Thermal decay of TEMPO in acidic media via an N-oxoammonium salt intermediate

Article abstract of DOI:10.1039/c1ob05475a

Disproportionation of TEMPO in acids leads to the formation of an N-oxoammonium salt, which can further decompose under thermal conditions, yielding the corresponding hydroxylamine, N₂O, CO₂ and a series of dimerisation products. Overall, acid-catalysed thermal decay of TEMPO leads to ca. 80% yield of hydroxylamine.

Full text of DOI:10.1039/c1ob05475a

View Article Online / Journal Homepage / Table of Contents for this issue
Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2011, 9, 5573
www.rsc.org/obc
PAPER
Thermal decay of TEMPO in acidic media via an N-oxoammonium salt
intermediate†
a
b
b
a
Yun Ma, Colin Loyns, Peter Price and Victor Chechik*
Received 25th March 2011, Accepted 12th May 2011
DOI: 10.1039/c1ob05475a
Disproportionation of TEMPO in acids leads to the formation of an N-oxoammonium salt, which can
further decompose under thermal conditions, yielding the corresponding hydroxylamine, N O, CO
and a series of dimerisation products. Overall, acid-catalysed thermal decay of TEMPO leads to ca.
0% yield of hydroxylamine.
2
2
8
Introduction
TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and its deriva-
1–5
tives are persistent free radicals widely used in chemistry and
6–8
biology. In particular, the redox cycle of TEMPO including the
corresponding hydroxylamine (TEMPOH) and oxoammonium
cation (Scheme 1) forms the basis of many applications such as
Scheme 2 Acid-catalysed disproportionation of TEMPO.
radicals in such conditions is therefore important; however litera-
ture data on thermal decomposition of nitroxides are scarce even
for neutral media. Thermal decomposition of 4-oxo-TEMPO was
1,9–12
13
catalytic oxidation of alcohols
or dismutation of superoxide.
17–18
reported by Murayama and co-workers.
It was suggested that
the presence of the carbonyl group makes the 3- and 5-hydrogen
atoms of 4-oxo-TEMPO more reactive, hence a self-reaction at
◦
1
10 C leads to the corresponding hydroxylamine and a nitroso-
compound, which eventually yields nitric oxide (NO) and phorone
Scheme 3). Alternatively, the enol form of the hydroxylamine can
be oxidised to the radical cation via single electron transfer (SET)
(
19
followed by dimerisation (Scheme 3).
Scheme 1 Inter-conversion of TEMPO, TEMPOH and oxoammonium
salt.
The reduced and oxidised forms of nitroxide radicals are
also involved in the acid catalysed disproportionation reaction.
Protonation of TEMPO is followed by the disproportionation,
1
4–15
yielding TEMPOH and oxoammonium cation (Scheme 2).
The
16
forward and reverse (i.e., comproportionation ) reactions are well
characterised at ambient temperature, including the measurement
14
of activation parameters.
However, some practical applications of nitroxide radicals in
acidic medium (e.g., mediation of living radical polymerisation of
acidic monomers) require high temperature. The fate of nitroxide
Scheme 3 Thermal decomposition and dimerisation of 4-oxo-TEMPO.
a
Department of Chemistry, University of York, Heslington, York, UK YO10
5
DD. E-mail: victor.chechik@york.ac.uk
However, these reactions depend on the keto–enol tautomerism.
b
Nufarm UK Limited, Wyke Lane, Wyke, Bradford, UK BD12 9EJ
17
TEMPO lacking the 4-keto group was reported to be stable under
the same conditions. For instance, TEMPO is stable in solvents
†
Electronic supplementary information (ESI) available. Kinetic mod-
elling and spectra of TEMPO disproportionation. See DOI:
◦
21
1
0.1039/c1ob05475a
lacking H-donating capacity until at least 150 C. In H-donating
This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 5573–5578 | 5573

View Article Online
solvents, TEMPO can be reduced to form the corresponding
hydroxylamine (TEMPOH) and amine (TEMPH) at temperature
above 150 C. Elimination of NO from nitroxides is observed
decay can be monitored by the disappearance of the UV peak at
476 nm (Fig. 2). The decay fits 1st order kinetics well; the rate
◦
◦
constant for reaction in water at 100 C was determined as 0.120 ±
-
1
under photolysis; this reaction leads to the formation of the
0.008 min . The thermal decomposition of oxoammonium salt is
not acid catalysed, as the reaction proceeds with a similar rate in
the pH range from -0.3 to 5. The reaction does not depend on the
presence of oxygen and light. We conclude therefore that stability
of TEMPO in acidic solutions at high temperature is determined
by the decomposition of oxoammonium salt.
