Asymmetric Transfer Hydrogenation of Aryl Heteroaryl Ketones using Noyori‐Ikariya Catalysts

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.

Asymmetric Transfer Hydrogenation of Cyclobutenediones

Four-membered carbocycles are fundamental substructures in bioactive molecules and approved drugs and serve as irreplaceable building blocks in organic synthesis. However, developing efficient protocols furnishing diversified four-membered ring compounds in a highly regio-, diastereo-, and enantioselective fashion remains challenging but very desirable. Here, we report the unprecedented asymmetric transfer hydrogenation of cyclobutenediones. The reaction can selectively afford three types of four-membered products in high yields with high stereoselectivities, and the highly functionalized products enable a series of further transformations to form more diversified four-membered compounds. Asymmetric synthesis of di-, tri-, and tetrasubstituted bioactive molecules has also been achieved. Systematic mechanistic studies and theoretical calculations have revealed the origin of the regioselectivity, the key hydrogenation transition state models, and the sequence of the double and triple hydrogenation processes. The work provides a new choice for the catalytic asymmetric synthesis of cyclobutanes and related structures and demonstrates the robustness of asymmetric transfer hydrogenation in the accurate selectivity control of highly functionalized substrates.

A general synthesis of aromatic and heteroaromatic lipoxin B4 analogues

Lipoxins are an important class of pro-resolving mediators that play a crucial role in the resolution of inflammation. Thus, the synthesis of more chemically and metabolically stable synthetic lipoxin analogues is an area of significant interest. Whereas synthetic analogues of lipoxin A4 (LXA4) have been well studied, analogues of lipoxin B4 (LXB4) have been the focus of considerably less attention. Herein we report the asymmetric synthesis of a focused library of LXB4 mimetics in which the triene core of the molecule has been replaced with different aromatic and heteroaromatic rings. The synthesis of each of these analogues was achieved by a general strategy in which the key steps were a Suzuki cross coupling between a common upper chain fragment and an aromatic lower chain, followed by a stereoselective ketone reduction.

Highly Enantioselective Iridium-Catalyzed Hydrogenation of o-Amidophenyl Ketones Enabled by 1,2-Diphenylethylenediamine-Derived P,N,N-Ligands with Tertiary Amine Terminus

A readily available and highly modular class of chiral P,N,N-ligands based on a structurally flexible nonchiral phosphine-amine framework with an optically active 1,2-diphenylethylenediamine unit bearing a tertiary amine terminus as the chiral source have been developed and successfully applied in the Ir-catalyzed asymmetric hydrogenation of o-amidophenyl ketones. These tridentate P,N,N-ligands exhibited excellent activity, enantioselectivity, and substrate tolerance, thus furnishing various optically active o-amidobenzhydrols in up to 99% yields and with >99% ee. The utility of this protocol has been proven by synthetically diverse product transformation and highly enantioselective production of a rice plant growth regulator, (S)-inabenfide.

Synthesis of N-alkoxy amines and hydroxylamines via the iridium-catalyzed transfer hydrogenation of oximes

Cationic iridium (Ir) complexes were found to catalyze the transfer hydrogenation of oximes to access N-alkoxy amines and hydroxylamines, and the reaction was accelerated by trifluoroacetic acid. The practical application of this protocol was demonstrated by a gram-scale transformation and two-step synthesis of the fungicide furmecyclox (BAS 389F) in overall yields of 92 and 85%, respectively. An asymmetric protocol using chiral Ir complexes to afford chiral N-alkoxy amines was demonstrated, but the low yields/ee obtained indicated that further development was required.

Asymmetric transfer hydrogenation of boronic acid pinacol ester (Bpin)-containing acetophenones

A series of Bpin-containing acetophenone derivatives were reduced by asymmetric transfer hydrogenation (ATH), using Noyori-Ikariya catalysts, with formic acid/triethylamine, to alcohols in high ee when the Bpin is in the para- or meta-position. Substrates containing ortho-Bpin groups were reduced in lower ee, with formation of a cyclic boron-containing group. The products were converted to substituted derivatives using Pd-catalysed coupling reactions. The results represent the first examples of ATH of Bpin-containing ketones.

Does the Configuration at the Metal Matter in Noyori-Ikariya Type Asymmetric Transfer Hydrogenation Catalysts?

Noyori–Ikariya type [(arene)RuCl(TsDPEN)] (TsDPEN, sulfonated diphenyl ethylenediamine) complexes are widely used C=O and C=N reduction catalysts that produce chiral alcohols and amines via a key ruthenium–hydride intermediate that determines the stereochemistry of the product. Whereas many details about the interactions of the pro-chiral substrate with the hydride complex and the nature of the hydrogen transfer from the latter to the former have been investigated over the past 25 years, the role of the stereochemical configuration at the stereogenic ruthenium center in the catalysis has not been elucidated so far. Using operando FlowNMR spectroscopy and nuclear Overhauser effect spectroscopy, we show the existence of two diastereomeric hydride complexes under reaction conditions, assign their absolute configurations in solution, and monitor their interconversion during transfer hydrogenation catalysis. Configurational analysis and multifunctional density functional theory (DFT) calculations show the λ-(R,R)S^Ru configured [(mesitylene)RuH(TsDPEN)] complex to be both thermodynamically and kinetically favored over its λ-(R,R)R^Ru isomer with the opposite configuration at the metal. Computational analysis of both diastereomeric catalytic manifolds show the major λ-(R,R)S^Ru configured [(mesitylene)RuH(TsDPEN)] complex to dominate asymmetric ketone reduction catalysis with the minor λ-(R,R)R^Ru [(mesitylene)RuH(TsDPEN)] stereoisomer being both less active and less enantioselective. These findings also hold true for a tethered catalyst derivative with a propyl linker between the arene and TsDPEN ligands and thus show enantioselective transfer hydrogenation catalysis with Noyori–Ikariya complexes to proceed via a lock-and-key mechanism.