Computational Investigations of the Effects of N-Heterocyclic Carbene Ligands on the Mechanism, Reactivity,
and Regioselectivity of Rh-Catalyzed Hydroborations
posted on 2020-03-09, 17:45authored byHuiling Shao, Yuening Wang, Christopher W. Bielawski, Peng Liu
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
RhH and RhB 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 RhH 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.