is trapped by nitroxide to give regioisomeric

cyclohexenes 16 and 18 as the major pathway. Alterna-tively, the addition of a nitroxide to olefin 14 generates

radical intermediate 15; hydrogen abstraction forms1,2-dideuteriocyclohex-2-ene (16). The ratio of cyclo-

hexenes 16 to 18 indicates that TEMPOL abstracts ahydrogen atom approximately 80% of the time. Both

mechanisms are operative for combinations of reac-tions of TEMPOL, PINO, and di-t-butyliminoxyl radi-cal with 1,2-dideuteriocyclohexene, 1,2-dideuteriocy-

clooctene, and trans-3,4-dideuteriohex-3-ene.128 DeBel-lis et al.129 also studied the reaction of TEMPOL with

cyclohexene. From product studies, they inferred a pre-dominant abstraction–addition mechanism. However,

they were unable to provide evidence for their postu-lated intermediates. Nonetheless, all these studies clearlyindicate that TEMPOL is capable of both hydrogen

abstraction and olefin addition at long reaction times attemperatures in the mid 60 to mid 80 8C range.Scheme 4

Scheme 3

700 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 44 (2006)

Nitroxides often decompose via their respective

oxammonium intermediates, which are formed by the

oxidative disproportionation of nitroxides. Oxidations

using oxammonium salts can be performed in acidic or

alkaline media, and a number of TEMPO analogues

can be employed. However, 4-oxo-2,2,6,6-tetramethyl-

piperidinyl-1-oxy (TEMPONE) or TEMPOL cannot be

used under alkaline conditions because they decompose

via the TEMPONE oxammonium (Scheme 7).130,131 In

oxidative media, TEMPOL is oxidized to TEMPONE,

which disproportionates to TEMPONE oxammonium

19. The deprotonation of the acidic a-keto hydrogen

results in elimination to form nitroso compound 20,which undergoes further decomposition.

Oxammonium salts have been used to oxidize

ketones to a-diketones, as first demonstrated by Golubev

and Miklyush;132 Hunter et al.133 showed that N-alkoxyamine 22, formed by the trapping of an a-car-bonyl radical with nitroxide, is an intermediate in this

oxidation (Scheme 8). Torii et al.134 isolated both the

a-keto aldehyde and the N-alkoxyamine while perform-

ing the oxidation of primary alcohols with a TEMPO

derivative, a ruthenium catalyst, and molecular oxygen.

It is tempting to propose that the mechanism for this

reaction is hydrogen atom abstraction followed by nitro-

xide trapping of the resulting radical. However, Ren

et al.135 proposed that these reactions occur by single

electron oxidation of an enol followed by trapping of the

resulting radical with nitroxide. The mechanism is illus-

trated in Scheme 9: the oxammonium of 4-methoxy-

2,2,6,6-tetramethylpiperidnyl-1-oxy (TEMPOL-Me) (25)oxidizes dienol 27 of b,c-unsaturated ketone 24 to

the corresponding enol radical cation 28 by single

electron transfer. The loss of a proton forms delocal-

ized radical 29, which is trapped at the least hin-

dered position by the resulting nitroxide to form N-alkoxyamine 26.

The slow decomposition of TEMPONE can also be

explained by enol oxidation. Murayama and co-

workers136,137 observed that the thermal decomposition

of TEMPONE gives the corresponding hydroxylamine

and trace amounts of phorone 34 (Scheme 10). When

TEMPONE was allowed to stand at room temperature

for 6 months, a diamagnetic decomposition product, N-alkoxyamine 37, was formed. A mechanism involving

hydrogen abstraction by nitroxide was proposed to

account for the observed products. However, an eno-

late oxidation mechanism also explains the formation

Scheme 6

Scheme 5

HIGHLIGHT 701

of these products. In this latter pathway, TEMPONE

disproportionates to the corresponding hydroxylamine

anion 30 and oxammonium cation 31. The hydroxyl-

amine enol 35 is oxidized by oxammonium 31 via sin-

gle electron transfer (SET) to the radical cation 36,which after proton loss is trapped by TEMPONE to give

N-alkoxyamine 37. Oxammonium enol 32 undergoes

b elimination to form nitroso intermediate 33. The enol

of the nitroso is oxidized by another equivalent of oxam-

monium to the radical cation, which eliminates nitric

oxide to give phorone 34. Thus, the decomposition of

TEMPONE can be explained without the invocation of

hydrogen abstraction under ambient conditions.

