Sc(III)-Catalyzed Carbonyl–Yne Reaction for the Diastereoselective Allene Synthesis: A Concerted Asynchronous Mechanism

The carbonyl-yne reaction, involving propargylic C-H bond cleavage for carbon-carbon bond formation, is significantly less developed than the classical Alder-ene reaction due to the linear geometry and relatively weak nucleophilicity of alkynes compared to alkenes. Herein, we report a Sc(OTf)3-catalyzed carbonyl-yne reaction enabling the diastereoselective synthesis of multisubstituted allenes from α-ketoamide-tethered alkynes with 100% atom economy and excellent diastereoselectivity (up to 20/1 dr). DFT calculations support a concerted, asynchronous mechanism involving C-C bond formation and propargylic C-H cleavage. Theoretical analysis suggests that the observed stereoselectivity arises from an interplay of electronic and entropic effects in the transition states.