Rhodium-Catalyzed Cycloismerization of 1,6-Enynes with an Intramolecular Halogen Shift: Reaction Scope and Mechanism.

The rhodium(I)-species-catalyzed cycloisomerization reaction of a wide spectrum of 1 ,6-enynes with an unusual intramolecular halogen shift was investigated. This Rh-catalyzed enyne cyclization reaction represents a new process for the synthesis of stereodefined α-halomethylene-γ-butyrolactones, lactams, tetrahydrofurans, pyrrolidines, and cyclopentanes. Coordinatively unsaturated rhodium species ([Rh(COD)- Cl]2 + dppb + AgSbF6) only catalyzes the reaction with enyne substrates bearing a Z-form double bond, while neutral rhodium species (RhCl(PPh3)3) could catalyze enyne substrates bearing a Z- or E-form double bond to form the desired products and has a wider substrate scopes. The mechanism of the reaction was studied by the employment of control experiments with different enyne isomers, and a π-allyl rhodium intermediate was suggested to explain the formation of the cyclic products with an intramolecular halogen shift. [ABSTRACT FROM AUTHOR]