Photocatalytic Cyclization of 2-Phosphinobiaryls to Trivalent Dibenzophospholes

This work demonstrates the photocatalytic cyclization of 2-phosphinobiaryls to dibenzophospholes. Using mild conditions, readily available 2-phosphinobiaryls were converted into trivalent P-alkyl- and P-aryl-dibenzophospholes. In this process, a P-substituent was lost from the starting phosphine in the form of a radical with preferential elimination of alkyl groups rather than aryl groups. Mechanistic experiments and quantum chemical calculations suggested that the 2-phosphinobiaryl undergoes a single electron oxidation by the excited photocatalyst to form a phosphine radical cation that then converts to the dibenzophosphole via a radical chain mechanism. Computational and experimental explorations of single electron transfer events uncovered two “hidden” radical-termination pathways, one regenerating the 2-phosphinobiaryl starting material and the other giving a phosphonium side product. These pathways explain the low quantum yield of the reaction and the need to use a photoredox catalyst with both oxidizing and reducing abilities.