Palladium(0) Complexes with Unsymmetric Bidentate Nitrogen Ligands for the Stereoselective Hydrogenation of 1-Phenyl-1-propyne to (<i>Z</i>)-1-Phenyl-1-propene

Abstract

A series of zerovalent palladium complexes Pd(NN)(alkene) of bi- or tridentate nitrogen ligands of the general formula 6-R‘ ‘-C5H3N-(C(R‘)NR)-2 (R‘ ‘ = H, Me, CHNR‘; R‘ = H, Me; R = alkyl, aryl, or amino group) and dimethylfumarate (dmfu) have been prepared and were subsequently employed as precatalysts in the homogeneous stereoselective semihydrogenation of 1-phenyl-1-propyne. An X-ray structure of Pd(C5H4N-(C(Me)Ni-Pr)-2)(dmfu) was obtained. Whereas only relative small changes in substituents apply, the various complexes show very different stabilities under hydrogenation conditions. The complex Pd(C5H4N-(C(H)N(CH2)2OH)-2)(dmfu) exhibits a good selectivity for the (Z)-alkene but decomposes just before full conversion of the alkyne, whereas the complex Pd(C5H4N-(C(H)Ni-Pr)-2)(dmfu) exhibits a good selectivity and stability under hydrogenation conditions and is a suitable catalyst for the stereoselective hydrogenation of 1-phenyl-1-propyne to (Z)-1-phenyl-1-propene.