An EPR study of the radical addition to 3-nitropentan-2-one as an archetype of α-carbonylnitroalkanes

Article abstract of DOI:10.1002/mrc.4063

Carbon, silicon, germanium, tin and lead-centered radicals were reacted with 3-nitropentan-2-one and 3-nitropentan-2-ol inside the cavity of an electron paramagnetic resonance spectrometer. In all cases, selective addition to the nitrogroup was observed with detection of the corresponding oxynitroxide radicals. In the case of the carbonyl substrate, alkyl acyl nitroxides were also detected because of α-photocleavage. The oxynitroxides decayed with a first order kinetics via fragmentation of the carbon-nitrogen bond (denitration). Unexpectedly, the activation parameters were fairly similar to those previously reported for the corresponding tert-butyl oxynitroxides and almost independent from the presence of a carbonyl or a hydroxyl group on the carbon adjacent to the one bearing the nitrogroup. Copyright

Full text of DOI:10.1002/mrc.4063

Research article
Received: 6 November 2013
Revised: 5 January 2014
Accepted: 25 February 2014
Published online in Wiley Online Library: 18 March 2014
(wileyonlinelibrary.com) DOI 10.1002/mrc.4063
An EPR study of the radical addition to
3-nitropentan-2-one as an archetype
of α-carbonylnitroalkanes
Mylène Campredona* and Angelo Albertib
Carbon, silicon, germanium, tin and lead-centered radicals were reacted with 3-nitropentan-2-one and 3-nitropentan-2-ol
inside the cavity of an electron paramagnetic resonance spectrometer. In all cases, selective addition to the nitrogroup was
observed with detection of the corresponding oxynitroxide radicals. In the case of the carbonyl substrate, alkyl acyl nitroxides
were also detected because of α-photocleavage. The oxynitroxides decayed with a first order kinetics via fragmentation of the
carbon–nitrogen bond (denitration). Unexpectedly, the activation parameters were fairly similar to those previously reported
for the corresponding tert-butyl oxynitroxides and almost independent from the presence of a carbonyl or a hydroxyl group
on the carbon adjacent to the one bearing the nitrogroup. Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: ESR; kinetics; oxynitroxides; α-carbonylnitroalkanes; 3-nitropentan-2-one; 3-nitropentan-2-ol
to the reduced 3-nitropentan-2-ol 2, i.e. the corresponding
unactivated derivative (Fig. 1).
The nitrogroup is one of the most important organic functionalities,
Introduction
and indeed, nitrocompounds have been largely exploited since the
early times of modern organic synthesis,[1] the success of these
derivatives being because of the relative easiness with which they
can be transformed into amines, oximes, imines or into compounds
characterized by a variety of different functionalities.[2] Even
more useful may be 1,2-difunctionalized and 1,4-difunctionalized
nitroderivatives; in particular, α-nitroketones have been proven es-
pecially valuable synthetic intermediates[3,4] to obtain natural prod-
ucts,[5,6] as well as many other derivatives such as pyrazines[7] and
benzoxazines.[8]
In many cases, these procedures involve as a key step a direct
or indirect reductive denitration of nitroalkanes used as reagents
for alkyl anion synthons;[2] on the other hand, denitration can
also be achieved through a radical process based on the use of
tributyltin hydride.[9] Actually, while the latter process takes place
smoothly and efficiently with tertiary nitroalkanes, secondary
nitroalkanes are generally very difficult to denitrate via radical
reactions.[10] However, activation of the nitrogroup by an α-electron
withdrawing substituent such as cyano, carbonyl or ester functions
makes the denitration of sec-nitroalkanes by tributyltin hydride a
rapid and efficient process.[11] Nitrogroups at benzylic or allylic
positions are also readily denitrated with Bu3SnH.[9]
While detailed studies have been focused on the denitration of
nitroxides originating from the addition of carbon, silicon, germa-
nium or tin-centered radicals to 2-methyl-2-nitropropane,[12,13] fewer
mechanistic investigations have been addressed to the correspond-
ing derivatives from secondary α-ketonitrocompounds.
We were therefore prompted to endeavor in a study of the
addition of carbon, silicon, germanium or tin-centered radicals
to 3-nitropentan-2-one 1, chosen as the archetype of secondary
α-carbonylnitroalkanes and of the decay of the resulting
oxynitroxides. For the sake of comparison, we extended the study
Results and Discussion
UV irradiation of oxygen-free benzene solutions of compound 1
or 2 and a number of radical precursors resulted in all cases in
the observation of intense electron paramagnetic resonance
(EPR) spectra, and the hyperfine parameters derived from their
analysis are collected in Tables 1 and 2, respectively. While inter-
pretable spectra were also observed when photolyzing a solution
of compound 1 as such, under similar conditions a solution of 2
only led to the detection of extremely weak spectra that could
not be rationalized.
