Mechanistic insights and the origin of regioselective borylation in an iridium-catalyzed alkyl C(sp³)–H bond functionalization

Abstract

Iridium-catalyzed ortho benzylic C(sp³)–H borylation of (2-propylphenyl)dimethylsilane, using bis(ethylene glycolato)diborane as borylating agent, is investigated using B3LYP-D3 density functional theory. The reaction is found to proceed, first, through a very facile oxidative addition of a Si–H bond at the Ir center. This is followed by reductive elimination of ethylene-glycolborane. The subsequent C–H activation step, accompanied by elimination of another molecule of ethylene-glycolborane, leads to formation of a racemic mixture of four diastereomeric chiral iradacycle intermediates. The ensuing chirality at the metal center is accompanied by stereodifferentiation of the two enantiotopic hydrogen atoms due to steric interaction between the alkyl group and the boryl ligands. Our calculations also correctly predict the experimentally observed regioselectivity. The propensity for C–H bond activation was found to be in the order benzylic C(sp³)–H > terminal alkyl C(sp³)–H > ortho C(sp²)–H of the aryl > secondary internal C(sp³)–H bonds. This is succeeded by oxidative addition of bis(ethylene glycolato)diborane at the Ir center. The resulting Ir(III) (bpy)trisboryl species then undergoes borylation at the benzyllic carbon. The relative free energies of the transition states for C–H activation and C–B bond formation are found to be comparable.