A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes

Frustrated Lewis pairs are compounds containing both Lewis acidic and Lewis basic moieties, where the formation of an adduct is prevented by steric hindrance. They are therefore highly reactive, and have been shown to be capable of heterolysis of molecular hydrogen, a property that has led to their use in hydrogenation reactions of polarized multiple bonds. Here, we describe a general approach to the hydrogenation of alkynes to cis -alkenes under mild conditions using the unique ansa -aminohydroborane as a catalyst. Our approach combines several reactions as the elementary steps of the catalytic cycle: hydroboration (substrate binding), heterolytic hydrogen splitting (typical frustrated-Lewis-pair reactivity) and facile intramolecular protodeborylation (product release). The mechanism is verified by experimental and computational studies. Frustrated Lewis pairs have been shown to be capable of heterolysis of strong covalent bonds such as those in molecular hydrogen, and have been used in the hydrogenation of polar multiple bonds. Here, a new type of ansa -aminohydroborane is shown to be active for the partial hydrogenation of alkynes under mild conditions.