Rhodium (III) porphyrins catalyze the decomposition of ethyl diazoacetate and the transfer of ethoxy-carbonylcarbene to alkenes to form cyclopropanes in moderate to high yields. When compared with other catalysts a large syn-selectivity was observed on reaction with cis- alkenes. This selectivity increased with the size of the substituents, and suggested a preferential direction of approach of the alkene towards a rhodium—carbene complex.
