Stereochemical Investigations of a Novel Class of Chiral Phosphapalladacycle Complexes Derived from 1-[(2,5-Dimethyl)phenyl]ethyldiphenylphosphine

The phosphapalladacycle derived from 1-(2‘,5‘-dimethylphenyl)ethyldiphenylphosphine has been prepared in the
optically active and racemic forms. The phosphine was synthesized as a racemate by the treatment of 1-chloro-1-(2‘,5‘-dimethylphenyl)ethane with sodium diphenylphosphide in THF. The racemic phosphapalladacycle was
subsequently obtained as the chloro-bridged dimer by the treatment of the phosphine with palladium(II) acetate
followed by anion metathesis with lithium chloride. Alternatively, the phosphine could be optically resolved via
metal complexation using (R,R)-bis(μ-chloro)bis{1-[1-(N,N-dimethylamino)ethyl]naphthyl-C2,N}dipalladium(II) as the
resolving agent. An efficient separation of the resulting diastereomeric complexes was achieved by silica gel
chromatography. The obtained optically resolved diastereomers were next subject to chemoselective removal of
the (R)-N,N-(dimethylamino)-1-(1-naphthyl)ethylaminate auxiliary by treatment with concentrated hydrochloric acid.
This process yielded the binuclear dimer complexes containing the resolved η1-P ligand. Cyclopalladation of the
coordinated phosphine could next be performed by treatment of its η1-P binuclear dimer with silver(I)
hexafluorophosphate(V) in a dichloromethane/water mixture followed by treatment with lithium chloride, giving rise
to a pair of optically pure enantiomeric dimers with [α]D −322 and +319° in CH2Cl2. Despite the possibilities of
the phosphine to attain a five- membered structure by ortho-palladation or a six-membered ring formation by aliphatic
C−H bond activation, only the former was observed. X-ray crystallographic data of the meso dimer and an
acetylacetonate derivative indicated that the phosphapalladacycle α-C* methyl substituent was axially located. The
2-D 1H−1H ROESY spectrum of the acetylacetonate derivative further revealed that the phosphapalladacycle was
conformationally rigid in CDCl3.