The oxidative decomposition of TEMIO during the

benzoyl peroxide initiated polymerization of styrene

produces a complex mixture of decomposition products

derived from an oxammonium intermediate (Scheme

11).138 Nitroxides enhance the rate of homolysis of

peroxide polymerization initiators by homosolvolysis

(induced decomposition) of peroxide initiators.139,140

The homosolvolysis of 1 equiv of benzoyl peroxide in

the presence of TEMIO produces 1 equiv of benzoyl-

oxyl radical and 1 equiv of N-alkoxyamine 38, whichcan also be represented as the oxammonium benzoate

salt 39 (Scheme 11). The oxammonium fragments to

nitroso 40, which traps radicals141 and monomer142,143

to give a complex mixture of products. If acetone is

added to the polymerization, N-alkoxyamine 41 is iso-

lated as the sole nitroxide decomposition product. Pre-

sumably, this is formed by the oxammonium oxidation

of the enol of acetone, followed by trapping of the

resulting radical cation with TEMIO. The oxidative

decomposition of TEMPO during the benzoyl peroxide

initiated polymerization of styrene has been found to

produce decomposition products by a similar mecha-

nism.144–146 This method of scavenging excess nitroxide

during NMP can improve the efficiency of the polymer-

ization by increasing the rate of monomer consumption.

Organic acids, organic salts, and anhydrides have

also been used as additives in TEMPO-mediated radi-

cal polymerizations to increase the polymerization rate.

Georges et al.147 demonstrated that the addition of

CSA to a benzoyl peroxide (BPO)-initiated, TEMPO-

mediated polymerization of styrene increased the

molecular weight and monomer conversion and

decreased the polydispersity. Because free-radical styr-

ene polymerization in the absence of nitroxide is not

affected by the addition of CSA, it was deduced that

CSA interacts with the nitroxide. Georges and co-

workers148,149 found that the addition of CSA increases

the consumption of TEMPO during the polymerization.

A number of groups have used CSA to increase poly-

merization rates,150,151 although no mechanism has

been offered. During the polymerization, an equili-

brium exists between the dormant polymer 42 and the

growing polymer chain 43 and the mediating nitroxide

(Scheme 12). Over the course of the polymerization,

chain-termination events cause a buildup of the

TEMPO concentration that forces this equilibrium to

the side of the dormant polymer 42, effectively inhibit-

ing chain elongation. The removal of the excess nitro-

xide shifts this equilibrium back to the side of the

active polymer radical, thus accelerating the polymer-

ization rate. Protonation of the nitroxide oxygen152 is

likely the first step, followed by acid-promoted dispro-

portionation of TEMPO to oxammonium salt 46 and

hydroxylamine 1 to remove the nitroxide from the

equilibrium process. Alternatively, several groups have

enhanced polymerization rates by the continuous addi-

tion of radical initiators to consume excess nitro-

Scheme 8

Scheme 7

702 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 44 (2006)

xide.153–157 The organic acid salt 2-fluoro-1-methylpyr-idinium p-toluenesulfonate has been used to trap themediating nitroxide as its oxammonium salt 47 andhydroxylamine anion salt 48 in much the same way asa Brønsted acid.158,159 The addition of acetic anhydridehas also been shown to increase TEMPO-mediated radi-cal polymerization rates.160 To account for this observa-tion, it has been proposed that the N-alkoxyamine nitro-gen becomes acylated.160 Given the very sterically hin-dered nature of this doubly neopentyl nitrogen, a muchmore likely explanation is the O-acylation of thehydroxylamine anion disproportionation product 45 toform 50 and TEMPO oxammonium acetate salt 49.Bakunov et al.161 independently demonstrated the O-acylation of a doubly neopentyl hydroxylamine withacetic anhydride. As can be seen, a variety of methodshave been developed to decompose the very smallamounts of excess nitroxide that form during NMP. Theuse of nitroxides that disproportionate is another strat-egy for maintaining a low but sufficient nitroxide con-centration under polymerization conditions.

DECOMPOSITION OF a-HYDROGENNITROXIDES

Ingold et al.162 used electron paramagnetic resonance

(EPR) techniques to study the kinetics of the self-reac-

tion of nitroxides bearing one or more hydrogen atomson at least one of the carbon atoms a to the nitroxidenitrogen atom. It was found that the rate of decomposi-tion of these a-hydrogen nitroxides depends on thesteric bulk. Dimethyl nitroxide and diethyl nitroxidedecompose 3900 and 1300 times faster, respectively,than diisopropyl nitroxide in dichlorodifluoromethane.In benzene, the rates of decomposition are 5100 and3300 times faster, respectively, than that of diisopropylnitroxide. These a-hydrogen nitroxides disproportionateto form nitrones and hydroxylamines as the primarydecomposition products.

