Manganese-Catalyzed and Base-Switchable Synthesis of Amines or Imines via Borrowing Hydrogen or Dehydrogenative Condensation

Remote C-H bond cooperation strategy enabled silver catalyzed borrowing hydrogen reactions

Metal-ligand cooperation (MLC) is an essential strategy in transition metal catalysis. Traditional NH-based and OH-based MLC catalysts, as well as the later developed (de)aromatization strategy, have been widely applied in atom-economic borrowing hydrogen/hydrogen auto-transfer (BH/HA) reactions. However, these conventional MLC approaches are challenging for low-coordination and low-activity coinage metal complexes, arising from the instability during (de)protonation on the coordination atom, the constraint in linear coordination, and possible poisoning due to extra functional sites. Herein, we demonstrate a remote C-H bond cooperation strategy that enables the unprecedented homogeneous Ag(i)-catalyzed BH/HA reaction. The covalent C-H bifunctional site well facilitates (de)hydrogenation, especially under the low coordination circumstances of d10 Ag(i). The strong electron-donating bis-N-heterocyclic carbene (NHC) ligand stabilizes the silver-hydride and stimulates the hydride activity on the trans-position of ligands. Mechanistic studies implicate the plausible remote assistance of the dissociative NHC arm in facilitating BH/HA reactions. Our findings emphasize the potential of the remote C-H bond cooperation strategy for low coordination metals in catalyzing BH/HA reactions and broadening MLC catalysts to d10 coinage metals.

Cu-UiO-66 Catalyzed Synthesis of Imines via Acceptorless Dehydrogenative Coupling of Alcohols and Amines

Herein, the Cu-UiO-66 catalyst was developed for acceptorless dehydrogenative coupling (ADC) between alcohols and amines to produce imines. The Cu-UiO-66 catalyst was synthesized by installing Cu2+ onto Zr-oxo clusters in UiO-66, and the catalyst efficiently catalyzes the ADC reaction under mild and environmentally friendly conditions with excellent selectivity. Mechanistic studies reveal that the O2⋅- radicals and porosity of formed in Cu-UiO-66 participate cooperatively during the catalytic cycle. Meanwhile, the only by-product of the system is environmentally benign water. Cycling tests and hot filtration tests showed that the Cu-UiO-66 catalyst exhibited excellent stability and catalytic activity during the reaction. Importantly, the Cu-UiO-66 catalyst might provide a promising strategy for the ADC reaction between alcohols and amines to produce imines.

Direct synthesis of partially ethoxylated branched polyethylenimine from ethanolamine

We report here a method to make a branched and partially ethoxylated polyethyleneimine derivative directly from ethanolamine. The polymerization reaction is catalysed by a pincer complex of Earth-abundant metal, manganese, and produces water as the only byproduct. Industrial processes to produce polyethyleneimines involve the transformation of ethanolamine to a highly toxic chemical, aziridine, by an energy-intensive/waste-generating process followed by the ring-opening polymerization of aziridine. The reported method bypasses the need to produce a highly toxic intermediate and presents advantages over the current state-of-the-art. We propose that the polymerization process follows a hydrogen borrowing pathway that involves (a) dehydrogenation of ethanolamine to form 2-aminoacetaldehyde, (b) dehydrative coupling of 2-aminoacetaldehyde with ethanolamine to form an imine derivative, and (c) subsequent hydrogenation of imine derivative to form alkylated amines.

Air-Stable Arene Manganese Complexes as Catalysts for the Syngas-Assisted Direct Reductive Amination, Cyanation of Aldehyde, and CO2 Fixation by Epoxide with High Functional Groups Tolerance

Manganese complexes [(arene)Mn(CO)3]+ were prepared in one step from arenes and Mn(CO)5Br. They were found to be efficient catalysts in the carbonyl cyanation with TMSCN, CO2 fixation by epoxides, and direct reductive amination in the presence of syngas. The amination reaction tolerated various reducible functional groups. The synergy of carbon monoxide and hydrogen in syngas provides high efficiency of the catalytic system. The developed protocols do not require an inert atmosphere, and the catalysts can be handled in air.

