Asymmetric Transfer Hydrogenation of 1,3-Alkoxy/Aryloxy Propanones Using Tethered Arene/Ru(II)/TsDPEN Complexes

Catalytic Asymmetric Transfer Hydrogenation of Acylboronates: BMIDA as the Privileged Directing Group

Developing a general, highly efficient, and enantioselective catalytic method for the synthesis of chiral alcohols is still a formidable challenge. We report in this article the asymmetric transfer hydrogenation (ATH) of N-methyliminodiacetyl (MIDA) acylboronates as a general substrate-independent entry to enantioenriched secondary alcohols. ATH of acyl-MIDA-boronates with (het)aryl, alkyl, alkynyl, alkenyl, and carbonyl substituents delivers a variety of enantioenriched α-boryl alcohols. The latter are used in a range of stereospecific transformations based on the boron moiety, enabling the synthesis of carbinols with two closely related α-substituents, which cannot be obtained with high enantioselectivities using direct asymmetric hydrogenation methods, such as the (R)-cloperastine intermediate. Computational studies illustrate that the BMIDA group is a privileged enantioselectivity-directing group in Noyori-Ikariya ATH compared to the conventionally used aryl and alkynyl groups due to the favorable CH-O attractive electrostatic interaction between the η6-arene-CH of the catalyst and the σ-bonded oxygen atoms in BMIDA. The work expands the domain of conventional ATH and shows its huge potential in addressing challenges in symmetric synthesis.

Iridium-catalyzed asymmetric transfer hydrogenation of aromatic ketones with a cinchona alkaloid derived NNP ligand

An iridium complex generated in situ from [Ir(COD)Cl]₂ and a cinchona alkaloid derived NNP ligand has been developed for the asymmetric transfer hydrogenation of aromatic ketones. In this study, 30 aromatic ketones and heteroaryl ketones were hydrogenated to produce valuable chiral alcohols with up to 99% ee using i-PrOH as the hydrogen source and the solvent. The easily prepared Ir(L8)(COD)Cl also exhibited excellent activity and enantioselectivity in asymmetric transfer hydrogenation of aromatic ketones with a high S/C ratio (up to 2000).

Chiral Vicinal Diamines Derived from Mefloquine

Novel 1,2-diamines based on the mefloquine scaffold prepared in enantiomerically pure forms resemble 9-amino-Cinchona alkaloids. Most effectively, 11-aminomefloquine with an erythro configuration was obtained by conversion of 11-alcohol into azide and hydrogenation. Alkylation of a secondary amine unit was needed to arrive at diastereomeric threo-11-aminomefloquine and to introduce diversity. Most of the substitution reactions of the hydroxyl group to azido group proceeded with net retention of the configuration and involved actual aziridine or plausible aziridinium ion intermediates. Enantiomerically pure products were obtained by the resolution of either the initial mefloquine or one of the final products. The evaluation of the efficacy of the obtained vicinal diamines in enantioselective transformations proved that erythro-11-aminomefloquine is an effective catalyst in the asymmetric Michael addition of nitromethane to cyclohexanone (up to 96.5:3.5 er) surpassing epi-aminoquinine in terms of selectivity.

Ru-TsDPEN catalysts and derivatives in asymmetric transfer hydrogenation reactions

This review summarizes current developments, novel synthetic routes for Ruthenium tethered chiral catalyst, and its derivatives along with its application in asymmetric synthesis. The review also covers derivatization in tethering unit, modification in N-monofunctionalized ligand as well as ligation of other ligand with Ru metal in chiral catalyst. Apparently, the effect of a modified tethered catalyst in the enantioselective synthesis of chiral products as well as in synthetic chemistry is also discussed in detail.

Sulfone Group as a Versatile and Removable Directing Group for Asymmetric Transfer Hydrogenation of Ketones

The sulfone functional group has a strong capacity to direct the asymmetric transfer hydrogenation (ATH) of ketones in the presence of [(arene)Ru(TsDPEN)H] complexes by adopting a position distal to the η6 -arene ring. This preference provides a means for the prediction of the sense of asymmetric reduction. The sulfone group also facilitates the formation of a range of reduction substrates, and its ready removal provides a route to enantiomerically enriched alcohols that would otherwise be extremely difficult to prepare by direct ATH of the corresponding ketones.

Rh(iii)-Catalyzed diastereoselective transfer hydrogenation: an efficient entry to key intermediates of HIV protease inhibitors

A highly efficient diastereoselective transfer hydrogenation of α-aminoalkyl α′-chloromethyl ketones catalyzed by a tethered rhodium complex was developed and successfully utilized in the synthesis of the key intermediates of HIV protease inhibitors.

Stereogenic cis-2-substituted-N-acetyl-3-hydroxy-indolines via ruthenium(ii)-catalyzed dynamic kinetic resolution-asymmetric transfer hydrogenation

Ruthenium(ii)-catalyzed dynamic kinetic resolution-asymmetric transfer hydrogenation of racemic 2-substituted-N-acetyl-3-oxoindolines to cis-2-substituted-N-acetyl-3-hydroxyindolines is reported. Using the homochiral {Ru[TfDPEN](p-cymene)} catalyst with S/C = 400 in a HCO2H/Et3N mixture, up to >99.9% ee and >99 : 1 dr are obtained with high yields (79-98%). This method provides the first example of preparing enantiomerically pure indolines through asymmetric transfer hydrogenation (ATH).

Accessible Bifunctional Oxy-Tethered Ruthenium(II) Catalysts for Asymmetric Transfer Hydrogenation

A concise synthesis of new oxy-tethered ruthenium complexes effective for the asymmetric transfer hydrogenation of aromatic ketones is described. The oxy-tether was constructed via a defluorinative etherification arising from an intramolecular nucleophilic substitution of a perfluorinated phenylsulfonyl substituent. The obtained tethered complexes exhibited desirable catalytic activity and selectivity, especially in the asymmetric transfer hydrogenation of functionalized aromatic ketones. The robustness and rigidity of the tether contribute to their superior catalytic performance relative to the nontethered prototype complex.

Highly enantioselective transfer hydrogenation of racemic α-substituted β-keto sulfonamides via dynamic kinetic resolution

An efficient enantioselective transfer hydrogenation of racemic α-substituted β-keto sulfonamides via dynamic kinetic resolution has been achieved, affording α-substituted β-hydroxyl sulfonamides in good yields and excellent diastereo- and enantioselectivities.