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

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 (Ar3P=O) 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 ((1)DCA*), producing the triarylphosphine radical cation Ar3P.+. This radical cation decays through radical coupling with O-2 to afford the peroxy radical cation Ar3P+-O-O-center dot, which we proposed to be the intermediate leading to the product Ar3P=O. 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 (Mes(3)P), the peroxy radical cation intermediate (Med(3)P(+)-O-O-center dot) had a lifetime long enough to be spectrophotometrically detected. The quantum yields of Ar3P=O 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-center dot undergoes radical attack upon the parent phosphine Ar3P that eventually produces the final product, Ar3P=O.