Effects of substituents on aryl groups during the reaction of triarylphosphine radical cation and oxygen

Abstract

In a previous report (S. Yasui, S. Tojo and T. Majima, J. Org. Chem., 2005, 70, 1276), we presented the results from the laser flash photolysis (LFP) and product analysis of the 9,10-dicyanoanthracene (DCA)-photosensitized oxidation of triarylphosphine (Ar₃P) in acetonitrile under air, which showed that the photoreaction results in the oxidation of Ar₃P to give the corresponding phosphine oxide (Ar₃PO) in a nearly quantitative yield, and that the reaction is initiated by the electron transfer (ET) from Ar₃P to DCA in the singlet excited state (¹DCA*), producing the triarylphosphine radical cation Ar₃P˙⁺. This radical cation decays through radical coupling with O₂ to afford the peroxy radical cation Ar₃P⁺–O–O˙, which we proposed to be the intermediate leading to the product Ar₃PO. We now examined this photoreaction in more detail using ten kinds of Ar₃P with various electronic and steric characteristics. The decay rate of Ar₃P˙⁺ measured by the LFP was only slightly affected by the substituents on the aryl groups of Ar₃P. During the photolysis of trimesitylphosphine (Mes₃P), the peroxy radical cation intermediate (Mes₃P⁺–O–O˙) had a lifetime long enough to be spectrophotometrically detected. The quantum yields of Ar₃PO increased with either electron-withdrawing or -releasing substituents on the aryl groups, suggesting that a radical center is developed on the phosphorus atom during the step when the quantum yield is determined. In addition, the o-methyl substituents in Ar₃P decreased the quantum yield. These results clearly indicated that Ar₃P⁺–O–O˙ undergoes radical attack upon the parent phosphine Ar₃P that eventually produces the final product, Ar₃PO.