Synthesis and the stereoelectronic properties of novel cyclopalladated complexes derived from enantiomerically pure (R/S)-N,N-dimethyl-1-(9-phenanthryl)ethylamine

Abstract

A series of enantiomerically pure organopalladium complexes containing the resolved orthometalated (R/S)-N,N-dimethyl-1-(9-phenanthryl)ethylamine ligand have been prepared. The absolute ring conformations of these novel organometallic phenanthrylamine chelates in the solid state and in solution have been investigated by X-ray structural analyses and 2D ROESY NMR spectroscopy, respectively. All the chiral ortho-metalated rings were found to be stereochemically rigid, with their absolute ring conformations being locked by the repulsive interactions between the methyl group on the chiral carbon and the adjacent aromatic proton H8 of the phenanthrylamine ring. The methyl substituents on the stereogenic carbons invariably take up the axial positions in the skewed five-membered rings such that the static δ and λ ring conformations are adopted by the R- and S-phenanthrylamine, respectively. Despite the enormous interchelate steric constraints, the monodentates PPh3 and DMPP are coordinated regiospecially to the ortho-palladated phenanthrylamine unit trans to the NMe2 group. The organometallic rings are able to adopt distorted tetrahedral geometries around their palladium atoms and even alter their conjugated phenanthryl planarities in order to accommodate bulky ligands including DMPP, PPh3, and N,N,N′,N′-tetramethyl-2,3-butanediamine. Compared with its naphthylamine analogue, the ortho-palladated phenanthrylamine unit shows a much higher stereoselectivity in the chiral template-promoted asymmetric cycloaddition reaction between DMPP and N,N-dimethylacrylamide.