Titanocene-Catalyzed Regioselective Carbomagnesation of Alkenes and Dienes.

A new method for regioselective carbomagnesation of alkenes and dienes has been developed by the use of a titanocene catalyst. This reaction proceeds efficiently at 0 °C in THF in the presence of Cp2TiCl2 by the combined use of organic halides (R−X; R = alkyl, aryl and vinyl) and n-BuMgCl to afford benzyl, α-silylalkyl, or allyl Grignard reagents, which were trapped with various electrophiles. The present reaction involves (i) addition of carbon radicals toward alkenes or dienes in the carbon−carbon bond-forming step and (ii) transmetalation on Ti of benzyl-, α-silylalkyl-, or allyltitanocene with n-BuMgCl in the carbon−magnesium bond-forming step. The scope and limitations of this reaction have also been examined.