Selective N-Alkylation of Aminobenzenesulfonamides with Alcohols for the Synthesis of Amino-(N-alkyl)benzenesulfonamides Catalyzed by a Metal-Ligand Bifunctional Ruthenium Catalyst

[(p-Cymene)Ru(2,2'-bpyO)(H2O)] was proven to be an efficient catalyst for the synthesis of amino-(N-alkyl)benzenesulfonamides via selective N-alkylation of aminobenzenesulfonamides with alcohols. It was confirmed that functional groups in the bpy ligand are crucial for the activity of catalysts. Furthermore, the utilization of this catalytic system for the preparation of a biologically active compound was presented.

A Single Terminal [NiII-OH] Catalyst for Direct Julia-Type Olefination and α-Alkylation Involving Sulfones and Alcohols

A terminal [NiII-OH] complex 1, supported by triflamide-functionalized NHC ligands, showed divergent reactivity for the reaction of sulfone with alcohol, contingent on base concentration, temperature, and time. Julia-type olefination of alcohols with sulfones was achieved using one equiv. of base, whereas lowering base loading to 0.5 equiv. afforded α-alkylated sulfones. Besides excellent substrate scope and selectivity, biologically active stilbene derivatives DMU-212, pinosylvin, resveratrol, and piceatannol were synthesized in high yield under Julia-type olefination conditions. An extensive array of controlled experiments and DFT calculations provide valuable insight on the reaction pathway.

Cp*Ir(III) complexes catalyzed solvent-free synthesis of quinolines, pyrroles and pyridines via an ADC strategy

A trio of Ir(III) complexes that are held together by a picolinamidato moiety were created. In our earlier research, we demonstrated the catalytic activity of the complexes for producing alpha-alkylated ketones from a ketone or secondary alcohol with a primary alcohol in the presence of a catalytic amount of a Cp*Ir(III) catalyst and tBuOK in toluene at 110 °C using the hydrogen-borrowing technique. Earlier many research groups had synthesized quinoline, pyrrole, and pyridine derivatives using 2-amino alcohol and ketone or secondary alcohol derivatives as starting materials, but in all those cases the reaction conditions are not suitable in terms of green synthesis like more catalyst loading, base loading, long reaction time, and high temperature. In addition, most of the reactions contain phosphine a hazardous by-product, along with the catalyst. Keeping in mind these shortcomings, we tried to expand the use of our catalysts after achieving an excellent result in our previous work, and we were successful in producing quinoline, pyrrole, and pyridine derivatives through acceptor-less dehydrogenative coupling (ADC) procedures at 90-110 °C under neat/solvent-free conditions and achieved good to exceptional yields of those nitrogen-containing heterocycles. This methodology is attractive because it is environmentally benign and allows for the "green" synthesis of nitrogen-containing heterocycles. All that is required is a modest quantity of catalyst and base, and the by-products are merely H2O and H2.

Synthesis of imines from the coupling reaction of alcohols and amines catalyzed by phosphine-free cobalt(II) complexes

Phosphine-free, air stable cobalt(II) based complexes (1a and 1b) consisting of ligands L1H2 and L2H2 (L1H2 = N,N'-((1,2-phenylenebis(azaneylylidene))bis(methaneylylidene))diphenol and L2H2 = N,N'-bis(4-diethylaminosalicylidene)-4,5-dichloro-1,2-phenylenediamine) were synthesized and utilized as catalysts in the coupling reaction of alcohols with amines into imines following an acceptorless dehydrogenative pathway. The reactions were carried out in the presence of t-BuOK base with low catalyst loading (1 mol%) in an open atmosphere. The corresponding imines were isolated in moderate to excellent yields. The methodology was screened with different substituted alcohols and amines. The proposed mechanistic pathway of this reaction was ascertained through intermediate mass and 1H NMR analyses. Most of the previously reported 3d transition metal catalysts used in imine synthesis reactions have a phosphine ligand environment, and the reactions were performed under inert conditions. Herein we have developed a sustainable route for the synthesis of imines from the coupling reaction of alcohols with amines under aerial reaction conditions using phosphine-free air stable cobalt catalysts.

