C-C bond cleavage in oxidation of aliphatic hydrocarbons under mild conditions in the Vv/H2O2/AcOH system

Full text of DOI:10.1023/A:1019230610870

Doklady Chemistry, Vol. 378, Nos. 4–6, 2001, pp. 150–153. Translated from Doklady Akademii Nauk, Vol. 378, No. 5, 2001, pp. 639–643.
Original Russian Text Copyright © 2001 by Gekhman, Stolyarov, Ershova, Moiseeva, Moiseev.
CHEMISTRY
This paper is dedicated to Prof. Brian James (University of British Columbia, Canada) in recognition
of his outstanding contribution to homogeneous catalysis and inorganic chemistry
C–C Bond Cleavage in Oxidation of Aliphatic Hydrocarbons
V
under Mild Conditions in the V /H O /AcOH System
2
2
Corresponding Member of the RAS A. E. Gekhman*, I. P. Stolyarov**, N. V. Ershova*,
N. I. Moiseeva**, and Academician I. I. Moiseev*
Received March 5, 2001
The oxidation of alkanes by H O in all available should be noted that conventional inhibitors of free rad-
2
2
II
III
Fenton systems for hydroperoxide activation (Fe /Fe , ical chain reactions—BHT, p-benzoquinone, or
IV
V
V
VI
II
III
V /V , Cr /Cr , Co /Co , etc.) results in alcohols anthracene derivatives (10 mol % of the substrate con-
and ketones [1–3]. In this paper, we report on the oxi- centration)—have no effect on both the substrate con-
dation of alkanes (using isooctane as an example) and sumption rate and composition of oxidation products.
methyl esters of fatty carboxylic acids accompanied by The oxidation rate of inhibitors, for example, of BHT,
the cleavage of a C–C bond between the atoms does not exceed the substrate consumption rate. p-Ben-
bearing no functional groups.
zoquinone is not oxidized at all under these conditions.
These observations point to a nonchain oxidation
V
The V /H O /AcOH system is capable of oxidizing
2
2
V
mechanism in the V /H O /AcOH system.
2
2
different substrates such as cyclohexane, substituted
alkenes, and anthracenes [4–10]. Parallel oxidation
pathways of cyclohexane yields cyclohexanol and
cyclohexanone, and a C–C bond therewith remains
intact [8]. Cyclohexane is oxidized more readily than
acyclic hydrocarbons [1]; thus, it might be expected
that the oxidation of alkanes in the system under con-
sideration either does not take place or proceeds with-
The products of n-alkane oxidation contained both
secondary alcohols and ketones with the same carbon
backbone as in the initial alkane (table); neither the oxi-
dative destruction nor isomerization of a carbon chain
was observed. This is consistent with the nonchain
character of the process. The terminal methyl groups of
an n-alkane are not involved in the reaction, and only
secondary carbon atoms are oxidized. For example, the
oxidation of n-heptane gave rise to 2-heptanol, 3-hep-
out C–C bond cleavage.
In the absence of a catalyst, alkanes and methyl tanol, 4-heptanol, 2-heptanone, 3-heptanone, and 4-
esters of higher fatty acids are stable to H O solutions heptanone. Neither hexanol-1 nor heptanal was found
2
2
in acetic acid at 30–50°C over a period of 3–5 h. Intro- among the reaction products.
–
2
ducing 10 M of VO(acac) into the solution catalyzes
2
The overall yield of alcohols and ketones was ~60%
at a substrate conversion of ~35%. The content of
ketones in the products of n-alkane oxidation was con-
siderably higher than that of the corresponding alcohols
the decomposition of H O , and the substrate is com-
2
2
1
pletely consumed in 30 min after mixing the reagents.
The conversion of n-alkanes increases in proportion
with the initial hydrogen peroxide concentration
(
table, figure). No hydroxy ketones were formed upon
[
H O ] ; for example, the 50% conversion of n-octane
2 2 0
the oxidation of n-heptane or lower alkanes.
is achieved at a 12-fold molar excess of the oxidant. It
Alcohols and ketones are the ultimate products of
alkane oxidation in the system under consideration;
alcohols are not intermediates in the formation of
1
Using ammonium vanadate instead of vanadyl acetylacetonate
has no effect on the composition of reaction products, as well as
on the conversion of a substrate. The EPR spectra of reaction ketones. For example, in the course of concurrent oxi-
solutions showed that, in the presence of either of these com-
dation of 0.01 M of 3-pentanol and 0.1 M of n-octane
IV
pounds as a catalyst, the signals that arose from V were not
(
[H O ] = 1 M), the alcohol was not consumed. Upon
2 2 0
observed until hydrogen peroxide was completely exhausted. This
permits the conclusion that the oxidative system is largely com-
posed of VV complexes, which catalyze the reactions observed.
