Synthesis of a Chiral Palladacycle and Its Application in Asymmetric Hydrophosphanation Reactions

A novel amine ligand, 1-(2,5-dimethylphenyl)-N,N,2,2-tetra-methylpropan-1-amine, was synthesized in six steps from commercially available p-xylene. Direct ortho-palladation of this amine ligand proceeded readily to form the racemic dimeric complex. The palladacycle structure and ring conformations of its triphenylphosphane derivative were thoroughly investigated by X-ray structural analysis in the solid state and 2D H-1-H-1 ROESY NMR spectroscopy in solution. This racemic palladacycle was efficiently resolved through the formation of its (S)-prolinato derivatives and an efficient separation of the resulting diastereomeric complexes was achieved by slow crystallization with the use of different solvent systems. The structure and absolute configuration of the two optically resolved palladium complexes were determined by X-ray diffraction. Both the (R,R) and (S,S)-di-mu-chlorido dimeric palladium complexes could be obtained chemoselectively by treating the corresponding prolinato derivatives with 1 M hydrochloric acid. The asymmetric hydrophosphanation reaction between diphenylphosphane and diethyl acetylenedicarboxylate was promoted by this newly synthesized palladacycle and resulted in the formation of a single isomer according to the P-31 NMR spectrum. The amine auxiliary could be subsequently removed chemoselectively from the palladium center by treatment with concentrated HCl. An optically pure C-2-symmetrical diphosphane ligand containing two ester functional groups at the two chiral carbon stereogenic centers was prepared by ligand displacement.