Enantioselective Metal-Free Hydrogenations of Disubstituted Quinolines

Stereocontrol via Propeller Chirality in FLP-Catalyzed Asymmetric Hydrogenation

Utilization of chiral frustrated Lewis pairs as catalysts in enantioselective hydrogenation of unsaturated molecules represents a promising approach in asymmetric synthesis. In our effort to improve our current understanding of the factors governing the stereoselectivity in these catalytic processes, herein we examined the mechanism of direct hydrogenation of aromatic enamines catalyzed by a binaphthyl-based chiral amino-borane. Our computational analysis reveals that only one particular conformer of the key borohydride reaction intermediate can be regarded as a reactive form of this species. This borohydride conformer has a well-defined chiral propeller shape, which induces facial selectivity in the hydride transfer to pro-chiral iminium intermediates. The propeller chirality of the reactive borohydride conformer is generated by the axially chiral binaphthyl scaffold of the amino-borane catalyst through stabilizing π-π stacking interactions. This new computational insight can be readily used to interpret the high degree of stereoinduction observed for these reactions. We expect that the concept of chirality relay could be further exploited in catalyst design endeavors.

Recent Advances in Asymmetric Catalysis Using p-Block Elements

The development of new methods for enantioselective reactions that generate stereogenic centres within molecules are a cornerstone of organic synthesis. Typically, metal catalysts bearing chiral ligands as well as chiral organocatalysts have been employed for the enantioselective synthesis of organic compounds. In this review, we highlight the recent advances in main group catalysis for enantioselective reactions using the p-block elements (boron, aluminium, phosphorus, bismuth) as a complementary and sustainable approach to generate chiral molecules. Several of these catalysts benefit in terms of high abundance, low toxicity, high selectivity, and excellent reactivity. This minireview summarises the utilisation of chiral p-block element catalysts for asymmetric reactions to generate value-added compounds.

Asymmetric catalysis with FLPs

Chiral catalysts play a crucial role in the realm of asymmetric catalysis. Since their breakthrough discovery in 2006, chiral frustrated Lewis pairs (FLPs) have risen as a novel catalyst category for a broad range of metal-free asymmetric reactions. This review provides an overview of the remarkable progress made in this field over the past 15 years. The design and synthesis of chiral FLPs and their applications in hydrogenation, hydrosilylation, transfer hydrogenation, and various other reactions are summarized and highlighted.

Asymmetric Reduction of Quinolines: A Competition between Enantioselective Transfer Hydrogenation and Racemic Borane Catalysis

A chiral phosphoric acid catalyzed asymmetric transfer hydrogenation of quinolines with regenerable dihydrophenanthridine derived by a borane-catalyzed hydrogenation of phenanthridine under H₂ has been successfully realized. Despite the competition of a racemic hydrogenation pathway, a variety of tetrahydroquinolines were furnished in high yields with up to 91% ee.

Chiral Frustrated Lewis Pair@Metal-Organic Framework as a New Platform for Heterogeneous Asymmetric Hydrogenation

Asymmetric hydrogenation, a seminal strategy for the synthesis of chiral molecules, remains largely unmet in terms of activation by non-metal sites of heterogeneous catalysts. Herein, as demonstrated by combined computational and experimental studies, we present a general strategy for integrating rationally designed molecular chiral frustrated Lewis pair (CFLP) with porous metal-organic framework (MOF) to construct the catalyst CFLP@MOF that can efficiently promote the asymmetric hydrogenation in a heterogeneous manner, which for the first time extends the concept of chiral frustrated Lewis pair from homogeneous system to heterogeneous catalysis. Significantly, the developed CFLP@MOF, inherits the merits of both homogeneous and heterogeneous catalysts, with high activity/enantio-selectivity and excellent recyclability/regenerability. Our work not only advances CFLP@MOF as a new platform for heterogeneous asymmetric hydrogenation, but also opens a new avenue for the design and preparation of advanced catalysts for asymmetric catalysis.

Asymmetric hydrogenation of TIPS-protected oximes with chiral boranes

An enantioselective metal-free hydrogenation of TIPS-protected oximes has been successfully realized for the first time by using chiral borane catalysts derived from chiral dienes and Piers' borane. A variety of hydroxylamine derivatives were afforded in 84-99% yields with 33-68% ees.

