Highly efficient synthesis of di-substituted allenes via copper-catalyzed SN2′- substitution reaction of organotitanium reagents with propargyl bromides

A simple and mild catalytic S_N2′-type displacement of propargyl halides with organotitanium reagents is reported. The S_N2′-type displacement of propargyl bromides with organotitanium reagents mediated by CuI (2 mol%)/MePPh₂(4 mol%) in dichloro methane afforded multi-substituted allenes in good yields (up to 95 %) at 40 °C for 5 h. The aryls bearing electron-donating or electron-withdrawing groups in aryltitanium reagents gave multi-substituted allenes in good yields. In addition, the di-substituted allenes bearing a naphthyl group was obtained in 54–99 % selectivities with isolated yields of 43–83 %. The process was simple and easily performed, and it provides an efficient method for the synthesis of di-substituted allene derivatives. On the basis of the experimental results, a possible catalytic cycle has been proposed.