Rate constants for cyclizations of a-hydroxy radical clocks

摘要

The l-hydroxy-l-methyl-6,6-diphenyl-5-hexenyl radical (4a) and the l-hydroxy-l-methyl-7,7-diphenyl-6-heptenyl radical (4b) were prepared from the corresponding PTOC esters (anhydrides of a carboxylic acid and N-hydroxypyridine-2-thione). The key step in the synthetic method for the precursors was a coupling reaction of the respective carboxylic acids with the thiohydroxamic acid, which was conducted for ca. 5 min and followed rapidly by chromatography. Rate constants for cyclizations of radicals 4a and 4b in acetonitrile and in THF were measured directly between -30 and 60 °C by laser flash photolysis methods. The Arrhenius functions in acetonitrile are log k = 9.9-2.6/2.303 RT and log k = 8.9-4.4/2.303 RT (kcal mol~(-1) for 4a and 4b, respectively. Rate constants for cyclizations at room temperature of 9 ×10~7 s~(-1) and 4 ×10~5s~(-1) are somewhat larger than the rate constants for cyclizations of analogous alkyl radicals. Crude rate constants at room temperature for H-atom trapping of 4a by thiophenol and 4b by t-butylthiol were k_T = 1.2×10~9 M~(-1) s~(-1) and k_T = 2×10~7 M~(-1)~ s~(-1), respectively, which are modestly larger than rate constants for reactions of alkyl radicals with the same trapping agents.