New Catalytic System for S−S and Se−Se Bond Addition to Alkynes Based on Phosphite Ligands

A new catalytic system for the Ar2E2 (E = S, Se) addition to terminal alkynes (HC⋮C−R) has been developed to synthesize bis-element-substituted alkenes Z-H(ArE)CC(EAr)R with high stereoselectivity and yields. Utilizing phosphite ligand P(OiPr)3 allowed solving two major problems of this catalytic reaction: (1) prevent catalyst polymerization and (2) simplify product purification procedures. Key intermediatestrans-[Pd(SPh)2(P(OiPr)3)2] and trans-[Pd2(SPh)4(P(OiPr)3)2]were synthesized by S−S oxidative addition reaction to Pd(0) and studied by X-ray analysis. The equilibrium between the mononuclear and dinuclear complexes in solution was established by 31P NMR spectroscopy. In addition to the advantages in the synthetic procedure, the isolation of the stable palladium complexes with phosphite ligand made possible a detailed mechanistic study of the catalytic reaction.