Highly Enantioselective Synthesis of Chiral Benzhydrols via Manganese Catalyzed Asymmetric Hydrogenation of Unsymmetrical Benzophenones Using an Imidazole-Based Chiral PNN Tridentate Ligand

Asymmetric Transfer Hydrogenation of Ketones Improved by PNN-Manganese Complexes

Chiral manganese(I) complexes that contain carbocyclic-fused 8-amino-5,6,7,8-tetrahydroquinolinyl groups that are appended with distinct para-R substituents have proven to be effective catalysts in the asymmetric transfer hydrogenation (ATH) of a wide range of ketones (48 examples). Notably, Mn2 proved to be the most productive catalyst, allowing an outstanding turnover number of 8300 with catalyst loadings as low as 0.01 mol %. Furthermore, this catalytic protocol shows considerable promise for applications in the synthesis of chiral drugs such as Lusutrombopag.

Highly Enantioselective Hydrogenation of Unsymmetrical Benzophenones via Iridium-f-phamidol Catalysis

A sequence of f-phamidol-based tetradentate phosphine ligands have been developed and successfully used in iridium-catalyzed enantioselective hydrogenation of benzophenones to deliver chiral benzhydrols in almost quantitative yields and with excellent enantioselectivities (up to >99% yield and up to >99% ee). Moreover, the catalytic system shows a broad substrate scope and functional group tolerance. The synthetic utilities of this methodology have been showcased by gram-scale experiments and the formal synthesis of levocetirizine.

Development of an imidazole-based N,N-bidentate ligand for the manganese catalyzed direct coupling of nitriles with alcohols

3d-Metal catalyzed borrowing hydrogen (BH) reactions represent powerful and environmentally friendly approaches for the direct coupling of alcohols with nitriles to assemble various important branched nitriles. The development of simple and efficient ligands is a crucial issue in this field. In this study, we designed a series of readily available N,N-bidentate ligands that demonstrated good efficiency in the Mn-catalyzed BH reaction of alcohols and nitrile derivatives, yielding the targeted nitriles in moderate to good yields. Remarkably, the mildness and practicality of this protocol were further demonstrated by the successful synthesis of anipamil via a two-cascade borrowing hydrogen procedure.

Impact of the Methylene Bridge Substitution in Chelating NHC-Phosphine Mn(I) Catalyst for Ketone Hydrogenation

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(Ph2PCH2NHC)Mn(CO)3Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000. Mechanistic investigation combining experimental and computational studies allowed to rationalize this beneficial effect as an enhanced stabilization of reaction intermediates including anionic hydride species fac-[(Ph2PC(Ph)NHC)Mn(CO)3H]- playing a crucial role in the hydrogenation process. These results highlight the interest of such carbon bridge substitution strategy being rarely employed in the design of chemically non-innocent ligands.

Manganese-Catalyzed Hydrogenation of Amides and Polyurethanes: Is Catalyst Inhibition an Additional Barrier to the Efficient Hydrogenation of Amides and Their Derivatives?

The hydrogenation of amides and other less electrophilic carbonyl derivatives with an N-C=O functionality requires significant improvements in scope and catalytic activity to be a genuinely useful reaction in industry. Here, we report the results of a study that examined whether such reactions are further disadvantaged by nitrogen-containing compounds such as aliphatic amines acting as inhibitors on the catalysts. In this case, an enantiomerically pure manganese catalyst previously established to be efficient in the hydrogenation of ketones, N-aryl-imines, and esters was used as a prototype of a manganese catalyst. This was accomplished by doping a model ester hydrogenation with various nitrogen-containing compounds and monitoring progress. Following from this, a protocol for the catalytic hydrogenation of amides and polyurethanes is described, including the catalytic hydrogenation of an axially chiral amide that resulted in low levels of kinetic resolution. The hypothesis of nitrogen-containing compounds acting as an inhibitor in the catalytic hydrogenation process has also been rationalized by using spectroscopy (high-pressure infrared (IR), nuclear magnetic resonance (NMR)) and mass spectrometry studies.

6aH-Benzo[α]fluorene: NMR evidence of the unexpected product of the reaction of butyryl chloride with 1,2-diphenylacetylene

The reaction of butyryl chloride with ethynylbenzene in the presence of AlCl3 afforded a mixture of the Z/E-isomers of 1-chloro-2-phenylhex-1-en-3-one. 1,2-Diphenylethyne under these conditions gave a novel polycarbocycle core, 6aH-benzo[a]fluorene. The chemical structure of 11-chloro-5,6-diphenyl-6a-propyl-6aH-benzo[a]fluorene was established by means of IE-MS, 1 H, 13 C NMR, COSY, HSQC, HMBC, and 2D INADEQUATE technique.

