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

Phosphino hydrazones derived from C-2-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)Cl-2] precatalysts [(P/N) = phosphino hydrazone] revealed a strong n-pi conjugation in the hydrazone moiety, identified by a high planarity degree at the pyrrolidine N(sp(3)) 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.