Phosphino Hydrazones as Suitable Ligands in the Asymmetric Suzuki–Miyaura Cross-Coupling
journal contributionposted on 18.05.2012, 00:00 by Abel Ros, Beatriz Estepa, Antonio Bermejo, Eleuterio Álvarez, Rosario Fernández, José M. Lassaletta
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.