Photolysis of 1 and 2
Photolysis of a benzene solution of nitroketone 1 afforded the
spectrum shown in Fig. 2 consisting of the signals from two
distinct aminoxyls that were identified as to two acyl-nitroxides
because of their low nitrogen hyperfine splitting (hfs) constant
(0.7 < aN < 0.8 mT) and their fairly high g-factor values
(g = 2.0067).[14]
One of these species (red dots) exhibited, in addition to that
with the nitrogen atom, coupling of the unpaired electron with
a hydrogen atom (I = one half), with an hfs constant value in line
*
Correspondence to: Mylène Campredon, Aix-Marseille Université, Centrale
Marseille, CNRS, Ism2-UMR 7313, 13397 Marseille, France. E-mail: mylene.campredon
@univ-amu.fr
a Aix-Marseille Université, Centrale Marseille, CNRS, Ism2-UMR 7313, 13397 Marseille,
France
b CNR-ISOF, Area della Ricerca di Bologna, Via P. Gobetti 101, I-40129 Bologna, Italy
Magn. Reson. Chem. 2014, 52, 289–297
Copyright © 2014 John Wiley & Sons, Ltd.

M. Campredon and A. Alberti
The formation of these two nitroxides is not obvious. Because
of the simultaneous presence of a carbonyl group and a
nitrogroup, the photobehavior of 1 may be fairly complex.
Indeed, the unfiltered light of the Hg lamp can excite several
intermediates leading to a large number of reactive species. Thus,
in Scheme 1, we outline some possible pathways leading to the
detected species, but we emphasize that they are markedly spec-
ulative and that while we believe them to represent a sensible
route to the resulting species, we have no definite evidence of
their actual relevance.
Figure 1. 3-Nitropentan-2-one 1 and 3-nitropentan-2-ol 2.
with the expectations for a sec-alkyl acyl-nitroxide; the second
species (blue stars) that only showed three lines because of the
nitrogen atom was instead identified as a tert-alkyl acyl-nitroxide.
Table 1. EPR spectral parameters of radicals from the photoreactions of 1 with different radical precursors
Radical precursor
X·
Rad.
aN/mT
aother/mT
g
—
—
3 or 4
5 or 6
1a
0.722
0.754
2.575
1.556
2.762
0.754
2.778
0.754
2.757
2.792
2.732
0.754
2.673
1.535
1.372
0.754
2.791
1.389
2.661
1.436
0.359 (1H)
—
2.00673
2.00675
2.00540
2.00594
2.00528
2.00675
2.00525
2.00675
2.00512
2.00545
2.00523
2.00675
2.00496
2.00611
2.00606
2.00675
2.00530
2.00580
2.00620
2.00579
tBuC(O)tBu
tBu
0.353 (1H)
—
7 or 8
1b
Et3SiH/tBuOOtBu
Ph3SiH/tBuOOtBu
(Me3Si)3SiH/tBuOOtBu
Ph3GeH/tBuOOtBu
Bu3SnSnBu3
Et3Si
0.428 (1H)
—
5 or 6a
1c
Ph3Si
0.400 (1H)
—
5 or 6a
1d
(Me3Si)3Si
Ph3Ge
Bu3Sn
Ph3Sn
cHex3Pb
Ph3Pb
0.482 (1H)
0.550 (1H)
0.472 (1H)
—
1e
1f
5 or 6a
1g
0.553 (1H)
0.312 (1H)
0.364 (1H)
—
9
Ph3SnSnPh3
10
5 or 6a
1i
cHex3PbPbcHex3
Ph3PbPbPh3
0.387 (1H)
0.439 (2H)
0.515 (1H)
0.244 (1H)
11
1k
12
aWeak signal after prolonged irradiation.
Table 2. EPR spectral parameters of radicals from the photoreactions of 2 with different radical precursors
Radical precursor
X·
Rad.
aN/mT
aother/mT
g
—
tBuC(O)tBu
—
tBu
Very weak signals
0.204(1H)
2a
7
2.592
1.579
2.751
1.438
2.755
2.766
1.438
2.735
2.700
1.438
2.722
1.438
2.776
1.460
2.00523
2.00574
2.00530
2.00585
2.00525
2.00525
2.00585
2.00528
2.00504
2.00585
2.00471
2.00585
2.00488
2.00580
—
0.270 (1H), 0.037 (m)a
Et3SiH/tBuOOtBu
Et3Si
2b
13b
2c
0.403 (2H)
Ph3SiH/tBuOOtBu
(Me3Si)3SiH/tBuOOtBu
Ph3Si
0.233 (1H)
0.286 (1H), 0.037 (m)a
(Me3Si)3Si
2d
13b
2f
0.403 (2H)
Ph3GeH/tBuOOtBu
Bu3SnSnBu3
Ph3Ge
Bu3Sn
0.304 (1H), 0.034 (m)a
0.378 (1H), 0.032 (m)a
0.403 (2H)
2g
13b
2h
13b
2i
Ph3SnSnPh3
Ph3Sn
0.354 (1H)
0.403 (2H)
cHex3PbPbcHex3
cHex3Pb
Ph3Pb
0.225 (1H)
14
0.300 (1H), 0.566 (1H)
Ph3PbPbPh3
Signal not interpreted
am stands for undefined multiplet.
bAfter prolonged irradiation.
wileyonlinelibrary.com/journal/mrc
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2014, 52, 289–297

An EPR study of the radical addition to 3-nitropentan-2-one
would result in G, an α-carbonyl-nitrosoderivative that by trapping
of acyl radicals A or C may account for the detected sec-alkyl acyl-
nitroxide 3 or 4. A third possibility is that photoexcitation of 1 re-
sults in the formation, via intermolecular hydrogen abstraction, of
the α-hydroxy-alkyl and α-nitro-alkyl radicals H and J, respectively,
which may recombine with nitric oxide to give the tert-alkyl
nitrosoderivatives K and L. The former can, in turn, trap acyl radicals
A or C to afford the detected tert-alkyl acyl-nitroxide 5 or 6, while a
similar process involving species L should be discarded as the
resulting acyl-nitroxides would be expected to exhibit, contrary to
what is found, a spectrum indicating the interaction of the unpaired
electron also with the nitrogen atom of the α-nitrogroup.
