Evidence for C-C coupling between two mutually cis carbon ligands, eta(3)-crotyl and aryl in organopalladium(II) complexes

Some (eta(3)-crotyl)(2,5-dichlorophenyl)palladium(II) complexes containing phosphine and phosphite ligands Pd(eta(3)-CH2CH-CHMe) (Ar)(PR(3)) (Ar=C6H3Cl2-2,5) were isolated as crystalline solids or generated in solution. These existed as a mixture of two geometrical isomers arising from a different way of relative disposition of the crotyl-methyl group and the aryl ligand. The electronic nature of the PR(3) ligand controrred the relative rates of the interconversion between the two isomers and the reductive elimination of the complexes which released MeCH=CHCH2Ar and CH2=CHCH(Me)(Ar). Electron-withdrawing phosphite ligands were particularly effective in enhancing the reductive elimination rate, making the contribution of the isomerization path almost negligible and allowing the formation of two coupling products to be followed separately by spectroscopic means. These observations demonstrated the occurrence of C-C coupling between mutually cis carbon ligands in (eta(3)-allyl)(hydrocarbyl)palladium(II) complexes. The eta(1)-crotyl complex, Pd(eta(1)-CH2CH=CHMe) (Ar)(dppen)(dppen = cisPh(2)PCH=CHPPh(2)) was isolated and shown to exist as a sore regio-isomer in solution. Reductive elimination of this eta(1)-crotyl complex gave MeCH=CHCH2Ar exclusively.