20
corresponding diene, similar to reactions in Scheme 3.
In the presence of acids, reactions of nitroxides at elevated
temperatures involve disproportionation (Scheme 2). However, the
thermal stability of TEMPO and oxoammonium salt has not been
studied under these conditions. Here, we report a mechanistic
study of TEMPO reactivity in aqueous acids at temperatures 80–
◦
1
00 C.
Results and discussion
Reversibility of TEMPO disproportionation at high temperature
In order to predict the reactivity of TEMPO in acidic medium
at elevated temperature, we built a kinetic model by adopting
the reported activation parameters of the disproportionation
14
reaction. TEMPO decay in dilute sulfuric acid (e.g. 0.1 M) was
then monitored by EPR spectroscopy using an oximetry fitting
22
method. Our experimental data at room temperature fit the
kinetic model precisely, but significant deviations were observed
for the data obtained at higher temperature (e.g. 80 C). Fur-
Fig. 2 Decomposition of 0.037 M oxoammonium chloride in 0.05 M
◦
H
2
SO
4
at 100 C monitored by UV-vis spectroscopy. Insert: Disappearance
◦
of the UV band at 476 nm over 21 min.
thermore, the disproportionation reaction is not fully reversible
under these conditions. At room temperature, increasing pH of
disproportionated reaction mixture to neutral values results in
fast comproportionation. For instance, after disproportionation at
room temperature (decay of TEMPO EPR signal is shown in Fig. 1
with crosses), the EPR signal of TEMPO can be brought back
to the original intensity upon neutralisation (Fig. 1, filled circles).
Oxoammonium salt as an intermediate in TEMPO decomposition
The rapid electron transfer between TEMPO and oxoammonium
cation can very significantly broaden the NMR spectra which com-
plicates product analysis of the oxoammonium salt decomposition
by NMR spectroscopy. Hence, phenylhydrazine was added to the
reaction mixture at different reaction times to reduce paramagnetic
intermediates (and other oxidising species including unreacted
oxoammonium salt). A controlled amount of phenylhydrazine was
also used as an internal reference to quantify the reaction product.
Interestingly, as determined by NMR spectroscopy, TEMPOH
was the only component with substantial concentration after
the decomposition of oxoammonium salt (i.e., after complete
disappearance of the UV band of oxoammonium salt). The yield
of TEMPOH from decomposition of an aqueous solution of
◦
However, we found that after the heating in acid at 80 C, TEMPO
cannot be recovered by neutralisation (Fig. 1, open circles).
◦
oxoammonium salt at 80 C was estimated to be ca. 60%. Similarly,
◦
TEMPO decay in 0.1 M sulfuric acid at 80 C consistently gave
ca. 80% yield of TEMPOH. This unexpectedly clean conversion
leaves only trace amounts of other products in the NMR spectra.
MS spectra of the reaction mixture also revealed TEMPOH as the
major product.
-
3
Fig. 1 EPR signal intensity of a solution of 6.4 ¥ 10 M TEMPO in 1 M
H
2
SO
4
after neutralisation to pH = 7 at room temperature (filled circles)
and 80 C (open circles). Crosses show EPR intensity of the reaction
mixture at room temperature prior to neutralisation.
◦
The main product of oxoammonium salt decomposition (and
TEMPO degradation in aqueous acid) is thus the corresponding
hydroxylamine TEMPOH. Since TEMPOH is a two-electron
reduction product of oxoammonium salt, the reaction mixture
must contain oxidation product(s) probably resulting from mul-
tiple electron transfers per molecule. It is also possible that the
reaction leads to a diversity of products with low concentra-
tion. Several possible mechanisms can be proposed to explain
the decomposition of oxoammonium salt. For instance, it was
Due to the high stability of TEMPO at temperatures below
00 C, these observations suggest that incomplete reversibility
of TEMPO disproportionation is due to degradation of one of
the disproportionation products, TEMPOH or oxoammonium
salt. For instance, TEMPOH can be readily oxidised. We found,
however, that under acidic conditions, protonated TEMPOH
= 6.6) has markedly high stability, showing no decompo-
sition/oxidation at 100 C within the time scale used in this study.