As already stated, and at variance with 1, photolysis of solu-
tions of nitroalcohol 2 only resulted in the detection of extremely
weak EPR signals that could not be interpreted. Because of the
lack of the carbonyl function, photolysis of 2 cannot obviously
result in the formation of acyl-nitroxides, while denitration is still
possible. In analogy to what is proposed for compound 1,
photofragmentation of 2 (Scheme 2) will lead to the alkyl radical
M and ·NO2 that may recombine to nitrite N, the decomposition
of which will afford nitric oxide and the alkoxy radical P.
Combination of ·NO and radical M would result in the formation
of the nitrosoderivative Q that might act as a trap for another
radical M to eventually give the di-sec-alkyl nitroxide R.
The fact that neither this nor other nitroxides could be
detected seems an indication that photodenitration of 2 is not
as efficient as that of 1 and that the presence of the electron-
withdrawing carbonyl group favors the loss of the nitrofunction
when the latter compound is photolyzed.
Figure 2. EPR spectrum observed at room temperature upon photolysis
of a benzene solution of compound 1. Red dots and blue stars identify the
lines of two acyl-nitroxides.
On one side, it is well established that UV–vis irradiation of al-
iphatic carbonyl compounds may result in the fragmentation of a
C(O)–C bond leading to alkyl and acyl radicals. While the cleavage
may take place on either side of the carbonyl group, it seems
reasonable that the cleavage leading to the more stable radical
be slightly favored. Thus, in the case of 1, the formation of
acetyl radical A, with the concomitant formation of a secondary
alkyl radical B, might be favored over that of acyl radical C and
of the less stable methyl radical.
It is also well known that aliphatic nitrocompounds are
photolabile species, their photolysis resulting in the loss of an
•NO2 and the formation of an alkyl radical, in the present case
D. Recombination of these two species does not necessarily lead
back to the starting compound: indeed, •NO2 may exist in differ-
Besides, as outlined in Scheme 3, the radical resulting from the
cleavage of 1 may receive additional resonance stabilization from
delocalization of the unpaired electron onto the oxygen atom,
stabilization instead impossible for the analogous radical from 2.
¨
ent mesomeric forms in one of which, namely •O–N=O, one of the
oxygen atoms bears the unpaired electron. Its recombination
with D would afford the unstable nitrite E that would further
decompose to give the alkoxy radical F and nitric oxide •NO.
The latter might also result from some photoexcited intermedi-
ates. Hydrogen abstraction by the latter may eventually lead to
pentan-3-ol-2-one, while recombination of nitric oxide with D
Radical addition to 1 and 2
When 1 or 2 were photolyzed in the presence of 2,2,4,4-
tetramethyl-3-pentanone (a photolytic source of tert-butyl
radicals) or of Group 14 organometallic hydrides in the presence
of di-tert-butyl peroxide, very intense and similar EPR spectra
Scheme 1. Suggested pathways leading to sec-alkyl acyl-nitroxides 3/4 and to tert-alkyl acyl-nitroxides 5/6.
Magn. Reson. Chem. 2014, 52, 289–297
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

M. Campredon and A. Alberti
Scheme 2. Hypothetical photoevolution of 3-nitropentan-2-ol, 2.
Scheme 3. Resonance stabilization of the radical resulting from photodenitration of 1 with respect to that originating from 2.
were detected. In all cases, these consisted in triplet of doublets
(Fig. 3a) characterized by g-values = 2.0050 and by fairly large
nitrogen splittings in the range 2.6 < aN < 2.8 mT. The only
notable difference in the spectra from the two compounds was
that the doublet splittings in the radicals from 2 were smaller
than those shown by the radicals from 1, the decrease of the
splitting being in the range from 30 to 45%.
Carbon-centered radicals can readily add to the nitrogroup of
nitroalkanes. In the case of 1, addition would lead to
oxynitroxides of general structure T in Scheme 4. On the other
hand, they normally react with ketones through addition to the
carbonyl carbon thus leading to alkoxy radicals, usually
undetectable under ‘normal’ EPR conditions, that is, in fluid
solution at ambient temperature. In the case of compound 2,
only addition to the nitrogroup is instead possible. This is
expected to result in oxynitroxides of general structure U having
spectral parameters very similar to those exhibited by radicals
akin to T. On this basis, the observed spectra were assigned to
tert-butoxynitroxides 1a and 2a. When reacting 1 or 2 with tbutyl,
along with the signals from 1a and 2a, an additional three-line
signal was also detected with spectral parameters typical of
di-tert-alkyl nitroxides. We attribute these three-line spectra to
di-tert-butyl nitroxide 7: indeed, as shown in Schemes 1 and 2,
nitric oxide is formed in the photodegradation of 1 and 2. This
t
adventitious ·NO might then combine with the butyl radicals
present in the system to give 2-methyl-2-nitrosopropane
[Me3C–N=O] that, in turn, would trap again a tbutyl radical to give
di-tert-butyl nitroxide 7 [Me3CN(Oꢀ)CMe3]. While in the case of 2,
this assignment seems unambiguous, this might not be the case
for 1, as its photodegradation may also lead to the tert-alkyl
t
nitrosoderivative K (Scheme 1) that by acting as a trap for butyl
radicals would eventually afford a tert-alkyl tert-butyl nitroxide
8 [EtCH(NO2)C(OH)(Me)–N(Oꢀ)CMe3] with spectral parameters
hardly distinguishable from those of di-tert-butyl nitroxide 7.
