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epoxide oxygen would be expected to be labelled.
Different ratios of inter- and intra-molecular chain propagation imply involvement
of a-keto hydroperoxides in the oxidation of ketones to CO2 and CO at 120±
155 C.216
The role of anthraquinones as mediators of one-electron transfer to molecular oxygen
has been studied by cyclic voltammetry in DMSO and DMF solution.217 The reduction
potentials of those anthraquinones containing OH groups were substantially shifted
towards more positive values in the presence of O2, whereas those without OH groups
202 Organic Reaction Mechanisms 1997
OH O O
R R R
O2
Me Me Me
HO
O O
OH OH
Vitamin KH2 (17)
O
O O
R
R R
O
O
Me
Me
Me
O
HO HO
O O
O
Vitamin oxide (18)(19)
Me Me Me Me
R =
Me
O O
R R
O O O OH
O2
(17)
Me Me
OH O
(20)
(18)
showed no such effect. Indicative of a signi®cant interaction with oxygen, this effect has
been explained, with the aid of theoretical calculations, in terms of formation of
hydroperoxide anion radicals which can be formed only by anthraquinones possessing
OH groups. A study of the oxidative deamination of benzylamine to benzaldehyde
catalysed by quinonoid cofactors supports the transamination mechanism of quinone-
catalysed aerobic deamination involving an aminophenol intermediate that is
5 Oxidation and Reduction 203
autoxidized to an iminoquinone during the catalytic cycle.218 Electrochemical results
suggest that an asymmetric orthoquinone structure is a requirement.
Ab initio and semiempirical molecular orbital calculations have been used, together
with charge-transfer theories, to investigate the structures of organodioxide anions and
related charge-transfer complexes between carbanions and molecular oxygen.219
1
The kinetics of quenching of O2 by the alkaloid boldine in a number of solvents
have been studied.220 Solvent-effect correlations of the quenching rate constant suggest
that the predominant mode of quenching is the formation of a charge-transfer complex
between the aromatic rings and the excited oxygen. Back-electron transfer of the
electron in such a complex regenerates boldine and ground-state oxygen (physical
quenching with no net chemical transformation) whilst combination leads to products
(chemical quenching). The rate of consumption of boldine (as measured by HPLC)
reveals that the latter accounts for up to 5% of the total quenching rate.
Atomic Oxygen, Triplet Oxygen, and Autoxidation
The formation of a biradical, involving the addition of an oxygen atom to the double
bond, is proposed to occur in the oxidation of acrylonitrile and crotononitrile by atomic
oxygen(3P).221
The oxidation of cyclohexene by means of molecular oxygen in the presence of
cobalt naphthenate, vanadyl acetylacetonate, and molybdenyl acetylacetonate as
catalysts has been studied.222 The mechanism of p-toluenesulfonic acid-catalysed
oxidation of styrene epoxide by O2 has been discussed, including the in¯uence of [O2]
on radical formation, drawing on literature data and comparison with the reaction in the
absence of O2.223 With the aim of acquiring a better understanding of the factors
responsible for knock in spark ignition engines, the oxidation mechanisms of pentane
and cyclopentane have been probed by oxidation at 873 K and chromatographic
determination of the product distribution according to the time of passage in the
reactor.224
Study of the oxidation of aromatics at high temperature is relevant to their use in
augmenting the octane rating of hydrocarbon fuels. Oxidation of anisole at 1000 K is
shown to proceed via the same pathway as pyrolysis under inert conditions.225
Modelling of the experimental kinetics and product mixtures indicates that the ®rst step
is cleavage of the OÐCH3 bond to generate phenoxy and methyl radicals, from which
cresols are obtained by attack of CH3 at positions ortho, meta, or para to the oxygen,
phenols by subsequent cleavage of the CÐCH3 bonds in the excited products, and
methylcyclopentadiene by elimination of CO. A symposium has dealt with the
mechanisms of oxidation of a range of aromatic and aliphatic hydrocarbons under
combustion conditions, together with the role of NO.226
In the oxidation of octan-2-one, undecan-4-one, 1,3-diphenylacetone, and 2-
phenylacetophenone, different ratios of intermolecular and intramolecular chain
propagation are proposed to lie behind the varying distribution of products, CO2,
CO, H2, hydroperoxides, and acids; a mechanism involving a-keto hydroperoxides was
proposed.227
204 Organic Reaction Mechanisms 1997
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