NHC–Cu-Catalyzed Addition of Propargyl­boron Reagents to Phos­phinoyl­imines. Enantioselective Synthesis of Tri­methyl­silyl-Substituted Homo­allenyl­amides and Application to the Synthesis of S‑(−)-Cyclooroidin

A catalytic method for the efficient and enantioselective addition of a 1-tri­methyl­silyl-substituted allene moiety to phos­phinoyl­imines is presented. Transformations are promoted by 5.0 mol % of a copper complex of an N-heterocyclic carbene in the presence of a propargylboron reagent that can be readily prepared in multigram quantities. Within 10 min of reaction, products are obtained in up to 91% yield, 98% allene (vs propargyl) selectivity, and 98:2 enantiomeric ratio. An assortment of aldimines serve as suitable substrates. The phosphinoyl and silyl groups can be removed efficiently and orthogonally. The silylallene moiety may be transformed to versatile derivatives that are difficult to access via nonsilylated allenes. The special features and utility of the approach are highlighted through a succinct enantioselective synthesis of marine alkaloid S-(−)-cyclooroidin.