Copper complexes of chiral, C2-symmetric semicorrin ligands were found to be efficient catalysts for the cyclopropane formation from olefins with diazo compounds. In the presence of 1 mode-% of catalyst, alkyl diazoacetates reacted smoothly with terminal olefins such as styrene, butadiene, and 1-heptene to give the corresponding optically active cyclopropanecarboxylic-acid derivatives (Table 1 Scheme 2). With one of the catalysts, enantioselectivities up to 97% ee were obtained (Table 2). Usually, the reactions were carried out using bis(semicorrinato)copper(II) complexes as precatalysts. In order to produce active catalysts, these complexes had to be activated first by heating in the presence of diazoacetate or by treatment with phenylhydrazine. Experiments with (semicorrinato)copper(I) complexes, prepared in situ from copper(I) tert-butoxide (Scheme 4), suggest that the actual catalyst is a [mono(semicorrinato)]copper(I).