Absolute Rate Constants for Alkoxycarbonyl Radical Reactions

PTOC oxalates (anhydrides of an oxalic acid monoester and N-hydroxypyridine-2-thione) were used in laser flash photolysis (LFP) kinetic studies. Irradiation of the PTOC oxalates gave the following alkoxycarbonyl radicals ROC(·)O:  2a, R = PhCH₂; 2b, R = (trans-2-phenylcyclopropyl)methyl; 2c, R = (2,2-diphenylcyclopropyl)methyl. Rate constants for decarboxylation of radical 2a measured by LFP were in agreement with those obtained previously by indirect kinetic methods. Rate constants for decarboxylation of radical 2c were measured by LFP over the temperature range 3−48 °C; the reaction is described by log k = 12.2−9.6/2.3RT (kcal/mol). Reactions of Bu₃SnH with radical 2c produced from the corresponding phenylseleno carbonate gave inconsistent kinetic results apparently due to the production of small amounts of PhSeH that reacted rapidly with 2c. An approximate rate constant at 2 °C for reaction of Bu₃SnH with 2c produced from the PTOC oxalate was obtained. Tin hydride reacts with the alkoxycarbonyl radical approximately 1 order of magnitude less rapidly than it reacts with alkyl radicals. Rate constants for 5-exo cyclizations of simple alkoxycarbonyl radicals were estimated from previous results and the approximate rate constant for tin hydride trapping; the cyclizations are slightly faster than 5-exo cyclizations of structurally related alkyl radicals.