During these experiments, Ingold and coworkersobserved the reversible formation of diamagnetic di-mers of diethyl163 and dibenzyl and 2,2,5,5-tetradeu-

teriopyrrolidine nitroxides.162 Bistrifluoromethyl nitrox-

ide, tetraphenylpyrrole nitroxide,164 Fremy’s salt,163 and

bicyclic a-hydrogen nitroxides have also been shown

to dimerize reversibly.165 However, dimethyl nitroxide,

pyrrolidine nitroxide, and piperidine nitroxide decom-

pose too rapidly for dimer formation to be detected.162

Although dimerization was not detected for diisopropyl

nitroxide or TEMPO, it is possible that dimer is formed

but is below the limit of detection by EPR. As a result

of these observations, two possible mechanisms were

proposed for the formation of the primary decomposi-

tion products nitrone 52 and hydroxylamine 53: rever-

Scheme 10

Scheme 9

HIGHLIGHT 703

sible nitroxide dimerization (k1) followed by intramo-

lecular hydrogen atom transfer via a concerted 5-cen-

tered transition state (k2) or direct abstraction of the

a-hydrogen atom by another molecule of the nitroxide

(k3; Scheme 13).

On the basis of the kinetics for the decomposition

of diethyl nitroxide, Ingold et al.162 deduced that the

disproportionation occurs via a two-step mechanism.

The rate of irreversible decomposition is slowed in

deuterium-substituted diethyl nitroxide, and this allows

the dimerization equilibrium (K1) to be accurately

determined. The overall rate and calculated values of

K1 and the Arrhenius parameter (A) support a two-step

mechanism over a single-step mechanism. The values

are similar to those of isoelectronic peroxyl radicals,

which disproportionate via a two-step mechanism.166

Deuterium-substituted diethyl nitroxide induces an iso-

tope effect that demonstrates that for nonsterically hin-

dered nitroxides, the removal of the hydrogen (k2) is

rate-limiting. However, for stable a-hydrogen nitrox-

Scheme 12

Scheme 11

704 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 44 (2006)

ides, the steric bulk decreases the rate of dimerization

(k1), so dimerization becomes the rate-limiting step in

the disproportionation.

Because the dimerization of sterically hindered a-hydrogen nitroxides such as diisopropyl nitroxide is

not observed by EPR,162 one might conclude that the

disproportionation of sterically hindered a-hydrogennitroxides occurs through direct a-hydrogen abstraction

(k3). On the basis of orbital overlap arguments, Aurich

et al.89 postulated that the activation energy minimum

(Emin) for a-hydrogen abstraction from a dialkyl nitrox-

ide occurs when the a-carbon/a-hydrogen bond and the

singly occupied molecular orbital (SOMO) of the

N��O bond are coplanar. In this case, the dihedral

angle / between the SOMO and the a-carbon/a-hydro-gen bond is 08 (Scheme 14). Conversely, the activation

energy for a-hydrogen abstraction is at a maximum

(Emax) when the a-carbon/a-hydrogen bond and SOMO

are orthogonal to each other. In this case, the a-carbon/a-hydrogen bond is in the nodal plane of the p system

of the N��O bond, and / is 908. Although ground-state

nitroxides do not readily abstract hydrogen atoms, the

stability of sterically hindered acyclic a-hydrogen nitro-

xides has been attributed to their tendency to remain

close to the Emax conformation and thus inhibit the

removal of the a-hydrogen by another nitroxide mole-

cule. For highly sterically hindered nitroxides, it is dif-

ficult to imagine that the removal of the a-hydrogen is

less sterically demanding than O��O dimerization.

Several groups have used Aurich’s hypothesis to ex-

plain the relative stability of particular a-hydrogen nitro-

xides. Reznikov et al.167 published a crystal structure in

which a-hydrogen nitroxide 54 (Fig. 1) lies close to

Aurich’s Emax conformation (/ � 778). The magnitude of

Scheme 14

Scheme 13

HIGHLIGHT 705

the EPR hyperfine coupling of the a-hydrogen was also

used to obtain a / value of approximately 808 in solution

for all a-hydrogen nitroxides investigated. In reference to

the crystal structure of nitroxide 55, Tordo et al.168 noted

that the a-carbon/a-hydrogen bond is in the nodal plane of

the N��O p system. These experimental findings of

stable a-hydrogen nitroxides adopting a conformation

close to the predicted Emax value has led credence to

the direct hydrogen abstraction mechanism, despite the

kinetic evidence for a two-step process.

