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S. K. Pandey, A. Bisai, and V. K. Singh
formation of ylide and toxicity associated with vapors of the latter. While
working on the reaction of activated DMSO with epoxides and other
strained rings,[10,11] we discovered that phosphonitrilic chloride in combi-
nation with DMSO is a good alternative for classical Swern oxidation.
Details of the work are described in this article.
At the outset, a simple primary alcohol was treated with a reagent
prepared from DMSO and phosphonitrilic chloride in CH2Cl2 at 2408C
followed by Et3N under the conditions of Swern oxidation. The ketone was
thus obtained in 84% isolated yield (entry 1). To show the scope of the
reaction, a variety of alcohols were oxidized with these reagents, and the
results are summarized in Table 1. All kinds of alcohols, such as primary,
secondary, allylic, and benzylic alcohols, could be oxidized to carbonyl
compounds in high yields. The worthy feature of the reaction was that an
alcohol that has a sulfur atom in the molecule could also be oxidized in
excellent yield (entry 11). The oxidation could also be carried out in a
molecule that has silyl ethers, which were inert to these conditions (entries
16, 17, 20, and 21). The THP ether is stable under the conditions used for
the oxidation reaction (entry 19). Oxidation of a sensitive substrate such as
geraniol was very smooth, giving an aldehyde in high yield (entry 15). The
oxidation of geraniol proceeded efficiently (87% yield) even with use of 0.5
equivalent of phosphonitrilic chloride. However, on the use of 0.33 equivalent
of the reagent, the yield dropped down to 60%. The yield increased to 95% if
the reaction was done at 2788C using 1 equivalent of the reagent. There was
no epimerization under these reaction conditions (entry 21).
In conclusion, we have described a mild method for oxidation of alcohols
using DMSO–phosphonitrilic chloride. Some functional groups, which
otherwise would have reacted with these reagents, remained unaffected
under the reaction conditions.
GENERAL PROCEDURE
DMSO (4 mmol) was added to a solution of phosphonitrilic chloride
(1.0 mmol) in CH2Cl2 (5 mL) at 2408C. The color of the solution turned
milky white. After 1 h, 1 equivalent of an alcohol solution in CH2Cl2
(2 mL) was added, and the reaction mixture was stirred at the same tempera-
ture for 45 min, when Et3N (5 mmol) was added. The reaction mixture was
slowly allowed to come to rt (8 h). The reaction mixture was taken in
CH2Cl2 and washed with water and brine. The organic layer was dried over
Na2SO4 and condensed. The crude product was chromatographed over
silica gel by column chromatography to provide pure ketones in high yields
(Table 1). All the carbonyl compounds are either commercially available or
known in literature. They were characterized by the usual spectral data and
compared with authentic samples.