the concurrent oxidation of 2-butanol (0.01 M) and n-
octane (0.1 M), the final 2-butanol concentration did
not exceed 0.001 M after all the H O ([H O ] = 1 M)
2
2
2
2 0
was consumed. The products accumulate without the
induction period, and the curve of alcohol accumula-
*
Kurnakov Institute of General and Inorganic Chemistry,
Russian Academy of Sciences, Leninskii pr. 31, Moscow, tion has no maximum (figure). These observations sug-
19991 Russia
gest that alcohols and ketones are formed from corre-
* Semenov Institute of Chemical Physics, Russian Academy sponding alkanes in parallel reactions. The oxidation of
of Sciences, ul. Kosygina 4, Moscow, 117977 Russia
linear alkanes resembles the oxidation of cyclohexane [8].
1
*
0
012-5008/01/0006-0150$25.00 © 2001 åÄIä “Nauka /Interperiodica”

C–C BOND CLEAVAGE
151
Indeed, as in the case of n-alkanes, the oxidation of
The reactivities of linear alkanes, other than n-pen-
V
cyclohexane in the V /H O /AcOH system results in tane, are slightly lower than the reactivity of cyclohex-
2
2
ane; the corresponding relative oxidation rate constants
range from 0.5 to 1.5. In other oxidative systems, cyclo-
hexane is also more active than n-alkanes [1–3].
cyclohexanol and cyclohexanone (formed in parallel
reactions), and no C–C bond cleavage is observed
8]. The only difference is that the ratio [ketone]/[alde-
[
hyde] ~ 1 is typical of the oxidation of cyclohexane,
whereas this ratio for alkanes slightly depends on the
position of a ruptured CH bond in a carbon chain but is
always more than unity. The suggested mechanism of
cyclohexane oxidation implies the formation of an
active oxidant in the rate-limiting stage; this active oxi-
dant can both oxidize the substrate and undergo deacti-
Unexpected results were obtained for the oxidation
V
of isooctane in the V /H O /AcOH system. The isooc-
2
2
tane molecule has only one methylene group; thus, by
analogy with the oxidation of cyclohexane and
n-alkanes, the oxidation of isooctane should result only
in 2,2,4-trimethyl-3-pentanone (1) and 2,2,4-trimethyl-
-pentanol (3) (Scheme 1). However, GC-MS evidence
showed the formation of the following oxidation prod-
3
vation to produce O . In this case, the reactivity of the
2
substrate varies in inverse proportion to the H O2 ucts: 2,2,4-trimethyl-3-pentanone (1) as a major product,
2
excess, all other things being the same. Given that this 2,4,4-trimethyl-1-pentene (2), 2,2,4-trimethyl-3-pen-
mechanism is also valid for the oxidation of linear tanol (3), 2,2,4-trimethyl-1-pentanol (4), 2,4,4-trime-
alkanes, relative reactivities of substrates can be esti- thyl-1-pentanol (5), 4,4-dimethyl-2-pentanone (6), 2,4-
mated based on the relative rate constants of n-alkane dimethyl-2-pentanol (7), and 4,4-dimethyl-2-pentanol
oxidation calculated from the data on competitive oxi- (8) (Scheme 1). In contrast to n-alkanes, isooctane is oxi-
dation of alkanes and cyclohexane [11].
dized to form, among other products, primary alcohols.
OH
H C
CH3
OH
OH
2
H C
CH3 H C
3
H C
CH2
3
3
CH3 CH3
(4)
H C
3
H C
3
CH3 CH3
3)
CH3 CH3
(5)
(
O
H C
CH3
H C
3
CH3
3
H C
CH3
CH3 CH3
3
H C
3
H C
3
H C
3
CH3 CH3
1)
CH3 CH2
(2)
(
H C
3
CH3
H C
3
CH3
H C
3
H C
3
H C
CH3
CH3 OH
8)
3
CH3 O
(
(6)
H C
3
OH CH3
7)
(
V
Scheme 1. Identified products of isooctane oxidation in the V /H O /AcOH system.