Organometallic radicals centered at a Group 14 element can
readily add to a carbonyl group or to a nitrogroup, leading to ketyl
radicals in the former case[15] and to oxynitroxides in the latter.[16]
Thus, when reacting compound 1 with Group 14 radical precur-
sors, the formation of these two kinds of radical adducts S and T
can be envisaged (Scheme 4), both of them being in principle
EPR detectable while characterized by very different spectral
parameters. Thus, adducts of general structure S are expected
to exhibit a quartet splitting because of a methyl group,
conformationally dependent additional splittings from a hydrogen
and a nitrogen atom and g-factor values in the range 2.0035 to
2.0045,[17] whereas the spectra of adducts having structure T
should predictably show a nitrogen splitting slightly smaller
a
b
2.0 mT
c
Figure 3. EPR spectra observed at room temperature upon photolysis of
t
(a) a benzene solution of compound 1, Et3SiH and BuOOtBu, (b) a ben-
t
zene solution of compound 1, (Me3Si)3SiH and BuOOtBu and (c) a ben-
zene solution of compound 1 and Ph3PbPbPh3.
Scheme 4. Possible radical adducts resulting from addition of ·MR3 radicals to compounds 1 or 2.
wileyonlinelibrary.com/journal/mrc
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2014, 52, 289–297

An EPR study of the radical addition to 3-nitropentan-2-one
than3.0mT, a hydrogen splitting in the range 0.2 to 0.6 mT and g-
factors in between 2.005 and 2.006.[14]
to 2-nitropropane.[18] In that case, the second species was identi-
fied as the adduct of a [trimethylsilyloxy-bis(trimethylsilyl)silyl
radical ·Si(SiMe3)2OSiMe3 resulting from a 1,2 silicon to oxygen
migration of a trimethylsilyl group in the tris(trimethylsilyl)silyloxy
radical ·OSi(SiMe3)3 deriving from the fragmentation of the nitro-
gen–oxygen bond of the primary adduct Me3CN(O·)OSi(SiMe3)3.
In this line, it seems therefore sensible to identify radical 1e as R′
N(O·)OSi(SiMe3)2OSiMe3, despite the fact that a similar radical 2e
was not observed.
It should be further mentioned that when photoreacting 1
with the organometals, a weak signal from acyl nitroxide 5 or 6
was observed along with the dominating signal of the organo-
metallic oxynitroxide 1b, 1c or 1f. Thus, although the addition
of Group 14 organometallic radicals to the nitrogroup is a fast
process, a small fraction of 1 underwent photofragmentation.
Finally, prolonged photolysis of solutions of 1 and a tin-centered
or lead-centered radical precursor, namely hexabutyl ditin,
hexaphenylditin, hexa-chexyldilead or hexaphenyldilead, resulted
in the appearance of other spectra (Fig. 2c) that exhibited spectral
parameters consistent with tert-alkyl-sec-alkyl nitroxides (1.4≤ aN ≤
1.5mT, 0.21 < aH < 0.35 mT, g = 2.006)[14] for radicals 9, 10 and 12
and with a di-sec-alkyl-nitroxide for radical 11. Photolysis of ditin or
dilead compounds [R3M–MR3] normally results in the cleavage of
the metal–metal bond (formation of ·MR3 radicals), but in some
instances, ·R radicals may be formed as well. In this light, radical
11 might be identified with di-chexyl nitroxide for which spectral
parameters in line with those presently measured have been
reported,[18] while the possibility that 11 be in fact a nitroxide V
resulting from the trapping of radical D by the nitrosoderivative
G (Scheme 5) seems unlikely as if this were the case, this species
should have been observed in all cases, independently of the
organometallic reagent being used.
On the basis of their spectral parameters and because of the
similarity of the spectra of the adducts from 1 and 2, the detected
radicals (1b–k and 2b–i in Tables 1 and 2) were therefore identi-
fied as oxynitroxides R3MON(O·)R′ with general structure T or U,
where M is a silicon, germanium, tin or lead atom, R is an alkyl
or aryl group and R′ is the pentanone or pentanol residue.
Because of the similarity of the nitrogen splitting in radicals
1b–i and 2b–i, the lower value of the α-hydrogen splittings
exhibited by the latter nitroxides should originate from different
conformational situations. This might, in principle, be the result
of a hydrogen bonding of the hydroxylic hydrogen by the
nitroxidic oxygen in species 2b–i, but it should be noted that
such interaction should also result in a stabilization of the polar
mesomeric form of the nitroxides that would lead to larger
nitrogen splitting actually not observed.
