Rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes: DFT analysis of origins of regio- and diastereo-selectivities

Rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes (ACPs) is an efficient method for late-transition-metal-mediated beta-sulfide elimination. The density functional theory (DFT) method was used to investigate the mechanism, and regio- and diastereo-selectivities of this type of reaction. The computational results showed that the unique control of the regio- and diastereo-selectivities of this reaction can be attributed to an unconventional reaction mechanism. Instead of the commonly accepted mechanism, which involves initial ring opening of the ACP, carbometallation, beta-sulfide elimination, and thioether migration, the Rh(i)-catalyzed ene-cycloisomerization reaction occurs via activation of the ACP double bond, beta-sulfide elimination, and the simultaneous thioether transfer and ring opening of the cyclopropyl group. Importantly, the calculation results explain why initial ACP double-bond activation was achieved with a Rh(i) catalyst but not with Pd(0) and Ni(0) catalysts. This mechanism does not occur with Pd(0) and Ni(0) catalysts because of steric effects.