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

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journal contribution
posted 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.