Electrochemically Driven and Acid-Driven Pyridine-Directed ortho-Phosphorylation of C(sp²)-H Bonds

The key intermediate palladacycles (mono-, bi-, and tetranuclear) with phosphonate, acetate, and other counterions in C(sp(2))H phosphonation are analyzed in terms of their redox properties, mutual transitions, reactivity, and reaction pathways for the selective production of the desired products. It was found that, in the presence of pyridine, the reaction proceeds through a mononuclear palladacycle, which at a relatively high electrolysis potential gives the product of ortho-phosphonation of the arene with a CP bond in good yield. Under acidic conditions, the process involves the tetrapalladium intermediate and leads to a product with a COP bond. Phosphorous acid gives inorganic phosphoric derivatives of 2-phenylpyridine. The electrochemical data on the redox properties of key palladacycles 4 and 5 and their potentials and energy gaps are confirmed by DFT calculations.