Synthesis of η²-Phosphonioalkene−Palladium(0) Complexes from Alkenylphosphonium Halides and Palladium(0) Species. Structure and Substitution Reactions of These Complexes

Treatment of Pd(dba)₂ (dba = dibenzylideneacetone) with (trans-RCHCHPPh₃)⁺Br^- in dichloromethane afforded [Pd(trans-RCHCHPPh₃)₂Br]⁺ (R = CO₂Et (4), CO₂Me (5), Ph (6)). As shown by the results of the X-ray diffraction, 4 adopts a distorted pentagonal geometry with two EtO₂CCHCH(PPh₃)⁺ moieties and a bromide ligand. The C−C double bond in the EtO₂CCHCH(PPh₃)⁺ moieties are π-bonded to the palladium(0) center and are coplanar with the bromide ligand and the metal center. Both (EtO₂CCHCHPPh₃)⁺ moieties are oriented with the two PPh₃ groups trans to each other and the olefin carbon that is attached to PPh₃ occupying the coordination site neighboring to the bromide ligand. Treatment of trans-[R¹CHCR²(PPh₃)]⁺Br^- with Pd(dba)₂ in the presence of 1 equiv of PPh₃ or P(OPh)₃ gave the corresponding palladium complexes Pd[trans-R¹CHCR²(PPh₃)](L)Br (L = PPh₃, R¹ = Ph, R² = H (3). L = PPh₃, R¹ = Me, R² = H (7); R¹ = CO₂Et, R² = H (8); R¹ = CO₂Me, R² = H (9); R¹ = H, R² = Me (10). L = P(OPh)₃, R¹ = CO₂Me, R² = H (11)) in 54−92% yields. Substitution studies showed complexes 3 and 9 react with dppe (1,2-bis(diphenylphosphino)ethane) in dichloromethane to give [Pd(RCHCH(PPh₃)(dppe)]⁺Br^- (R = Ph (12); R = CO₂Me (13)), but treatment of 9 and 11 with PPh₃ and P(OPh)₃, respectively, afforded only the original complexes on isolation.