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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