Definitive mechanistic studies of rhodium-catalyzed cyclopropanation are lacking. However, the mechanism has been rationalized based on product distribution and stereoselectivity. Attack of the diazo compound on the metal center generates a zwitterionic metal alkyl complex, which expels nitrogen gas to afford a metal carbene intermediate. Concerted addition of the metal carbene to the olefin (without direct coordination of the olefin to the metal) generates the observed cyclopropane product. The configuration of the olefin is retained throughout the process; however, metal carbenes with heterotopic faces may generate a mixture of diastereomers, as shown at the right of Eq. (2).
The configuration of the product is determined by the trajectory of approach of the olefin to the metal carbene. In reactions of monosubstituted metal carbenes with terminal olefins, the olefin likely approaches "end-on" (with the carbon-carbon double bond of the olefin nearly parallel to the metal-carbon double bond of the carbene) with the olefin R group pointed away from the substituent of the carbene. A second transition state model has been proposed for reactions of vinyl-substituted carbenes. In this model, the olefin approaches "side-on" (with the carbon-carbon double bond of the olefin perpendicular to the metal-carbon double bond of the carbene) with the olefin R group far from the vinyl group.