Pd(OAc)2-Catalyzed C-H Activation/C-O Cyclization: Mechanism, Role of Oxidant-Probed by Density Functional Theory

A series of density functional theory determinations have been carried out to characterize Pd(OAc)(2)-catalyzed C H activation and subsequent intramolecular C-O bond-coupling of phenyl-tert-butanol in perfluorobenzene (C6F6) solvent. Full, nontruncated models of the real chemical transformations were studied, with structures in agreement with recent X-ray determinations. Conformational analyses have provided thermodynamic validity of the geometric structures used. The B3LYP/DZVP and B3LYP/BS1 methods (BS1 = TZVP((H,C,O)) + SDD(Pd,I)) were comparatively employed, with C6F6 solvent contributions accounted for by the IDSCRF method; key transition states were confirmed by intrinsic reaction coordinate determinations. The novel reaction mechanism proposed was divided into the following four steps: C H activation, oxidation, reductive elimination, catalyst recovery. Two competing reaction routes were quantitatively compared, differing in the oxidation state of Pd (+2 vs +4). Results reveal the pathway involving Pd(IV) intermediates to be more spontaneous and, therefore, more probable than the Pd(II) path, the latter hindered by a kinetically inaccessible reductive elimination step, with total energy and free energy barriers of 41.0 and 38.6 kcal.mol(-1), respectively. The roles played by the oxidant and Pd(IV) species have also been addressed through Bader's atoms-in-molecules wave function analyses, providing a quantitative electronic metric for C-H activation chemistry.