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

摘要

Aryl-substituted alkenes(ArHC=CH_2)react with alkyl halides(R-X,X=Br or Cl)in the presence of a catalytic amount of [Cp_2TiCl_2]and"BuMgCl in Et_2O 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 beta-hydrogen elimination from benzyltitanocene intermediates gives alkylated alkenes.A possibility that titanocene activates alkenes as radical accepters was also proposed.