It is difficult to accept that sterically hindered nitro-

xides decompose by a different mechanism than their

less sterically hindered counterparts. As a result, sev-

eral groups have applied conformational arguments to

Ingold’s original two-step mechanism. After the forma-

tion of the oxygen–oxygen dimer, the concerted hydro-

gen atom transfer would presumably be a pericyclic

reaction in which the nitrogen lone pair, C��H bond,

and breaking O��O bond form an overlapping six-elec-

tron array. This would require an O��N��C��H dihe-

dral angle of approximately 908. Regarding the crystal

structure of N-alkoxyamine 56, Hawker et al.169 noted

that / between the O��N��C��H is 167.548; thus, thea-carbon/a-hydrogen bond is almost transoid to the

N/O bond. With reference to Ingold et al.’s early

work,162 it was noted that ‘‘because disproportionation

is thought to occur via a five-membered transition state

with the O and the H-atoms cisoid, the tendency of

nitroxides . . . to undergo disproportionation is signifi-

cantly reduced.’’ In this conformation, the a-hydrogenwould have to rotate substantially to undergo removal.

The five-centered transition state involves the oxygen–

oxygen dimerization of two nitroxides, whereas this X-

ray depicts the crystal structure of an N-alkoxyamine.

However, the N-alkoxyamine may be the best dimer

analogue, as both the postulated O��O dimer and the

N-alkoxyamine contain sp3 nitrogens. The crystal struc-

ture of N-alkoxyamine 58 derived from the stable a-hydrogen nitroxide TIPNO shows the same transoid

conformation (Fig. 2).36 In reference to the recently

published crystal structure of a-hydrogen nitroxide 57,Studer et al.86 made similar stability arguments. How-

ever, one should be somewhat cautious in comparing the

structure of this nitroxide to the structure of its oxygen–

oxygen dimer because the nitroxide nitrogen is sp2 pla-

nar, whereas the nitrogen of the dimer is sp3 tetrahedral.

Ingold et al.162 showed that for diethyl nitroxide,

the removal of the hydrogen is rate-limiting. It is thus

tempting to explain the stability of sterically hindered

a-hydrogen nitroxides by the slow rate of removal of

the a-hydrogen, whether by a-hydrogen abstraction (k3)or via a five-centered transition state (k2). However, a-hydrogen removal is only rate-limiting in the dispro-

portionation of nonsterically hindered nitroxides, in

which dimerization is facile.

The structure of the nitroxide–nitroxide dimer is un-

clear. Dimers of a-hydrogen nitroxides have not been

isolated, and EPR data provide no insight into the

structure of the dimer other than the paramagnetic char-

acter. The gain in energy from the delocalization of the

unpaired electron has been calculated to be approxi-

mately 30 kcal/mol. The gain in energy from the forma-

Figure 2. Crystal structure of TIPNO-based N-alkoxyamine.

Figure 1. Examples of compounds with relevant crystal structures.

706 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 44 (2006)

tion of an oxygen–oxygen r bond has been calculated to

be approximately 35 kcal/mol. Thus, the gain in energy

from O��O dimerization (35 kcal/mol) is insufficient to

compensate for the loss of the energy of delocalization

for the two nitroxide molecules (2 � 30 kcal/mol), mak-

ing O��O dimerization thermodynamically unfavorable.89

Ingold et al.163 noted the energetic cost of the O��O rdimer but originally favored this dimer because of an

analogy to the reported r dimer of Fremy’s salt.170

Fremy’s salt forms crystals from aqueous solutions

in two distinct forms.171 Crystallization at temperatures

below 30 8C produces the common dimer in the form

of diamagnetic yellow-orange needles, which was orig-

inally postulated to exist as the oxygen–oxygen bonded

structure 59 (Scheme 15). Crystallization above 30 8Cproduces slightly paramagnetic orange-brown plates

that possess on average one unpaired electron per dimer

unit. Because a paramagnetic dimer pair should contain

two free electrons per unit, a weak covalent bond be-

tween the monomer units is assumed. X-ray analysis of

this form shows two N��O groups arranged in a four-

center heat-to-tail dimer (60). Blackstock172 recently

obtained high-resolution crystal structures that show that

the low-temperature crystalline form of Fremy’s salt is

actually an isomorph of this head-to-tail dimer rather than

the oxygen–oxygen bonded structure 59. Lajzerowicz et

al.173 obtained X-ray data that show that the dimer of

bicyclic nitroxide (61) has the same head-to-tail arrange-

ment (62) as the two isomorphic dimers of Fremy’s salt.

