Benzylic oxidation and photooxidation by air in the presence of graphite and cyclohexene

Article abstract of DOI:10.1016/j.tetlet.2007.03.072

Graphite is introduced as a convenient catalyst for cyclohexene-promoted photooxidation of p-xylene, ethylbenzene, and cumene by air. Availability of the reagent (air), lack of chemical waste, low toxicity, and reusability of the catalyst make the process a good green alternative of oxidation of these industrially important hydrocarbons.

Full text of DOI:10.1016/j.tetlet.2007.03.072

Tetrahedron Letters 48 (2007) 3417–3421
Benzylic oxidation and photooxidation by air in the presence
of graphite and cyclohexene
Grigoriy Sereda* and Vikul Rajpara
The University of South Dakota, Department of Chemistry, 414 E. Clark Street, Vermillion, SD 57069, United States
Received 26 July 2006; revised 9 March 2007; accepted 9 March 2007
Available online 15 March 2007
Abstract—Graphite is introduced as a convenient catalyst for cyclohexene-promoted photooxidation of p-xylene, ethylbenzene, and
cumene by air. Availability of the reagent (air), lack of chemical waste, low toxicity, and reusability of the catalyst make the process
a good green alternative of oxidation of these industrially important hydrocarbons.
Ó 2007 Elsevier Ltd. All rights reserved.
Unmodified graphite is receiving increased attention as a
mild catalyst for acylation,1 alkylation,2 and cleavage of
ethers.3 Development of efficient catalytic systems for
oxidation of aromatic hydrocarbons to peroxy- hydr-
oxy-, and carbonyl compounds is an area of current
interest because of its significant practical potential.
Research in this field is driven by the involvement of
expensive catalysts and unsatisfactory control of the
synthetic outcome for current reaction conditions. Thus,
use of a cobalt(II)-chelated copolymer was suggested to
improve the well-known cumene peroxidation.4 For
conversion of alkylbenzenes to alcohols, aldehydes,
and acids there are recently patented procedures, involv-
ing tungsten, palladium-based catalysts,5,6 and photoox-
idation.7,8 Here we report oxidation and photooxidation
of the benzylic carbon in p-xylene, ethylbenzene, and
cumene by air, as well as the influence of unmodified
graphite and cyclohexene on the reaction outcome. Pass-
ing air at the rate of 1 mL/min through refluxing
p-xylene 1 with graphite in the presence of cyclohexene
under the ambient light leads to a mixture of 4-methyl-
benzyl hydroperoxide 2,9a 4-methylbenzyl alcohol 3,10
4-methylbenzaldehyde 4,11 and 4-methylbenzoic acid
512 at the molar ratio of 4.5:1.5:1:1.5 (Scheme 1).
After the reaction was complete, the catalyst that was
filtered out and kept on air completely regained its
activity.
Graphite does not seem to require a substantial activa-
tion time before the reaction starts. A sample taken
CH3
CH3
CH2OOH
CH2OH
CHO
COOH
O
O
Graphite, air
cyclohexene
reflux, 24 h
+
+
+
+
CH3
CH3
CH3
CH3
CH3
CH3
1
2
3
4
5
6
Scheme 1. Oxidation of p-xylene.
*
Corresponding author. Tel.: +1 605 677 6190; fax: +1 605 677 6397; e-mail: gsereda@usd.edu
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.072

