Cp2TiCl2-Catalyzed Grignard exchange reactions with acetylenes. A convenient method for preparation of E-alkenyl Grignard reagents

Disubstituted acetylenes react with isobutylmagnesium halide in the presence of a catalytic amount of Cp₂TiCl₂ in ether to afford E-alkenyl Grignard reagents selectively and in almost quantitative yields. The regiochemistry of this hydromagnesation reaction is high for alkylarylacetylenes and silylacetylenes giving E-ArC(MgBr)CHR from alkylarylacetylenes, E-ArC(MgBr)CH(SiMe₃) from arylsilylacetylenes, and ECHRC(MgBr)(SiMe₃) from alkylsilylacetylenes, respectively. Thanks to the high reactivity of the Grignard reagent, the present reaction offers a novel, selective and operationally simple route for preparation of trisubstituted olefins.