Mechanism for carbon-oxygen bond-forming reductive elimination from palladium(IV) complexes

Carbon-oxygen bond-forming reductive elimination from transient Pd(IV) aryl/acetate complexes was recently implicated as the product release step in Pd(II)-catalyzed arene oxygenation reactions. The mechanistic details of C-O bond formation from these Pd(IV) intermediates remain elusive and, therefore, are subjected to a systematic theoretical investigation in the present study. Three proposed mechanisms are examined including (A) pre-equilibrium dissociation of a benzoate ligand followed by reductive elimination from the resulting five-coordinate Pd(IV); (B) direct reductive elimination from the six-coordinate Pd(IV); and (C) dissociation of a pyridyl arm of one cyclometalated ligand followed by internal coupling. Through density functional theory calculations it is suggested that mechanism B is favored over the other two mechanisms. This conclusion is supported by the success of the theoretical model based on mechanism B to reproduce the experimental activation free energy barriers. The same theoretical model also can reproduce the small effect of the solvent polarity and the negative Hammett reaction constant associated with the reductive elimination rates. All of these results suggest that mechanism A should not be involved in the reductive elimination. Furthermore, our calculations explain why the rate of reductive elimination from the Pd(IV) complex of bisphenylpyridine is significantly faster than that from the Pd(IV) complex of bisbenzo[h]quinoline.