A Phosphine-Oxide Cobalt(II) Complex and Its Catalytic Activity Studies toward Alcohol Dehydrogenation Triggered Direct Synthesis of Imines and Quinolines

Herein, a new pincer-like amino phosphine donor ligand, H2L1, and its phosphine-oxide analog, H2L2, were synthesized. Subsequently, cobalt(II) complexes 1 and 2 were synthesized by the reaction of anhydrous Co(II)Cl2 with ligands H2L1 and H2L2, respectively. The ligands and complexes were fully characterized by various physicochemical and spectroscopic characterization techniques. Finally, the identity of the complexes 1 and 2 was confirmed by single crystal X-ray structure determination. The phosphine ligand containing complex 1 was converted to the phosphine oxide ligand containing complex 2 in air in acetonitrile solution. Both complexes 1 and 2 were investigated as precatalysts for alcohol dehydrogenation-triggered synthesis of imines in air. The phosphine-oxide complex 2 was more efficient than the phosphine complex 1. A wide array of alcohols and amines were successfully reacted in a mild condition to result in imines in good to excellent yields. Precatalyst 2 was also highly efficient for the synthesis of varieties of quinolines in air. As H2L2 in 2 has side arms that can be deprotonated, we investigated complex 2 for its base (KOtBu) promoted deprotonation events by various spectroscopic studies and DFT calculations. These studies have shown that mono deprotonation of the amine side arm attached to the pyridine is quite feasible, and deprotonation of complex 2 leads to a dearomatized pyridyl ring containing complex 2a. The mechanistic investigations of the catalytic reaction, by a combination of experimental and computational studies, have suggested that the dearomatized complex, 2a acted as an active catalyst. The reaction proceeded through the hydride transfer pathway. The activation barrier of this step was calculated to be 26.5 kcal/mol, which is quite consistent with the experimental reaction temperature under aerobic conditions. Although various pincer-like complexes are explored for such reactions, phosphine oxide ligand-containing complexes are still unexplored.

Manganese-catalyzed C-C and C-N bond formation with alcohols via borrowing hydrogen or hydrogen auto-transfer

Transition-metal-mediated "borrowing hydrogen" also known as hydrogen auto-transfer reactions allow the sustainable construction of C-C and C-N bonds using alcohols as hydrogen donors. In recent years, manganese complexes have been explored as efficient catalysts in these reactions. This review highlights the significant progress made in manganese-catalyzed C-C and C-N bond-formation reactions via hydrogen auto-transfer, emphasizing the importance of this methodology and manganese catalysts in sustainable synthesis strategies.

A Phosphine-Free Air-Stable Mn(II)-Catalyst for Sustainable Synthesis of Quinazolin-4(3H)-ones, Quinolines, and Quinoxalines in Water

The synthesis, characterization, and catalytic application of a new phosphine-free, well-defined, water-soluble, and air-stable Mn(II)-catalyst [Mn(L)(H2O)2Cl](Cl) ([1]Cl) featuring a 1,10-phenanthroline based tridentate pincer ligand, 2-(1H-pyrazol-1-yl)-1,10-phenanthroline (L), in dehydrogenative functionalization of alcohols to various N-heterocycles such as quinazolin-4(3H)-ones, quinolines, and quinoxalines are reported here. A wide array of multisubstituted quinazolin-4(3H)-ones were prepared in water under air following two pathways via the dehydrogenative coupling of alcohols with 2-aminobenzamides and 2-aminobenzonitriles, respectively. 2-Aminobenzyl alcohol and ketones bearing active methylene group were used as coupling partners for synthesizing quinoline derivatives, and various quinoxaline derivatives were prepared by coupling vicinal diols and 1,2-diamines. In all cases, the reaction proceeded smoothly using our Mn(II)-catalyst [1]Cl in water under air, affording the desired N-heterocycles in satisfactory yields starting from cheap and readily accessible precursors. Gram-scale synthesis of the compounds indicates the industrial relevance of our synthetic strategy. Control experiments were performed to understand and unveil the plausible reaction mechanism.

