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