Enantioselectivity and cis/trans-Selectivity in Dirhodium(II)-Catalyzed addition of diazoacetates to olefins

The Rh11‐catalyzed carbenoid addition of diazoacetates to olefins was investigated with [Rh2{(4S)‐phox}4] (1;phox = tetrakis[(4S)‐tetrahydro‐4‐phenyloxazol‐2‐one]), [Rh2{(2S)‐mepy}4] (2; mepy = tetrakis[methyl (2S)‐tetrahydro‐5‐oxopyrrole‐2‐carboxylate]), and [Rh2(OAc)4] (3). While catalysis with 2 and 3 afford preferentially trans‐cyclopropanecarboxylates, the cis‐isomers are the major products with 1. In general, the enantioselectivities achieved with 1 and 2 are comparable. Additions catalyzed by 1 are strongly sensitive to steric effects. Highly substituted olefins afford cyclopropanes in only poor yield. The preferential cis‐selectivity observed in reactions catalyzed by 1 is attributed to dominant interactions between the ligand of the catalyst and the substituents of both olefin and diazoacetate, which overrule the steric interactions between olefin and diazoacetate in the transition state for carbene transfer.