Manganese-Catalyzed Mono-N-Methylation of Aliphatic Primary Amines without the Requirement of External High-Hydrogen Pressure

The synthesis of mono-N-methylated aliphatic primary amines has traditionally been challenging, requiring noble metal catalysts and high-pressure H2 for achieving satisfactory yields and selectivity. Herein, we developed an approach for the selective coupling of methanol and aliphatic primary amines, without high-pressure hydrogen, using a manganese-based catalyst. Remarkably, up to 98 % yields with broad substrate scope were achieved at low catalyst loadings. Notably, due to the weak base-catalyzed alcoholysis of formamide intermediates, our novel protocol not only obviates the addition of high-pressure H2 but also prevents side secondary N-methylation, supported by control experiments and density functional theory calculations.

Efficient and chemoselective imine synthesis catalyzed by a well-defined PN3-manganese(II) pincer system

The highly efficient reductive amination of aldehydes with ammonia (NH3) and hydrogen (H2) to form secondary imines is described, as well as the dehydrogenative homocoupling of benzyl amines. Using an air-stable, well-defined PN3-manganese(II) pincer complex as a catalyst precursor, various aldehydes are easily converted directly into secondary imines using NH3 as a nitrogen source under H2 in a one-pot reaction. Importantly, the same catalyst facilitates the dehydrogenative homocoupling of various benzylamines, exclusively forming imine products. These reactions are conducted under very mild conditions, without the addition of any additives, yielding excellent selectivities and high yields of secondary imines in a green manner by minimizing wastes.

Nickel Pincer Complexes Catalyzed Sustainable Synthesis of 3,4-Dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxides via Acceptorless Dehydrogenative Coupling of Primary Alcohols

We report the atom-economic and sustainable synthesis of biologically important 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxide (DHBD) derivatives from readily available aromatic primary alcohols and 2-aminobenzenesulfonamide catalyzed by nickel(II)-N∧N∧S pincer-type complexes. The synthesized nickel complexes have been well-studied by elemental and spectroscopic (FT-IR, NMR, and HRMS) analyses. The solid-state molecular structure of complex 2 has been authenticated by a single-crystal X-ray diffraction study. Furthermore, a series of 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxide derivatives have been synthesized (24 examples) utilizing a 3 mol % Ni(II) catalyst through acceptorless dehydrogenative coupling of benzyl alcohols with benzenesulfonamide. Gratifyingly, the catalytic protocol is highly selective with the yield up to 93% and produces eco-friendly water/hydrogen gas as byproducts. The control experiments and plausible mechanistic investigations indicate that the coupling of the in situ generated aldehyde with benzenesulfonamide leads to the desired product. In addition, a large-scale synthesis of one of the thiadiazine derivatives unveils the synthetic usefulness of the current methodology.

Borrowing Hydrogen β-Phosphinomethylation of Alcohols Using Methanol as C1 Source by Pincer Manganese Complex

Herein, we report a manganese-catalyzed three-component coupling of β-H containing alcohols, methanol, and phosphines for the synthesis of γ-hydroxy phosphines via a borrowing hydrogen strategy. In this development, methanol serves as a sustainable C1 source. A variety of aromatic and aliphatic substituted alcohols and phosphines could undergo the dehydrogenative cross-coupling process efficiently and deliver the corresponding β-phosphinomethylated alcohol products in moderate to good yields. Mechanistic studies suggest that this transformation proceeds in a sequential manner including catalytic dehydrogenation, aldol condensation, Michael addition, and catalytic hydrogenation.