Iridium-Catalyzed Asymmetric Hydrogenation of Simple Ketones with Tridentate PNN Ligands Bearing Unsymmetrical Vicinal Diamines

An iridium catalytic system with a ferrocene-based phosphine ligand bearing a modular and tunable unsymmetrical vicinal diamine scaffold was developed for the asymmetric hydrogenation of aryl ketones. This approach provided a powerful tool for the enantioselective synthesis of diverse chiral alcohols with excellent reactivity and enantioselectivity (up to 99% yield, up to 99% ee, and up to 50,000 turnover number). The substituents and chirality of unsymmetrical diamines in ligands played an important role in the satisfactory results.

Manganese-Catalyzed Hydrogenative Desulfurization of Thioamides

Earth-abundant transition metal catalysis has emerged as an important alternative to noble transition metal catalysis in hydrogenation reactions. However, there has been no Earth-abundant transition metal catalyzed hydrogenation of thioamides reported so far, presumably due to the poisoning of catalysts by sulfur-containing molecules. Herein, we described the first manganese-catalyzed hydrogenative desulfurization of thioamides to amines or imines. The key to success is the use of MnBr(CO)₅ instead of commonly-employed pincer-manganese catalysts, together with simple NEt₃ and CuBr. This protocol features excellent selectivity on sole cleavage of the C=S bond of thioamides, in contrast to the only known Ru-catalyzed hydrogenation of thioamides, and unprecedented chemo-selectivity tolerating vulnerable functional groups such as nitrile, ketone, aldehyde, ester, sulfone, nitro, olefin, alkyne and heterocycle, which are usually susceptible to common hydride-type reductive protocols.

Rational Design of a Facially Coordinating P,N,N Ligand for Manganese-Catalysed Enantioselective Hydrogenation of Cyclic Ketones

DFT calculations on the full catalytic cycle for manganese catalysed enantioselective hydrogenation of a selection of ketones have been carried out at the PBE0-D3PCM //RI-BP86PCM level. Mn complexes of an enantiomerically pure chiral P,N,N ligand have been found to be most reactive when adopting a facial coordination mode. The use of a new ligand with an ortho-substituted dimethylamino-pyridine motif has been calculated to completely transform the levels of enantioselectivity possible for the hydrogenation of cyclic ketones relative to the first-generation Mn catalysts. In silico evaluation of substrates has been used to identify those likely to be reduced with high enantiomer ratios (er), and others that would exhibit less selectivity; good agreements were then found in experiments. Various cyclic ketones and some acetophenone derivatives were hydrogenated with er's up to 99 : 1.

Room temperature chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds by using a well-defined mixed donor Mn(i) pincer catalyst

Chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds in α,β-unsaturated ketones, aldehydes and imines is accomplished at room temperature (27 °C) using a well-defined Mn(i) catalyst and 5.0 bar H₂. Amongst the three mixed-donor Mn(i) complexes developed, κ³-(^R2PN³N^Pyz)Mn(CO)₂Br (R = Ph, ⁱPr, ^t Bu); the ^t Bu-substituted complex ( ^tBu2PN³N^Pyz)Mn(CO)₂Br shows exceptional chemoselective catalytic reduction of unsaturated bonds. This hydrogenation protocol tolerates a range of highly susceptible functionalities, such as halides (-F, -Cl, -Br, and -I), alkoxy and hydroxy, including hydrogen-sensitive moieties like acetyl, nitrile, nitro, epoxide, and unconjugated alkenyl and alkynyl groups. Additionally, the disclosed method applies to indole, pyrrole, furan, thiophene, and pyridine-containing unsaturated ketones leading to the corresponding saturated ketones. The C[double bond, length as m-dash]C bond is chemoselectively hydrogenated in α,β-unsaturated ketones, while the aldehyde's C[double bond, length as m-dash]O bond and imine's C[double bond, length as m-dash]N bond are preferentially reduced over the C[double bond, length as m-dash]C bond. A detailed mechanistic study highlighted the non-innocent behavior of the ligand in the ( ^tBu2PN³N^Pyz)Mn(i) complex and indicated a metal-ligand cooperative catalytic pathway. The molecular hydrogen (H₂) acts as a hydride source, whereas MeOH provides a proton for hydrogenation. DFT energy calculations supported the facile progress of most catalytic steps, involving a crucial turnover-limiting H₂ activation.

