Mechanism and Origins of Ligand-Controlled Linear Versus Branched Selectivity of Iridium-Catalyzed Hydroarylation of Alkenes
† Department
of Chemistry, School of Science, Tianjin
University, and Collaborative Innovation Center of Chemical Science
and Engineering (Tianjin), Tianjin 300072, People’s Republic
of China
‡ Department
of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
ACS Catal., 2016, 6, pp 809–820
DOI: 10.1021/acscatal.5b02201
Publication Date (Web): December 21, 2015
Copyright © 2015 American Chemical Society
Abstract
The
iridium-catalyzed carbonyl-directed hydroarylation of monosubstituted
alkenes developed by Bower and co-workers [Crisenza, G. E. M.;
McCreanor, N. G.; Bower, J. F. J. Am. Chem. Soc. 2014,
136, 10258–10261] provides an efficient strategy for highly
branched-selective hydroarylation of both aryl- and alkyl-substituted
alkenes. Density functional theory calculations in the present study
revealed that the unique regiochemical control in this reaction is due
to an unconventional modified Chalk–Harrod-type mechanism. Instead of
the commonly accepted Chalk–Harrod-type mechanism of transition
metal-catalyzed hydroarylation that involves C–H oxidative addition,
olefin migratory insertion into the Ir–H bond, and C–C reductive
elimination, the Ir-catalyzed reaction occurs via migratory insertion of
the olefin into the Ir–aryl bond and C–H reductive elimination. The
experimentally observed ligand-controlled selectivity is attributed to a
combination of electronic and steric effects in the
selectivity-determining olefin migratory insertion step. Ligand steric
contour maps show that, in reactions with large-bite-angle bisphosphine
ligands, such as dFppb, the steric repulsions between the
substrate and the aryl substituents on the ligand lead to complete
branched selectivity, and the linear selectivity in reactions with
small-bite-angle ligands is due to electronic effects that favor
2,1-olefin migratory insertions.
