Hydrogen Peroxide Oxidation of Mustard-Model Sulfides
J . Org. Chem., Vol. 66, No. 23, 2001 7589
Sch em e 1
hydroxylation are considered. In contrast to the alkene
and alkane reactions, only a few reports on sulfide and
sulfoxide oxidations have been published9,10 since the
pioneer work of Oae et al.11 and Ando et al.12 These
preliminary investigations show that alkene epoxidation
can be a source of inspiration for designing catalytic
systems convenient for sulfide decontamination. More-
over, they show that, depending on the metal of the
catalyst, sulfoxides and/or sulfones are obtained readily,
whereas disulfides arising from the coupling of radical
cations are not observed, except when the sulfide bears
an acidic R-hydrogen atom.11b The use of hydrogen
peroxide as the oxidant, which was not very efficient
because of the competition between the catalase activity
(destruction of the oxidant by dismutation) of the cata-
lyst, the ligand oxidation (destruction of the catalyst), and
the oxygen transfer (oxygenation activity) to the sub-
strate, is now under control for alkene epoxidation,13-15
although the mechanism and the nature of the reactive
intermediate are still under discussion.16-19
sulfide oxidation rates are not significantly substituent-
dependent,20much less than their corresponding hydroly-
sis rates.5a
Moreover, for large-scale procedures as required for
decontamination, the efficiency of a catalytic system has
to be evaluated not only from the efficiency of the
substrate oxidation but also from the oxygen transfer of
the oxidant to the substrate and from the stability of all
the components of the system, which is not necessarily
considered in usual lab-scale investigations. In this
paper, we show that iron and manganese tetraarylpor-
phyrins provide efficient catalytic systems for sulfide
oxidations into sulfones or sulfoxides by hydrogen per-
oxide, in ethanol, without significant bleaching of the
catalyst and marked oxidant dismutation when the
metal, the substituents of the porphyrinic ligand, and the
cocatalyst are appropiately chosen.
On the other hand, the lipophilicity of 1b, a property of
interest in view of forthcoming extensions to aqueous
microheterogeneous media, is close to that of 1c, while
that of 1a is markedly greater.4 With the same objective
in mind, ethanol rather than acetonitrile is the solvent,
and we have shown previously that these two solvents
provide similar efficiencies in sulfide oxidations when
tetraphenyl porphyrin (TPP) is the catalyst ligand.10
The catalysts are manganese- and iron-meso-tetra-
arylporphyrins (Scheme 1) with various substituents in
their phenyl rings. This so-called second generation of
porphyrins21 is investigated since TPP was shown to
achieve sulfide oxidation successfully but can be readily
bleached in the presence of hydrogen peroxide.10 More-
over, phenyl-substituted metalloporphyrins are well known
to catalyze efficiently H2O2 epoxidation of alkenes13-15
whose reactivity toward oxidation is likely in the same
range as that of the investigated sulfides. The third
generation of still more robust porphyrins21 with sub-
stituents in the pyrrole â-positions, which are mainly
used for the hydroxylation of poorly oxidizable alkanes,
is not considered since sulfides are expected to be reactive
substrates. Finally, a cocatalyst, imidazole14,22,23 or am-
monium acetate,23,24 is added in some experiments, in
agreement with various catalytic systems previously
designed for epoxidation and for some sulfide oxi-
dations.9c,10,11b
Resu lts a n d Discu ssion
Dibenzyl sulfide (1a ) and phenyl-2-chloroethyl sulfide
(1b) are used as models of mustard (1c). On one hand,
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Eds.; Plenum Press: New York, 1993; p 147. (c) Metalloporphyrin
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The effect of the catalytic systems on the sulfide
conversion and product distribution (sulfoxide versus
sulfone) is measured in experiments carried out at room
temperature, according to the usual standard procedure,
i.e., by progressive additions of H2O2 (35% in water) to
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