3,3‘-Diphosphoryl-1,1‘-bi-2-naphthol−Zn(II) Complexes as Conjugate Acid−Base Catalysts for Enantioselective Dialkylzinc Addition to Aldehydes

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Abstract
A highly enantioselective dialkylzinc (R22Zn) addition to a series of aromatic, aliphatic, and heteroaromatic aldehydes (5) was developed based on conjugate Lewis acid−Lewis base catalysis. Bifunctional BINOL ligands bearing phosphine oxides [P(O)R2] (7), phosphonates [P(O)(OR)2] (8 and 9), or phosphoramides [P(O)(NR2)2] (10) at the 3,3‘-positions were prepared by using a phospho-Fries rearrangement as a key step. The coordination of a NaphO−Zn(II)−R2 center as a Lewis acid to a carbonyl group in a substrate and the activation of R22Zn(II) with a phosphoryl group (PO) as a Lewis base in the 3,3‘-diphosphoryl-BINOL−Zn(II) catalyst could promote carbon−carbon bond formation with high enantioselectivities (up to >99% ee). Mechanistic studies were performed by X-ray analyses of a free ligand (7) and a tetranuclear Zn(II) cluster (21), a 31P NMR experiment on Zn(II) complexes, an absence of nonlinear effect between the ligand (7) and Et-adduct of benzaldehyde, and stoichiometric reactions with some chiral or achiral Zn(II) complexes to propose a transition-state assembly including monomeric active intermediates.

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