om6b00513_si_001.pdf (1.56 MB)
Mechanistic Insights and the Origin of Regioselective Borylation in an Iridium-Catalyzed Alkyl C(sp3)–H Bond Functionalization
journal contribution
posted on 2016-09-08, 14:49 authored by Chandan Patel, Vibin Abraham, Raghavan B. SunojIridium-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.