(2,4,6-TRIMETHYLBENZOYL) DIPHENYLPHOSPHINE OXIDE PHOTOCHEMISTRY - A DIRECT TIME-RESOLVED SPECTROSCOPIC STUDY OF BOTH RADICAL FRAGMENTS

The photochemi stry of (2,4,6-trimethylbenzoyl) diphenylphosphine oxide (1) has been inve stigated using time-resolved infrared (TRIR) and time-resolved UV spectroscopy (laser flash photolysis (LFP)), in addition to conventional photochemical steady-state techniques and product analysis. Direct photolysis of 1 in dichloromethane (CH2Cl2) or n-heptane solution causes alpha-cleavage to afford the corresponding 2,4,6-trimethylbenzoyl (2) and diphenylphosphonyl (3) radicals. The 2,4,6-trimethylbenzoyl radical (2) is readily detected using TRIR spectroscopy, exhibiting carbonyl IR absorption maxima at 1805 and 1797 cm(-1) in n-heptane and CH2Cl2 solution, respectively. The diphenylphosphonyl radical (3) is readily detected by LFP, exhibiting a strong UV absorption in the 300-350 nm region. Rate constants for the reactions of radicals 2 and 3 with bromotrichloromethane (BrCCl3), thiophenol (PhSH), and benzhydrol (Ph(2)CHOH) have been determined. Both radicals react with BrCCl3 and PhSH via atom abstraction with rate constants in the 10(7)-10(8) M(-1) s(-1) range. Photolysis of 1 in the absence of a radical scavenger affords diphenyl[(2,4,6-trimethylbenzoyl)oxy]phosphine (9) in addition to the expected free radical-derived products. Compound 9 is also produced upon photolysis of 1 in the presence of 0.05 M BrCCl3, indicating that it is formed mainly via cage recombination of radicals 2 and 3. The reactivity of radicals 2 and 3 is discussed.