ABSOLUTE RATE CONSTANTS FOR ALPHA-AMIDE RADICAL REACTIONS

The 1-(diethylcarbamoyl)-6,6-diphenyl-5-hexenyl radical (4a), the 1-(d iethylcarbamoyl)-7,7-diphenyl-6-heptenyl radical (4b), and the (diethy lcarbamoyl)-1-methyl-6,6-diphenyl-5-hexenyl radical (4c) were produced from the corresponding PTOC esters (anhydrides of the carboxylic acid and N-hydroxypyridine-2-thione) by laser flash photolysis methods. Th e kinetics of cyclizations of radicals 4a and 4b were measured at vari ous temperatures, and that of cyclization of 4c was measured at ambien t temperature. Radicals 4a and 4b were employed as radical clocks in i ndirect kinetic studies to determine rate constants for reaction of se condary alpha-amide radicals with Bu3SnH. The calibrated tin hydride t rapping reaction was then employed to determine rate constants for cyc lization of the 1-(diethylcarbamoyl)-5-hexenyl radical (12). The rate constants for 5-exo cyclizations of secondary alpha-amide radicals are similar to those of their isostructural alkyl radical analogues. The rate-constants for the 5-exo cyclization of tertiary alpha-amide radic al 4c and the 6-exo cyclization of the secondary alpha-amide radical 4 b are smaller than those of the analogous alkyl radicals and alpha-est er substituted radicals, apparently due to steric effects. The rate co nstants for tin hydride trapping of secondary alpha-amide radicals are similar to those for reactions with secondary alpha-ester radicals.