Spontaneous formation and stabilization of radicals during the cocrystallization of alicyclic and peroxyalicyclic dicarboxylic acids

Article abstract of DOI:10.1016/j.mencom.2008.05.003

The cocrystallization of di(cis-2-carboxycyclohexylcarbonyl) peroxide and cyclohexane-1,2-dicarboxylic acid from aqueous solution affords stable 1-carboxycyclohexyl radicals, which were identified by EPR spectroscopy.

Full text of DOI:10.1016/j.mencom.2008.05.003

Mendeleev
Communications
Mendeleev Commun., 2008, 18, 123–125
Spontaneous formation and stabilization of radicals during the
cocrystallization of alicyclic and peroxyalicyclic dicarboxylic acids
Evgenii K. Starostin,* Andrei V. Lalov, Anatolii V. Ignatenko and Gennady I. Nikishin
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 499 135 5328; e-mail: nika@ioc.ac.ru
DOI: 10.1016/j.mencom.2008.05.003
The cocrystallization of di(cis-2-carboxycyclohexylcarbonyl) peroxide and cyclohexane-1,2-dicarboxylic acid from aqueous
solution affords stable 1-carboxycyclohexyl radicals, which were identified by EPR spectroscopy.
Radicals are generally generated by physical (thermolysis,
photolysis, radiolysis or electric discharge) or chemical (RedOx
systems) action.1
The IR spectra show absorption bands at 1080 and 1060 cm–1
assigned to vibrations of the peroxide (O–O) group.7
The 13C NMR spectrum of peroxide 2a shows two signals of
the carboxy [d 174.2 (s, COOH)] and peroxide [d 174 (s, COOO)]
groups. The 13C NMR spectrum of peroxide 2b also has two
signals at d 174.2 (s, COOH) and 169.7 (s, COOO). Note
that the 13C NMR spectrum of peroxide 2b exhibits two signals
of the C=C group at d 125 and 126, whereas the spectrum of
dicarboxylic acid 3b shows one singlet of the C=C bond at
d 126. The observed nonequivalence is, apparently, associated
with the influence of the peroxide bond of 2b, resulting in a
lowering of the molecular symmetry.
In the 1980s, we discovered the formation and stabiliza-
tion of carbon-centered radicals during the cocrystallization
of aliphatic peroxydicarboxylic (peroxysuccinic and peroxy-
glutaric) acids with dicarboxylic (succinic, glutaric and fumaric)
acids without any additional action on the resulting crystalline
system. The radicals thus generated occur in a solid matrix,
which hinders diffusion. In such matrices, these radicals remain
intact at room temperature for several months.2–4
These radicals are of scientific and practical interest because
they are initiators for low-temperature polymerization, telo-
merization and vulcanization. We believe that this phenomenon
is of a general character and can be extended to other organic
peroxides. To confirm this expectation, we performed the reac-
tions of anhydrides 1a,b with hydrogen peroxide in an alkaline
medium to prepare di(cis-2-carboxycyclohexylcarbonyl) peroxide
2a and di(cis-2-carboxycyclohexenylcarbonyl) peroxide 2b
(Scheme 1), respectively, and then carried out the cocrystalliza-
tion of these products with cis-cyclohexane-1,2-dicarboxylic 3a5
and cis-cyclohex-4-ene-1,2-dicarboxylic 3b6 acids, respectively.†
Peroxides 2a and 2b were characterised by IR, 1H and
13C NMR spectroscopy and elemental analysis; acids 3a and
3b, by NMR spectroscopy.‡
Earlier, X-ray diffraction studies showed that the structural
similarity of the starting compounds is one of the main pre-
requisites for the formation of stabilised radicals during the
cocrystallization of peroxysuccinic and succinic acids or peroxy-
glutaric and glutaric acids.8,9 The peroxide molecule, which is
bulkier than the corresponding two dicarboxylic acid molecules,
is inserted into the crystalline lattice of the dicarboxylic acid
matrix, and the O–O bond cleavage in peroxide occurs as a
result of deformation strain in the crystalline lattice.4
The EPR study showed that the cocrystallization of peroxide
2a with dicarboxylic acid 3a§ affords radical species. The
primary EPR spectrum of the cocrystallization product is a
superposition of an unresolved singlet with gi = 2.0053 and a
‡
O
O
O
O
The NMR spectra were recorded on a Bruker AC-200 spectrometer
O
(200.13 MHz for 1H and 50.32 MHz for 13C). The IR spectra were
measured on a Specord M-82 instrument.
