Palladium(II) Acetate-Tris(triphenylphosphine)rhodium(I) Chloride: A Novel Catalytic Couple for the Intramolecular Heck Reaction.

A series of crotyl ethers underwent intramolecular cyclizations in the presence of a novel catalytic couple, composed of tris(triphenylphosphine)rhodium(I) chloride and palladium(II) acetate, under Heck conditions initially described by Jeffery.(1) The data indicated that the combination of these two metal catalysts formed a synergistic interaction that mediated intramolecular couplings in a superior manner as compared to the use of palladium(II) acetate alone. When tris(triphenylphosphine)rhodium(I) chloride was added to the system, a distinct increase in rate and selectivity for the endocyclic form of the bicyclic ether products was generally observed. In addition, changing parameters such as dilution and intial pot temperature appeared to significantly affect rate and selectivity in the experiments involving the catalytic couple, but not as drastically with those that were done only in the presence of palladium(TI) acetate. Substitution of bis(triphenylphosphine)palladium(II) acetate for palladium(II) acetate provided an outcome that slightly favored the endocyclic form of the bicyclic ether, but when tris(triphenylphosphine)rhodium(I) chloride was added as a cocatalyst, the ratio clearly favored the exocyclic isomer.