Synthesis of Thiophene-Substituted Ketones via Manganese-Catalyzed Dehydrogenative Coupling Reaction

This study reports an efficient and green one-step method for synthesizing thiophene-substituted ketones from 2-thiophenemethanol and ketones via dehydrogenative coupling using manganese complexes as catalysts. The manganese complex demonstrated a broad applicability under mild conditions and extended the range of usable substrates. Utilizing this strategy, we carried out an efficient and diverse reaction of ketones with 2-thiophenemethanol, and successfully synthesized a series of thiophene-substituted saturated ketones and α, β-unsaturated ketones in good isolated yields.

Ce4+/Ce3+ Redox Effect-Promoted CdS/CeO2 Heterojunction Photocatalyst for the Atom Economic Synthesis of Imines under Visible Light

The employment of stoichiometric alcohols and amines for imine synthesis under mild and green reaction conditions is still a challenge in the field. In this work, based on our research foundation in the thermocatalytic synthesis of imines over ceria, a CdS/CeO2 heterojunction photocatalyst was constructed and successfully realized the atom-economic synthesis of imines under visible light without additives at room temperature. Mechanistic experiments and corresponding characterizations indicated that the CdS/CeO2 heterojunction can improve the separation efficiency of photogenerated carriers, which can be further enhanced by the Ce4+/Ce3+ redox pair by rapidly combining photogenerated e-. The in situ-reduced Ce3+ can better activate O2 to form Ce-O-O·, which, together with h+, efficiently accelerates alcohol oxidation, which is the rate-determined step for the synthesis of imines via oxidative coupling reaction of alcohol and amine. In addition, our photocatalyst exhibited fairly decent reusability and substrate universality. This work solves problems of using base additives and excess amine or alcohol in the reported photocatalytic systems and provides new insight for designing CeO2-based photocatalytic oxidation catalysts.

Benzimidazol-2-ylidene ruthenium complexes for C-N bond formation through alcohol dehydrogenation

A low temperature hydrogen borrowing approach to generate secondary amines using benzimidazole-based N-heterocyclic carbene (BNHC) ruthenium complexes is reported. A series of the piano-stool complexes of the type [(η6-p-cymene)(BNHC)RuCl2] (1a-g) were synthesized via one-pot reaction of the NHC salt precursor, Ag2O, and [RuCl2(p-cymene)]2 and characterized using conventional spectroscopic techniques. The geometry of two precursors, [(η6-p-cymene)(Me4BnMe2BNHCCH2OxMe)RuCl2] (1f) and [(η6-p-cymene)(Me5BnMe2BNHCCH2OxMe)RuCl2] (1g), was studied by single crystal X-ray diffraction. These catalysts were found to dehydrogenate alcohols efficiently at temperatures as low as 50 °C to allow Schiff-base condensation and subsequent imine hydrogenation to afford secondary amines. Notably, this ruthenium-based procedure enables the N-alkylation of aromatic and heteroaromatic primary amines with a wide range of primary alcohols in excellent yields of up to 98%. The present methodology is green and water is liberated as the sole byproduct.

Ionic-Liquid Hydrogen-Bonding Promoted Alcohols Amination over Cobalt Catalyst via Dihydrogen Autotransfer Mechanism

Higher amines are important high-valuable chemicals with wide applications, and amination of alcohols is a green route to them, which however generally suffers from harsh reaction conditions and use of equivalent base. Herein, we report an ionic-liquid (IL) hydrogen-bonding promoted dihydrogen autotransfer strategy for amination of alcohols to higher amines over cobalt catalyst under base-free conditions. Co(BF4 )2  ⋅ 6 H2 O complexed with triphos and IL (e. g., tetrabutylphosphonium tetrafluoroborate, [P4444 ][BF4 ]) shows high performances for the reaction and is tolerant of a wide scope of amines and alcohols, affording higher amines in good to excellent yields. Mechanism investigation indicates that the [BF4 ]- anion activates the alcohol via hydrogen bonding, promoting transfer of both hydroxyl H and α-H atoms of alcohol to the cobalt catalyst to form an aldehyde intermediate and cobalt dihydride complex, which are involved in the subsequent reductive amination. This strategy provides a green and effective route for alcohol amination, which may have promising applications in alcohol-involved alkylation reactions.