Recyclable Mn(I) Catalysts for Base‐Free Asymmetric Hydrogenation: Mechanistic, DFT and Catalytic Studies

We report here a mechanistic, DFT and catalytic study on a series of Mn(I) complexes 1, 2(a–d), 3, 4. The studies apprehended the requirements for Mn(I) complexes to be active in both asymmetric direct (AH) and transfer hydrogenations (ATH). The investigations disclosed 6 vital factors accelerating the formation of a resting species, which plays a significant role in lowering the activities of the Mn(I) complex 1 in ATH and AH, respectively. In addition, we also report here a base free Mn(I) catalyzed ATH of aryl alkyl ketones with high enantioselectivity (up to 98 % ee) and improved activity. More significantly, a novel and simple single‐step process for recycling the resting species from the catalytic leftover has been discovered. Notably, the studies provide evidence for the existence of two different temperature dependent mechanisms for AH and ATH, in contrast to previous studies on related systems.

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.

Rh-Catalyzed Highly Enantioselective Hydrogenation of Functionalized Olefins with Chiral Ferrocenylphosphine-Spiro Phosphonamidite Ligands

A highly efficient Rh-catalyzed hydrogenation of functionalized olefins has been realized by a new family of highly rigid chiral ferrocenylphosphine-spiro phosphonamidite ligands. Excellent enantiocontrol (>99% ee in most cases) was achieved with a wide range of α-dehydroamino acid esters and α-enamides. This practicable catalytic system was further applied in the scalable synthesis of highly optically pure key intermediates of cinacalcet and d-phenylalanine.

A theoretical study of asymmetric ketone hydrogenation catalyzed by Mn complexes: from the catalytic mechanism to the catalyst design

Herein, a density functional theory (DFT) study was performed to investigate asymmetric ketone hydrogenation (AKH) catalyzed by Mn complexes, from the catalytic mechanism to the catalyst design. The calculated results indicated that the Mn(CO)₂-PSiNSiP (A1, PSiNSiP = P(Ph)₂Si(CH₃)₂NSi(CH₃)₂P(Ph)₂) pincer complex has potential high catalytic activity for ketone hydrogenation. The Mn(CO)-LYB (B, LYB = P(Ph)₂Si(CH₃)₂NSi(CH₃)₂P(Me)₂) pincer complex was then designed to catalyze AKH with good stereoselectivity. The hydrogen transfer (HT) step is the chirality-determining step. To avoid the enantiomer of Mn(CO)₂-LYB, which could eliminate the high stereoselectivity during AKH, novel Mn complexes with quadridentate ligands, such as Mn(CO)-LYC (C, LYC = P(CH₃)₂CH₂Si(CH₃)NSi(CH₃)(Si(CH₃)CH₂P(CH₃)₂)CH₂P(Ph)₂) and Mn(CO)-LYD (D, LYD = P(CH₃)₂CH₂Si(CH₃)NSi(CH₃)(Si(CH₃)CH₂P(CH₃)₂)CH₂P(Cy)₂), were designed to drive AKH with medium stereoselectivity. In order to increase the stereoselectivity of AKH, Mn(CO)-LYE (E, LYE = PH₂CH₂Si(CH₃)NSi(CH₃)(Si(CH₃)CH₂P(CH₃)₂)CH₂P(Ph)₂) and Mn(CO)-LYF (F, LYF = PH₂CH₂Si(CH₃)NSi(CH₃)(Si(CH₃)CH₂P(CH₃)₂)CH₂P(Cy)₂) were further designed and showed very good stereoselectivity, which is due to the lower deformation energy and stronger interactions between the ketone substrates and catalysts. This work may shed light on the design of cheap metal catalysts with a new ligand framework for the asymmetric hydrogenation (AH) of CX bonds (X = O, N).

