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.
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