No Transition Metals Required - Oxygen Promoted Synthesis of Imines from Primary Alcohols and Amines under Ambient Conditions

The synthesis of imines denotes a cornerstone in organic chemistry. The use of alcohols as renewable substituents for carbonyl-functionality represents an attractive opportunity. Consequently, carbonyl moieties can be in situ generated from alcohols upon transition-metal catalysis under inert atmosphere. Alternatively, bases can be utilized under aerobic conditions. In this context, we report the synthesis of imines from benzyl alcohols and anilines, promoted by KOt Bu under aerobic conditions at room temperature, in the absence of any transition-metal catalyst. A detailed investigation of the radical mechanism of the underlying reaction is presented. This reveals a complex reaction network fully supporting the experimental findings.

Sustainable Synthesis of α-Hydroxycarboxylic Acids by Manganese Catalyzed Acceptorless Dehydrogenative Coupling of Ethylene Glycol and Primary Alcohols

Herein, we report a straightforward synthesis of valuable α-hydroxycarboxylic acid molecules via an acceptorless dehydrogenative coupling of ethylene glycol and primary alcohols. A bench-stable manganese complex catalyzed the reaction, which is scalable, with the product being isolated with high yields and selectivities under mild conditions. The protocol is environmentally benign, producing water and hydrogen gas as the only byproducts. Methanol can also be used as a C1 source for producing the platform molecule lactic acid, with a high turnover of >10⁴ . The methodology was also used to functionalize alcohols derived from natural products and fatty acids. Furthermore, it was applied for synthesizing α-amino acid, α-thiocarboxylic acid, and several drugs and bioactive molecules, including endogenous metabolites, Danshensu, Enalapril, Lisinopril, and Rosmarinic acid. Preliminary mechanistic studies were performed to shed light on the mechanism involved in the reaction.

Manganese-Catalyzed Chemoselective Coupling of Secondary Alcohols, Primary Alcohols and Methanol

Herein, we report a manganese-catalyzed three-component coupling of secondary alcohols, primary alcohols and methanol for the synthesis of β,β-methylated/alkylated secondary alcohols. Using our method, a series of 1-arylethanol, benzyl alcohol derivatives, and methanol undergo sequential coupling efficiently to construct assembled alcohols with high chemoselectivity in moderate to good yields. Mechanistic studies suggest that the reaction proceeds via methylation of a benzylated secondary alcohol intermediate to generate the final product.

Rational design of N-heterocyclic compound classes via regenerative cyclization of diamines

The discovery of reactions is a central topic in chemistry and especially interesting if access to compound classes, which have not yet been synthesized, is permitted. N-Heterocyclic compounds are very important due to their numerous applications in life and material science. We introduce here a consecutive three-component reaction, classes of N-heterocyclic compounds, and the associated synthesis concept (regenerative cyclisation). Our reaction starts with a diamine, which reacts with an amino alcohol via dehydrogenation, condensation, and cyclisation to form a new pair of amines that undergoes ring closure with an aldehyde, carbonyldiimidazole, or a dehydrogenated amino alcohol. Hydrogen is liberated in the first reaction step and the dehydrogenation catalyst used is based on manganese.

