Silylium-Ion-Initiated Disulfidation of Unactivated Alkenes with Disulfides: Methodology and Isolation of the Sulfenium-Ion Carriers.

A metal-free and operationally simple method for the two-component disulfidation of unactivated alkenes enabled by silylium-ion initiation is reported. The protocol promotes intermolecular coupling of alkenes with readily available disulfides under mild conditions, providing access to vicinal dithioethers with broad substrate scope and excellent functional-group tolerance. More than 30 combinations of alkenes and disulfides demonstrate the generality and practicality of the transformation. Mechanistically, the highly electrophilic silylium ion is proposed to activate the disulfide in the form of a sulfonium ion, thereby turning it into a sulfenium-ion carrier. Both an example of an initially formed sulfonium ion [Et3Si(EtSSEt)][HCB11H5Br6] and a subsequently generated electrophile [MeS(MeSSMe)][HCB11H5Br6] were isolated and characterized by X-ray diffraction. Transfer of the sulfenium-ion electrophile from these cations to the C-C double bond, followed by attack of another disulfide molecule as a sulfur nucleophile, allows for the formation of two C-S bonds without the need for a transition-metal catalyst or an external oxidant.