Reactions of trifluoromethyl bromide and related halides: part 9. Comparison between additions to carbonyl compounds, enamines, and sulphur dioxide in the presence of zinc

A Barbier procedure, under moderate pressure, was used for the trifluoromethylation of various carbonyl compounds, starting from trifluoromethyl bromide and zinc in pyridine. Trifluoromethyl methanols were obtained from aldehydes and trifluoromethyl ketones from activated esters. Ethyl benzoate, or acetone, induced the formation of the solvated trifluoromethylzinc derivatives which did not react with carbonyl compounds. Consequently, the Barbier condensation in that case was considered to involve nascent organometallics reacting near the zinc surface. The reaction with sulphur dioxide, leading to trifluoromethanesulphinate, showed striking differences from that of carbonyl compounds. It was shown that the main pathway occurred in solution. This condensation was interpreted by the initial formation of sulphur dioxide radical anion, which reacts with trifluoromethyl bromide by a single-electron-transfer process. Attempts to condense iminium salts failed when a hydrogen atom was lacking in the α position. When the iminium ion can be transformed in situ to an enamine, a reaction occurred, leading to α-trifluoromethyl ketones. This condensation was interpreted by a chain mechanism involving trifluoromethyl radicals.