These results provide evidence that Group 14 organometallic
radicals react with nitroketones via selective addition to the
nitrogroup. Although, in principle, an initial addition of the ·MR3
radicals to the carbonyl group followed by trapping of the
resulting S ketyl radicals by another molecule of 1 to give
oxynitroxides T should not be discarded ‘a priori’, it seems that
such a possibility can be safely discarded as all the resulting
oxynitroxides would basically have almost identical structures
and should therefore be characterized by the same spectral
parameters, which was not the case as indicated by the data
collected in Table 1.
It is worth noting that in the case of the reaction of 1 with tris
(trimethylsilyl)silyl radical, the spectra showed the presence of
two species, 1d and 1e, with very similar spectral parameters
(Fig. 2b). Because of the large bulk of the tris(trimethylsilyl)silyl
group, it is envisageable that rotation of the OMR3 residue
around the N–O bond may become hindered, and the two
species might be tentatively identified as different conformers
of the corresponding oxynitroxide, characterized by almost equal
nitrogen and slightly different hydrogen splittings. It is also
possible that the presence of two conformers be favored by
some sort of coordination of a silicon atom by the carbonyl
oxygen, an interaction that in the case of 2, for which only the
adduct 2d was detected, might be hampered by the presence of
the hydroxylic hydrogen atom. On the other hand, the detection
of two species with very similar EPR spectroscopic parameters
has been reported for the addition of tris(trimethylsilyl)silyl radicals
As for species 9, 10 and 12, in principle, it is possible to envis-
age the formation of radicals akin to species Y according to two
different pathways shown in Scheme 5. One of these only
involves fragmentation of nitroketone 1, eventually leading to
the tert-alkyl sec-alkyl nitroxide W. On the other hand, this
assignment would collide with the fact that 9, 10 and 12 exhibit
different spectral parameters and, secondly, that these nitroxides
were not detected when photolyzing solutions of 1 alone. A
second possibility is that for tin and lead-centered radicals, addi-
tion to the carbonyl group leading to adduct S may compete
with addition to the nitrogroup. Then, a coupling of these ketyl
radicals with ·NO from the concomitant photofragmentation of
Scheme 5. Possible route to radicals 9, 10 and 12.
Magn. Reson. Chem. 2014, 52, 289–297
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

M. Campredon and A. Alberti
1 would result in the tertiary nitrosoderivative X that, upon
trapping of a secondary radical D, would eventually afford the
tert-alkyl sec-alkyl nitroxide Y. We then attribute to radicals 9,
10 and 12 the general structure Y that would account for the
slight but definite differences of their spectral parameters with
the nature of the MR3 residue and with the fact that these species
are only observed in the presence of ditin or dilead compounds.
Prolonged photoreaction of 2 with the organometals eventu-
ally resulted in the disappearance of the spectra from
oxynitroxides 2b–h that were in all cases replaced by that of a
symmetric di-sec-alkyl nitroxide 13 to which we assign the
structure R. Because this species is not observed when photolyz-
ing compound 2 alone, it cannot be formed as shown in
Scheme 2, and in Scheme 6 we outline a different possible route
to radical 13.
decay of tert-alkyl oxynitroxides has been previously investigated,
particular attention having been addressed to Me3CN(O·)OMR3
species, where M is a carbon, a silicon, a germanium or a tin atom,
and it has been shown that these oxynitroxides decay via cleavage
of the nitrogen–oxygen bond when MR3 is a tert-butyl[12] or a tris
(trimethylsilyl)silyl group,[19] whereas when MR3 is a tributyltin
group, the decay involves cleavage of the nitrogen–carbon bond,
and accordingly, tributyltin hydride has been widely exploited for
the reductive denitration of tertiary aliphatic nitrocompounds.[20]
It has been subsequently shown that SiPh3, GePh3 tert-alkyl
oxynitroxides also decay via cleavage of the N–O bond.[13]
Not being aware of detailed studies of the decay of sec-alkyl
oxynitroxides, we have studied the decay of oxynitroxides 1c, f
and g and 2c, f and g in the wider possible temperature range,
i.e. 253 to 323 K. The radicals were generated by photolysis of so-
Thus, decay of 2b–h would lead to radical M that may subse-
quently combine with nitric oxide to give the nitrosoderivative
Q, which, in turn, would eventually afford 13 through the
trapping of another radical M. It cannot be, however, excluded
that in the decay of 2h, the fragmentation of the N–O bond
may compete to some extent with that of the N–C bond,
resulting again in the formation of the nitrosoderivative Q.
Coming eventually to radical 14, its spectral parameters are
indicative of an asymmetric di-sec-alkyl nitroxide that we identify
as 2-hydroxypent-3-yl ckexyl nitroxide resulting from the trapping
of a chexyl radical by the nitrosoderivative Q.
lutions of 1 or 2 containing di-tert-butylperoxide and
triphenylsilyl hydride (1c and 2c) or triphenylgermyl hydride (1f
and 2f) or simply hexabutylditin (1g and 2g) at the desired tem-
perature. Once the signal had reached a stationary level, irradia-
tion was switched off and the signal decrease of a clean
spectral line was monitored in time.