O
O
HO
O
O
OH
i, H2O2, NaOH
ii, H2SO4
2a: mp 94–95 °C. 1H NMR [(CD3)2CO] d: 1.5 (m, 4H, CH2CH2), 1.85
(m, 4H, CH2CHCOOH, CH2CHCOO), 2.9 (m, 1H, CHCOOH), 3.1 (m,
1H, CHCOO). 13C NMR {H} [(CD3)2CO] d: 174.2 (s, COOH), 174.0 (s,
COOO), 42.4 (s, CHCOOH), 40.6 (s, CHCOO), 26.4 (s, CH2CHCOOH),
24.0 (s, CH2CHCOOO), 23.95 (s, CH2CH2). IR (vaseline oil, n/cm–1):
1804, 1776, 1696, 1080, 1060. Found (%): C, 56.18; H, 6.40. Calc. for
C16O8H22 (%): C, 56.14; H, 6.43.
X
X
X
1a,b
2a,b
a X = CH2–CH2
b X = CH=CH
Scheme 1
†
The starting cis-cyclohexane-1,2-dicarboxylic anhydride 1a and
2b: mp 89–90 °C. 1H NMR [(CD3)2CO] d: 2.2–2.7 (br. m, 4H,
CH2CHCOOH, CH2CHCOO), 3.08 (m, 1H, CHCOO), 3.23 (m, 1H,
CHCOO), 5.67 (m, 2H, CH=CH). 13C NMR {H} [(CD3)2CO] d: 174.19
(s, COOH), 169.7 (s, COOO), 126.16 and 124.88 (2s, CH=CH), 39.76
(s, CHCOOH), 37.06 (s, CHCOO), 26.39 and 26.19 (2s, 2CH2).
IR (vaseline oil, n/cm–1): 1804, 1776, 1760, 1660, 1080, 1060. Found
(%): C, 56.69; H, 5.58. Calc. for C16O8H18 (%): C, 56.69; H, 5.46.
3a: 1H NMR [(CD3)2CO] d: 1.5 (m, 2H, CH2), 2.1–1.8 (m, 2H,
CH2CHCOOH), 2.85 (m, 1H CHCOOH). 13C NMR {H} [(CD3)2CO] d:
174.95 (s, COOH), 42.47 (s, CHCOOH), 26.7 (s, CH2CHCOOH), 26.19
(s, CH2).
cis-cyclohex-4-ene-1,2-dicarboxylic anhydride 1b (Aldrich) were used
without additional purification. A finely ground powder of the anhydride
(0.01 mol) was added with vigorous stirring to 3% H2O2 (30 ml) and
10% NaOH (15 ml) at –3–4 °C in such a way that the temperature was
no higher than 2 °C. The reaction mixture was filtered, and the solution
was acidified with 10% H2SO4 to pH ~ 2. The peroxide was separated
by filtration, washed with cold distilled water and dried in a vacuum
desiccator. The yields of 2a and 2b were 15 and 10%, respectively. Both
peroxides are readily soluble in water, methanol and acetone and are poorly
soluble in benzene. Acids 3a and 3b were synthesised by the reactions of
anhydrides 1a and 1b, respectively, with a 20% NaOH solution at 5 °C
followed by acidification with 10% H2SO4 to pH ~ 2. The resulting acids
were filtered off, washed with cold distilled water and dried.
3b: 1H NMR [(CD3)2CO] d: 2.3–2.6 (m, 2H, CH2), 3.02 (m, 1H,
CHCOOH), 5.64 (m, 2H, CH=CH). 13C NMR {H} [(CD3)2CO] d:
175.08 (s, COOH), 126.00 (s, CH=CH), 26.60 (s, CH2).
