The first vanadium-catalyzed oxidation of aryl allylic selenides with in situ
[2,3] sigmatropic rearrangement†
Rich G. Carter* and T. Campbell Bourland
Department of Chemistry and Biochemistry, University of Mississippi, Oxford, MS 38677 USA.
E-mail: rgcarter@olemiss.edu
Received (in Corvallis, OR, USA) 28th July 2000, Accepted 1st September 2000
First published as an Advance Article on the web 28th September 2000
The efficient synthesis of a series of 2° allylic alcohols from
a vanadium-catalyzed oxidation of suitably disposed allylic
selenides with tandem [2,3] sigmatropic rearrangement is
outlined.
The rapid growth of metal-catalyzed transformations in recent
decades has contributed to the construction of evermore
complex synthetic targets. For example, the catalytic oxidation
of sulfides is well documented using several transition metal
catalysts.1 This methodology has proven useful in both racemic
and enantioselective pathways. Recently, Bolm and co-workers
showcased the use of vanadium for the enantioselective
oxidation of prochiral sulfides.2 In addition, the Ellman
laboratory has reported an elegant use of a similar catalytic
system for the enantioselective oxidation of alkyl disulfides.3
Despite these encouraging results, relatively little attention has
been paid to the corresponding oxidation of selenium. In
particular, the tandem oxidation of suitably disposed allylic
selenides with in situ [2,3] sigmatropic rearrangement has been
limited to stoichiometric oxidants such as oxaziridines, peracids
or peroxides.4 Scattered asymmetric methods have also been
developed for these transformations utilizing primarily stoi-
chiometric titanium–tartrate complexes5 or chiral oxaziridines.6
In addition, several groups have studied the effect of a chiral
auxiliary on selenium.7 None of these systems, however, have
been exploited in a sub-stoichiometric sense for this transforma-
tion.8 We herein report the first catalytic method for oxidation
of allylic selenides with tandem in situ [2,3] sigmatropic
rearrangement.
Our initial exploits in this area were directed toward the use
of titanium-mediated oxidation based on previously described
conditions;9 however, this proved to be ineffective in our hands
due to lack of generality, slow reaction times, and the need for
stoichiometric amounts of the metal catalyst. Our interest then
turned to the use of vanadyl(IV) acetylacetonate as a potential
candidate for the desired transformation. Synthesis of the
selenide precursors was readily accomplished from the corre-
sponding alcohol using tributylphosphine and o-nitrophenyl
selenocyanate in high yield.10
The standard reaction conditions involved a 0.3 M solution of
the selenide 1 in methylene chloride with vanadyl(IV) acet-
ylacetonate (10 mol%) in the presence of powdered 4 Å
molecular sieves (Scheme 1). This green solution was then
cooled to 210 °C in an ice–acetone bath and cumene
hydroperoxide (1.8 equiv.) was added to the stirred solution.
After the reaction was judged to be complete, tributylphosphine
(1.2 equiv.) was added to convert the intermediate selenenate 3
to the desired allylic alcohol 4. This efficient and rapid method
for cleavage of the selenenate species 3 is noteworthy because
of its tolerance for sterically demanding systems such as the
adamantyl selenide 1c. Previous described methods for selene-
nate cleavage (such as pyridine and water) proved to be slow
and capricious in our hands. The vanadium metal catalyst is
Scheme 1 Selenide oxidation, sigmatropic rearrangement and selenenate
cleavage sequence.
critical for the dual reaction sequence to proceed; the blank
experiment without VO(acac)2 resulted in no conversion of the
selenide under the prescribed reaction conditions.11 It has been
previously reported that phenyl selenides can be oxidized using
tert-butyl hydroperoxide in the absence of a metal catalyst;12
however, the decreased reactivity of the selenium to oxidation
that is observed may be due to the electron withdrawing nature
of the o-nitro substituent.
A variety of substituents on the alkene were studied in order
to investigate the utility of this methodology as shown in Table
1. These results suggest that the oxidation and rearrangement
occur rapidly, regardless of the steric environment located at the
allylic position. In addition, the vanadium-mediated conditions
occur equally effectively with conjugated and non-conjugated
olefins. These results are in contrast to preliminary studies in
our laboratory with titanium-mediated reactions in which
substitution on the alkene function had a dramatic impact on the
efficiency of the reaction. Furthermore, the rearrangement does
not seem to be influenced by a stereogenic center located in the
allylic position. For example, treatment of selenides 1f and 5c
yielded the resultant allylic alcohol in nearly 1+1 selectivity.
This result is in good agreement with Davis and co-workers who
have shown that the stereochemical outcome of the resultant
alcohol in acyclic substrates appears to be determined by the
oxidation of the selenide.6 Finally, the results in Table 2
demonstrate that this methodology is equally effective for the
cis olefin geometry.
In conclusion, the first vanadium-catalyzed method for
oxidation of selenides is reported with tandem [2,3] sigmatropic
rearrangement. This methodology is effective on a broad range
of substrates, regardless of steric environment, thereby expand-
ing the scope and utility of selenoxide rearrangements.
Furthermore, tributylphosphine is shown to be a rapid and
† Electronic supplementary information (ESI) available: typical experi-
b0/b006278m/
DOI: 10.1039/b006278m
Chem. Commun., 2000, 2031–2032
This journal is © The Royal Society of Chemistry 2000
2031
Carter