In contrast, the oxoammonium salt decays rapidly at 100 C; the
◦
21
1
23
(
pK
a
suggested that oxidation of H O by oxoammonium salt leads to
2
◦
the formation of hydroxyl radical and TEMPO. In acid, TEMPO
would disproportionate to form additional hydroxylamine and
◦
5
574 | Org. Biomol. Chem., 2011, 9, 5573–5578
This journal is © The Royal Society of Chemistry 2011

View Article Online
oxoammonium salt. As a result, this mechanism would lead to the
accumulation of hydroxylamine in the product.
a large scale (e.g. several grams). As further degradation of the
ring-opening product may lead to volatile molecules, the reaction
was carried out in a closed system. Interestingly, formation of
gas bubbles was observed during the reaction, alongside a small
amount of green oily substance. The reaction products thus are
distributed between the aqueous, organic and gas phases, which
were analysed separately.
However, this mechanism is unlikely to be correct. Firstly,
oxoammonium salt decomposition was also observed in an-
hydrous conditions. This suggests that thermal decomposition
of oxoammonium salt does not depend on the presence of
24
25
H
2
O. Secondly, the redox potentials of reactions (1) and (2)
(
Scheme 4) suggest that oxidation of water (to hydroxyl radical)
Analysis of the aqueous phase using NMR spectroscopy after
reduction with phenylhydrazine showed TEMPOH as the only
product with a yield of ca. 60%. In order to detect NMR
silent components, the aqueous product was titrated using di-
lute NaOH. The resulting titration curve (Fig. 4) showed two
components present in the aqueous reaction product. The first
component corresponds to a strong acid with a yield of 41.4%.
by oxoammonium cation is physically impossible. Therefore,
oxidation of water was ruled out as a possible reaction pathway.
This strong acid could be HCl and HNO . Considering ca. 4%
3
nitrate was detected by ion-exchange chromatography, this means
ring opening of each molecule of oxoammonium chloride releases
a proton, which combines with the chloride anion. The second
2
Scheme 4 Oxidation of H O by oxoammonium salt and the corres-
ponding redox potentials.
component has pK 6.6 and a yield of 60.2%, which corresponds
to TEMPOH. The assignment was confirmed by titration of a
solution of TEMPOH in sulfuric acid with NaOH, which gave
a
Alternatively, ring opening of oxoammonium salt (Scheme 5)
could lead to a carbocation which would further decompose to a
series of products. Several observations favour the ring-opening
route. Careful analysis of the oxoammonium salt decomposition
mixture revealed some minor components that can be associated
with the ring-opening process. For instance, a small amount of
nitrate (ca. 4%) which could result from oxidation of the ni-
troso derivative was detected using ion-exchange chromatography.
very similar pK
obtained from quantitative NMR studies. Therefore, the products
in the aqueous phase are TEMPOH and HNO /HCl.
a
value. The yield is very consistent with the yield
3
Thermal decomposition of oxoammonium salt in H O leads to
2
the formation of a minor species with a UV band at 670 nm,
which slowly diminishes in the presence of acid (Fig. 3). This
is consistent with the n
which could form via ring opening of the oxoammonium cation.
N
→p* transition of C-nitroso monomers
26
These observations are thus consistent with the ring-opening
mechanism of oxoammonium salt decomposition. However the
oxidation products were only detected in small amount, which
leaves a missing piece in the mass balance.
Fig. 4 Titration curve of the aqueous phase of an aqueous solution of
-4
◦
2
.51 ¥ 10 mol oxoammonium chloride heated at 100 C for 30 min
and titrated with 0.0207 M NaOH (aq). Left: titration curve. Right:
1
st-derivative of the titration curve.
The gas phase product is colourless, which leaves a limited
Scheme 5 Ring opening reaction of oxoammonium cation.
2
x
number of possible components, including CO, CO , NO and
27
volatile hydrocarbons. A GC-FID method was adopted to detect
small molecule hydrocarbons, which yielded negative results. The
IR spectrum of the gas product (Fig. 5) has distinctive doublets
at 2212/2236 cm and 1272/1299 cm . These distinctive peaks
are consistent with the asymmetric and symmetric stretches of
-
1
-1
28
-1
nitrous oxide (N
suggests the presence of CO
of this peak is much greater than the background CO
order to estimate the amount of the gas product, we synthesised
CO and N O gases and mixed them in a 1 : 1 (v/v) ratio. The
2
O). In addition, the peak at ca. 2350 cm
in the gas product (the intensity
signal). In
2
2
2
2
IR spectrum of the mixture (Fig. 5) is remarkably similar to the
experimentally observed spectrum of the gas phase. Using these
data and considering the solubility of the gases in water, the yield
Fig. 3 UV spectrum of the reaction mixture containing 0.057 M aqueous
oxoammonium salt heated at 100 C for 20 min.