Towards the Development of Frustrated Lewis Pair (FLP) Catalyzed Hydrogenations of Tertiary and Secondary Carboxylic Amides

The development of the frustrated Lewis pair catalyzed hydrogenation of tertiary and secondary amides is reviewed. Detailed insight into our strategies in order to overcome challenges during the reaction development process is provided. Furthermore, the developed chemistry is extended to the hydrogenation of polyamides and of trifluoroacetamides for the convenient introduction of trifluoroethyl groups into organic molecules.

Chiral FLP-catalyzed asymmetric hydrogenation of 3-fluorinated chromones

The asymmetric hydrogenation of fluorinated olefins is an efficient pathway towards the synthesis of chiral fluorine-containing compounds. This paper described metal-free asymmetric hydrogenation of 3-fluorinated chromones with the use of readily available achiral borane and chiral oxazoline as an FLP catalyst for the first time. A variety of optically active 3-fluorochroman-4-ones were obtained in high yields with up to 88% ee.

Recent Developments in Enantio- and Diastereoselective Hydrogenation of N-Heteroaromatic Compounds

The enantioselective and diastereoselective hydrogenation of N-heteroaromatic compounds is an efficient strategy to access chirally enriched cyclic heterocycles, which often possess highly bio-active properties. This strategy, however, has only been...

Iridium-Catalyzed Asymmetric Transfer Hydrogenation of Quinolines in Biphasic Systems or Water

An asymmetric transfer hydrogenation (ATH) of quinolines in water or biphasic systems was developed. This ATH reaction proceeds smoothly without the need for inert atmosphere protection in the presence of a water-soluble iridium catalyst, which bears an easily available aminobenzimidazole ligand. This ATH system can work at a catalyst loading of 0.001 mol % (S/C = 100 000, turnover number (TON) of up to 33 000) under mild reaction conditions. The turnover frequency (TOF) value can reach as high as 90 000 h-1. A variety of quinoline and N-heteroaryl compounds are transformed into the desired products in high yield and up to 99% enantiomeric excess (ee).

Iridium-Catalyzed Transfer Hydrogenation for Construction of Quinolines from 2-Aminobenzyl Alcohols with Enones in Water

Herein, we describe a method for the synthesis of functionalized quinolines from 2-aminobenzyl alcohols with α,β-unsaturated ketones. This method exhibits tolerance to various functional groups and high efficiency, is environmentally benign, and can be performed on a gram scale. Control experiments suggest that this transformation is accomplished by iridium complex catalyzed transfer hydrogenation, which is then followed by Friedländer cyclization. The results display that alkali is essential for the high selectivities of this catalytic system.

Relay Catalysis by Achiral Borane and Chiral Phosphoric Acid in the Metal-Free Asymmetric Hydrogenation of Chromones

A strategy of relay catalysis by achiral borane and chiral phosphoric acid was successfully developed for the asymmetric hydrogenation of chromones, giving the desired products in high yields with up to 95% ee. Achiral borane and chiral phosphoric acid are highly compatible in this reaction. The achiral borane acts as a Lewis acid for the first-step hydrogenation, and the chiral phosphoric acid acts as an effective chiral proton shuttle to control the enantioselectivity.

Asymmetric Synthesis of Hydroquinolines with α,α-Disubstitution through Organocatalyzed Kinetic Resolution

The first kinetic resolution of hydroquinoline derivatives with α,α-disubstitution has been achieved through asymmetric remote aminations with azodicarboxylates enabled by chiral phosphoric acid catalysis. Mechanistic studies suggest a monomeric catalyst pathway proceeding through rate- and enantio-determining electrophilic attack promoted by a network of attractive non-covalent interactions between the substrate and catalyst. Facile subsequent removal and transformations of the newly introduced hydrazine moiety enable these protocols to serve as powerful tools for asymmetric synthesis of N-heterocycles with α,α-disubstitution.

Recent Advances in Homogeneous Catalysts for the Asymmetric Hydrogenation of Heteroarenes

The asymmetric hydrogenation of heteroarenes has recently emerged as an effective strategy for the direct access to enantioenriched, saturated heterocycles. Although several homogeneous catalyst systems have been extensively developed for the hydrogenation of heteroarenes with high levels of chemo- and stereoselectivity, the development of mild conditions that allow for efficient and stereoselective hydrogenation of a broad range of substrates remains a challenge. This Perspective highlights recent advances in homogeneous catalysis of heteroarene hydrogenation as inspiration for the further development of asymmetric hydrogenation catalysts, and addresses underdeveloped areas and limitations of the current technology.

