The first example of direct oxidation of sulfides to sulfones by an osmate molecular oxygen system

Article abstract of DOI:10.1039/b212749k

Osmate-exchanged Mg-Al layered double hydroxides catalysed the delivery of two oxygen atoms simultaneously via a 3 + 1 cycloaddition to sulfide to form sulfone directly for the first time, reminiscent of 3 + 2 cycloaddition in asymmetric dihydroxylation reactions.

Full text of DOI:10.1039/b212749k

The first example of direct oxidation of sulfides to sulfones by an osmate
molecular oxygen system†
Boyapati M. Choudary,* Chinta Reddy V. Reddy, Billakanti V. Prakash, Mannepalli L. Kantam and
B. Sreedhar
Indian Institute of Chemical Technology, Hyderabad 500007, India. E-mail: choudary@iict.ap.nic.in;
Fax: 91-40-27160921
Received (in Cambridge, UK) 2nd January 2003, Accepted 6th February 2003
First published as an Advance Article on the web 20th February 2003
Osmate-exchanged Mg–Al layered double hydroxides cata-
lysed the delivery of two oxygen atoms simultaneously via a
3 + 1 cycloaddition to sulfide to form sulfone directly for the
first time, reminiscent of 3 + 2 cycloaddition in asymmetric
dihydroxylation reactions.
recently and their homogeneous analogue were used in the
direct oxidation of thioanisole to methyl phenyl sulfone using
molecular oxygen as stoichiometric oxidant at 55 °C in
phosphate buffer and t-butanol (Table 1, entry 1) and no
sulfoxide was detected by GC. LDH–OsO4 (cat 1) exhibits
faster activity over the resin–OsO4 (cat 2) and K2OsO4·2H2O
catalysts in oxidation of thioanisole (Fig. 1). The large positive
electric potential and spatial organisation of the LDH–OsO4
may be responsible for the superior performance; this result is in
consonance with asymmetric dihydroxylation reactions8 and
oxidative bromination.9
LDH–OsO4, the best-evolved catalyst, was further subjected
to the oxidation of various aliphatic and aromatic sulfides‡ and
the results are summarised in Table 1. All the substrates
irrespective of the nature of the substituents present on the
phenyl ring as shown in Table 1 afforded excellent yields
without formation of sulfoxide. It is interesting to note that
The oxidation of sulfides to sulfones has been the subject of
extensive studies, since sulfones are useful synthons for the
construction of various chemically and biologically significant
molecules.1,2 The reaction path for the oxidation of sulfides to
sulfones involves formation of sulfoxides as intermediates that
contaminate the sulfone products, which are difficult to
separate.2b With inherent advantages such as ready availability
and low price, molecular oxygen is widely used as an oxidant in
many organic transformations, but reports on the oxidation of
sulfides are still scarce.3,4 These include the in situ generated
peracid from the O2–aldehyde and cobalt system,3 the singlet
oxygen produced under photochemical conditions.3 Recent
disclosure of the deactivation of thioether mustard analogue by
the oxidation of its sulfide to sulfoxide using an Au(III)-based
homogeneous catalyst represents the sole catalytic protocol
using molecular oxygen directly.5 Therefore, there is an
incentive to develop direct oxidation of sulfides to sulfones. We
chose an osmium catalyst for the direct oxidation of sulfides to
sulfones, since it is known to activate molecular oxygen6 and
deliver two oxygen atoms simultaneously to the olefin in the
dihydroxylation reactions.7
In this communication, we report the effective oxidation of
sulfides directly to sulfones without sulfoxide contamination in
excellent yields at a faster rate for the first time using molecular
oxygen as the stoichiometric oxidant and osmate exchanged
layered double hydroxide (LDH–OsO4) as a recyclable catalyst.
The examination of transient surface intermediate species,
prepared on interaction of LDH–OsO4 with the sulfides, by
using XPS and TGA–DTA–MS and kinetic studies, provides
evidence for the simultaneous delivery of two oxygen atoms via
3 + 1 cycloaddition (Scheme 1).
Table 1 Oxidation of sulfides to sulfones catalysed by LDH–OsO4 (cat 1)
using molecular oxygen as oxidanta
Entry
1
RA
RB
Time/h
Yield (%)b
C6H5
CH3
8
24
5
99 (93)c
NRd
96
95
96
95
96
95
2
3
4
5
6
7
p-Cl-C6H4
p-MeO-C6H4
C6H5
p-Cl-C6H4
p-Br-C6H4
n-C10H21
CH3
CH3
C2H5
C2H5
CH3
CH3
5
16
15
10
12
a Reaction conditions as exemplified in the ESI. b Isolated yields. c Yield
after 4th cycle. d No reaction without catalyst.
Exchanger–OsO4 catalysts immobilized on Mg–Al-LDH and
resin8 by the ion-exchange technique designed and developed
Scheme 1 Plausible mechanism for the exchanger–OsO4 catalyzed
oxidation of sulfides using molecular oxygen as stoichiometric oxidant.
† Electronic supplementary information (ESI) available: experimental
details; TGA–DTA–MS thermogram of LDH-OsO4® with thioanisole. See
http://www.rsc.org/suppdata/cc/b2/b212749k/
Fig. 1 Selectivity as a function of time in the oxidation of thioanisole using
different osmate catalysts.
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CHEM. COMMUN., 2003, 754–755
This journal is © The Royal Society of Chemistry 2003