Nitroxides have an inherent dipolar head-to-tail

attraction. Mendenhall and Ingold165 estimated this

dipolar attractive force of crystalline bicyclic nitrox-

ide 61 with a point charge calculation to be 4.9 kcal/

mol. Although it was admitted that the calculation

made some gross assumptions, it was noted that the

dipolar attraction could provide ‘‘a very significant

fraction of the enthalpy for dimerization of unhin-

dered nitroxides in solution.’’ This dipolar attraction

creates a continuum between loosely packed dimeric

paramagnetic species and tightly packed diamagnetic

dimers. The position of a nitroxide dimer pair along

this continuum depends both on the steric bulk and

temperature. At 25 8C, crystalline bicyclic nitroxide

61 is yellow and diamagnetic.165 The yellow color of

61 decreases on cooling. At temperatures above 74 8C,61 is orange to red and gives a strong EPR signal.

Thus, a number of nonsterically hindered nitroxides

appear to form dimers with a head-to-tail arrangement

of nitroxide NO groups. In solution at room tempera-

ture, the stable sterically hindered nitroxide TEMPO is

orange to red and gives a strong EPR signal. The

reversible dimerization of TEMPO is not observed by

EPR. However, TEMPO is known to spontaneously

disproportionate to the corresponding oxammonium

and hydroxylamine in the presence of an acid. Single

electron transfer must occur via the interaction of the

NO p systems of two molecules of TEMPO. On the

basis of known crystal structures of nitroxide dimers

and Ingold’s dipolar attraction calculation, it is reason-

able to assume that the NO group interaction required

for single electron transfer occurs in a head-to-tail ori-

entation. This assumption is supported by the accepted

mechanism of oxammonium oxidation in which the

alcohol to be oxidized adds to the nitrogen and not the

oxygen of the oxammonium salt.22,93,102,174–176 This

form of head-to-tail nitroxide dimerization was first

proposed by Martinie-Hombrouck and Rassat177 to

explain the reversible dimerization and subsequent

decomposition of aryl methyl nitroxides. The proposed

dimer structure is a paramagnetic ion pair with a loose

head-to-tail structure analogous to the known nitroxide

crystal structures 60 and 62. By analogy, nitroxides are

also known to form charge-transfer complexes in solu-

tion with electron acceptors such as 2,3-dichloro-5,6-

dicyano-1,4-benzoquinone.7

PROPOSED a-HYDROGEN NITROXIDEDECOMPOSITION MECHANISM

Through the postulation of the head-to-tail dimer struc-

ture as part of Ingold’s two-step nitroxide decomposi-

Scheme 15

HIGHLIGHT 707

tion, a new mechanism for the decomposition of steri-

cally hindered acyclic a-hydrogen nitroxides emerges,

illustrated here with TIPNO (Scheme 16). In this mecha-

nism, the head-to-tail nitroxide dimer undergoes reversi-

ble SET to form the ion-pair oxammonium 65 and

hydroxylamine anion 64. The oxammonium species tau-

tomerizes, and a proton is transferred to form nitrone 67and hydroxylamine 66. The latter can be reoxidized to

TIPNO in the presence of oxygen. The nitrone undergoes

further decomposition (discussed later).

The formation of the primary decomposition prod-

uct nitrone 67 by the oxidative disproportionation of

TIPNO through the oxammonium intermediate 65 is

supported by literature precedents. Ali and Wazeer178

and others179 have shown that oxammonium com-

pounds are intermediates in the preparation of nitrones

by the oxidation of the corresponding hydroxylamines.

Rychnovsky et al.18 showed that TEMPO can be oxi-

dized to the corresponding oxammonium and reduced

to the corresponding hydroxylamine anion electro-

chemically. This is the common pattern observed with

bisneopentyl nitroxides in which the corresponding

oxammonium cannot undergo tautomerization.

To determine if the disproportionation products 64and 65 could be formed during the decomposition of

TIPNO in a manner similar to the disproportionation

of TEMPO, we examined the redox behavior of

TIPNO by cyclic voltammetry. As a control, the cyclic

voltammogram (CV) of TEMPO was first recorded

with a procedure based on the protocol of Rychnovsky

et al.18 The CV of TIPNO was recorded (Fig. 3). This

CV shows an oxidation peak at �750 mV. The very

small reduction peak at �675 mV indicates that the

oxidized TIPNO decomposes almost completely and

that this reaction is almost completely irreversible. The

reduction peak at �0 mV is likely due to the reduction

of protons. The source of the protons is assumed to be

those derived from the tautomerization of the oxammo-

nium to the nitrone. These cyclic voltammetry experi-

ments show that TIPNO can be oxidized electrochemi-

cally and that the oxammonium species decomposes

rapidly. It is also possible to chemically oxidize TIPNO

to nitrone 67. The oxidation of TIPNO with hydrogen

peroxide and catalytic sodium tungstate180 provided the

nitrone in a 27% yield after flash chromatography (56%

based on the recovered nitroxide starting material;

Scheme 17).