Because in all cases very clean spectra were observed with vir-
tually no signal from the adventitious dialkyl nitroxides that
might result from a combination of the two aforementioned de-
cay processes, it was concluded that the oxynitroxides
fragmented preferentially at the nitrogen–carbon bond, as indi-
cated in Scheme 7. The decay process can therefore be described
by reactions 1 and 2 in the case of the tin derivatives 1g and 2g,
whereas reaction 3 should also be considered for the silyloxy
nitroxides and germyloxy nitroxides 1c, 1f, 2c and 2f (where X·
is either radical D or M shown in Scheme 5):
Decay of oxynitroxides from 1 and 2
All oxynitroxides 1a–k and 2a–i were fairly persistent species,
with half-life times ranging from several seconds to a few mi-
nutes. The decay of these radicals, which reflects their fragmenta-
tion to a diamagnetic molecule and a radical fragment, may
involve two different processes: the cleavage of the carbon–nitro-
gen bond results in the formation of a sec-alkyl radical and a
nitrite (O=N–OMR3), whereas the cleavage of the nitrogen–oxy-
gen bond leads to a nitrosoderivative and an ·OMR3 radical. The
XNðOꢀÞOMR3→Xꢀ þ O ¼ NOMR3
Xꢀ þ XNðOꢀÞOMR3→XNðOXÞOMR3
(1)
(2)
Scheme 6. Possible routes to radical 13.
Scheme 7. Formation and decay reactions of radicals 1c, f and g and 2c, f and g.
wileyonlinelibrary.com/journal/mrc
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2014, 52, 289–297

An EPR study of the radical addition to 3-nitropentan-2-one
Based on Eqn 4, first-order decay kinetics were expected for
the six investigated oxynitroxides, and this was what we actually
found (Fig. 4) provided that the critical maximum concentration
of the organometallic hydride was respected. Indeed, a mixing
of different order kinetics were instead observed if [HMR3] > 10ꢁ1
M were used.
Xꢀ þ HMR3→XH þ ꢀMR3
(3)
The decay process of 1g and 2g can therefore be expressed as
ꢁd½XNðOꢀÞOMR3ꢂ=dt ¼ 2k1½XNðOꢀÞOMR3ꢂ
(4)
An Arrhenius treatment of the kinetic data obtained at the dif-
ferent temperatures led to the activation parameters collected in
Table 3, where those taken from the literature for the correspond-
ing tert-butyl organometaloxynitroxides are also reported. The
first feature to emerge from an analysis of Table 3 is that the ac-
tivation parameters determined in this study are fairly similar to
those previously determined for the corresponding tert-butyl
oxynitroxides, being about half a kcal molꢁ1 larger when M is sil-
icon, about half a kcal molꢁ1 smaller when M is germanium and
ca 1 kcal molꢁ1 smaller when M is tin. The easiness of the cleav-
age of the C–N bond should reflect the stability of the resulting
radical and diamagnetic species. In the present case, the latter
fragment is the same, i.e. an organometallic nitrite R3MO–N=O,
and therefore, the easiness of the cleavage should reflect the
stability of the resulting carbon-centered radicals.
while in the case of 1c, 1f, 2c and 2f, it becomes
2
ꢁd½XNðOꢀÞOMR3ꢂ=dt ¼ 2k2k1½XNðOꢀÞOMR3ꢂ =fk3½HMR3ꢂ (5)
þk2½XNðOꢀÞOMR3ꢂg
It should, however, be taken into account that the recombina-
tion reaction 2 must be very fast, and the value of k2 can be safely
estimated as ~109 ꢁ1 sꢁ1; besides, although the rates of hydrogen
M
abstraction from triphenylsilane and triphenylgermane by second-
ary alkyl radicals are not available, by combining kinetic data avail-
able in the literature for reactions involving these species,[21,22] it
can be estimated that the k3 value should not exceed 103 ꢁ1 sꢁ1
M
for Ph3SiH and 2 × 104 ꢁ1sꢁ1 for Ph3GeH. Thus, for oxynitroxide
M
concentrations not lower ~10ꢁ5 M, as it was the case in our exper-
iments, and for low organometallic hydride concentrations as
those we have been using ([HMR3] ≤ 10ꢁ1 M), in Eqn (5), the term
k3[HMR3] can be disregarded as negligible with respect to k2[XN
(O·)OMR3], and Eqn 5 reverts to Eqn (4) also for oxynitroxides 1c
and f and 2c and f.
On this basis, we would have expected for the fragmentation
of the sec-alky oxynitroxides from 1 and 2 activation energy
values larger than those found for the corresponding tert-butyl
derivatives. In particular, we would have expected 1c, f and g
and 2c, f and g to exhibit Ea values up to ca 3 kcal molꢁ1 larger
than those of the corresponding tert-alkyl derivatives, a bond
c
t
Figure 4. Time profile of the EPR signal observed upon a short UV irradiation of benzene solutions of compound 1, Ph3GeH and BuOOtBu at 275 K
t
(upper trace), and of compound 2, Ph3SiH and BuOOtBu at 365 K (lower trace). In the inserts, the Arrhenius plots for the corresponding adducts 1f
and 2c.
