Allene formation by gold catalyzed cross-coupling of masked carbenes and vinylidenes

Abstract

Addition of a sterically demanding cyclic (alkyl)(amino)carbene (CAAC) to AuCl(SMe₂) followed by treatment with [Et₃Si(Tol)]⁺[B(C₆F₅)₄]⁻ in toluene affords the isolable [(CAAC)Au(η²-toluene)]⁺[B(C₆F₅)₄]⁻ complex. This cationic Au(I) complex efficiently mediates the catalytic coupling of enamines and terminal alkynes to yield allenes and not propargyl amines as observed with other catalysts. Mono-, di-, and tri-substituted enamines can be used, as well as aryl-, alkyl-, and trimethylsilyl-substituted terminal alkynes. The reaction tolerates sterically hindered substrates and is diastereoselective. This general catalytic protocol directly couples two unsaturated carbon centers to form the three-carbon allenic core. The reaction most probably proceeds through an unprecedented “carbene/vinylidene cross-coupling.”