Titanium-Catalyzed Cycloisomerization of 1,6-Dienes. Regio- and Stereoselective Synthesis of exo-Methylenecycloalkanes

Cycloisomerization of 1,6-dienes 1 in the presence of a catalytic amount of n -BuMgBr and a variety of organotitanium complexes such as Cp 2 TiCl 2, Cp(MeO)TiCl 2, Cp(Me 2 N)TiCl 2, or CpTi[(TMS) 2 NC(NCy) 2 )Cl 2 provided the corresponding 2-methyl-1-methylenecyclopentanes 2 with high efficiency. In contrast, catalysis using the more sterically encumbered complex ethylenebis(tetrahydroindenyl)titanium dichloride led to the predominant formation of methylenecyclohexanes 3 .