Asymmetric Hydrogenation of Ketones and Enones with Chiral Lewis Base Derived Frustrated Lewis Pairs

The concept of frustrated Lewis pairs (FLPs) has been widely applied in various research areas, and metal-free hydrogenation undoubtedly belongs to the most significant and successful ones. In the past decade, great efforts have been devoted to the synthesis of chiral boron Lewis acids. In a sharp contrast, chiral Lewis base derived FLPs have rarely been disclosed for the asymmetric hydrogenation. In this work, a novel type of chiral FLP was developed by simple combination of chiral oxazoline Lewis bases with achiral boron Lewis acids, thus providing a promising new direction for the development of chiral FLPs in the future. These chiral FLPs proved to be highly effective for the asymmetric hydrogenation of ketones, enones, and chromones, giving the corresponding products in high yields with up to 95 % ee. Mechanistic studies suggest that the hydrogen transfer to simple ketones likely proceeds in a concerted manner.

Highly Diastereo- and Enantioselective Ir-Catalyzed Hydrogenation of 2,3-Disubstituted Quinolines with Structurally Fine-Tuned Phosphine-Phosphoramidite Ligands

A highly diastereo- and enantioselective Ir-catalyzed hydrogenation of unfunctionalized 2,3-disubstituted quinolines, especially 3-alkyl-2-arylquinolines, has been realized. The success of this hydrogenation is ascribed to the use of a structurally fine-tuned chiral phosphine-phosphoramidite ligand with a (S_a)-3,3'-dimethyl H₈-naphthyl moiety and (R_c)-1-phenylethylamine backbone. The hydrogenation displayed broad functional group tolerance, thus furnishing a wide range of optically active 2,3-disubstituted tetrahydroquinolines in up to 96% ee and with perfect cis-diastereoselectivity.

Hydrogenation of N-Heteroarenes Using Rhodium Precatalysts: Reductive Elimination Leads to Formation of Multimetallic Clusters

A rhodium-catalyzed method for the hydrogenation of N-heteroarenes is described. A diverse array of unsubstituted N-heteroarenes including pyridine, pyrrole, and pyrazine, traditionally challenging substrates for hydrogenation, were successfully hydrogenated using the organometallic precatalysts, [(η⁵-C₅Me₅)Rh(N-C)H] (N-C = 2-phenylpyridinyl (ppy) or benzo[h]quinolinyl (bq)). In addition, the hydrogenation of polyaromatic N-heteroarenes exhibited uncommon chemoselectivity. Studies into catalyst activation revealed that photochemical or thermal activation of [(η⁵-C₅Me₅)Rh(bq)H] induced C(sp²)-H reductive elimination and generated the bimetallic complex, [(η⁵-C₅Me₅)Rh(μ₂,η²-bq)Rh(η⁵-C₅Me₅)H]. In the presence of H₂, both of the [(η⁵-C₅Me₅)Rh(N-C)H] precursors and [(η⁵-C₅Me₅)Rh(μ₂,η²-bq)Rh(η⁵-C₅Me₅)H] converted to a pentametallic rhodium hydride cluster, [(η⁵-C₅Me₅)₄Rh₅H₇], the structure of which was established by NMR spectroscopy, X-ray diffraction, and neutron diffraction. Kinetic studies on pyridine hydrogenation were conducted with each of the isolated rhodium complexes to identify catalytically relevant species. The data are most consistent with hydrogenation catalysis prompted by an unobserved multimetallic cluster with formation of [(η⁵-C₅Me₅)₄Rh₅H₇] serving as a deactivation pathway.

