Synthesis, Optical Resolution, and Stereochemical Properties of a Rationally Designed Chiral C–N Palladacycle

Abstract

A novel racemic tertiary amine, 1-(2,5-diisopropylphenyl)-N,N-dimethylethanamine, was synthesized from 2,5-diisopropylbenzaldehyde via a multistep approach in high overall yield. The ortho palladation of this ligand was found to be sensitive to the reaction conditions and the palladating reagents employed. The metal complexation process could thus generate a cyclopalladated complex in high yield, lead to an unexpected N-demethylated amine palladium(II) complex, or both. Both products have been isolated and characterized crystallographically in the solid state and spectroscopically in solution. The racemic cyclopalladated complex could be efficiently resolved via the formation of (S)-prolinato derivatives. The absolute stereochemistries of the resolved diastereomeric complexes were determined by single-crystal X-ray crystallography in the solid state and by 1H–1H rotating frame Overhauser effect (ROESY) NMR spectroscopy in solution. An evaluation of the sterically hindered resolved cyclopalladated units as chiral auxiliaries was conducted in the endo-cycloaddition reaction between 3,4-dimethyl-1-phenylphosphole (DMPP) and ethyl vinyl ketone. The two expected phosphanorbornene adducts were generated with moderate stereoselectivity.