Lewis base catalyzed enantioselective sulfenoamination of alkenes

Abstract

The concept of Lewis base activation of Lewis acid has been successfully applied to the enantioselective sulfenoamination of olefins. The unreactive, achiral Lewis acidic sulfenylating agent, N-arylthiophthalimide, is activated by the coordination of a chiral Lewis base, binaphthyl-derived selenophosphoramide, in presence of a Brønsted acid as a co-catalyst. The Lewis base-acid adduct exhibits a strong sulfenylating ability towards various olefins with formation of enantioenriched thiiranium ion intermediates. These configurationally stable thiiranium ions are stereospecifically captured by amines and anilines to afford nitrogen-containing heterocycles, such as piperidines, azepanes, and tetrahydroquinolines. In the course of developing an enantioselective carbosulfenylation of alkenes, a seemingly contradicting phenomenon of a catalyst inhibiting a stoichiometric reaction was observed. In the absence of catalyst, the background reaction rates were comparable to or greater than the catalyzed process, despite the observation of highly enantioenriched product when a chiral, nonracemic catalyst was employed. Detailed kinetic and spectroscopic studies revealed that the conversion of the Lewis base pre-catalyst to the catalytically active species was responsible for the observed comparable reactivity. Specifically, the equimolar formation of the byproducts of the catalyst activation, sulfonate ion and phthalimide, buffered the Brønsted acid, resulting in inhibition of the uncatalyzed racemic pathway. Therefore, the operating background reaction under catalytic conditions cannot be represented by simply omitting the catalyst.