Unmasking the Ligand Effect in Manganese-Catalyzed Hydrogenation: Mechanistic Insight and Catalytic Application

Manganese-catalyzed hydrogenation reactions have attracted broad interest since the first report in 2016. Among the reported catalytic systems, Mn catalysts supported by tridentate PNP- and NNP-pincer ligands have most commonly been used. For example, a number of PNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, aldimines, ketones, nitriles, and esters. Furthermore, various NNP-Mn pincer catalysts have been shown to be active in the hydrogenation of less-reactive substrates such as amides, carbonates, carbamates, and urea derivations. These observations indicated that Mn catalysts supported by NNP-pincer ligands exhibit higher reactivity in hydrogenation reactions than their PNP counterparts. Such a ligand effect in Mn-catalyzed hydrogenation reactions has yet to be confirmed. Herein, we investigated the origin and applicability of this ligand effect. A combination of experimental and theoretical investigations showed that NNP-pincer ligands on the Mn complexes were more electron-rich and less sterically hindered than their PNP counterparts, leading to higher reactivity in a series of Mn-catalyzed hydrogenation reactions. Inspired by the ligand effect on Mn-catalyzed hydrogenations, we developed the first Mn-catalyzed hydrogenation of N-heterocycles. Specifically, NNP-Mn pincer catalysts hydrogenated a series of N-heterocycles (32 examples) with up to 99% yields, and the corresponding PNP-Mn pincer catalysts afforded low reactivity under the same conditions. This verified that such a ligand effect is generally applicable in hydrogenation reactions of both carbonyl and noncarbonyl substrates based on Mn catalysis.

B(C6F5)3-Catalyzed Highly Stereoselective Hydrogenation of Unfunctionalized Tetrasubstituted Olefins

A metal-free hydrogenation of unfunctionalized tetrasubstituted olefins were successfully realized using a combination of B(C₆F₅)₃ and Ph₂NMe catalyst. The corresponding products were afforded in 58-98% yields with up to >99:1 cis/trans selectivity.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

FLP catalysis: main group hydrogenations of organic unsaturated substrates

This article is focused on recent developments in main group mediated hydrogenation chemistry and catalysis using "frustrated Lewis pairs" (FLPs). The broading range of substrates and catalyst systems is reviewed and the advances in catalytic reductions and the development of stereoselective, asymmetric reductions made since 2012 is considered.

Design considerations for chiral frustrated Lewis pairs: B/N FLPs derived from 3,5-bicyclic aryl piperidines

Herein, 3,5-bicyclic aryl piperidines are derivatized to generate chiral B/N FLPs. Initially, the twofold symmetric amine C6H2F2(C5H8NiPr) 1 was converted in a series of synthetic steps to the styrene-derivative C6HF2(C5H8NiPr)(CH[double bond, length as m-dash]CH2) 4. Efforts to hydroborate the vinyl fragment proved challenging as a result of the strongly basic nitrogen, although the species C6HF2(C5H8N(H)iPr)(CH2CH2B(OH)(C6F5)2) 5 was crystallographically characterized. Modification of the system was achieved by conversion of the amine C6H2F2(C5H8NH) 6 to C6HF2(C5H8NPh)(CH[double bond, length as m-dash]CH2) 9. Hydroboration of 9 with 9-BBN or HB(C6F5)2 gave C6HF2(C5H8NPh)(CH2CH2BBN) 10 or C6HF2(C5H8NPh)(CH2CH2B(C6F5)2) 11, respectively. The latter species was derivatized by complexation of PPh3 to give C6HF2(C5H8NPh)(CH2CH2B(C6F5)2)(PPh3) 12. The Lewis acidities of 10 and 11 were assessed by the Gutman-Beckett test and by computations of the FIA and GEI. While 10 did not effect HD scrambling or hydrogenation of N-phenylbenzylimine, 11 was effective in HD scrambling. Despite this, no reduction of N-t-butylbenzylimine or N-phenylbenzylimine was achieved. These data demonstrate that 10 lacks the threshold combination of Lewis acidity and basicity to activate H2, while 11 lacks the steric demands about boron to preclude classical Lewis acid-base bond formation with imine substrates.

Asymmetric hydrogenation of imines with chiral alkene-derived boron Lewis acids

With the aim of developing easily accessible chiral Lewis acids for asymmetric hydrogenation, a variety of binaphthyl-based chiral alkenes were prepared in one step from the corresponding diols. Using the in situ generated chiral boron Lewis acids through the hydroboration of chiral alkenes with Piers' borane, metal-free asymmetric hydrogenations of imines were realized to furnish the desired amine products in high yields with up to 89% ee.