2
2
The formation of 2,4,4-trimethyl-1-pentene (2) points tertiary carbon atom, such as, for example, 2,4,4-trime-
to the oxidative dehydrogenation of isooctane. Similar thyl-2-pentanol.
reactions were observed for the oxidation of cyclooctane
H C
CH3
H C
CH3
III
3
3
by the GIF system (Fe salts/ tert-butyl-hydroperoxide)
12]. In this context, the formation of 4,4-dimethyl-2-
(
1)
H C
3
H C
3
OH
[
CH CH
CH CH
3 3
3
3
pentanone (6) (Scheme 1) could be explained by the oxi-
dative dehydrogenation of isooctane and by the oxidative
splitting of 2,4,4-trimethyl-1-pentene (2) forming in the
V /H O /AcOH system [9, 10]. However, the formation
of 2,4-dimethyl-2-pentanone (7) and 4,4-dimethyl-2-pen-
tanone (8) strongly suggests the C–C bond cleavage.
This alcohol or its acetate may presumably undergo
dehydration or AcOH elimination to produce 2,4,4-trim-
ethyl-1-pentene (2) found among the reaction products.
V
2
2
The most surprising thing in the oxidation of isooc-
V
tane in the V /H O /AcOH system is the oxidative C−C
2
2
bond rupture. First, oxidizable impurities that may
The surprising thing is that the reaction mixture attend the initial isooctane were suspected to be respon-
lacked the products of hydroxylation of isooctane at the sible for the occurrence of C hydrocarbons (such as
7
DOKLADY CHEMISTRY Vol. 378 Nos. 4–6 2001

1
52
GEKHMAN et al.
V
V
–3
Oxidation of n-alkanes in the V /H O /AcOH system at 30°C, [V ] = 4 × 10 M, and [H O ] = 1 M
2
2
2
2 0
4
× 103,
total
M
[
alcohol]total × 10 , [ketone]
No.
n-Alkane
Pentane
Hexane
Heptane
Octane
Concentration, M Conversion, %
M
45a
40
25
18
20
b
1
2
3
4
5
6
7
8
9
0.1
49
32
32
30
31
29
21
16
15
11
47
c
d
f
0.1
0.1
0.1
37
24
32
e
g
h
Nonane
Decane
0.05
0.05
0.05
0.05
0.05
0.05
0.1
18
22
33
23
18
21
43
9i
8
6
5
5
3
Tetradecane
Pentadecane
Hexadecane
Nonadecane
Pentane
1
1
0
1
38
BHT
0.01
Note: 2-pentanol/3-pentanol = 1.6/1; b 2-pentanone/3-pentanone = 1.9/1; c 2-hexanol/3-hexanol = 0.7/1; 2-hexanone/3-hexanone = 0.8/1;
a
e
h
d
f
g
2
2
-heptanol/3-heptanol/4-heptanol = 0.8/2/1; 2-heptanone/3-heptanone/4-heptanone = 1.4/1.6/1; 2-octanol/3-octanol/4-octanol = 0.3/1/2;
i
-octanone/3-octanone/4-octanone = 0.7/1.2/1; 2-nonanone/3-nonanone/(4-nonanone + 5-nonanone) = 1.3/1/1.
3
4
,4-dimethyl-2-pentanone (6), 2,4-dimethyl-2-pen- in the form of trimethylsilyl esters. In addition to the
tanol (7), and 4,4-dimethyl-2-pentanol (8)) among the products with the same length of the hydrocarbon chain
reaction products. However, the purity of the starting as in the starting FAME, the oxidation products of
isooctane was higher than 99.8%, and the content of methyl octanoate contain at least three isomers of
2
,2-dimethylpentane and 2,4-dimethylpentane was no hydroxyheptanoic acid, three isomers of hydroxyhex-
2
more than 0.02%. These minute amounts of C alkanes anoic acid, three isomers of hydroxypentanoic acid,
7
are insufficient to explain the formation of noticeable and two isomers of hydroxybutanoic acid. Hydroxy
amounts (no less than 2% of the stating isooctane) of 6, acids and keto acids were found among the oxidation
7
, and 8. Correspondingly, the formation of 4,4-dime- products of other FAMEs.
thyl-2-pentanone (6), 2,4-dimethyl-2-pentanol (7), and
A good solubility of the reaction products in water
and their tendency toward polycondensation make the
quantitative analysis of a reaction mixture difficult.
However, the chromatograms of the oxidation products
of all FAMEs show a peak corresponding to a glycolic
acid ester. In some experiments, the ester is a major prod-
4
,4-dimethyl-2-pentanol (8) cannot be attributed to the
admixtures of isomeric hexanes in the starting substrate.
Therefore, all available data indicate that the C alcohols
7
and ketones originate from isooctane, and its oxidation
involves the oxidative destruction of a C–C bond.
Another evidence in favor of the C–C bond cleavage uct, as judged from the chromatographic peak intensity.
