Issue 2, 2011

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 kT = 1.2 × 109 M−1s−1 and kT = 2 × 107 M−1s−1, respectively, which are modestly larger than rate constants for reactions of alkyl radicals with the same trapping agents.

Graphical abstract: Rate constants for cyclizations of α-hydroxy radical clocks

Supplementary files

Article information

Article type
Paper
Submitted
16 Aug 2010
Accepted
16 Sep 2010
First published
08 Nov 2010

Org. Biomol. Chem., 2011,9, 516-522

Rate constants for cyclizations of α-hydroxy radical clocks

C. B. DeZutter, J. H. Horner and M. Newcomb, Org. Biomol. Chem., 2011, 9, 516 DOI: 10.1039/C0OB00588F

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