Gold-Catalyzed Asymmetric Thioallylation of Propiolates via Charge-Induced Thio-Claisen Rearrangement

A gold­(I)-catalyzed enantioselective
thioallylation of propiolates
with allyl sulfides is described. The key mechanistic element is a
sulfonium-induced Claisen rearrangement which helps minimize the allyl
dissociation and render higher enantioselectivity. This protocol features
remarkable scope of the allyl moiety, allowing enantiocontrolled synthesis
of all-carbon quaternary centers, and exhibits exceptional functional
group compatibility with many Lewis bases and π-bonds. This
intermolecular variant of Claisen rearrangement forges both C–S
and C–C bonds concomitantly, providing efficient access to
interesting optically active organosulfur compounds which can be transformed
further through the vinyl sulfide as a functional handle. The rate
of the reaction was zeroth order with respect to allyl sulfides, which
suggested a reversible inhibition, providing a resting state for the
catalyst. The Hammett plot displayed a correlation with σp values, suggesting a turnover-limiting sigmatropic rearrangement
where decreased electron-density on sulfur accelerated the rearrangement.