Remarkable success in this reaction has been achieved using chiral oxazaborolidines prepared from (semi)synthetic aminoalcohols derived from natural amino acids, terpenoids, etc. Among the various well-known approaches, borane reduction has been demonstrated to allow fine-tuning for attaining a higher degree of selectivity. These results contribute to better understanding the complexity of the processes occurring in the reaction mixture prior to the possible oxazaborolidine formation, which play a crucial role on the degree of enantioselectivity achieved in the borane reduction of α-chloroacetophenone.Īsymmetric reduction of ketones still attracts significant interest since it is one of the most efficient ways of generating enantiomerically pure secondary alcohols as important intermediates or chiral building blocks for the synthesis of biologically relevant structures.
A mechanistic insight on the in situ formation and stability at room temperature of intermediates generated from ligands and borane as possible precursors of the oxazaborolidine-based catalytic system has been obtained by 1H DOSY and multinuclear 1D and 2D ( 1H, 10/11B, 13C, 15N) NMR spectroscopy of equimolar mixtures of borane and selected ligands. In all cases, the aminoalcohols demonstrated better efficiency (up to 94% ee), while only poor asymmetric induction was achieved with the corresponding squaramides. A series of squaric acid amides (synthesized in 66–99% isolated yields) and a set of chiral aminoalcohols were comparatively studied as ligands in a model reaction of reduction of α-chloroacetophenone with BH 3
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