Solvent effects in the synthesis of trifluoromethyl sulfones from sodium trifluoromethanesulfinate

Abstract

As reported by J.B. Hendrickson, primary alkyl ‘triflones’ are known to be formed cleanly, but slowly, from potassium trifluoromethanesulfinate and corresponding alkyl bromides in refluxing acetonitrile. Under these conditions, secondary bromides are not reactive and allyl bromides deliver allyl triflones without significant isomerization into 1-propenyl triflones. We investigated the behaviour of sodium trifluoromethanesulfinate (easily available from CF 3Br and sodium dithionite) towards alkyl bromides in solvents other than acetonitrile. In N,N-dimethylacetamide, the reaction rates increase dramatically and the following features are noteworthy: 1. both secondary and primary triflones can be obtained in fair yields, 2. primary and secondary α-bromo esters and ketones produce the corresponding sulfones within a few hours, 3. however, under more drastic conditions or in the presence of diisopropylethylamine, primary α-bromo carbonylated compounds yield secondary and tertiary triflone intermediates, which eliminate trifluoromethane-sulfinic acid in situ to produce α,β-unsaturated γ- dicarbonylated products such as dibenzoyl ethylene (from bromoacetophenone) or ethyl fumarate and ethyl aconitate (from ethyl bromoacetate). On the other hand, allyl bromide yields, in N,N-dimethylacetamide and other solvents, a mixture of allyl triflone and 1-propenyl triflone, the composition of which is dependent on the nature of the solvent. All these results seem to be directly related to the solubility of sodium bromide in the solvent studied and to its solvating properties.