An Efficient New Methodology for the Synthesis of 1-Functionalized 2-Halo-2-alkenes via Hydrohalogenation Reaction of Electron-Deficient Allenes

The hydrohalogenation reactions of electron-deficient allenes, such as 1,2-allenic ketones, carboxylic acids, esters, amides, nitriles, sulfoxides (with the use of AlX₃/H₂O), sulfones, and phosphine oxides with MX (X = I, Br, Cl, M = Na, Li) in different solvents, such as HOAc, CF₃COOH-HOAc (1 : 1), CF₃COOH and THF were studied. The reaction is nucleophilic conjugate addition in nature and provides efficient entries to 1-functionalized 2-halo-2-alkenes. The β,γ- over α,β-selectivity and stereoselectivity depend on the solvent(s) and the temperature applied. In most cases, the Z- and E-isomers can be easily separated with chromatography on silica gel. The subsequent cross coupling reaction with organometallic reagents and terminal alkynes showed the synthetic potential of these products.

electron-deficient allenes - hydrohalogenation - regioselectivity - stereoselectivity - solvent effect