cs3007878_si_001.pdf (1.89 MB)
Mechanistic Investigation of Palladium-Catalyzed Allylic C–H Activation
journal contribution
posted on 2016-02-19, 20:22 authored by Casper Engelin, Thomas Jensen, Sergio Rodriguez-Rodriguez, Peter FristrupThe mechanism for the palladium-catalyzed allylic C–H
activation
was investigated using a combination of experimental and theoretical
methods. A Hammett study revealed a buildup of a partial negative
charge in the rate-determining step, and determination of the kinetic
isotope effect (KIE) indicated that the C–H bond is broken
in the turnover-limiting transition state. These experimental findings
were further substantiated by carrying out a detailed density functional
theory (DFT)-based investigation of the entire catalytic cycle. The
DFT modeling supports a mechanism in which a coordinated acetate acts
as a base in an intramolecular fashion during the C–H activation
step. The reoxidation of palladium was found to reach an energy level
similar to that of the C–H activation. Calculations of turnover
frequencies for the entire catalytic cycle for the C–H alkylation
were used to acquire a better understanding of the experimental KIE
value. The good correspondence between the experimental KIE and the
computed KIE values allows discrimination between scenarios where
the acetate is acting in an intramolecular fashion (C–H alkylation)
and an intermolecular fashion (C–H acetoxylation and C–H
amination).