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

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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 1a determined in this work. The results suggest that rate constants for reactions of primary alkyl radicals with t-BuSH are about 20% smaller than those previously reported. In the case of PhSH, the results are in good agreement with one previously reported set of rate constants but about 35% smaller than another set of rate constants that was subsequently incorporated into fast alkyl radical kinetics. The rate constants for alkyl radical reactions calibrated by competition against reaction with PhSeH and PhSH apparently are 30−40% smaller than those previously reported, and the derived rate constants for the fast radical reactions should be adjusted. An especially noteworthy example is ring opening of the cyclopropylcarbinyl radical, the Arrhenius function for which was determined in part from PhSH trapping results. Using the adjusted rate constants for PhSH and recalculating the Arrhenius parameters for the cyclopropylcarbinyl radical ring opening gives log(k/s-1) = (13.045 ± 0.10) − (6.99 ± 0.09)/θ (kcal/mol, errors at 2σ); the rate constant at 20 °C of 6.7 × 107 s-1 is about 13% smaller than that previously calculated.

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