Laser Flash Photolysis and Time-Resolved ESR Study of Phosphinoyl Radical Structure and Reactivity

Abstract

The photochemistry of two phosphine oxides and the rate constants of reaction of their daughter radicals with several alkenes, halocarbons, and oxygen have been determined. Photolysis of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (1) and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (4) in each case affords a phosphinoyl and a benzoyl radical. The phosphinoyl radicals are readily detected by laser flash photolysis and exhibit absorption maxima at 325 and 450 nm for the diphenylphosphinoyl (3) and 2,6-dimethoxybenzoyl-2,4,4-trimethylpentylphosphinoyl (6) radicals, respectively. The rate constants for reaction of the phosphinoyl radicals with alkyl halides, alkenes, and oxygen range from 104 to 109 M-1 s-1. Radical 3 is 2−6 times more reactive than radical 6. For example, 3 adds to methyl methacrylate with a rate constant of (11 ± 2) × 107 M-1 s-1 whereas 6 has an addition rate constant for the same reaction of (2.3 ± 0.3) × 107 M-1 s-1. The rate constants for reaction with alkyl halides decrease with increasing C−X bond strength, while the rate constants for quenching by acrylates decrease with increasing methyl substitution on the β-carbon. The 2,6-dimethoxybenzoyl (5) and phosphinoyl (6) radicals derived from 4 are readily detected by time-resolved ESR (TR ESR); benzoyl radical 5 appears as a singlet and phosphinoyl radical 6 appears as a doublet of triplets (A(P) = 285 G, A(H) = 4.8 G). The CIDEP patterns of 5 and 6 indicate that the radicals are formed from α-cleavage of the triplet excited state of 4. TR ESR has also proved useful in the direct detection of the polarized benzyl radicals formed from addition of phosphinoyl radicals 3 and 6 to styrene and 2,4,6-trimethoxystyrene. The lower reactivity of 6 compared to 3 is attributed to its more planar structure and lower degree of spin localization in a s-orbital on phosphorus.