Evidence of a Wheland Intermediate in Carboxylate-Assisted C(sp2)−H Activation by Pd(IV) Active Catalyst Species Studied via DFT Calculations

Abstract

Evidence of a Wheland intermediate in carboxylate-assisted C−H activation was found using DFT calculations when the Pd(IV) catalyst species was postulated as the active catalyst species (ACS). In order to delineate the reaction mechanism of Pd-catalyzed bisarylation of 3-alkylbenzofuran, five hypothetical catalyst species, [Pd(OAc)(PMe3)(Ph)] (I), [Pd(OAc)2] (II), [Pd(OAc)2(PMe3)] (III), [Pd(OAc)2(Ph)]+ (IV) and [Pd(OAc)2(PMe3)(Ph)]+ (V) were tested as potential ACS candidates. The catalyst species I, previously reported as an ACS in the context of ambiphilic metal−ligand assistance or a concerted metalation-deprotonation mechanism, was unsuccessful, with maximum activation barriers (ΔG‡max) for the C(sp2)−H and C(sp3)−H activations of 33.3 and 51.4 kcal/mol, respectively. The ΔG‡max values for the C(sp2)−H and C(sp3)−H activations of II−V were 23.8/28.7, 32.0/49.6, 10.9/10.9, and 36.0/36.0 kcal/mol, respectively, indicating that ACS is likely IV. This catalyst species forms an intermediate state (IV_1) before proceeding to the transition state (IV_TS1,2) for C(sp2)−H activation, in which C(2) atom of 3-methylbenzofuran has a substantial σ-character. The degree of σ-character of the IV_1 state was further evaluated quantitatively in terms of geometric parameters, partial charge distribution, and activation strain analysis. The analysis results support the existence of a Wheland intermediate, which has long been recognized as the manifestation of the electrophilic aromatic substitution mechanism yet never been identified computationally.