Mechanistic Basis for Efficient, Site-Selective, Aerobic Catalytic Turnover in Pd-Catalyzed C-H Imidoylation of Heterocycle-Containing Molecules

A recently reported Pd-catalyzed method for oxidative imidoylation of C-H bonds exhibits unique features that have important implications for Pd-catalyzed aerobic oxidation catalysis: (1) The reaction tolerates heterocycles that commonly poison Pd catalysts. (2) The site selectivity of C-H activation is controlled by an N-methoxyamide group rather than a suitably positioned heterocycle. (3) A Pd-0 source, Pd-2(dba)(3) (dba = dibenzylideneacetone), is superior to Pd(OAc)(2) as a precatalyst, and other Pd-II sources are ineffective. (4) The reaction performs better with air, rather than pure O-2. The present study elucidates the origin of these features. Kinetic, mechanistic, and in situ spectroscopic studies establish that Pd-II-mediated C-H activation is the turnover-limiting step. The (BuNC)-Bu-t substrate is shown to coordinate more strongly to Po-II than pyridine, thereby contributing to the lack of heterocycle catalyst poisoning. A well-defined Pd-II-peroxo complex is a competent intermediate that promotes substrate coordination via proton-coupled ligand exchange. The effectiveness of this substrate coordination step correlates with the basicity of the anionic ligands coordinated to Pd-II, and Pd-0 catalyst precursors are most effective because they selectively afford the Pd-II-peroxo in situ. Finally, elevated O-2 pressures are shown to contribute to background oxidation of the isonitrile, thereby explaining the improved performance of reactions conducted with air rather than 1 atm O-2. These collective results explain the unique features of the aerobic C-H imidoylation of N-methoxybenzamides and have important implications for other Pd-catalyzed aerobic C-H oxidation reactions.