◦
of CO
2
and N O was estimated to be 8.9% and 6.4%.
2
The organic phase product proved to be a complicated mixture
of several components. The green colour of the organic phase
suggests absorbance in the visible region of UV light. Indeed,
the UV spectrum of this mixture showed the previously observed
Oxoammonium salt decomposition: product analysis
In order to identify the minor reaction products, the decompo-
sition of aqueous oxoammonium chloride was investigated on
This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 5573–5578 | 5575

View Article Online
indicates that dimerised products may have formed by hydrogen
abstraction from oxoammonium salt followed by addition of
TEMPO or another molecule.
Mechanism of TEMPO decay in acid
With the identification of the main products of the oxoammonium
salt degradation, the mechanism for the overall reaction can be
proposed. The major reaction products were TEMPOH, CO
2
,
N
2
O and dimerisation products such as C18 33NO. We propose
H
that ring-opening of oxoammonium salt leads to a carbocation
followed by elimination of nitroxyl, HNO. Nitroxyl is known to
quickly dimerise to hyponitrous acid, H
2
N
2
O
2
, which dehydrates
The carbocation formed can
be deprotonated to yield isomers of 2,6-dimethylhepta-1,5-diene
(C 16). The dienes can be oxidised under strong conditions to
form CO and water. Since the dimerisation products in the organic
29–30
to form nitrous oxide, N O.
2
Fig. 5 Infrared spectra of the gas phase product (solid line) and a model
mixture of CO + N O (dashed line).
H
2
2
9
2
phase were formed in small amounts, the dimerisation pathway
probably accounts for a small proportion of the total decomposi-
tion. The hydroxylamine (reduction product) comproportionates
with oxoammonium salt to form a nitroxide radical (e.g. TEMPO).
Hydrogen abstraction from oxoammonium salt by TEMPO forms
an alkyl radical on the piperidine ring, which can dimerise
by reacting with another TEMPO molecule. Alternatively, both
band at 670 nm (Fig. 3), which is consistent with the presence
of a nitroso compound. TLC and GC traces revealed more than
eight components in the mixture. Since the overall yield of the
organic products accounts for only 3–4% (w/w) of the starting
material, isolation of the components is difficult. GC-MS (EI)
of the organic product did not yield exact structures for each
component. Similar ions were detected for several components on
the gas chromatogram. For instance, the ion at 124 m/z appeared
as molecular ion in six peaks on the gas chromatogram. This ion
31–33
34–35
TEMPO
and oxoammonium salt
can add to the double
bond on the dehydrogenated oxoammonium salt, and thus form
the dimerisation products. The overall mechanism is depicted in
Scheme 6.
can be assigned to C H16 isomers, probably dienes or cyclic alkenes.
9
It is likely that several isomers of this compound are present in the
organic phase.
In order to detect less stable products, the organic mixture
was also analysed using MS (ESI) without chromatographic
separation. The resulting mass spectrum provided rich informa-
tion on the reaction products and indication of the reaction
mechanism. The ions at 280, 298 and 327 could correspond
to a series of dimerisation products (suggested structures are
shown in Fig. 6). The compound with m/z 327 is consistent with
a nitroso derivative. We note that a C-nitroso compound was
earlier identified by the UV band at 670 nm. In addition, the ion
at 154 is consistent with the product of oxoammonium cation
dehydrogenation. The presence of this molecule in the product
Conclusions
In summary, thermal decomposition of TEMPO in acidic medium
proceeds via disproportionation reaction giving stable hydroxy-
lamine (in the protonated form) and oxoammonium salt which
undergoes a ring-opening at high temperature. This is followed
by elimination of HNO from the carbocation and formation of
isomers of C
formation of N
salt leads to TEMPOH, CO
The main product of this reaction is TEMPOH with ca. 80% yield.
9
H
16 diene. Dimerisation of HNO eventually leads to
O gas. Oxidation of the dienes by oxoammonium
and trace amount of other products.
2
2
Fig. 6 Mass spectrum (+ESI) of the organic mixture (in CHCl
576 | Org. Biomol. Chem., 2011, 9, 5573–5578
3
).