The products derived from the direct thermolysis of

TIPNO were next examined. When neat TIPNO was

heated to 120 8C for 12 h, oxime 68 was isolated as a

1:1 mixture of E/Z isomers in a 39% yield in addition

to 10% of the recovered nitroxide (Scheme 18). Simi-

lar results were obtained when the nitroxide was

refluxed for 12 h in toluene. The identification of

oxime 68 was confirmed by authentic synthesis by the

condensation of isobutyrophenone with hydroxylamine.

No other decomposition products were isolated. It was

not surprising that hydroxylamine 66 was not isolated,

as dialkyl hydroxylamines oxidize easily in air to the

corresponding nitroxides.

The isolation of oxime 68 as the sole decomposition

product from the thermolysis of TIPNO is intriguing.

One might be tempted to postulate an a-scission proc-

ess to account for the loss of the t-butyl group, as ascission is thought to occur in the decomposition of

monosubstituted alkyl and aryl nitroxides.181 However,

Scheme 16

708 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 44 (2006)

DTBN is thermally robust and has been used in

NMPs.182–188 One might also propose the loss of the t-butyl cation from oxammonium intermediate 65. Thisis also unlikely, as DTBN has been used catalytically

in the oxammonium-mediated oxidations of alco-

hols.109 Thus, oxime 68 is most likely formed from

nitrone 67 by the loss of the t-butyl cation followed by

proton transfer.

To test the thermal stability of nitrone 67, an

authentic sample was prepared. The titanium tetra-

chloride catalyzed condensation of t-butyl amine and

isobutyrophenone afforded the ketimine, which was

reduced with sodium borohydride to give the secon-

dary amine 69 in an 88% yield after flash chromato-

graphy (Scheme 19). With Goti’s rhenium metal cata-

lyzed oxidation conditions,189–192 the secondary amine

was oxidized with urea hydrogen peroxide (UHP) and

methyl trioxorhenium (MTO) to give an easily separa-

ble mixture of products. Flash chromatography af-

forded nitrone 67 in a 58% yield and TIPNO in a 33%

yield. With nitrone 67 in hand, it was subjected to

refluxing for 12 h in deuterated toluene. Oxime 68 was

the only product detected in the 1H NMR spectrum. As

a control, TIPNO was also refluxed for 12 h in deuter-

ated toluene. The only products detected (by thin-layer

chromatography) were oxime 68 and residual nitroxide.

The intermediate nitrone 67 was not detected. The

complete conversion of nitrone 67 to oxime 68 upon

thermolysis strongly supports the intermediacy of nitrone

67 during the decomposition of TIPNO. Because nitrone

67 was not observed in the TIPNO decomposition

experiments, the conversion of nitrone to oxime must

proceed faster than nitrone formation. Indeed, the

decomposition of nitrone 67 to oxime went to comple-

Scheme 17

Figure 3. CV of TIPNO recorded with a Pt electrode and a saturated calomel electrode refer-

ence in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate as the electrolyte and

1 mM TIPNO as the analyte. The instrument was calibrated with TEMPO.

Scheme 18

HIGHLIGHT 709

tion under conditions in which TIPNO was not com-

pletely consumed.

Thus, it seems very likely that TIPNO decomposes

by slow, reversible disproportionation followed by the

rapid conversion of nitrone 67 to the sole decomposi-

tion product, oxime 68. Both Briere and Rassat193 and

Ingold et al.163 observed the formation of various

decomposition products resulting from radical addition

to nitrone intermediates. In the TIPNO decomposition

studies presented here, no nitrone trapping products

were observed. One must conclude either that they

formed but were thermally labile or that the rate of

decomposition of nitrone 67 is very fast.

Phenyl t-butyl nitrone (BPN) is a common spin trap

that is stable at typical NMP temperatures. In general,

aryl nitrones are more stable and less reactive than

their aliphatic counterparts because the C¼¼NO p sys-

tem of the nitrone is stabilized by overlap with the aryl

p system. A simple MOPAC energy minimization of

BPN predicts the phenyl ring p system to be nearly

coplanar with the C¼¼NO p system. In contrast, a sim-

ple MOPAC energy minimization of nitrone 67 (Fig.

