Phosphino Hydrazones as Suitable Ligands in the Asymmetric Suzuki–Miyaura Cross-Coupling

Phosphino hydrazones derived from C2-symmetric hydrazines exhibit excellent catalytic activity
and provide good enantioselectivities in the asymmetric Suzuki–Miyaura
cross-coupling to axially chiral biaryls, in particular for the most
challenging reactions of monocyclic, functionalized aryl bromides
and triflates. X-ray analysis of preformed [Pd­(P/N)­Cl2]
precatalysts [(P/N) = phosphino hydrazone] revealed a strong n–π conjugation in the hydrazone moiety, identified
by a high planarity degree at the pyrrolidine N­(sp3) atom,
that makes rotations around N–N bonds inconsequential. The
complexes are also characterized by an envelope-like conformation
with the Pd atom placed at the opposite side to the 2-phenyl group
on the nearest stereogenic center of the pyrrolidine group. The isolation
and structural analysis of oxidative addition intermediates indicate
that the configurational stability of Pd–C­(Ar) bonds is dependent
on the substitution pattern in the aryl bromide.