Computational study of cyclopropanation reactions with halodiazoacetates.

The mechanism of rhodium(II)-catalyzed cyclopropanation reactions with ethyl bromo-, chloro-, and iododiazoacetate has been studied with density functional theory calculations. The halodiazoacetates were shown to be remarkably kinetically active compared to ethyl diazoacetate, as demonstrated experimentally in a study of reaction rates and supported by the calculated low potential energy barriers for the rate-determining loss of dinitrogen. In the rhodium carbenoids formed from the halodiazoacetates, pi-interactions between the halogen, the carbenoid carbon, and one rhodium atom were found. These interactions provide an explanation for the relatively high stability of these carbenoids and, consequently, the existence of small but significant potential energy barriers for the cyclopropanation step. The predicted diastereomeric ratios correspond well with the experimental results. In addition to transition states in which the alkene approaches the carbenoid in an end-on manner, as described in computational studies of cyclopropanations with other diazo compounds, side-on trajectory transition states were found to be of importance. The relative energies of the side-on trajectory transition states compared to the end-on trajectory transition states were shown to be affected by both the substrate alkene and the carbenoid substituents, a fact that should be taken into consideration when using models to explain and predict the stereochemical outcome of cyclopropanation reactions.