NCN Pincer Palladium Complexes: Their Preparation via a Ligand Introduction Route and Their Catalytic Properties

A wide range of NCN pincer palladium complexes, [4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chloropalladium (alkyl = n-butyl, benzyl, cyclohexyl, tert-butyl, adamantyl, phenyl, 4-methoxyphenyl), were
readily prepared from trans-(4-tert-butyl-2,6-diformylphenyl)chlorobis(triphenylphosphine)palladium via
dehydrative introduction of the corresponding alkylimino ligand groups (ligand introduction route) in excellent
yields (71−98%). NMR studies on this route for forming pincer complexes revealed the intermediacy of
[4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chlorobis(triphenylphosphine)palladium which is in equilibrium with
the corresponding NCN pincer complexes via coordination/dissociation of the intramolecular imino groups
and triphenylphosphine ligands. A series of chiral NCN pincer complexes bearing pyrroloimidazolone units
as the trans-chelating donor groups, [4-tert-butyl-2,6-bis{(3R,7aS)-2-phenylhexahydro-1H-pyrrolo[1,2-c]imidazol-1-on-3-yl}phenyl]chloropalladium, were also prepared from the same precursor via condensation
with proline anilides in high yields. The catalytic properties of the NCN imino and the NCN pyrroloimidazolone
pincer palladium complexes were examined in the Heck reaction and the asymmetric Michael reaction to
demonstrate their high catalytic activity and high enantioselectivity.