The chiral boronate-catalyzed asymmetric transfer hydrogenation of various aromatic ketones to high-value alcohols: Preparation and spectroscopic studies

Abstract

This work deals with the synthesis, spectroscopic studies and catalytic evaluation of the novel chiral salen (L1H2) and (L2H2) ligands and their chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes. Initially, the reaction of 5-azidomethyl salicylaldehyde and (R)-(−)-2-amino-1-butanol in absolute ethanol afforded a new chiral salen ligand (L1H2). Then, a novel chiral salen ligand (L2H2) have been prepared from chiral salen ligand (L1H2) for the synthesis of boronate [L2(B1-4)] complexes through click reaction approach under ambient conditions. The reaction of chiral salen (L1H2) and (L2H2) ligands with various boronic acids afforded a new tetra-coordinated mononuclear chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes. All the compounds are remarkably stable crystalline solids and were obtained in good yields. For the full characterization of newly synthesized chiral salen ligands and their boronate complexes, the FT-IR, UV–Vis, NMR (1H, 13C, and 11B), LC-MS, and elemental analysis techniques have been used. The well-shaped chiral boronate compounds were investigated as catalyst for the asymmetric transfer hydrogenation (ATH) of aromatic ketones under appropriate settings. Particularly, it was proved that the ferrocene-based boronate compounds can afford an efficient catalytic conversion compared to the other boronate complexes in the asymmetric transfer hydrogenation catalytic studies.