Synthesis and Structure of η²-Phosphonioalkene−Palladium(0) Complexes. A Catalyst Intermediate in the Palladium-Mediated Synthesis of Alkenylphosphonium Halides

Abstract

Treatment of Pd(PPh3)4 with PhCHCHBr in THF at 45 °C gave [(trans-PhCHCHPPh3)Pd(PPh3)2]+Br- (1), which was found to be converted from Pd(PPh3)2(CHCHPh)Br. X-ray structural analysis shows that 1 adopts a distorted-square-planar geometry with two PPh3 ligands and a (PhCHCHPPh3)+ moiety. The C−C double bond in the latter is π-bonded to the palladium(0) center and occupies two of the four square-planar coordination sites. Under similar reaction conditions, {Pd[trans-PhCHCHP(p-tolyl)3][P(p-tolyl)3]2}+Br- (2) was formed from Pd[P(p-tolyl)3]4 and PhCHCHBr, while [(trans-MeCHCHPPh3)Pd(PPh3)2]+Br- (3) was isolated from the reactions of Pd(PPh3)4 with MeCHCHBr. Treating PhCHCHBr with trans-Pd(PPh3)2(C6H4O-p-CH3)I in THF at ambient temperature led to the formation of (trans-PhCHCHPPh3)Pd(PPh3)X (5) and (p-CH3OC6H4PPh3)+X- (X, mixture of Br and I). Complex 5 (X = Br) can be prepared by treating trans-PhCHCH(PPh3)+Br- with Pd(dba)2 and 1 equiv of PPh3 in CH2Cl2. X-ray analysis shows 5 adopts a distorted-square-planar structure consisting of PPh3, X, and PhCHCH(PPh3)+ as ligands. The PPh3 ligand is trans to the olefin carbon that is connected to the PPh3 group in PhCHCH(PPh3)+, while the halide ligand is trans to the olefin carbon bonded to a phenyl moiety. Reaction of 5 (X = Br) with 1 equiv of PPh3 at ambient temperature afforded 1 in high yield. Pd(PPh3)4 or Pd(OAc)2 catalyzes the reactions of PPh3 with PhCHCHBr, MeCHCHBr, MeCHC(Me)Br (mixtures of isomers), CH2C(Me)Br, cis-(EtO2C)CHCHBr, and cis-(MeO2C)CHCHBr to give the corresponding trans-RCHCR‘(PPh3)+Br-. During the reaction of PPh3 with PhCHCHBr, 1 was found to be a catalyst intermediate in the reaction solution. All the phosphonioalkene−palladium(0) complexes and alkenylphosphonium halides isolated are trans in the (RCHCR‘PPh3)+ moiety.