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

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.