3232
J . Org. Chem. 2001, 66, 3232-3234
Oxid a tion of Or ga n ic Su lfid es by Br 2 a n d
H2O2. Electr op h ilic a n d F r ee-Ra d ica l
P r ocesses
Anna Bravo,† Barbara Dordi,† Francesca Fontana,*,‡ and
Francesco Minisci†
The selectivity is determined by the hydrogen abstrac-
tion from C-H bonds of alcohols or aldehydes, which is
either endothermic or, in the most favorable cases,
thermoneutral (eq 6).
Dipartimento di Chimica del Politecnico, via Mancinelli 7,
20131 Milano MI, Italy, and Universita` di Bergamo,
Dipartimento di Ingegneria, viale Marconi 5,
24044 Dalmine BG, Italy
fontana@unibg.it
Received December 7, 2000
In these cases, a low stationary concentration of HBr
is very useful in order to shift the equilibrium of eq 6 to
the right. The different behavior of aliphatic and benzyl
alcohols is the consequence of the different electronic
configurations of alkyl and acyl radicals, which are
reflected on the energies of the involved C-H bonds and
on the polar effects. With nonbenzylic alcohols, the
strength of RC(O)-H bonds is much lower than that of
R-CH(OH)-H bonds, so that aldehydes, being much
more reactive than the corresponding alcohols, are never
formed, even at low conversions. With benzylic alcohols,
the strength of ArC(O)-H bonds is slightly higher
compared to ArCH(OH)-H, as a result of the conjugation
of the R-hydroxybenzyl radical (π-type) with the aromatic
ring and the nonconjugation of the acyl radical (σ-type).
Aromatic aldehydes are therefore less reactive than the
corresponding alcohols, also because the polar effects in
hydrogen abstraction by Br. are more marked with the
alcohols.3,4
Bromine can be a selective oxidant for organic com-
pounds; it can operate by an electrophilic or a free-radical
mechanism. Its use, however, is not widespread because
it is an expensive, environmentally unfriendly reagent;
in addition, the formation of HBr as reduction product
can affect the selectivity or even the occurrence of the
oxidation itself.
We have recently introduced two developments that
make Br2 more attractive as an oxidant: (i) the use of
an aqueous-organic two-phase system, in which the
oxidation by Br2 takes place in the organic phase while
HBr is extracted by the aqueous phase, thus minimizing
its possible negative effects; and (ii) the use of aqueous
H2O2 to rapidly reoxidize HBr to Br2, to keep the
stationary concentration of HBr very low and to render
the oxidation catalytic in Br2 (the actual oxidant is, in
this case, H2O2, much more convenient than Br2 from
every point of view).
In the past1,2 we used Br2 as electrophilic catalyst in
the oxidation of 2,3,6-trimethylphenol by H2O2 to the
corresponding trimethylbenzoquinone, an intermediate
for the synthesis of vitamin E (eqs 1 and 2).
We have now investigated the behavior of organic
sulfides toward the Br2-catalyzed oxidation. The results,
summarized in the Table 1, show that both electrophilic
and free-radical processes are involved and that the
behavior is strongly depending on the structure of the
sulfides.
Diaryl sulfides do not react with H2O2 and a catalytic
amount of Br2; the latter is consumed by electrophilic
aromatic substitution and no further reaction occurs. By
using stoichiometric amounts of Br2, 4,4′-dibromodiphen-
yl sulfide was obtained in almost quantitative yield from
diphenyl sulfide.
Dibenzyl sulfide reacts with H2O2 and a catalytic
amount of Br2, leading mainly to dibenzyl sulfoxide and
minor amounts of benzyl bromide, benzylsulfonic acid,
benzyl alcohol, and benzaldehyde. Benzyl bromide and
benzylsulfonic acid were obtained in almost quantitative
yield by using stoichiometric amounts of Br2 in the
absence of H2O2. Under the same conditions also dibenzyl
sulfoxide and stoichiometric amounts of Br2 led to benzyl
bromide and benzylsulfonic acid.
The overall stoichiometry is, therefore, shown by eq 3:
Another example, in which Br2 acts as free-radical
catalyst, involves the selective oxidation of primary
alcohols to esters or aldehydes3,4 (eqs 4 and 5).
Our interpretation of these results involves both elec-
trophilic and free-radical reactions. Initially the electro-
philic attack of Br2 on sulfur in the organic phase leads
to the sulfoxide (eq 7):
* Fax: +39-035-562779.
† Dipartimento di Chimica del Politecnico.
‡ Universita` di Bergamo.
(1) Minisci, F.; Citterio, A.; Vismara, E.; Fontana, F.; De Bernar-
dinis, S. J . Org. Chem. 1989, 54, 728.
(2) Minisci, F.; Citterio, A.; Vismara, E.; De Bernardinis, S.; Cor-
reale, M.; Neri, C. Italian Patent (Enichem) (18.5.1986) no. 20754 A/86.
(3) Amati, A.; Dosualdo, G.; Zhao, L.; Bravo, A.; Fontana, F.; Minisci,
F.; Bjørsvik, H.-R. Org. Proc. Res. Dev. 1998, 2, 261.
(4) Minisci, F.; Fontana, F. Chim. Ind. 1998, 80, 1309.
10.1021/jo0017178 CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/11/2001
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