4) predicts the phenyl ring to be twisted nearly 908 outof the plane of the nitrone C¼¼NO p system. Indeed,

X-ray crystal structures of sterically hindered C-aryl-

substituted nitrones show that the aryl groups are

twisted out of the plane of the C¼¼NO p system.194–204

To probe the conformations, degrees of conjugation,

and thermal stabilities of sterically hindered aryl nitro-

nes, a series of substituted nitrones was prepared. BPN

was prepared by the simple condensation of t-butylhydroxylamine (generated in situ by zinc metal reduc-

tion of 2-methyl-2-nitropropane) and benzaldehyde in a

42% yield after flash chromatography. Secondary

amines were prepared by the titanium tetrachloride cata-

lyzed condensation of t-butylamine and acetophenone,

propiophenone, or isobutyrophenone, followed by the

reduction of the resulting ketimines with sodium boro-

hydride. Flash chromatography provided the product

secondary amines in 44, 39, and 88% yields, respec-

tively. The amines were oxidized to the corresponding

nitrones 70, 71, and 67 in 50, 42, and 58% yields,

respectively, with UHP and MTO.190 Because all these

nitrones are liquids at room temperature, analysis by

X-ray crystallography was not an option. Thus, the

extent of ring twisting was probed through the evalua-

tion of the conjugation by UV spectroscopy. The

wavelength of maximum absorbance (kmax) values

decrease with the steric bulk (Fig. 5) and approach

kmax of an isolated phenyl ring (254 nm). In compari-

son, Emmons205 reported a kmax value of 249 nm for

oxaziridine 72, in which the phenyl group enjoys no poverlap with a neighboring functionality.

Multitemperature 1H NMR analysis was used to com-

pare the rates of decomposition in this series of nitrones.1H NMR spectra were recorded for five 5-mg samples

in 0.700 mL of deuterated toluene each for three nitro-

nes: R ¼ H, Et, or iPr. Five samples each of these three

nitrones were sealed and heated for 2 h at 70, 80, 90,

100, or 110 8C. The samples were allowed to cool to

room temperature, and 1H NMR spectra were recorded.

The decomposition percentage was determined by inte-

gration, and the results are reported in Table 1.

The data show that the rate of decomposition

increases with increased steric bulk of the substituents.

The decomposition of BPN (R ¼ H) was not observed

at 70 and 80 8C. Only a small amount of decomposi-

tion of BPN was observed at 90, 100, and 110 8C. Thesame trend was observed for the ethyl-substituted

nitrone 71, with the decomposition percentage increas-

Scheme 19

Figure 4. Nitrones and a related oxaziridine.

710 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 44 (2006)

ing at each temperature. Isopropyl-substituted nitrone

67 decomposed to some extent at all of the tempera-

tures tested, and the decomposition percentage increased

with an increase in the temperature. An increase in the

steric bulk from ethyl to isopropyl at 110 8C led to an

almost sixfold increase in the amount of decomposition.

Thus, it appears that an increase in the steric bulk of

the nitrone substituents prevents conjugation of the

phenyl group with the nitrone p system and presents a

driving force for the loss of the t-butyl cation to form

the planar, conjugated oxime.

Finally, the thermal decomposition of TIPNO in sol-

vent systems of various polarities was monitored by

ultraviolet–visible (UV–vis) spectroscopy with the goal

of probing the structure of the nitroxide dimer 63. Theformation of a covalently bound dimer would be

favored by nonpolar solvents, whereas the formation of

an ion-pair dimer would be favored by polar solvents.

The decomposition of TIPNO was monitored at

120 8C in bromobenzene, 90:10 bromobenzene/dime-

thylformamide (DMF), 50:50 bromobenzene/DMF, and

DMF at 435 nm. TIPNO has an absorbance at 435 nm

due to its yellow-orange color, whereas the decomposi-

tion products (nitrone 67 and oxime 68) do not.

Decomposition was accelerated in nonpolar solvents,

and this indicates that the dimer is closer to a covalen-

tly bonded head-to-tail dimer than an ion pair (Fig. 6).

When this dimer dissociates, it can either reform the

two nitroxides or form the stable oxammonium species

and hydroxylamine anion via SET.

CONCLUSIONS

Herein a new mechanism for the decomposition of a-hydrogen nitroxides is proposed. In this postulated

mechanism, the transient head-to-tail nitroxide dimer

63 undergoes disproportionation via reversible single

electron transfer to form hydroxylamine anion 64 and

oxammonium 65. The oxammonium ion tautomerizes,

and a proton is transferred to give nitrone 67 and

hydroxylamine 66, the later of which is reoxidized by

oxygen to TIPNO. The nitrone 67 rapidly decomposes

to oxime 68 by the loss of the t-butyl cation followed

by proton transfer.

