Absolute Rate Constants for Iminyl Radical Reactions

The kinetics of cyclization of the 2-methyl-6,6-diphenyl-5-hexeniminyl radical (3) were measured by laser flash photolysis methods from 0 to 50°C, and the kinetics of trapping of radical 3 by thiophenol and p-chlorothiophenol were determined between 4 and 83°C by indirect methods using the unimolecular rate constants as the basis values. Radical 3 cyclizes with a rate constant of 2.2 × 106 s^-1 at 25°C and reacts with thiophenol and p-chlorothiophenol at this temperature with rate constants of 0.6 and 1.4 × 10⁷ M^-1 s^-1, respectively. Bu₃SnH reacted with 3 too slowly to prevent nearly complete cyclization, but a crude rate constant for the trapping reaction was estimated. The cyclization and trapping reactions of iminyl radical 3 are slower than the corresponding reactions of alkyl radicals.