Titanocene-Catalyzed Alkylation of Aryl-Substituted Alkenes with Alkyl Halides

Aryl-substituted alkenes (ArHC=CH2) react with alkyl halides (R-X, X = Br or Cl) in the presence of a catalytic amount of [Cp 2TiCl2] and BuMgCl in Et2O to give alkylated alkenes (ArHC=CHR). This reaction proceeds regio- and stereoselectively under mild conditions to afford E-olefins. Primary and secondary alkyl bromides and secondary alkyl chlorides can be used as suitable alkylating reagents. The reactions of aliphatic alkenes, such as 1-octene and internal alkenes, were sluggish. When t-alkyl halides are employed, alkylative dimerization of alkenes proceeds exclusively to give symmetrical vic-diarylalkanes. These reactions involve addition of alkyl radicals to arylalkenes to form benzyl radicals as a carbon-carbon bond-forming step. Dimerization of thus formed benzyl radicals affords symmetrical alkanes and β-hydrogen elimination from benzyltitanocene intermediates gives alkylated alkenes. A possibility that titanocene activates alkenes as radical accepters was also proposed.