Frustrated Lewis Pairs Catalyzed Asymmetric Metal-Free Hydrogenations and Hydrosilylations

The use of frustrated Lewis pairs is an extremely important approach to metal-free hydrogenations. In contrast to the rapid growth of catalytic reactions, asymmetric hydrogenations are far less developed due to a severe shortage of readily available chiral frustrated Lewis pair catalysts with high catalytic activities and selectivities. Unlike the stable Lewis base component of frustrated Lewis pairs, the moisture-sensitive boron Lewis acid component is difficult to prepare. The development of convenient methods for the quick construction of chiral boron Lewis acids is therefore of great interest. In this Account, we summarize our recent studies on frustrated Lewis pair-catalyzed, asymmetric metal-free hydrogenations and hydrosilylations. To address the shortage of highly active and selective catalysts, we developed a novel strategy for the in situ preparation of chiral boron Lewis acids by the hydroboration of chiral dienes or diynes with Piers' borane without further purification, which allows chiral dienes or diynes to act like ligands. This strategy ensures the construction of a useful toolbox of catalysts for asymmetric metal-free hydrogenations and hydrosilylations is rapid and operationally simple. Another strategy is using combinations of readily available Lewis acids and bases containing hydridic and acidic hydrogen atoms, respectively, as a novel type of frustrated Lewis pairs. Such systems provide a great opportunity for using simple chiral Lewis bases as the origins of asymmetric induction. With chiral diene-derived boron Lewis acids as catalysts, a broad range of unsaturated compounds, such as imines, silyl enol ethers, 2,3-disubstituted quinoxalines, and polysubstituted quinolines, are all viable substrates for asymmetric metal-free hydrogenations and give the corresponding products in good yields with high enantioselectivities and/or stereoselectivities. These chiral catalysts are very effective for bulky substrates, and the substrate scope for these metal-free asymmetric hydrogenations has been dramatically expanded. Chiral alkenylboranes were designed to enhance the rigidity of the framework and modify the Lewis acidity through the resulting double bonds. Frustrated Lewis pairs of chiral alkenylboranes and phosphines are a class of highly effective catalysts for asymmetric Piers-type hydrosilylations of 1,2-dicarbonyl compounds, and they give the desired products in high yields and enantioselectivities. Moreover, asymmetric transfer hydrogenations of imines and quinoxalines with ammonia borane as the hydrogen source have been achieved with frustrated Lewis pair of Piers' borane and (R)-tert-butylsulfinamide as the catalyst. Mechanistic studies have suggested that the hydrogen transfer occurs via an 8-membered ring transition state, and regeneration of the reactive frustrated Lewis pair with ammonia borane occurs through a concerted 6-membered ring transition state.

Enantioselective Hydrogenation of Imidazo[1,2-a]pyridines

The enantioselective synthesis of tetrahydroimidazo[1,2-a]pyridines by direct hydrogenation was achieved using a ruthenium/N-heterocyclic carbene (NHC) catalyst. The reaction forgoes the need for protecting or activating groups, proceeds with complete regioselectivity, good to excellent yields, enantiomeric ratios of up to 98:2, and tolerates a broad range of functional groups. 5,6,7,8-Tetrahydroimidazo[1,2-a]pyridines, which are found in numerous bioactive molecules, were directly obtained by this method, and its applicability was demonstrated by the (formal) synthesis of several functional molecules.

B(C6F5)3-catalyzed transfer hydrogenations of imines with Hantzsch esters

Transfer hydrogenations of imines with Hantzsch esters were realized using 0.1 mol% of B(C₆F₅)₃, and up to 38% ee was obtained for asymmetric reactions.

β-Hydroxy-tetrahydroquinolines from Quinolines Using Chloroborane: Synthesis of the Peptidomimetic FISLE-412

A new synthetic protocol provides a simple and direct method to generate functionalized β-hydroxy-tetrahydroquinolines (THQs). Hydroboration of quinolines using chloroboranes followed by oxidation with NaBO3 ⋅H2 O led to the formation of functionalized β-hydroxy THQs. High regio- and diastereoselectivities were observed in α and γ substituted quinolines and the trans diastereomer of the β-hydroxy-THQ was the major isostere. This new protocol was utilized to build the novel antibody-targeted lupus peptidomimetic, FISLE-412.

B(C6F5)3-Promoted hydrogenations of N-heterocycles with ammonia borane

A transition-metal-free method for the B(C₆F₅)₃-promoted hydrogenations of N-heterocycles using ammonia borane under mild reaction conditions has been developed. The reaction affords a broad range of hydrogenated products in moderate to good yields. The enantioselective versions for the corresponding products were also investigated via our approach, showing good feasibility.