Absolute Rate Constants for α-Amide Radical Reactions

The 1-(diethylcarbamoyl)-6,6-diphenyl-5-hexenyl radical (4a), the 1-(diethylcarbamoyl)-7,7-diphenyl-6-heptenyl radical (4b), and the 1-(diethylcarbamoyl)-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. The kinetics of cyclizations of radicals 4a and 4b were measured at various temperatures, and that of cyclization of 4c was measured at ambient temperature. Radicals 4a and 4b were employed as radical clocks in indirect kinetic studies to determine rate constants for reaction of secondary α-amide radicals with Bu₃SnH. The calibrated tin hydride trapping reaction was then employed to determine rate constants for cyclization of the 1-(diethylcarbamoyl)-5-hexenyl radical (12). The rate constants for 5-exo cyclizations of secondary α-amide radicals are similar to those of their isostructural alkyl radical analogues. The rate constants for the 5-exo cyclization of tertiary α-amide radical 4c and the 6-exo cyclization of the secondary α-amide radical 4b are smaller than those of the analogous alkyl radicals and α-ester substituted radicals, apparently due to steric effects. The rate constants for tin hydride trapping of secondary α-amide radicals are similar to those for reactions with secondary α-ester radicals.