3418
G. Sereda, V. Rajpara / Tetrahedron Letters 48 (2007) 3417–3421
from the reaction mixture after 1 h has shown the pres-
ence of compounds 2–5 (by NMR). Products 4 and 5
were isolated by column chromatography in order to
obtain an additional evidence of their identity.
alysts and produces a mixture of 1-phenylethyl hydro-
peroxide 8,16 1-phenylethanol 9,17 and acetophenone
1018 (Scheme 2) at the molar ratio of 2:0.2:1. (Table 3,
entry 2). The distilled out unreacted ethylbenzene did
not contain any impurities (by NMR).
Protection from light does not affect the overall extent of
conversion, but significantly impacts the reaction prod-
uct distribution (Table 1, entries 2 and 3). Ester 6 was
the major product of oxidation in the dark, while hydro-
peroxide 2 was not detected among the products (entry
2). The presence of ester 613 in the reaction mixture
was additionally confirmed by mass-spectrometry. In a
control experiment, no autooxidation of p-xylene was
observed in the absence of carbonaceous catalysts under
the ambient light (Table 1, entry 1). Cyclohexene plays a
significant role in the oxidation of p-xylene. No reaction
takes place without cyclohexene, or a complex mixture
of unidentified products is formed (Table 2). The con-
tent of phenolic products in the reaction mixture was
We believe that hydroperoxide 8 is formed by the com-
mon radical mechanism followed by decomposition as
outlined in Scheme 3. The reaction was almost com-
pletely suppressed when run with light protection
(source of free radicals, Table 3, entry 1), or in the pres-
ence of cyclohexene (interceptor of free radicals, Table
4, entry 2). Interestingly, cyclohexene has the opposite
effect on oxidation of p-xylene (Tables 1,2), where it
seems to play an essential role in the formation of free
radicals. Black particles of graphite absorb the ambient
light and, therefore, stop oxidation of ethylbenzene
(Table 3, entries 5 and 6).
1
estimated by the integral intensity of a H multiplet at
6.8–6.9 ppm.
Interestingly, simultaneous action of light, cyclohexene,
and graphite leads to significant oxidation of ethylbenz-
ene to practically important acetophenone 10 as the
major reaction product (Table 4, entry 6). This reaction
results from the unique combination of controversial ef-
fects of cyclohexene (generator and interceptor of free
radicals) and graphite (generator, interceptor of free
radicals, absorbent of light and, possibly, reaction inter-
mediates). The exact reason of such synchronous behav-
ior remains unknown.
The known tendency of cyclohexene to form relatively
stable allylic radicals upon irradiation or in the presence
of peroxides14 and the ability of graphite to produce
hydroperoxy-surface groups upon oxidation by air15 of-
fer an explanation of their synchronous catalytic activity
toward free-radical oxidation of p-xylene with air. The
exact mechanism of this reaction is yet unclear, however,
lack of light may allow for accumulation of the surface
peroxy-groups, which, in turn, induce decomposition of
hydroperoxide 2 (Table 1, entry 2).
Because of the formation of a stable tertiary benzylic free
radical, cumene 11 was more active toward oxidation
than ethylbenzene 7 (Tables 3 and 4). After refluxing
for 24 h with or without light protection, the reaction
led to mixtures of products 14,19 12,20 and 10 (Scheme 4).
As opposed to p-xylene 1, oxidation of ethylbenzene 7
under ambient light proceeds without carbonaceous cat-
Table 1. Oxidation of p-xylene in the presence of cyclohexene
Catalyst
Light protection
Peroxide
Alcohol
Aldehyde
Acid
Ester
Total yield (g)
Entry
None
Graphite
No
Yes
No
0
0
4.5
0
1
1.5
0
1
1
0
1.7
1.5
0
1.7
0
0
0.23
0.23
1
2
3
Table 2. Oxidation of p-xylene (24 h, no light protection)
Catalyst
Peroxide
Alcohol
Aldehyde
Acid
Ester
Phenolic products
Total yield (g)
None
Graphite
0
0.5
0
0.5
0
1
0
0.5
0
0
0
2
0
0.06a
a Substantial amounts of unidentified products were formed.
CH3
HO
O
CH3
CH3
CH3
HO
O
Graphite, air
cyclohexene
reflux, 24 h
+
+
7
8
9
10
Scheme 2. Oxidation of ethylbenzene.