®
Fig. 2 XPS spectra of Os 4f and S 2p of LDH–OsO4
.
molecular oxygen served as a better stoichiometric oxidant for
aliphatic and aromatic sulfides than H2O2. The catalyst was
used for four cycles in the oxidation of thioanisole, which shows
consistent selectivity and a small decrease in activity. Osmium
was not present even in traces in solution after the reaction.
Sulfoxide, which is generally initially formed in the oxida-
tions of sulfide, is tranformed into sulfone on further oxidation
during the reaction. The higher nucleophilicity of sulfide allows
nucleophilic reaction with the oxygen to form sulfoxide, while
the sulfoxide undergoes electrophilic reaction with the oxidant
to form sulfone.10 An unprecedented direct oxidation of sulfide
to sulfone without formation of sulfoxide as intermediate is
demonstrated here: the oxidation of thioanisole was studied as a
function of time using LDH–OsO4, resin–OsO4, and K2OsO4
catalysts. Sulfoxide was not formed even in the initial stages of
the reaction (Fig. 1).
candidate and practical alternative to the currently practiced
process.
Ch. V. R. and B. V. P. thank the Council of Scientific and
Industrial Research, India, for their fellowships.
Notes and references
‡ LDH–OsO4 (10 mg, 0.0134 mmol) was taken in a round-bottomed flask
containing aqueous buffer solution (12.5 mL, pH 10.4) and tBuOH (5 mL)
and stirred at 1 bar O2, 55 °C in an oil bath. Then sulfide (1 mmol) was
added in one portion and the reaction mixture was stirred vigorously. After
completion of the reaction (as monitored by TLC), the catalyst was filtered
and washed with ethyl acetate. After removing the solvent, the crude
material was chromatographed on silica gel to afford the corresponding
sulfone.
1 (a) S. Patai and Z. Rappoport, Synthesis of sulfones, sulfoxides, and
cyclic sulfides, John Wiley, Chichester, 1994; (b) M. Madesclaire,
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Dyszlewski, J. Org. Chem., 1990, 55, 955.
®
The XPS of the transient surface intermediate LDH–OsO4
obtained by the interaction of LDH–OsO4 with thioanisole in
the presence of molecular oxygen in anhydrous acetonitrile
(55–60 °C) shows Os 4f7/2 lines at 53.378 and 54.386, Os 4f5/2
at 55.988 and 56.886 eV, respectively. This suggests the
possible presence of a surface Os–sulfone complex along with
the unreacted LDH–OsO4 (Scheme 1, intermediate 3) in +VI
oxidation state.11 The XPS of LDH–OsO4® shows S 2p3/2 and
2p1/2 lines at 167.8 and 168.9 eV, respectively, characteristic of
the formation of sulfone (Fig. 2).12 The reported binding
energies for S–O of sulfoxide and metallic sulfides are 2–7 eV
lower than the corresponding sulfone binding energies. There-
fore, the higher binding energies observed are presumably
ascribed to the formation of an Os–sulfone complex (Scheme 1,
intermediate 3) on the surface, which is indeed confirmed by
TGA–DTA–MS. The m/z values 79, 77, and 64 amu† observed
are assigned to radical cations of CH3SO2, C6H5, SO2,
fragments from the surface Os–sulfone intermediate (Fig. A,
ESI;† Scheme 1, intermediate 3) obtained as above. The XPS of
LDH–OsO4®, which does not show any direct Os–S bond,
indicates that the possible reaction path via complex 2 by 2 + 1
cycloaddition is ruled out. Thus, kinetic, XPS and TGA–DTA–
MS studies unambiguously establish the concerted 3 + 1
cycloaddition via the delivery of two oxygens simultaneously as
detailed in Scheme 1, similar to the 3 + 2 cycloaddition for
asymmetric dihydroxylation and heterogeneity of the reac-
tion.6
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The LDH–OsO4 catalyst is successfully employed for
oxidation of sulfides to sulfones with excellent yields for the
first time. The simple ecofriendly procedure, easily recoverable
and reusable catalytic system described here is a potential
CHEM. COMMUN., 2003, 754–755
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