SHORT PAPER
335
Rapid, Efficient and Chemoselective Deoxygenation of Sulfoxides to
Thioethers Using NaBH4/I2
D
eoxygenation o
a
S
ulfoxide
b
s
to Thioethe
a
rs k Karimi,* Daryoush Zareyee
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), P. O. Box 45195-159, Gava Zang, Zanjan, Iran
Fax +98(241)4249023; E-mail: karimi@iasbs.ac.ir
Received 1 October 2002; revised 9 December 2002
Inspection of the data in Table 1 clearly shows that the
method is equally applicable for a variety of structurally
different sulfoxides. Highly chemoselective deoxygen-
ation of sulfoxide can be achieved when it is present along
Abstract: Sodium borohydride in the presence of iodine in anhy-
drous THF converts a range of structurally different sulfoxides to
their thioethers in excellent yields. It has been found that chemose-
lective deoxygenation of sulfoxides can be achieved in the presence
of other reducible functional groups such as esters, nitriles and dou- with an olefin moiety without the formation of any detect-
ble bonds.
able hydroboration products (Table 1, entry 3). Moreover,
efficient deoxygenation of dibenzyl and benzyl phenyl
sulfoxides to their thioethers without cleavage of C–S
bond shows the usefulness of the presented protocol.
(Table 1, entries 14, 15). The method can be selectively
applied for the chemoselective reduction of sulfoxide in
the presence of nitriles and esters which was previously
reported to be effectively reduced using NaBH4/I2 system
(Scheme 2).5
Key words: sulfoxides, deoxygenation, reduction, NaBH4, iodine,
sulfides
Sulfoxides are important intermediates in a variety of syn-
thetic transformation, especially they are used as chiral
auxiliary in asymmetric synthesis.1 This accordingly de-
mands techniques for removal of the residual sulfoxide
moiety to generate the corresponding sulfide,2 which
could be further desulfurized by treatment with Raney
nickel or a reducing low valent metal system such as lith-
ium in liquid ammonia.1,3 Therefore, much effort has been
devoted, of late, to the development of milder conditions
where the presence of various sensitive and/or reducible
functional groups can be tolerated.
O
S
S
Ph
Ph
Me
100
0
Ph
Ph
Me
OH
NaBH4 (1 equiv), I2 (1.2 equiv)
anhyd THF, r.t., 5 min
O
OMe
Although there are a good number of methods available
for the deoxygenation of sulfoxide to thioethers, there still
remain the important problems of reaction, i.e. many of
them need rather drastic conditions and/or long reaction
time.
O
S
S
Ph
Me
100
0
Ph
Me
NaBH4 (1 equiv), I2 (1.2 equiv)
anhyd THF, r.t., 5 min
Ph CN
Ph CH2NH2
Therefore, a search for new improved methods based on
readily available reagents is still pursued. Recently, sever-
al publications describing the reduction of various types
of functional groups such as carboxylic acids,4 carboxylic
esters, amides and nitriles, and hydroboration of alkenes,5
and amino acids 6 using NaBH4/I2 systems have appeared.
However, to the best of our knowledge there is no report
on the use of this reagent for deoxygenation of sulfoxide.
Scheme 2
This observation clearly suggests that the method can be
applied for the chemoselective reduction of sulfoxides in
the presence of other reducible functional groups such as
esters and amides in multifunctional molecules.
We now report that NaBH4/I2 system is an efficient and On the other hand substitution with one nitro group as in
powerful reagent for rapid conversion of sulfoxide to their the case of 4-nitrophenyl phenyl sulfoxide gave somewhat
thioethers (Scheme 1).
lower yield of the corresponding sulfide together with un-
identified side product (Table 1, entry 16). This is due to
the reduction of nitro group leading to a complex mixture
of the corresponding amine and azo compounds.2i
O
NaBH4 / I2
anhyd THF, r.t., 3-18 min
S
S
R1
R2
R1
R2
In summary, we have demonstrated that NaBH4/I2 system
is an inexpensive and excellent reagent for rapid deoxy-
genation of a variety of structurally diverse sulfoxides to
the corresponding sulfides. The present procedure shows
good chemoselectivity, and the reaction conditions are
quiete mild. Further application of this reagent is currently
in progress in our laboratory.
Scheme 1
Synthesis 2003, No. 3, Print: 28 02 03.
Art Id.1437-210X,E;2003,0,03,0335,0336,ftx,en;Z12902SS.pdf.
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