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© 2008 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2008, 18, 123–125
7
6
5
4
3
2
1
(a)
0
10
20
30
40
(b)
t/days
Figure 2 Plot of the concentration of the radicals vs. time.
also confirmed by quantum chemical calculations, as well as
small (£1.7 G) splitting constants for all other protons. All other
possible radicals definitely have completely different calculated
splitting constants.
3410
3450
3490
3530
G
Therefore, the mechanism of formation of radical 6a by the
cocrystallization of peroxide 2a with dicarboxylic acid 3a can
be represented by Scheme 2.
Figure 1 (a) Simulated spectrum for 5a and 6a radicals (line width = 7,
L/G = 1) and (b) experimental EPR spectrum.
Radicals 4a that are initially generated from peroxide 2a are
rearranged into more stable EPR-detectable radicals 6a via the
1,2-migration of the hydrogen atom.
Radicals 6a abstract hydrogen from the matrix substance,
dicarboxylic acid 3a, thus being transformed into cyclohexane-
carboxylic acid 7a (Scheme 3). The latter was extracted from
the reaction mixture with hexane and then methylated with diazo-
methane. The methyl ester was identified by chromatography–
mass spectrometry.
triplet of triplets with the parameters gi = 2.0029, Ai(1H) = 32 G,
Ai(1H) = 10 G and the linewidth of about 7 G, which is apparently
attributed to interactions with the matrix substance. The central
singlet disappears in a few hours, and the triplet of triplets
becomes more pronounced, the parameters remain unchanged
(Figure 1).
Most likely, the singlet, whose intensity decreases much
more rapidly than that of the triplet of triplets, belongs to
acyloxy radical 5a, which is additionally confirmed by the
value of gi. According to PBE calculations10 with the TZ2P
basis set11 using the PRIRODA program,12 radical 5a has small
splitting constants (£ 1.5 G), which do not allow us to observe
the resolved signal. The second spectrum, the triplet of triplets,
is assigned to 1-carboxycyclohexyl radical 6a. In the crystalline
lattice, these radicals are packed so that there are two pairs of
nonequivalent (axial and equatorial) β-protons with the hyperfine
coupling constants Ai(1H) = 32 G and Ai(1H) = 10 G. This is
COOH
COOH
3a
+
5a
6a
7a
Scheme 3
To study the cocrystallization products of dicarboxylic acid
3a with peroxydicarboxylic acid 2a by EPR, six samples with
different ratios between the starting compounds (4–90%) were
prepared. For these samples, the disappearance rate of radicals
was followed for 1.5 months. The molar concentration of the
peroxide optimal for stabilization of the radicals in the solid
matrix was found to be 40% with respect to dicarboxylic acid.
The measured initial concentration of the radical species in this
sample was about 1014 g–1. Figure 2 presents the concentration
vs. time for a sample stored at room temperature.
Solution
Solid matrix
COO
O
O
O
O
O
O
COOH
5a
– CO2
OH HO
2a
cocrystallization
The cocrystallization products of peroxide 2b and peroxy-
dicarboxylic acid 3b (4–90% 3b) were also studied by EPR
spectroscopy. In this case, the spectrum range corresponds to
that of radical 5a. However, a pronounced multiplet pattern was
not observed.
COOH
COOH
4a
HOOC
COOH
3a
We are grateful to B. N. Shelimov for helpful discussions
and comments. This study was supported by the Council on
Grants of the President of the Russian Federation (Programme
for Support of Leading Scientific Schools of the Russian
Federation, grant no. NSh 5022.2006.3).
6a
Scheme 2
§
Cocrystallization. Peroxydicarboxylic acid 2a (2b) and dicarboxylic
acid 3a (3b) were completely dissolved in a required amount of distilled
water, the solvent was distilled off and the solid residue was dried in a
vacuum desiccator. The peroxide content was determined by iodometric
titration.
The EPR spectra were recorded on a Bruker EMX 6/1 spectrometer
equipped with an ER-4102ST resonator (9.8 GHz) and an ER 4111 VT
temperature controlling unit; dpph was used as a standard for the deter-
mination of the g factors.
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Received: 4th December 2007; Com. 07/3052
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