Thermal and Photocontrol of the Equilibrium between a 2-Phosphinoazobenzene and an Inner Phosphonium Salt

The reversible conversion between a phosphine and a phosphonium salt has been achieved by external stimuli of light and heat. Two 2-phosphinoazobenzenes were successfully synthesized by desulfurization of the corresponding phosphine sulfides. One of the phosphines bearing an azo group was in equilibrium with an inner phosphonium salt and showed thermochromism, which is derived from the change of the equilibrium constant depending on the temperature. While the 2-phosphinoazobenzene reacted as a usual triarylphosphine, its reaction with water gave phosphine oxide bearing a hydrazine moiety via a mechanism similar to the Mitsunobu reaction. The 2-phosphinoazobenzene bearing a methyl group at the 4‘-position of azobenzene was isomerized to the Z-isomer by irradiation. The Z-isomer was neither in equilibrium with an inner phosphonium salt nor hydrolyzed, in contrast to the E-isomer, because its geometry is difficult for an intramolecular nucleophilic attack. Photoisomerization caused the switching of the unique reactivity toward water. Such phosphines in equilibrium with the inner phosphonium salts are expected to be useful to control organic reactions by taking advantage of the photoisomerization of the azobenzene moiety.