Palladacyclic Imidazoline–Naphthalene Complexes: Synthesis and Catalytic Performance in Pd(II)-Catalyzed Enantioselective Reactions of Allylic Trichloroacetimidates

A new family of air- and moisture-stable enantiopure C,N-palladacycles (PIN-acac complexes)
were prepared
in good overall yield in three steps from 2-iodo-1-naphthoic acid
and enantiopure β-amino alcohols. Three of these PIN complexes
were characterized by single-crystal X-ray analysis. As anticipated,
the naphthalene and imidazoline rings of PIN-acac complexes 18a and 18b were canted significantly from planarity
and projected the imidazoline substituents R1 and R2 on opposite faces of the palladium square plane. Fifteen
PIN complexes were evaluated as catalysts for the rearrangement of
prochiral (E)-allylic trichloroacetimidate 19 (eq 2) and the SN2′ allylic substitution
of acetic acid with prochiral (Z)-allylic trichloroacetimidate 23. Although these complexes were kinetically poor catalysts
for the Overman rearrangement, they were good catalysts for the allylic
substitution reaction, providing branched allylic esters in high yield.
However, enantioselectivities were low to moderate and significantly
less than that realized with palladacyclic complexes of the COP family.
Computational studies support an anti-acetoxypalladation/syn-deoxypalladation mechanism analogous to that observed
with COP catalysts. The computational study further suggests that
optimizing steric influence in the vicinity of the carbon ligand of
a chiral C,N-palladacycle, rather
than near the nitrogen heterocycle, is the direction to pursue in
future development of improved enantioselective catalysts of this
motif.