Rhodium-catalyzed cycloisomerization of 1,6-enynes with an intramolecular halogen shift: Reaction scope and mechanism

The rhodium(1)-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 alpha-halomethylene-gamma-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 pi-allyl rhodium intermediate was suggested to explain the formation of the cyclic products with an intramolecular halogen shift.