G. Sereda, V. Rajpara / Tetrahedron Letters 48 (2007) 3417–3421
Table 3. Oxidation of ethylbenzene and cumene
3419
Catalyst
None
Substrate
Light protection
t (h)
Peroxide
Alcohol
PhAc
Total yield (g)
Entry
Ethylbenzene
Yes
No
Yes
No
Yes
No
Yes
No
18
24
24
24
23
24
24
20
10
2
0.7
0
0
0
5
1
1
1
1
0
0
1
1
0.03
1.2
0.08a
2.0
1
2
3
4
5
6
7
8
0.2
0.7
1.5
0
0
1
Cumene
Graphite
Ethylbenzene
Cumene
0
0
1
0.25
0.07b
0.66
1
a 0.7 equiv of 2,2-dimethyl-2,3-diphenylbutane 13 is also formed.
b 1 equiv of 2,2-dimethyl-2,3-diphenylbutane is 13 also formed.
CH3
.
O
CH3
CH3
O
.
HO
O
CH3
R.
O2
RH
- R.
- RH
7
8
CH3
HO
- OH.
Heat
.
CH3
O
RH
- R.
H
9
CH3
O
R.
- RH
10
Scheme 3. Mechanism of oxidation of ethylbenzene by air.
Table 4. Oxidation of ethylbenzene and cumene in the presence of cyclohexene
Catalyst
None
Substrate
Light protection
Peroxide
Alcohol
PhAc
Phenolic products
Total yield (g)
Entry
Ethylbenzene
Yes
No
Yes
No
Yes
No
Yes
No
1
20
0.3
0
1
1
1
1
1
1
0
1
—
1
0
0
0
0.1
0
0.1
—
0
0.13
0.1
0.97
1.84
0
0.79
0.09a
2.20
1
2
3
4
5
6
7
8
Cumene
1.8
0.7
0
0.3
—
0.3
Graphite
Ethylbenzene
Cumene
0
0.06
—
0
a Only unidentified products were formed.
As opposed to the common acid-catalyzed rearrange-
ment of hydroperoxides involving migration of the phe-
nyl group, homolytical cleavage of 14, followed by the
loss of the methyl radical, led to acetophenone 10.
Transfer of the methyl radical at the last step of the
C–C bond cleavage takes place in the known reactions
of oxidation of cumene 11 to acetophenone.21 This reac-
tion proceeds through cumene hydroperoxide 14, gener-
ated by Mn(III)-catalyzed oxidation of 11 by
iodosobenzene. We determined the molar ratio of the
hydroperoxide 14 and alcohol 12 by integration of the
inverse-gated broad band decoupled 13C NMR spec-
trum with the relaxation delay of 2 s. The inverse-gated
decoupling was employed to prevent influence of
the NOE-effect on the integral intensities of the signals.
To insure practically complete relaxation, the sp3
quaternary carbons in 14 and 12 were integrated in the
array of experiments with different relaxation delays
until the results became consistent, according to the
known technique.22

3420
G. Sereda, V. Rajpara / Tetrahedron Letters 48 (2007) 3417–3421
H3C
CH3
HO
O
CH3
CH3
CH3
CH3
CH3
HO
O
Graphite, air
cyclohexene
reflux, 24 h
+
+
11
14
12
10
Scheme 4. Oxidation of cumene.
Oxidation of cumene was significantly suppressed when
run with light protection (similarly to ethylbenzene) and
produced a significant amount (0.7 equiv with respect to
acetophenone, Table 3, entries 3 and 4) of 2,3-dimethyl-
2,3-diphenylbutane 13,23 the product of dimerization of
the a,a-dimethylbenzyl radical. This observation indi-
cates that in the presence of light, the concentration of
a,a-dimethylbenzyl radicals significantly drops, proba-
bly, due to the hydrogen abstraction from the oxidation
products 10, 12, and 14. Similarly to ethylbenzene,
graphite retards oxidation of cumene in the presence
of light, but does not suppress it completely (Table 3,
entry 8).
tribution of photooxidation products of ethylbenzene
and cumene is significantly affected by cyclohexene
and the graphite catalyst.
Acknowledgments
We thank the State of South Dakota (2010 Research ini-
tiative and 2010 Individual SEED Grant) and NSF
(URC Grant 0532242) for financial support of this
work.
Supplementary data
In the presence of cyclohexene, cumene 11 is efficiently
oxidized by air regardless of the exposure to light (Table
4, entries 3 and 4). Oxidation of cumene in the presence
of cyclohexene in the dark was suppressed by graphite
(Table 4, entries 3 and 7), which emphasizes the role
of graphite as an interceptor of radicals as opposed to
a plain light absorbent.
Experimental procedures for oxidation, for synthesis of
carbon nanofibers, 1H and NMR spectra for all reaction
products. 1H and 13C NMR spectra of hydroperoxide 8.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.03.072.
References and notes
Similarly to ethylbenzene, the retarding effect of graph-
ite on photooxidation of cumene (Table 3, entries 4 and
8) was lifted in the presence of cyclohexene (Table 4,
entries 4 and 8), and the composition of the reaction
mixture was significantly shifted toward acetophenone.
Therefore, the synchronous action of light, cyclohexene
and graphite on the oxidation of cumene again led to a
practical procedure for the preparation of acetophenone
(Table 4, entry 8).
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