A Heteroleptic Dirhodium Catalyst for Asymmetric Cyclopropanation with alpha-Stannyl alpha-Diazoacetate. 'Stereoretentive' Stille Coupling with Formation of Chiral Quarternary Carbon Centers

摘要

The heteroleptic dirhodium paddlewheel catalyst7with a chiral carboxylate/acetamidate ligand sphere is uniquely effective in asymmetric [2+1] cycloadditions with alpha-diazo-alpha-trimethylstannyl (silyl, germyl) acetate. Originally discovered as a trace impurity in a sample of the homoleptic parent complex [Rh-2((R)-TPCP)(4)] (5), it is shown that the protic acetamidate ligand is quintessential for rendering7highly enantioselective. The -NH group is thought to lock the ensuing metal carbene in place via interligand hydrogen bonding. The resulting stannylated cyclopropanes undergo "stereoretentive" cross coupling, which shows for the first time that even chiral quarternary carbon centers can be made by the Stille-Migita reaction.