Rate constants for cyclizations of α-hydroxy radical clocks

Abstract

The 1-hydroxy-1-methyl-6,6-diphenyl-5-hexenyl radical (4a) and the 1-hydroxy-1-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 = 9.9–2.6/2.303RT and log k = 8.9–4.4/2.303RT (kcal mol⁻¹) for 4a and 4b, respectively. Rate constants for cyclizations at room temperature of 9 × 10⁷s⁻¹ and 4 × 10⁵s⁻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 × 10⁹ M⁻¹s⁻¹ and k_T = 2 × 10⁷ M⁻¹s⁻¹, respectively, which are modestly larger than rate constants for reactions of alkyl radicals with the same trapping agents.