Mechanism and Origins of Enantioselectivity of Rhodium-Catalyzed [4+3] Annulation of Benzosilacyclobutenes with 7-Oxabenzonorbornadienes.

Rhodium-catalyzed asymmetric [4+3] annulation of benzosilacyclobutenes with 7-oxabenzonorbornadienes provides an efficient route to enantioenriched seven-membered sila-heterocycles. In this study, density functional theory calculations were performed to elucidate the reaction mechanism and the origins of the enantioselectivity. The catalytic cycle is initiated by C─Si oxidative addition to generate the Rh(III) metallacyclic intermediate. However, the subsequent migratory insertion followed by the β-O elimination pathway was found to be kinetically infeasible. Instead, the computations reveal that the Rh(III) metallacyclic intermediate undergoes Si─Cl reductive elimination to generate the Rh(I)-aryl species, from which the annulation proceeds through migratory insertion, β-O elimination, and σ-bond metathesis. The C─Si oxidative addition step constitutes the rate-determining step, whereas the migratory insertion step is responsible for controlling the enantioselectivity. Structural and distortion/interaction analyses demonstrate that the stereochemical outcome is predominantly governed by the steric repulsion between the chiral ligand and the 7-oxabenzonorbornadiene substrate.