Wittig/B─H insertion reaction: A unique access to trisubstituted -alkenes.

The Wittig reaction, which is one of the most effective methods for synthesizing alkenes from carbonyl compounds, generally gives thermodynamically stable -alkenes, and synthesis of trisubstituted -alkenes from ketones presents notable challenges. Here, we report what we refer to as Wittig/B─H insertion reactions, which innovatively combine a Wittig reaction with carbene insertion into a B─H bond and constitute a promising method for the synthesis of thermodynamically unstable trisubstituted -boryl alkenes. Combined with the easy transformations of boryl group, this methodology provides efficient access to a variety of previously unavailable trisubstituted -alkenes and thus provides a platform for discovery of pharmaceuticals. The unique selectivity of the reaction is determined by the maximum overlap of the orbitals between the B─H bond of the borane adduct and the alkylidene carbene intermediate in the transition state.