V
in reactions catalyzed by the V /H O /AcOH system
was provided by the oxidation of fatty acids methyl
esters (FAMEs).
2
2
Glycolic acid originates from an FAME rather from
the solvent (AcOH). Thus, the oxidation of methyl
isobutanoate does not result in the formation of glycolic
As in n-alkane oxidation, the FAME conversion acid. In addition, the GC-MS analysis of the silylated
monotonically increases with an increase in the initial products of the oxidation of methyl stearate in
H O concentration ([H O ] ). The limiting substrate CD COOD showed that the reaction mixture contains
3
2
2
2
2 0
conversion (η )—the conversion achieved after all only the nondeuterated trimethylsilyl glycolate
∞
H O was exhausted—constitutes 90% at a tenfold (CH
) SiOCH
3
COOSi(CH
) . The lack of deuterium
3 3
2
2
3
2
molar excess of H O . Our findings indicate that the rel- atoms in the OCH
COO moiety of glycolic acid
2
2
2
ative FAME reactivity increases with an increase in the excludes the formation of it from the solvent.
number of carbon atoms in the FAME chain: the higher
V
The oxidation of FAMEs in the V /H O /AcOH
2
2
the molecular weight of the ester, the lower the H O
2
2
system involves oxidative destruction, as demonstrated
by the formation of glycolic acid and its esters and
other products with a shortened hydrocarbon chain
compared to a starting FAME.
excess required to achieve the desired η value. The
∞
oxidation of FAMEs results in alcohols and ketones
functionalized at different sites of the aliphatic chain of
an initial ester. For example, the oxidation of methyl
octanoate yields three octanoic acid isomers identified
3
The reaction mixture of the FAME oxidation was silylated with
bis(trimethylsilyl)trifluoroacetamide at room temperature, and the
resulting trimethysilyl esters were extracted with chloroform and
analyzed by GC-MS.
2
As determined by GC-MS (a 100-m SPB-1 capillary column
0
.25 mm in i.d.).
DOKLADY CHEMISTRY Vol. 378 Nos. 4–6 2001

C–C BOND CLEAVAGE
153
3
Concentration, 10 M
ecule to coordinate vanadium(V) can influence both the
rate and selectivity of oxidation. This argument should
be taken into account to rationalize the surprising easi-
ness of C–C bond cleavage in the course of FAME oxi-
dation.
5
4
3
2
1
0
0
0
0
0
1
2
Our findings indicate that the oxidation of FAMEs
V
in the V /H O /AcOH system does not involve free
2
2
radical chain reactions. However, it is conceivable that
the C–C bond cleavage involves electron transfer from
hydrocarbon C atoms to the vanadium(V) complex.
Similar transformations were observed in the course of
II
III
Co /Co -catalyzed alkane oxidation by dioxygen in a
CF COOH solution [13, 14].
3
ACKNOWLEDGMENTS
0
50
100
150
200
250
Time, min
This work was supported by the Russian Foundation
for Basic Research, project nos. 99-03–33248 and 99–
3–32291, and by the Federal Program for Supporting
Leading Scientific Schools, project no. 00–15–97429.
Accumulation of (1) pentanone and (2) pentanol in the
course of oxidation of n-pentane in the V /H O /AcOH
0
V
2
2
V
–3
system at 30°C, [V ] = 4 × 10 M, [S]0 = 0.1 M, and
H O ] = 1 M.
[
2
2 0
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H
H
OH
H
–
2e
C
+ H O
C
+ 2H+,
(2)
3)
2
H
–
4e
C O + 4H+.
(
C
+ H O
2
H
4
A similar scheme can be suggested for C–C bond
splitting:
5
6
7
8
9
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OH
H
–2e
+
+
C
+ H O
C
+ H + [CH ]. (4)
3
2
H
+
[
CH ] in reaction (4) symbolizes a cationic species
3
trapped by a solvent molecule in the transition state,
which further transforms to methyl acetate.
The methyl CH bonds in isooctane can be oxidized
to produce primary alcohols; however, their yield is
small. Five CH groups of the isooctane molecule con-
3
tain 15 CH bonds per two methylene bonds and one CH
bond at the tertiary C atom. Therefore, the low yield of
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1
1
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2
The electronegative CH OOC groups in FAME
3
molecules might be expected to suppress the reactivity
of the aliphatic chain in oxidation. However, the FAME
oxidation rates do not significantly differ from the oxi-
dation rates of cyclohexane and alkanes. Similar to
alkanes, the FAME molecule is oxidized to produce the
corresponding hydroxy and keto acids and their esters.
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1
1
1
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DOKLADY CHEMISTRY Vol. 378 Nos. 4–6 2001