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.