Unravelling the mechanism of cobalt (II) catalyzed O-arylation reaction between aryl halides and phenols: A DFT study

Abstract

A detailed theoretical investigation into the mechanism of the cobalt (II) catalyzed O-arylation reaction of phenols with aryl iodides has been carried out with the aid of the Density Functional Theory method at the UB3LYP-D3 level of theory. The naturally occurring amino acid, l-valine was taken as the ancillary ligand for modeling the reaction mechanism. A tetrahedral, l-valine ligated cobalt (II) phenoxide complex in the quartet state, was found to be the active catalytic species. Unlike the Cu-catalyzed C-X (X = O, S, N) cross-coupling and Co-catalyzed CN coupling, the reaction proceeds through a facile σ – bond metathesis mechanism through the concerted breaking of Csp2 – I bond and the formation of Csp2 – O bond. The high reaction barrier (33.60 kcal/mol) obtained for the model system agrees with the experimental requirement of an elevated temperature [1]. To rationalize the experimentally observed selectivity of electron-deficient aryl iodides, we have investigated the electronic effects of various functional groups in the substrate molecules on the activation barrier of the reaction. This was contemplated by using Hammett correlation studies and Frontier Molecular Orbital analysis (FMO). The FMO analysis suggested that substitution by electron-withdrawing groups (EWG) at the para-position of the aryl iodides, decreases the LUMO energy, thereby decreasing the activation barrier of the reaction. However, in the case of substitution at the para-position of the phenolic moiety in the cobalt-phenoxide complex, the electron-rich groups are more favorable than the electron-withdrawing groups due to the rise in the HOMO energy of the substrate by the former. Thus, the present study can throw light on designing better catalytic systems for O-arylation reactions.