Cyclic voltammetry of TIPNO indicates that it

undergoes oxidation in a similar manner to TEMPO.

Figure 5. Decomposition of nitrones as monitored by UV–vis.

Table 1. Nitrone Decomposition Data

Nitrone

(R)

Decomposition after 2 h (%)

70 8C 80 8C 90 8C 100 8C 110 8C

H 0a 0a 4b 6b 8b

Et 0a 0a 9c 14c 16c

iPr 27d 36d 47d 64d 91d

a No decomposition was observed at this temperature.b As determined by the integration of the sp2 nitrone proton at d

¼ 7.8 ppm.c As determined by the integration of the methylene of the ethyl

group at d ¼ 2.8 ppm.d As determined by the integration of the isopropyl methine mul-

tiplet at d ¼ 3.6 ppm.

HIGHLIGHT 711

However, this oxidation product rapidly decomposes to

such an extent that the electrochemical oxidation is

essentially irreversible. A secondary reduction peak

characteristic of the reduction of a proton indicates that

a proton is liberated in the decomposition. Overall, the

cyclic voltammetry experiment supports the postulate

that TIPNO is oxidized to oxammonium 65, which rap-

idly decomposes to nitrone 67. The formation of oxam-

monium 65 is likely due to single electron transfer in

head-to-tail nitroxide dimer 63.In separate experiments, TIPNO and nitrone 67

were both found to thermally decompose to oxime 68as the only detectable product. These experiments sup-

port the postulate that nitrone 67 is an intermediate in

the decomposition of TIPNO and that oxime 68 is

derived directly from nitrone 67.Evidence is presented that the phenyl ring of steri-

cally congested nitrone 67 is twisted out of the plane

of the nitrone C¼¼NO p system. This loss of conjuga-

tion causes nitrone 67 to decompose at a faster rate

than less sterically congested analogues. The nitrone

decomposes at a faster rate than the parent nitroxide

TIPNO. The rapid irreversible conversion of nitrone 67to oxime 68 at typical polymerization temperatures

provides an irreversible pathway for the removal of

excess free nitroxide during NMP. As the first step of

nitroxide decomposition is a bimolecular dimerization,

the steric hindrance of TIPNO controls the rate of sin-

gle electron transfer to give oxammonium 65 and

hydroxylamine anion 64. Reversible equilibration of

oxammonium 65 to nitrone 67 followed by rapid irre-

versible conversion to oxime 68 ensues. Thus, part of

the success of TIPNO as an excellent NMP mediator is

the result of a fine balance between the steric bulk to

moderate the rate of dimerization (and thus single elec-

tron transfer) and nitrone instability.

Nitrones that are not sterically impacted do not

effectively decompose. Thus, they can equilibrate back

to oxammonium (and ultimately nitroxide) or act as

spin traps during the polymerization. Nitroxides such

as TIPNO and DEPN,46,168 which bear a phenyl group

or phosphonate ester, can form stabilized conjugate psystems in the oxime. However, they are prevented

from enjoying p stabilization in the nitrone by steric

congestion and thus are particularly subject to rapid

nitrone decomposition by the loss of the t-butyl cation.This postulate suggests that other substituents that

extend the p system of the oxime but are sterically

prevented from doing so in the nitrone would make good

candidates in the design of new nitroxides for NMP.

These studies provide a basis for predicting the

required elements in designing improved nitroxides for

NMP. In addition to a low BDE of the parent N-alkoxyamine, the nitroxide should bear an a-hydrogento allow for tautomerization of the N-oxammonium

species to the nitrone. The nitrogen should have a terti-

ary N-alkyl that can be lost as a stabilized cation to

irreversibly form oxime. Lastly, the a-carbon should

bear a group capable of providing stability to the

oxime via conjugation, provided that the steric bulk of

the second substituent at the a-carbon is suitably tuned.

Thus, both the BDE of the C��O bond of the transient

N-alkoxyamine and the nitroxide stability at the poly-

merization temperatures are important in the design

and development of future nitroxides for use in NMP.

The authors thank the National Science Foundation (CHE-

0078852) for providing financial support for this project. They

thank Keith Ingold and Silas Blackstock for insightful com-

Figure 6. Decomposition of TIPNO at 120 8C as monitored by UV–vis.

712 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 44 (2006)

ments. They also thank Chris Grant and Adam Schwartzberg of

the Jin Zhang research group at the University of California at

Santa Cruz for their help with cyclic voltammetry.

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