Adjusting the Top End of the Alkyl Radical Kinetic Scale. Laser Flash Photolysis Calibrations of Fast Radical Clocks and Rate Constants for Reactions of Benzeneselenol

Abstract

Rate constants for 5-exo cyclizations of the 6,6-diphenyl-5-hexenyl radical (1a), the 1-methyl-6,6-diphenyl-5-hexenyl radical (1b), and the 1,1-dimethyl-6,6-diphenyl-5-hexenyl radical (1c) were measured by laser flash photolysis methods, and Arrhenius parameters for these cyclizations were determined. Relative rate constants for cyclizations of radicals 1 and reactions with benzeneselenol were determined by indirect kinetic methods, and the relative Arrhenius parameters for the competing reactions were combined with the parameters for the cyclization reactions to give absolute Arrhenius parameters for the PhSeH reactions. At 20 °C, PhSeH reacts with the 1°, 2°, and 3° radicals 1 with nearly the same rate constants, (1.2 ± 0.1) × 109 M-1 s-1. Absolute Arrhenius parameters for reactions of PhSH and t-BuSH with the primary alkyl radical 1a were calculated using literature values for the competition between cyclization of 1a and reactions with the appropriate thiol and the absolute values for cyclization of 1...