5
This journal is © The Royal Society of Chemistry 2011

View Article Online
Scheme 6 Proposed mechanism of TEMPO decay at high temperature.
Experimental
MS and GC-MS
Mass spectra were recorded on a Bruker micro-TOF spectrometer
equipped with LCQ ion trap with the indicated ion sources.
Gas Chromatography was performed on a VARIAN CP-3800
Gas chromatograph prior to MS analysis where indicated. The
GC column used was a Zebron ZB-5HT inferno column. The
Chemicals and materials
All chemicals were purchased from Sigma–Aldrich and used
as received without further purification unless stated otherwise.
TEMPO oxoammonium chloride was obtained from Nufarm
UK Ltd.
◦
temperature program was held at 60 C for 2 min, then ramped at
◦
-1
◦
5
1
2
C min from 60 to 350 C followed by an isothermal period of
◦
0 min at 350 C. Helium was used as carrier gas at a flow rate of
.2 ml min . EI (70 eV) was operated at 10 unit mass resolution.
Monitoring TEMPO decay by EPR spectroscopy
-1
-1
-
4
ESI was operated in positive ion mode with a digital resolution of
Typically a 5 ¥ 10 M TEMPO in H
2
SO solution (e.g. 0.1 M
4
-
4
10 unit mass.
H
2
SO ) was freshly prepared. Around 70 mL of this solution was
4
transferred to a glass Pasteur pipette, with the bottom end flame
sealed. The top end was sealed with lab film to prevent solvent
evaporation. The glass pipette was then placed in the EPR cavity
Gas phase IR
Infra-red spectra were recorded on a JASCO FT/IR-400 spec-
trometer. For the gas phase IR measurements, gas samples were
generated as follows. The reaction flask was charged with the
reaction mixture, then degassed using freeze–pump–thaw method
and EPR measurements were undertaken at a desired temperature
◦
(
e.g. 80 C) using a variable temperature setup. Typical EPR
parameters were: modulation amplitude 0.5 G, microwave power
.00 mW, acquisition time 160 s.
1
and re-filled with N . Gas formation was estimated by the readings
2
of an attached gas syringe. The obtained gas samples were injected
into a gas-tight IR cuvette cell (KBr) for measurements.
Monitoring oxoammonium salt decomposition by UV-vis
spectroscopy
Acknowledgements
A solution of oxoammonium chloride (0.037 M oxoammonium
salt in 0.1 M H
2
SO
4
) was heated at the desired temperature (e.g.
The authors thank Nufarm UK Ltd. and the University of York
(Wild Fund) for funding. Dr Marco Conte is thanked for the
helpful discussions. Dr James Hopkins and Miss Shalini Punjabi
are thanked for their help with the headspace analysis. Mr Gareth
◦
100 C). At different reaction times, aliquots of the reaction
mixture were taken and the UV-vis spectra were recorded on a
double beam Hitachi U3000 spectrophotometer.
This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 5573–5578 | 5577

View Article Online
Moody is thanked for the GC analysis. Angela Glossop (Nufarm)
is thanked for performing the IC analysis.
17 K. Murayama and T. Yoshioka, Bull. Chem. Soc. Jpn., 1969, 42, 2307–
2
309.
1
8 T. Yoshioka, S. Higashid, S. Morimura and K. Murayama, Bull. Chem.
Soc. Jpn., 1971, 44, 2207.
Notes and references
19 A. Nilsen and R. Braslau, J. Polym. Sci., Part A: Polym. Chem., 2006,
4, 697–717.
4
1
2
3
4
5
6
7
8
9
A. Dijksman, A. Marino-Gonzalez, A. M. I. Payeras, I. Arends and
R. A. Sheldon, J. Am. Chem. Soc., 2001, 123, 6826–6833.
P. P. Borbat, A. J. Costa-Filho, K. A. Earle, J. K. Moscicki and J. H.
Freed, Science, 2001, 291, 266–269.
M. Pouliot, P. Renaud, K. Schenk, A. Studer and T. Vogler, Angew.
Chem., Int. Ed., 2009, 48, 6037–6040.
M. Z. Zhao, J. Li, E. Mano, Z. G. Song, D. M. Tschaen, E. J. J.
Grabowski and P. J. Reider, J. Org. Chem., 1999, 64, 2564–2566.
