LASER FLASH-PHOTOLYSIS AND TIME-RESOLVED ESR STUDY OF PHOSPHINOYL RADICAL STRUCTURE AND REACTIVITY

The photochemistry of two phosphine oxides and the rate constants of r eaction of their daughter radicals with several alkenes, halocarbons, and oxygen have been determined. Photolysis of (2,4,6-trimethylbenzoyl )-diphenylphosphine oxide (1) and 6-dimethoxybenzoyl)-2,4,4-trimethylp entylphosphine oxide (4) in each case affords a phosphinoyl and a benz oyl radical. The phosphinoyl radicals are readily detected by laser fl ash photolysis and exhibit absorption maxima at 325 and 450 nm for the diphenylphosphinoyl (3) and -dimethoxybenzoyl-2,4,4-trimethylpentylph osphinoyl (6) radicals, respectively. The rate constants for reaction of the phosphinoyl radicals with alkyl halides, alkenes, and oxygen ra nge from 10(4) to 10(9) M(-1) s(-1). Radical 3 is 2-6 times more react ive than radical 6. For example, 3 adds to methyl methacrylate with a rate constant of (11+/-2) x 10(7) M(-1) s(-1) whereas 6 has an additio n rate constant for the same reaction of (2.3+/-0.3) x 10(7) M(-1) s(- 1). The rate constants for reaction with alkyl halides decrease with i ncreasing C-X bond strength, while the rate constants for quenching by acrylates decrease with increasing methyl substitution on the beta-ca rbon. The 2,6-dimethoxybenzoyl (5) and phosphinoyl (6) radicals derive d from 4 are readily detected by time-resolved ESR (TR ESR); benzoyl r adical 5 appears as a singlet and phosphinoyl radical 6 appears as a d oublet of triplets (A(P)=285 G, A(H)=4.8G). The CIDEP patterns of 5 an d 6 indicate that the radicals are formed from alpha-cleavage of the t riplet excited state of 4. TR ESR has also proved useful in the direct detection of the polarized benzyl radicals formed from addition of ph osphinoyl 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 phos phorus.