Magn. Reson. Chem. 2014, 52, 289–297
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

M. Campredon and A. Alberti
Table 3. Activation parameters for the cleavage of the nitrogen–carbon bond for some selected alkyl organometalloxy nitroxides
Radical/#
MR3
Ea/kcal molꢁ1
Log (A/sꢁ1
)
Reference
D–N(Oꢀ)OMR3/1c
D–N(Oꢀ)OMR3/1f
D–N(Oꢀ)OMR3/1g
M–N(Oꢀ)OMR3/2c
M–N(Oꢀ)OMR3/2f
M–N(Oꢀ)OMR3/2g
Me3C–N(Oꢀ)OMR3
Me3C–N(Oꢀ)OMR3
Me3C–N(Oꢀ)OMR3
SiPh3
19.36 0.55
17.06 0.32
6.66 0.37
19.40 0.95
17.37 1.25
6.96 0.15
18.80 0.92
17.86 0.72
8.11 0.56
13.37 0.39
12.71 0.24
5.00 0.29
11.52 0.58
9.69 0.76
4.42 0.10
11.13 0.64
9.86 0.48
5.58 0.36
This work
This work
This work
This work
This work
This work
Ref. [13]
GePh3
SnBu3
SiPh3
GePh3
SnBu3
SiPh3
GePh3
SnBu3
Ref. [13]
Ref. [13]
Residues D and M as defined in Scheme 7.
dissociation energy difference of 2.7 kcal molꢁ1 having been
dedicated data station for the acquisition and manipulation of
the spectra, an NMR-Gaussmeter for the calibration of the
magnetic field and a Systron-Donner frequency counter for the
determination of g-factors that were corrected with respect to
that of perylene radical cation in concentrated sulfuric acid
(g = 2.00258).[25] The temperature of the sample was controlled
estimated for Me3C–H (93.9 kcal molꢁ1
)
and Me2HC–H
(96.3 kcal molꢁ1).[23] In principle, one might infer that the
C–N bond cleavage of oxynitroxides 1c, f and g would lead
to a radical fragment featuring a carbonyl group adjacent to
the radical center. As shown in the left-hand side of Scheme 3,
this may result in resonance stabilization and hence in lower-
than-expected fragmentation activation energy values, similar to
those measured for the processes leading to a tert-alkyl radical.
While this may appear a sensible proposition, it has to be stressed
that for the decay of radicals 2c, f and g, activation energy values
almost equal to those determined for 1c, f and g were obtained,
which collides with the lack of resonance stabilization in the
radicals from the former derivatives. At this moment, we are
therefore unable to offer a clear-cut explanation to account for
the similarity of the activation energy values of fragmentations
of oxynitroxides leading to sec-alkyl and tert-alkyl radicals.
through
a standard variable temperature accessory and
monitored with a chromel–alumel thermocouple inserted inside
the sample tube. During photolysis, the unfiltered light from a
200-W Hg–Xe lamp (Hamamatsu LC8-06) was focused on the
spectrometer cavity by means of a fused silica light guide.
In a typical experiment, 200 μl of a benzene solution of the
substrate (~10ꢁ3 M), the radical precursor (~10ꢁ2 M) and di-tert-
butylperoxide (~10ꢁ3 M, if needed) was thoroughly degassed in
a quartz sample tube (i.d. 4 mm) that was subsequently inserted
inside the EPR cavity. In the decay kinetics experiments, after
choosing the field value corresponding to the top of a ‘clean’
spectral line, the irradiation was switched off, and the decrease
of the signal was followed operating the spectrometers in the
time-sweep mode.
A further point that cannot go unnoticed is provided by the
frequency factor values found for the decay of oxynitroxides 1g
and 2g, values unusually small for unimolecular reactions. Similar
values have been found when the apparent rate constant for
decay contains the monomer–dimer equilibrium constant, and
it might not be by chance that a similar effect was also observed
previously also for the decay of the Me3CN(O·)OSnBu3
oxynitroxide.[13] To check this possibility, temperature jump
experiments were carried out during the decay of 1g and 2g,
but no effects were observed in the available temperature range.
When not straightforward, the interpretation of the EPR
spectra was based on computer simulations obtained through
the use of a custom-made simulation program based on a Monte
Carlo self-minimization procedure.[26]
References
[1] N. Ono, in The Nitro Group in Organic Synthesis (Eds: John Wiley & Sons),
New York, 2001.
[2] G. Rosini, R. Ballini. Synthesis 1988, 833–847.
Experimental
[3] R. H. Fischer, H. M. Weit. Synthesis 1980, 261–264.
[4] R. Ballini, L. Barboni, F. Fringuelli, A. Palmieri, F. Pizzo, L. Vaccaro.
Green Chem. 2007, 9, 823–838.
Reagents
[5] R. Ballini, in Studies in Natural Products Chemistry, vol. 19 (Ed: Atta-
ur-Rahma), Elsevier, Amsterdam, 1997, pp. 117–184.
[6] R. Ballini, M. Petrini, G. Rosini. Molecules 2008, 13, 319–330.
[7] N. Sato, T. Matsura. J. Chem. Soc., Perkin Trans. I 1996, 2345–2350.
[8] M. J. Joung, J. H. Ahn, H. M. Kim, K. W. Kim, J. H. Kang, S. J. Lee, N. M.
Yoon. J. Heterocycl. Chem. 2002, 39, 1065–1069.
