(2,4,6-Trimethylbenzoyl)diphenylphosphine Oxide Photochemistry. A Direct Time-Resolved Spectroscopic Study of Both Radical Fragments

The photochemistry of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) has been investigated 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 α-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^-1in n-heptane and CH₂Cl₂solution, 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 (Ph₂CHOH) have been determined. Both radicals react with BrCCl₃and PhSH via atom abstraction with rate constants in the 10⁷—10⁸M^-1s^-1range. 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.