NOVEL IODINE REAGENT SYSTEM
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Scheme 1. Transformation of epoxide to alcohol.
carbonium ion. After considerable exploration, a combination of diphosphorus
tetraiodide and a catalytical amount of tetraethyl ammonium bromide was demon-
strated to offer the most reliable and convenient reductive system. To the best of our
knowledge, there are no reports on P2I4-mediated regioselective reductive transform-
ation of epoxides to alcohols (Scheme 1).
In this communication, we report on our preliminary result on regioselective
cleavage of epoxides using P2I4 and TEAB to form higher substituted alcohols.
RESULTS AND DISCUSSION
For initial studies, we selected styrene epoxide as a model substrate. We
observed that without TEAB, the reaction does not take place. Instead of
TEAB, we tried other quaternary ammonium salts such as tetraethylammonium
chloride and tetraethylammonium iodide as catalyst, but no reaction was observed.
Screening with alternative solvents led us to conclude that dichloromethane was
the most suitable solvent for the conversion. Although complete conversion was
also observed in carbon disulfide, it was not selected because of its toxicity and
flammability.
It has been well studied for reduction of cyclic and acyclic dithioacetals using
P2I4 and found that the proton required for the reduction comes from the moisture
present in the solvent.[9,10] In this case, the transformation required traces of moist-
ure, which comes from solvent used in the reaction mixture. We confirmed this by
conducting the reaction in absolute dry conditions, where reduction to correspond-
ing alcohol was not observed in either dry carbon disulfide or dry dichloromethane.
To explore the reaction scope, a variety of aromatic, aliphatic epoxides were
converted into alcohols, and results are presented in Table 1. Table 1 clearly shows
that reactions completed within 5–6 h with good yield. It was found that either
electron-rich or electron-deficient aromatic epoxides were suitable for this reaction,
giving desired alcohols in good yield (Table 1, entries 2–7). It was noted that a
variety of functional group tolerated these reaction conditions (Table 1, entries 8
and 9). Reductive cleavage of epoxide occurs with retention of configuration, pro-
ducing (1R)-1-phenylehtanol exclusively from (2R)-2-phenyloxirane and (1R)-1-
(4-chlorophenyl) ethanol from (2R)-2-(4-chlorophenyl)oxirane (Table 1, entries 10
and 11).
The major advantages of this methodology are availability of this reagent, safer
reaction conditions, and complete regioselectivity in the reductive cleavage of
epoxide compared with existing reagents. Moreover, with the combination of the
excellent methods available for the synthesis of optically active epoxides, the present
method can provide easy access to optically active alcohols with retention of
configuration.