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

The emerging field of organometallic catalysis has shifted towards research on Earth-abundant transition metals due to their ready availability, economic advantage, and novel properties. In this case, manganese, the third most abundant transition-metal in the Earth's crust, has emerged as one of the leading competitors. Accordingly, a large number of molecularly-defined Mn-complexes has been synthesized and employed for hydrogenation, dehydrogenation, and hydroelementation reactions. In this regard, catalyst design is based on three pillars, namely, metal-ligand bifunctionality, ligand hemilability, and redox activity. Indeed, the developed catalysts not only differ in the number of chelating atoms they possess but also their working principles, thereby leading to different turnover numbers for product molecules. Hence, the critical assessment of molecularly defined manganese catalysts in terms of chelating atoms, reaction conditions, mechanistic pathway, and product turnover number is significant. Herein, we analyze manganese complexes for their catalytic activity, versatility to allow multiple transformations and their routes to convert substrates to target molecules. This article will also be helpful to get significant insight into ligand design, thereby aiding catalysis design.

Manganese-Catalyzed Asymmetric Hydrogenation of 3H-Indoles

The asymmetric hydrogenation (AH) of 3H-indoles represents an ideal approach to the synthesis of useful chiral indoline scaffolds. However, very few catalytic systems based on precious metals have been developed to realize this challenging reaction. Herein, we report a Mn-catalyzed AH of 3H-indoles with excellent yields and enantioselectivities. The kinetic resolution of racemic 3H-indoles by AH was also achieved with high s-factors to construct quaternary stereocenters. Many acid-sensitive functional groups, which cannot be tolerated when using a state-of-the-art ruthenium catalyst, were compatible with manganese catalysis. This new process expands the scope of this transformation and highlights the uniqueness of earth-abundant metal catalysis. The reaction could proceed with catalyst loadings at the parts per million (ppm) level with an exceptional turnover number of 72 350. This is the highest value yet reported for an earth-abundant metal-catalyzed AH reaction.

Proton-Promoted Nickel-Catalyzed Asymmetric Hydrogenation of Aliphatic Ketoacids

A robust and highly active homogeneous chiral nickel-phosphine complex for the asymmetric hydrogenation of aliphatic γ- and δ-ketoacids has been discovered. The hydrogenation could proceed smoothly in the presence of 0.0133 mol% catalyst loading (S/C=7500). The coordination chemistry and catalytic behavior of Ni(OTf)₂ with (S,S)-Ph-BPE were explored by ¹ H NMR and HRMS. The mechanistic studies revealed that a proton promoted the activation of the substrate C=O bond and controlled the stereoselectivity through hydrogen bonds. A series of chiral γ- and δ-alkyl substituted lactones were obtained in high yields with excellent enantioselectivities (up to 98 % yield and 99 % ee). In addition, this catalytic system also demonstrated that levulinic acid produced from a biomass feedstock was converted into chiral γ-valerolactone without loss of ee value.

Iridium/f-diaphos catalyzed asymmetric hydrogenation of 2-imidazolyl aryl/alkyl ketones

The iridium/f-diaphos L1, L5 or L12 catalyzed asymmetric hydrogenation of 2-imidazolyl aryl/alkyl ketones to afford two enantiomers of the desired chiral alcohols with high conversions (up to 99% yield) and moderate to excellent enantioselectivities (61% - >99% ee) was realized for the first time. This protocol could be easily conducted on a gram-scale with a TON of 9700.

Nickel-catalyzed enantioselective umpolung hydrogenation for stereoselective synthesis of β-amido esters

Nickel complexes ligated by strongly donating diphosphines catalyze enantioselective hydrogenation for the preparation of acyclic and cyclic β-amido esters. A combination of acetic acid and indium powder provides protons and electrons to form nickel hydrido complexes under umpolung hydrogenation conditions.

Mn(I) phosphine-amino-phosphinites: a highly modular class of pincer complexes for enantioselective transfer hydrogenation of aryl-alkyl ketones

A series of Mn(I) catalysts with readily accessible and more π-accepting phosphine-amino-phosphinite (P'(O)N(H)P) pincer ligands have been explored for the asymmetric transfer hydrogenation of aryl-alkyl ketones which led to good to high enantioselectivities (up to 98%) compared to other reported Mn-based catalysts for such reactions. The easy tunability of the chiral backbone and the phosphine moieties makes P'(O)N(H)P an alternative ligand framework to the well-known PNP-type pincers.

