Enantioselective Hydroboration of Ketones Catalyzed by Rare-Earth-Metal Complexes Supported with Phenoxy-Functionalized TsDPEN Ligands

Abstract

Six novel chiral rare-earth-metal complexes bearing the phenoxy-functionalized TsDPEN ligand H3L1 (H3L1 = N-((1R,2R)-2-((3,5-di-tert-butyl-2-hydroxybenzyl)amino)-1,2-diphenylethyl)-4-methylbenzenesulfonamide) were synthesized successfully and well characterized. The solid-state structures of four tetranuclear rare-earth-metal complexes [RE2L13]2 (RE = Nd (1), Sm (2), Eu (3), Gd (4)) and the dual-core yttrium complex Y2L13 (5) were determined by X-ray diffraction, respectively. The structure of lanthanum complex 6 was speculated by the 1H DOSY spectroscopy in THF-d8 together with DFT calculations. Complexes 1–5 were employed to catalyze the enantioselective hydroboration of ketones and α,β-unsaturated ketones using pinacolborane (HBpin) as a reductant, and complex 1 gave better outcomes in comparison to the others. The corresponding secondary alcohols were obtained in excellent yields and moderate ee values. The same results were also achieved using the combined catalyst system of the neodymium amide Nd[N(SiMe3)2]3 with the phenoxy-functionalized TsDPEN ligand H3L1 in a 1:1.5 molar ratio.