Highly Efficient Kinetic Resolution of Aryl-Alkenyl Alcohols by Ru-Catalyzed Hydrogen Transfer

No matter through asymmetric reduction of ketones or kinetic resolution of secondary alcohols, enantioselective synthesis of the corresponding secondary alcohols is challenging when the two groups attached to the prochiral or chiral centers are spatially or electronically similar. For examples, dialkyl (sp3 vs. sp3), diaryl (sp2 vs. sp2), and aryl-alkenyl (sp2 vs. sp2) alcohols are difficult to produce with high enantioselectivities. By exploiting our recently developed Ru-catalysts of minimal stereogenicity, we reported herein a highly efficient kinetic resolution of aryl-alkenyl alcohols through hydrogen transfer. This method enabled such versatile chiral building blocks for organic synthesis as allylic alcohols, to be readily accessed with excellent enantiomeric excesses at practically useful conversions.

Chiral Aminoalcohols and Squaric Acid Amides as Ligands for Asymmetric Borane Reduction of Ketones: Insight to In Situ Formed Catalytic System by DOSY and Multinuclear NMR Experiments

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 BH3•SMe2. In all cases, the aminoalcohols demonstrated better efficiency (up to 94% ee), while only poor asymmetric induction was achieved with the corresponding squaramides. 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. 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.

Insights Into The Origin of Selectivity For [2+2] Cycloaddition Step Reaction Involved In The Mechanism of Enantioselective Reduction of Ketones With Borane Catalyzed By A B-Methoxy Oxazaborolidine Catalyst Derived From (–)-β-Pinene. A DFT And Combined Topological ELF, NCI And QTAIM Study

Theoretical studies on [2+2] cycloaddition step involved in the Enantioselective Reduction of ketones with borane catalyzed by a B-MethoxyOxazaborolidine Catalyst Derived from Pinene has been performed by means of the Density Functional Theory method (DFT) at MPWB1K /6-31G (d,p). The formation of the M5a(S) complexes via transition state TSa(S) was the more favorable pathway among other [2+2]cycloaddition competing steps. The explanation of the formation of O-B and N-B through two-stage one-step mechanism was allowed by means of the electron localization function (ELF) topological analysis. NCI and QTAIM analysis of the two computed transition states TSa(S) and TSa(R) indicate that the difference between both in term of stability comes mainly from the orientation of the methanediyl group inside the pinene skeleton, which implies that CH-H…O interaction found at TSa(S) is the great factor that makes it more stable than TSa(R).

1,3,4-oxadiazole-functionalized α-aminophosphonates as ligands for the ruthenium-catalyzed reduction of ketones

Three α-aminophosphonates, namely diethyl[(5-phenyl-1,3,4-oxadiazol-2-ylamino)(4-trifluoromethylphenyl) methyl]phosphonate (3a), diethyl[(5-phenyl-1,3,4-oxadiazol-2-ylamino)(2-methoxyphenyl)methyl]phosphonate (3b) and diethyl[(5-phenyl-1,3,4-oxadiazol-2-ylamino)(4-nitrophenyl)methyl]phosphonate (3c), were synthetized via the Pudovik-type reaction between diethyl phosphite and imines, obtained from 5-phenyl-1,2,4-oxadiazol-2-amine and aromatic aldehydes, under microwave...