Asymmetric Diels–Alder Reactions with Chiral Acetylenic Carbene Complexes as Dienophiles

A new synthetic method is developed for the asymmetric Diels–Alder reactions of acetylenic dienophiles. Acetylenic Fischer carbene complexes with chiral pyrrolidines as the heteroatom stabilizing substituent were anticipated to block three of the four possible approaches of a diene and lead to selective asymmetric cycloadditions. A series of complexes of the type (CO)5MC((C4H7N)CR2OMe)CCH (M=Cr, W; R=H, Me, Ph) were prepared by aminolysis of the corresponding methoxy complexes that have the terminal acetylene protected as a silane. Michael addition is completely suppressed if the alkyne is protected as a triisopropylsilyl group and after aminolylsis by 1,2-addition of a pyrrolidine the terminal alkynyl carbene complexes can be obtained predominately as E-isomers upon protodesilylation. The cycloadditions of the E-isomers (but not Z-isomers) of these complexes occur with significant asymmetric induction with 2-triisopropylsiloxy-1,3-pentadiene (66–73% de) but not with cyclopentadiene and α-triisopropylsiloxyvinyl cyclohexene. A model is presented to account for the observed stereoselectivities.