M. S. Maji, T. Pfeifer and A. Studer, Angew. Chem., Int. Ed., 2008, 47,
20 J. F. W. Keana and F. Baitis, Tetrahedron Lett., 1968, 9, 365.
21 M. V. Ciriano, H. G. Korth and W. B. V. Scheppingen, J. Am. Chem.
Soc., 1999, 121, 6375–6381.
22 M. Conte, Y. Ma, C. Loyns, P. Price, D. Rippon and V. Chechik, Org.
Biomol. Chem., 2009, 7, 2685–2687.
23 V. A. Golubev, E´ . G. Rozantsev and M. B. Neiman, Bull. Acad. Sci.
USSR, Div. Chem. Sci. (Engl. Transl.), 1965, 14, 1898–1904.
24 W. H. Koppenol and J. F. Liebman, J. Phys. Chem., 1984, 88, 99–101.
25 J. L. Hodgson, M. Namazian, S. E. Bottle and M. L. Coote, J. Phys.
Chem. A, 2007, 111, 13595–13605.
9
547–9550.
S. M. Vaz and O. Augusto, Proc. Natl. Acad. Sci. U. S. A., 2008, 105,
191–8196.
P. J. Wright and A. M. English, J. Am. Chem. Soc., 2003, 125, 8655–
665.
26 Y. L. Chow, J. N. S. Tam and K. S. Pillay, Can. J. Chem., 1973, 51,
8
2477–2485.
27 J. R. Hopkins, A. C. Lewis and K. A. Read, J. Environ. Monit., 2003,
5, 8–13.
8
J. Fuchs, N. Groth, T. Herrling and G. Zimmer, Free Radical Biol.
28 T. M. Miller and V. H. Grassian, J. Am. Chem. Soc., 1995, 117, 10969–
Med., 1997, 22, 967–976.
10975.
T. Fey, H. Fischer, S. Bachmann, K. Albert and C. Bolm, J. Org. Chem.,
29 M. N. Hughes and H. G. Nicklin, J. Chem. Soc. A, 1971, 164–168.
30 P. S. Y. Wong, J. Hyun, J. M. Fukuto, F. N. Shirota, E. G. DeMaster,
D. W. Shoeman and H. T. Nagasawa, Biochemistry, 1998, 37, 18129–
18129.
2
001, 66, 8154–8159.
1
0 X. Wang, R. Liu, Y. Jin and X. Liang, Chem.–Eur. J., 2008, 14, 2679–
2
685.
1
1
1 R. A. Sheldon and I. Arends, Adv. Synth. Catal., 2004, 346, 1051–1071.
31 F. Aldabbagh, W. K. Busfield, I. D. Jenkins and S. H. Thang,
Tetrahedron Lett., 2000, 41, 3673–3676.
2 A. E. J. Denooy, A. C. Besemer and H. Vanbekkum, Tetrahedron, 1995,
5
1, 8023–8032.
32 J. E. Babiarz, G. T. Cunkle, A. D. DeBellis, D. Eveland, S. D. Pastor
and S. P. Shum, J. Org. Chem., 2002, 67, 6831–6834.
33 A. D. Allen, B. Cheng, M. H. Fenwick, W.-w. Huang, S. Missiha, D.
Tahmassebi and T. T. Tidwell, Org. Lett., 1999, 1, 693–696.
34 P. P. Pradhan, J. M. Bobbitt and W. F. Bailey, Org. Lett., 2006, 8,
5485–5487.
1
1
3 A. Samuni, S. Goldstein, A. Russo, J. B. Mitchell, M. C. Krishna and
P. Neta, J. Am. Chem. Soc., 2002, 124, 8719–8724.
4 V. A. Golubev, V. D. Sen, I. V. Kulyk and A. L. Aleksandrov, Bull. Acad.
Sci. USSR, Div. Chem. Sci. (Engl. Transl.), 1975, 24, 2119–2126.
5 V. D. Sen and V. a. Golubev, J. Phys. Org. Chem., 2009, 22, 138–143.
6 A. Israeli, M. Patt, M. Oron, A. Samuni, R. Kohen and S. Goldstein,
Free Radical Biol. Med., 2005, 38, 317–324.
1
1
35 K. M. Church, L. M. Holloway, R. C. Matley and R. J. Brower,
Nucleosides, Nucleotides Nucleic Acids, 2004, 23, 1723–1738.
5
578 | Org. Biomol. Chem., 2011, 9, 5573–5578
This journal is © The Royal Society of Chemistry 2011