3-Nitro-2-pentanone (1) was prepared as described in the
literature[24] from 3-nitro-2-pentanol (2) that was commercially
available (Columbia Chemicals, USA), as were triethylsilane,
triphenylsilane, tris(trimethylsilyl)silane, triphenylgermanium
hydride, bis(tributyltin), bis(triphenyltin), di-tert-butylperoxide
(the Sigma-Aldrich Group, USA) and bis(triphenyllead) (Strem
Chemicals, Inc., USA), while bis(cyclohexyllead) was a gift from
the Inorganic Chemistry group at Sussex University, UK.
[9] N. Ono, H. Miyake, R. Tamura, A. Kaji. Tetrahedron Lett. 1981, 22,
1705–1708.
[10] N. Kornblum, S. C. Carlson, R. G. Smith. J. Am. Chem. Soc. 1979, 101,
647–657.
[11] N. Ono, H. Miyake, A. Kamimura, I. Hamamoto, R. Tamura, A. Kaji. Tet-
rahedron 1985, 41, 4013–4024.
Apparatus and procedures
[12] M. J. Perkins, B. P. Roberts. J. Chem. Soc., Perkin Trans. II 1974, 297–304.
[13] M. Lucarini, G. F. Pedulli, A. Alberti, M. Benaglia. J. Am. Chem. Soc.
1992, 114, 9603–9607.
[14] a) A. R. Forrester, in Landolt-Börnstein – Magnetic Properties of Free
Radicals (Eds: H. Fischer, K.-H. Hellwege), Springer Verlag,
Electron paramagnetic resonance spectra were recorded by
means of an upgraded ER200D/ESP 300 Bruker spectrometer
operating in the X-band (~9.3 GHz) and equipped with a
wileyonlinelibrary.com/journal/mrc
Copyright © 2014 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2014, 52, 289–297

An EPR study of the radical addition to 3-nitropentan-2-one
Heidelberg, 1979, II/9c1; b)A. R. Forrester, in Landolt-Börnstein –
Magnetic Properties of Free Radicals (Ed: H. Fischer), Springer Verlag,
Heidelberg, 1989, II/17d1; c)A. Alberti, in Landolt-Börnstein –
Magnetic Properties of Free Radicals (Ed: H. Fischer), Springer Verlag,
Heidelberg, 2005, II/26d.
Magnetic Properties of Free Radicals (Ed: H. Fischer), Springer Verlag,
Heidelberg, 1987, II/17b.
[18] M. Ballestri, C. Chatgilialoglu, M. Lucarini, G. F. Pedulli. J. Org. Chem.
1992, 57, 948–952.
[19] a) A. H. M. Kayen, K. De Boer, T. J. De Boer. Recl. Trav. Chim. 1977,
96, 1–8; b)H.-G. Korth, R. Sustmann, J. Dupuis, B. Giese. Chem. Ber.
1987, 120, 1197–1202.
[20] D. D. Tanner, D. J. Harrison, J. Chen, A. Kharrat, D. D. M. Wayner, D.
Griller, D. J. McPhee. J. Org. Chem. 1990, 55, 3321–3325.
[21] K. B. Clark, D. Griller. Organometallics 1991, 10, 746–750.
[22] C. Chatgilialoglu, J. Dickhaut, B. Giese. J. Org. Chem. 1991, 56, 6399–6403.
[23] D. Griller, J. M. Kanabus-Kaminska, A. Maccoll. J. Mol. Struct.
THEOCHEM 1988, 163, 125–131.
[15] a) A. Alberti, A. Hudson. J. Chem. Soc., Perkin Trans. II 1978, 1098–1102; b)A.
Alberti, A. Hudson. J. Organomet. Chem. 1979, 164, 219–225; c)A. Alberti,
M. Guerra, G. F. Pedulli. J. Chem. Soc., Perkin Trans. II 1979, 1568–1573;
d) A. Alberti, C. Chatgilialoglu. Tetrahedron 1990, 46, 3963–3972.
[16] a) A. Alberti, M. Guerra, G. F. Pedulli, M. Tiecco. Gazzetta 1974, 104,
1301–1305; b)A. Alberti, G. Martelli, G. F. Pedulli. J. Chem. Soc., Perkin
Trans. II 1977, 1252–1255; c)A. Alberti, G. F. Pedulli, F. Ciminale.
Tetrahedron 1982, 38, 3605–3608; d)M. Campredon, G. Giusti, R.
Guglielmetti, A. Samat, G. Gronchi, A. Alberti, M. Benaglia. J. Chem.
Soc., Perkin Trans. II 1993, 2089–2094.
[24] G. Rosini, R. Ballini, P. Sorrenti, M. Petrini. Synthesis 1984, 607–608.
[25] J. E. Wertz, J. R. Bolton, Electron Spin Resonance, McGraw-Hill,
New York, 1972.
[26] M. Lucarini, B. Luppi, G. F. Pedulli, B. P. Roberts. Chem. Eur. J. 1999,
5, 2048–2054.
[17] a) H. Fischer, H. Paul, in Landolt-Börnstein – Magnetic Properties of
Free Radicals (Eds: H. Fischer, K.-H. Hellwege), Springer Verlag,
Heidelberg, 1977, II/9b; b)F. A. Neugebauer, in Landolt-Börnstein –
Magn. Reson. Chem. 2014, 52, 289–297
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc