Rational Design of a Novel Chiral Palladacycle: Synthesis, Optical Resolution, and Stereochemical Evaluation

Abstract

A novel palladacycle was designed and prepared by direct cyclopalladation of the ligand 1-(3,6-dimethylnaphthalen-1-yl)-N,N-dimethylethanamine, which was synthesized by a multistep sequence starting from 2,7-dimethylnaphthalene. The palladacycle structure and ring conformations of its triphenylphosphane derivative was investigated by X-ray structural analysis in the solid state and 2D ¹H–¹H ROESY NMR spectroscopy in solution. The ortho-palladated ring is stereochemically rigid, and the five-membered ring conformation is locked by repulsive interactions between the methyl group on the prochiral carbon atom and its neighboring naphthylene proton. The racemic cyclopalladated complex could be efficiently resolved through the formation of its (S)-prolinato derivatives, and efficient separation of the resulting diastereomeric complexes was achieved by simple silica-gel chromatography. The structures and absolute configurations of the two optically resolved palladium complexes were determined by X-ray crystallography. Both the (R,R)- and (S,S)-di-μ-chloride dimeric palladium complexes could be obtained chemoselectively by treating the corresponding prolinato derivatives with 1 M hydrochloric acid. Despite the severe interchelate steric constraints within this new organopalladium complex, the bulky monodentate ligand 3,4-dimethyl-1-phenylphosphole (dmpp) was coordinated regiospecifically to the ortho-palladated amine unit trans to the NMe₂ group. In comparison to its naphthylamine analogue, the new palladacycle showed a much higher stereoselectivity in the chiral template-promoted asymmetric Diels–Alder reaction of coordinated dmpp and N,N-dimethylacrylamide.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)