Manganese Promoted (Bi)carbonate Hydrogenation and Formate Dehydrogenation: Toward a Circular Carbon and Hydrogen Economy

We report here a feasible hydrogen storage and release process by interconversion of readily available (bi)carbonate and formate salts in the presence of naturally occurring α-amino acids. These transformations are of interest for the concept of a circular carbon economy. The use of inorganic carbonate salts for hydrogen storage and release is also described for the first time. Hydrogenation of these substrates proceeds with high formate yields in the presence of specific manganese pincer catalysts and glutamic acid. Based on this, cyclic hydrogen storage and release processes with carbonate salts succeed with good H₂ yields.

A binuclear Cu(II) complex as an efficient photocatalyst for N-alkylation of aromatic amines

Visible-light driven photoreactions using transition metal complexes as catalysts are currently a research hotspot in developing environmentally friendly sustainable processes. To develop a potential copper-based photocatalyst, a binuclear Cu(II) complex has been synthesized using a Mannich base ligand viz. 2,4-dichloro-6-((4-(2-hydroxyethyl)piperazin-1-yl)methyl)phenol (H₂L). The photocatalyst has been characterized using ESI-MS and single crystal X-ray diffraction. Under the irradiation of visible light, the catalyst can catalyze hydrogen auto-transfer in N-alkylated amine formation and benzyl alcohol oxidation reactions with excellent conversion. A plausible mechanistic pathway for catalytic reactions has been explored through ESI-MS spectrometric, UV-Vis spectroscopic and computational studies.

Well-Defined Phosphine-Free Manganese(II)-Complex-Catalyzed Synthesis of Quinolines, Pyrroles, and Pyridines

Herein, we report a simple, phosphine-free, and inexpensive catalytic system based on a manganese(II) complex for synthesizing different important N-heterocycles such as quinolines, pyrroles, and pyridines from amino alcohols and ketones. Several control experiments, kinetic studies, and DFT calculations were carried out to support the plausible reaction mechanism. We also detected two potential intermediates in the catalytic cycle using ESI-MS analysis. Based on these studies, a metal-ligand cooperative mechanism was proposed.

Ruthenium-Catalyzed Divergent Acceptorless Dehydrogenative Coupling of 1,3-Diols with Arylhydrazines: Synthesis of Pyrazoles and 2-Pyrazolines

Herein, the divergent transformations of 1,3-diols with arylhydrazines via acceptorless dehydrogenative coupling reactions to selectively synthesize pyrazoles and 2-pyrazolines were reported, which were based on Ru₃(CO)₁₂/NHC-phosphine-phosphine catalytic systems. The reactions featured low catalyst loading, high selectivity, wide substrate scope, and good yields, with only water and hydrogen gas (H₂) as the byproducts.

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 Synthesis of Imines from Primary Alcohols and Aromatic Amines

Herein, we describe the synthesis of a wide variety of imines through oxidative coupling of alcohols and (hetero)aromatic amines catalyzed by Mn complexes bearing N^N triazole ligands. A wide variety of imines in excellent yields (up to 99%) have been prepared. Mn-based catalysts proved to be highly efficient and versatile, allowing for the first time, the preparation of several imines containing N-based heterocycles.

Disarming the alkoxide trap to access a practical FeCl3 system for borrowing-hydrogen N-alkylation

Disarming the alkoxide trap using an in situ reduction strategy to access a practical FeCl3 and N-heterocyclic carbene system for borrowing-hydrogen N-alkylation.

Hydrazine Hydrate Accelerates Neocuproine-Copper Complex Generation and Utilization in Alkyne Reduction, a Significant Supplement Method for Catalytic Hydrogenation

Diimine (HN═NH) is a strong reducing agent, but the efficiency of diimine oxidized from hydrazine hydrate or its derivatives is still not good enough. Herein, we report an in situ neocuproine-copper complex formation method. The redox potential of this complex enable it can serve as an ideal redox catalyst in the synthesis of diimine by oxidation of hydrazine hydrate, and we successfully applied this technique in the reduction of alkynes. This reduction method displays a broad functional group tolerance and substrate adaptability as well as the advantages of safety and high efficiency. Especially, nitro, benzyl, boc, and sulfur containing alkynes can be reduced to the corresponding alkanes directly, which provides a useful complementary method to traditional catalytic hydrogenation. Besides, we applied this method in the preparation of the Alzheimer's disease drug CT-1812 and studied the mechanism.

