SYNTHESIS AND STRUCTURE OF ETA(2)-PHOSPHONIOALKENE-PALLADIUM(0) COMPLEXES - A CATALYST INTERMEDIATE IN THE PALLADIUM-MEDIATED SYNTHESIS OF ALKENYLPHOSPHONIUM HALIDES

Treatment of Pd(PPh3)(4) with PhCH=CHBr in THF at 45 degrees C gave [( trans-PhCH=CHPPh3)Pd(PPh3)(2)]Br-+(-) (1), which was found to be conve rted from Pd(PPh3)(2)(CH=CHPh)Br. X-ray structural analysis shows that 1 adopts a distorted-square-planar geometry with two PPh3 ligands and a (PhCH=CHPPh3)(+) moiety. The C-C double bond in the latter is pi-bo nded to the palladium(0) center and occupies two of the four square-pl anar coordination sites. Under similar reaction conditions, ns-PhCH=CH P(p-tolyl)(3)][P(p-tolyl)(3)](2)}Br-+(-) (2) was formed from Pd[P(p-to lyl)(3)](4) and PhCH=CHBr, while [(trans-MeCH=CHPPh3)Pd(PPh3)(2)]Br-+( -) (3) was isolated from the reactions of Pd(PPh3)4 with MeCH=CHBr. Tr eating PhCH=CHBr with trans-Pd(PPh3)(2)(C6H4O-p-CH3)I in THF at ambien t temperature led to the formation of (trans-PhCH=CHPPh3)Pd(PPh3)X (5) and (p-CH3OC6H4PPh3)X-+(-) (X, mixture of Br and I). Complex 5 (X Br) can be prepared by treating trans-PhCH=CH(PPh3)Br-+(-) with Pd(dba)(2 ) and 1 equiv of PPh3 in CH2Cl2. X-ray analysis shows 5 adopts a disto rted-square-planar structure consisting of PPh3, X, and PhCH=CH(PPh3)( +) as ligands. The PPh3 ligand is trans to the olefin carbon that is c onnected to the PPh3 group in PhCH=CH(PPh3)(+), while the halide ligan d 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 PhCH=CHBr, MeCH=CHBr, MeCH=C(Me)Br (mixtures of isomers), CH2=C(M e)Br, cis-(EtO2C)CH=CHBr, and cis-(MeO2C)CH=CHBr to give the correspon ding trans-RCH=CR'(PPh3)Br-+(-). During the reaction of PPh3 with PhCH -CHBr, 1 was found to be a catalyst intermediate in the reaction solut ion. All the pho sphonioalkene-palladium(0) complexes and alkenylphosp honium halides isolated are trans in the (RCH-CR'PPh3)(+) moiety.