The Synthesis of Glycols by Mercury-Photosensitized Alcohol Dehydrodimerization

Article abstract of DOI:10.1021/jo00067a021

A variety of alcohols can be dehydrodimerized to give 1,2-diols on a multigram scale at 1 atm pressure and reflux temperature on photolysis (254 nm) in the presence of a trace of Hg vapor.Initial C-H bond breaking is followed by recombination of the resulting α-centered radicals, which normally leads to C-C bond formation α to oxygen.The reaction rate and selectivity can be increased by operating at lower temperatures under H2, in which case H atoms replace Hg* as the principal abstracting reagent and H atom abstraction from the α-CH bond leads directly to the α-C-centered radical.Under H atom conditions, unsaturated alcohols also react, in which case diols other than the 1,2-isomer can be formed selectively.The product can be rationalized on the basis of H atom addition to the C=C double bond to give the most stable radical which then dimerizes.For the special case of t-BuOH, H atom abstraction from the t-BuOH β-CH group under H atom conditions leads to the β-centered radical, which dimerizes to the 1,4-diol.Radical disproportionation accounts for some of the byproducts observed.The following previously unknown C-H bond strengths (kcal/mol) were determined from the results, assuming the literature BDE for the α-C-H of 2-propanol (91.0 +/-1.0); n-butanol, 92.8 +/- 1.0( α), 95.2 +/- 1.0 (β), and 94.3 +/- 1.0 (γ); n-propanol, 93.1 +/- 1.0 (α), and 95.0 +/- 1.0 (β), respectively.