Computational Investigations of the Effects of N-Heterocyclic Carbene Ligands on the Mechanism, Reactivity, and Regioselectivity of Rh-Catalyzed Hydroborations

Density functional theory calculations were performed to study the effects of N-heterocyclic carbene (NHC) ligands on the Rh-catalyzed hydroboration of styrene and to identify factors controlling reactivity and regioselectivity. Our computational mechanistic investigations revealed that branched and linear hydroboration products are formed via mechanisms that involve the migratory insertions of styrene into RhH and RhB bonds, respectively. Such reaction mechanisms are fundamentally different from those calculated for hydroborations catalyzed by Rh–phosphine complexes in which the styrene prefers to insert into the RhH bond regardless if the linear or branched product is formed. The calculated steric and electronic effects exhibited by the NHC ligands on the corresponding reaction rates and regioselective outcomes revealed that stronger electron-donor ligands promote reactivity, and the steric bulk of the NHC ligands effectively controll the regioselectivity of the hydroboration reaction. Generally, bulkier NHC ligands favor the formation of linear products and less sterically demanding NHC ligands favor the formation of branched products.