Mechanistic Investigation of Palladium-Catalyzed Allylic C–H Activation

Abstract

The 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).

O 2 -promoted allylic acetoxylation of alkenes: Assessment of “push” versus “pull” mechanisms and comparison between O 2 and benzoquinone

“…We attribute this effect to ground-state destabilization of the π-allyl-Pd II species by electron-withdrawing substituents, resulting in more facile reduction of Pd II to Pd 0 and nucleophilic attack by acetate on the π-allyl ligand. 16 …”

In Situ Spectroscopic Investigation of the Rhenium‐Catalyzed Deoxydehydration of Vicinal Diols

“…[14] To provide further support for this mechanistic proposal, a calculation of the theoretical KIE, should the cleavage of the CÀH bond indeed be the rate-determining step, was performed. [53][54][55] This resulted in a difference in activation energy of 4.0 kJ mol À1 , which corresponds to a relative reactivity of 2.9 if we assume a Boltzmann distribution. At this relatively high temperature, the contribution from tunneling should be insignificant.…”

“…Subsequently, the thermodynamic data were recalculated using the Hessian from the frequency calculation at the experimental temperature (T = 453 K). The mass of the hydrogen abstracted was set to either 1 or 2 amu in two separate calculations for both ground-state and transitionstate structures in line with earlier work, [53][54][55] and the resulting theoretical KIE was derived using a Boltzmann expression. …”

The mechanism of the Pd-catalyzed formation of coumarins: a theoretical study

“…23 There is already support for this mechanism borne out by Kinetic Isotope Effect (KIE) data, which suggests that this mechanism may be the most probable option in the production of coumarin. 23,24 The CMD mechanism in the formation of coumarin is shown in Fig. 4, for the Pd(OAc) 2 catalyst, with the stationary points shown in the following order: non-bound Pd(OAc) 2 and phenylpropriolate (cI), minima of bound Pd(OAc) 2 and phenylpropriolate with non-cleaved aryl C-H (cNC), intermediate formed upon cleavage of the aryl C-H bond (cC), intermediate where the coumarin product is still bound to the Pd(OAc) 2 catalyst (cP), and the final state corresponding to the unbound Pd(OAc) 2 and coumarin product (cRP).…”