The title compounds are effective catalysts for intermolecular cyclopropenation reactions of 1-alkynes with diazo esters and diazo amides. Diastereoselectivities achieved from the appropriate match with d- or l-menthyl diazoacetate are 77 to ≥94% de. Enantioselectivities up to ≥94% ee with 3-methoxy-1 -propyne and 3,3-diethoxy-1 -propyne have been obtained. Variations in these values for metal carbene additions to alkynes are associated with electronic and/or steric influences from the alkyne or carbene substituents. N,N-Dimethyldiazoacetamide provides a higher level of enantiocontrol than do diazo esters. The absolute configurations of the cyclopropene products have been established; dirhodium(II) tetrakis [methyl 2-oxopyrrolidine-5(S)-carboxylate], Rh₂(5S,-MEPY)₄, produces 2-substituted-2-cyclopropene-1-carboxylates having the (S)-configuration whereas use ofRh₂(5R-MEPY)₄ provides these cyclopropene products in the (R)-configuration. Diimide reduction of these cyclopropenecarboxylates produces the cis-disubstituted cyclopropane products exclusively