Synthesis of N-Heterocycles via Oxidant-Free Dehydrocyclization of Alcohols Using Heterogeneous Catalysts

N-Heterocycles, such as pyrroles, pyrimidines, quinazolines, and quinoxalines, are important building blocks for organic chemistry and the fine-chemical industry. For their synthesis, catalytic borrowing hydrogen and acceptorless dehydrogenative coupling reactions of alcohols as sustainable reagents have received significant attention in recent years. To overcome the problems of product separation and catalyst reusability, several metal-based heterogeneous catalysts have been reported to achieve these transformations with good yields and selectivity. In this Minireview, we summarize recent developments using both noble and non-noble metal-based heterogeneous catalysts to synthesize N-heterocycles from alcohols and N-nucleophiles via acceptorless dehydrogenation or borrowing hydrogen methodologies. Furthermore, this Minireview introduces strategies for the preparation and functionalization of the corresponding heterogeneous catalysts, discusses the reaction mechanisms and the roles of metal electronic states, and the influence of support Lewis acid-base properties on these reactions.

Catalytic Formal Hydroamination of Allylic Alcohols Using Manganese PNP-Pincer Complexes

Several manganese-PNP pincer catalysts for the formal hydroamination of allylic alcohols are presented. The resulting γ-amino alcohols are selectively obtained in high yields applying Mn-1 in a tandem process under mild conditions.

Advancements in multifunctional manganese complexes for catalytic hydrogen transfer reactions

Catalytic hydrogen transfer reactions have enormous academic and industrial applications for the production of diverse molecular scaffolds. Over the past few decades, precious late transition-metal catalysts were employed for these reactions. The early transition metals have recently gained much attention due to their lower cost, less toxicity, and overall sustainability. In this regard, manganese, which is the third most abundant transition metal in the Earth's crust, has emerged as a viable alternative. However, the key to the success of such manganese-based complexes lies in the multifunctional ligand design and choice of appropriate ancillary ligands, which helps them mimic and, even in some cases, supersede noble metals' activities. The metal-ligand bifunctionality, achieved via deprotonation of the acidic C-H or N-H bonds, is one of the powerful strategies employed for this purpose. Alongside, the ligand hemilability in which a weakly chelating group tunes in between the coordinated and uncoordinated stages could effectively stabilize the reactive intermediates, thereby facilitating substrate activation and catalysis. Redox non-innocent ligands acting as an electron sink, thereby helping the metal center in steps gaining or losing electrons, and non-classical metal-ligand cooperativity has also played a significant role in the ligand design for manganese catalysis. The strategies were not only employed for the chemoselective hydrogenation of different reducible functionalities but also for the C-X (X = C/N) coupling reactions via HT and downstream cascade processes. This article features multifunctional ligand-based manganese complexes, highlighting the importance of ligand design and choice of ancillary ligands for achieving the desired catalytic activity and selectivity for HT reactions. We have also discussed the detailed reaction pathways for metal complexes involving bifunctionality, hemilability, redox activity, and indirect metal-ligand cooperativity. The synthetic utilization of those complexes in different organic transformations has also been detailed.

Ambient Synthesis of Tricyclic Naphthalenes via Stepwise Styryl-yne Dearomative Diels-Alder Cyclization

A cascade of styrylynols promoted by MnO₂ allows the synthesis of fused tricycles with a naphthalene core. The reaction occurs under ambient conditions, offering a practical synthetic tool because of the inexpensive and abundant manganese species. The method affords products through the sequential oxidation of a propargyl alcohol, stepwise Diels-Alder cyclization, and finally rearomatization. According to density functional theory, the usually unfavorable stepwise Diels-Alder mechanism is instead a general tool for eliciting otherwise challenging dearomative annulation.