Enantio- and Regioselective CuH-Catalyzed Conjugate Reduction of Yne-Allenones

A new asymmetric catalytic conjugate reduction of yne-allenones to synthesize enantioenriched cyclobuta[a]naphthalen-4(2H)-ones has been established that uses copper-bisphosphine complexes as catalysts and gives excellent regio- and enantioselectivities (≥99% ee) in most cases. This protocol tolerates a broad scope of substrates, exhibits high compatibility with various substituents, and gives excellent stereoselectivity, providing a catalytic and efficient entry to fabrication of synthetically important chiral 6-6-4 tricarbocyclic scaffolds.

Chiral Tridentate Ligands in Transition Metal-Catalyzed Asymmetric Hydrogenation

Asymmetric hydrogenation (AH) of double bonds has been one of the most effective methods for the preparation of chiral molecules and for the synthesis of important chiral building blocks. In the past 60 years, noble metals with bidentate ligands have shown marvelous reactivity and enantioselectivity in asymmetric hydrogenation of a series of prochiral substrates. In recent years, developing chiral tridentate ligands has played an increasingly important role in AH. With modular frameworks and a variety of functionalities on the side arms, chiral tridentate ligand complexes enable both reactivities and stereoselectivities. Although great achievements have been made for noble metal catalysts with chiral tridentate ligands since the 1990s, the design of chiral tridentate ligands for earth abundant metal catalysts has still been in high demand. This review summarizes the development of chiral tridentate ligands for homogeneous asymmetric hydrogenation. The philosophy of ligand design and the reaction mechanisms are highlighted and discussed as well.

Enantioselective synthesis of functionalized 1,4-dihydropyrazolo-[4',3':5,6]pyrano[2,3-b]quinolines through ferrocenyl-phosphine-catalyzed annulation of modified MBH carbonates and pyrazolones

An enantioselective synthesis of highly functionalized 1,4-dihydropyrazolo[4',3':5,6]pyrano[2,3-b]quinolines from modified MBH carbonates and pyrazolones via a chiral phosphine-mediated alkylation/annulation sequence has been realized. The chiral dihydropyrano[2,3-c]pyrazoles bearing bio-active condensed heterocycles were facilely formed in good chemical yields and with high to excellent enantioselectivity by utilizing low catalyst loading.

Robust and efficient hydrogenation of carbonyl compounds catalysed by mixed donor Mn(I) pincer complexes

Any catalyst should be efficient and stable to be implemented in practice. This requirement is particularly valid for manganese hydrogenation catalysts. While representing a more sustainable alternative to conventional noble metal-based systems, manganese hydrogenation catalysts are prone to degrade under catalytic conditions once operation temperatures are high. Herein, we report a highly efficient Mn(I)-CNP pre-catalyst which gives rise to the excellent productivity (TOF° up to 41 000 h^-1) and stability (TON up to 200 000) in hydrogenation catalysis. This system enables near-quantitative hydrogenation of ketones, imines, aldehydes and formate esters at the catalyst loadings as low as 5-200 p.p.m. Our analysis points to the crucial role of the catalyst activation step for the catalytic performance and stability of the system. While conventional activation employing alkoxide bases can ultimately provide catalytically competent species under hydrogen atmosphere, activation of Mn(I) pre-catalyst with hydride donor promoters, e.g. KHBEt₃, dramatically improves catalytic performance of the system and eliminates induction times associated with slow catalyst activation.

Enantioselective direct, base-free hydrogenation of ketones by a manganese amido complex of a homochiral, unsymmetrical P–N–P′ ligand

Base-free direct hydrogenation of ketones using a Mn(PNP′)(CO)₂ complex is more enantioselective than that of a related base-activated iron complex.

Asymmetric hydrogenation catalyzed by first-row transition metal complexes

This review focuses on asymmetric direct and transfer hydrogenation with first-row transition metal complexes. The reaction mechanisms and the models of enantiomeric induction were summarized and emphasized.

Design of chiral ferrocenylphosphine-spiro phosphonamidite ligands for ruthenium-catalyzed highly enantioselective coupling of 1,2-diols with amines

A series of chiral ferrocene-backbone phosphines-spiro phosphonamidite ligands was developed for ruthenium-catalyzed enantioselective access to a broad range of β-amino alcohols from 1,2-diols and amines via the borrowing-hydrogen prciniple.

Ruthenium catalyzed α-methylation of sulfones with methanol as a sustainable C1 source

Methylation of sulfones bearing an α-CH bond can be easily achieved via a one-step, Ru(ii) catalyzed redox neutral reaction using methanol as a sustainable C1 building block.