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.

通过光与热的外部刺激，已实现膦（phosphine）与鏻盐（phosphonium salt）之间的可逆转化。通过对相应膦硫化物进行脱硫反应，成功合成了两种2-膦基偶氮苯（2-phosphinoazobenzenes）。其中一种带有偶氮基团的膦与内鏻盐（inner phosphonium salt）处于平衡状态，并表现出热致变色现象——该现象源于平衡常数随温度变化而改变。尽管该2-膦基偶氮苯可作为普通三芳基膦（triarylphosphine）参与反应，但它与水反应时，会通过类似光延反应（Mitsunobu reaction）的机理生成带有肼基片段的膦氧化物（phosphine oxide）。在偶氮苯4’位带有甲基的2-膦基偶氮苯经辐照可异构为Z型异构体（Z-isomer）。与E型异构体（E-isomer）不同，该Z型异构体既无法与内鏻盐形成平衡，也不能发生水解反应，原因在于其空间构型难以发生分子内亲核进攻（intramolecular nucleophilic attack）。光异构化实现了对水相反应活性的调控。这类可与内鏻盐形成平衡的膦，有望借助偶氮苯片段的光异构化特性，用于有机反应的精准调控。