Mechanistic Study of the Rhodium‐Catalyzed [3+2+2] Carbocyclization of Alkenylidenecyclopropanes with Alkynes

Abstract

A systematic theoretical study has been performed on the recently reported Rh(I)-catalyzed [3+2+2] carbocyclization reactions between alkenylidenecyclopropanes (ACPs) and alkynes. With the aid of theoretical calculations, two possible mechanisms, that is, alkene-carbometalation-first and alkyne-carbometalation-first mechanisms, are examined in this study. In the oxidative addition step, the possibility of reaction on either the distal or proximal C-C bond of the cyclopropane group has been evaluated. The calculations indicate that the alkene-activation-first mechanism is more favored for the overall catalytic cycle. This mechanism involves four steps, that is, oxidative addition of the distal (rather than the proximal) C-C bond of cyclopropane group, alkene carbometalation, alkyne carbometalation, and reductive elimination. The rate-determining step in the overall catalytic cycle is the carbometalation of the alkyne (i.e., the alkyne-insertion step) and this step also determines the regioselectivity. Finally, the origin of the regioselectivity is determined by the steric effect (i.e., the steric crowding between the electron-withdrawing group on alkyne and other ligands on the rhodium center) in the alkyne-insertion step.