Electronic dependence of C-O reductive elimination from palladium (aryl)neopentoxide complexes

Thermal decomposition of the palladium (aryl)neopentoxide complexes [P-P]Pd(Ar)OCH2CMe3 [P-P = Tol-BINAP or BINAP; Ar = p-C6H4CHO (1b), p-C6H4COPh (1c), p-C6H4NO2 (1d), o-C6H4NO2 (1e), o-C6H4CN (1f)] possessing substituents on the palladium-bound aryl group suitable for delocalization of negative charge led to quantitative (greater than or equal to 95%) formation of aryl ether without detectable beta-hydride elimination. Thermal decomposition of 1b-f obeyed first-order kinetics, and the rate of reductive elimination decreased in the order o-NO2 > p-NO2 > p-CHO > p-COPh > o-CN. Conversely, thermal decomposition of the related derivatives [P-P]Pd(Ar)OCH2CMe3 [P-P = Tol-BINAP or BINAP; Ar = p-C6H4Cl (1g), m-C6H4NO2 (1h), m-C6H4CN (1i)] which did not possess a resonance stabilizing group on the palladium-bound aryl group led to no detectable formation of aryl ether. These and related data point to the buildup of negative charge in the palladium-bound aryl group in the transition state for C-O reductive elimination and are consistent with a mechanism initiated by inner-sphere nucleophilic attack of the alkoxide ligand at the ipso-carbon atom of the palladium-bound aryl group through a zwitterionic Meisenheimer-type intermediate or transition state.