A new class of highly enantioselective oxazaborolidine-catalyzed reductions of achiral ketones is reported which depends on stereoelectronic effects involving p-substituted non-planar aromatic ketones, or pi-coordinated transition-metal containing ketones, or strained ring ketones, as exemplified in Table 1. The discovery of these reactions was guided by the transition-state model 1, for which they provide experimental support. Because high enantioselectivities (> 30:1) are achievable, these reductions define an excellent method for the synthesis of for example, chiral benzhydrols, chiral propargylic, or chiral allylic alcohol. Lower enantioselectivities observed with CH2Cl2 as solvent, relative to toluene as solvent, are consistent with the transition-state model I and indicate that CH2Cl2 hydrogen bonds to the donor groups in the pi-electron-rich carbonyl substituent (R(L) in 1) thereby diminishing electron supply.
Efficient Access to Chiral Benzhydrols via Asymmetric Transfer Hydrogenation of Unsymmetrical Benzophenones with Bifunctional Oxo-Tethered Ruthenium Catalysts
A concise asymmetric transfer hydrogenation of diaryl ketones, promoted by bifunctional Ru complexes with an etherial linkage between 1,2-diphenylethylenediamine (DPEN) and η(6)-arene ligands, was successfully developed. Because of the effective discrimination of substituents at the ortho position on the aryl group, unsymmetrical benzophenones were smoothly reduced in a 5:2 mixture of formic acid and
Chiral C₂-symmetric diamines have emerged as versatile auxiliaries or ligands in numerous asymmetric transformations. Chiral 2,2'-bispyrrolidine-based salan ligands were prepared and applied to the asymmetric aryltransfer to aldehydes with arylboronic acids as the source of transferablearyl groups. The corresponding diarylmethanols were obtained in high yields with moderate to good enantioselectivitives
手性 C 2 对称二胺已成为众多不对称转化中的通用助剂或配体。制备了基于手性 2,2'-双吡咯烷的 Salan 配体,并将其应用于以芳基硼酸作为可转移芳基来源的醛的不对称芳基转移。相应的二芳基甲醇以高产率获得,对映选择性高达 83% ee。