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The amide functional group is ubiquitous in living organisms, and is of particular importance in bioactive compounds and pharmaceuticals. Because of the prevalence and significance of the amide bond, considerable efforts have been invested throughout the years in developing new synthetic methodologies for its formation. Nevertheless, amide synthesis still largely relies on variants of the traditional condensation of carboxylic acids and amines, mediated by stoichiometric coupling reagents. This poses a sustainability challenge, since such reactions suffer from unfavorable atom and step economies, involve harmful chemicals and produce chemical waste. Hence, establishing sustainable approaches to amide synthesis is of great importance. Over the last two decades, we have developed homogeneous catalytic reactions for sustainable synthetic transformations, primarily based on transition metal complexes of pincer ligands. A considerable portion of these efforts has been devoted to acceptorless dehydrogenative coupling, including that of alcohols and amines through ruthenium-catalyzed reactions. These latter processes generate amides without resorting to coupling reagents and typically produce no waste, with their only byproduct being H2 gas, which is itself a valuable resource. In the present review, we chronicle our progress in this area of research since 2014. This includes the use of water and ammonia as amidation reagents, expanding the scope of amidation substrates and target amides, achieving milder reaction conditions, development of amidation-based liquid organic hydrogen carrier systems, and introduction of manganese-based catalysts. [ABSTRACT FROM AUTHOR]

Liquid organic hydrogen carriers (LOHCs) are powerful systems for the efficient unloading and loading molecular hydrogen. Herein, a liquid‐to‐liquid organic hydrogen carrier system based on reversible dehydrogenative coupling of ethylene glycol (EG) with ethanol catalysed by ruthenium pincer complexes is reported. Noticeable advantages of the current LOHC system is that both reactants (hydrogen‐rich components) and the produced esters (hydrogen‐lean components) are liquids at room temperature, and the dehydrogenation process can be performed under solvent and base‐free conditions. Moreover, the hydrogenation reaction proceeds under low hydrogen pressure (5 bar), and the LOHC system has a relatively high theoretical gravimetric hydrogen storage capacity (HSC>5.0 wt %), presenting an attractive hydrogen storage system. [ABSTRACT FROM AUTHOR]

A novel liquid organic hydrogen carrier (LOHC) system, with a high theoretical hydrogen capacity, based on the unpresented hydrogenation of ethylene urea to ethylenediamine and methanol, and its reverse dehydrogenative coupling, was established. For the dehydrogenation only a small amount of solvent is required. This system is rechargeable, as the H2‐rich compounds could be regenerated by hydrogenation of the resulting dehydrogenation mixture. Both directions for hydrogen loading and unloading were achieved using the same catalyst, under relatively mild conditions. Mechanistic studies reveal the likely pathway for H2‐lean compounds formation. Renewable energy carrier: The first example of ethylene urea hydrogenation to ethylenediamine and methanol was developed. Based on this, a rechargeable hydrogen‐storage system was established. [ABSTRACT FROM AUTHOR]

Abstract: We have developed unprecedented methods for the direct transformation of primary alcohols to alkenes in the presence of hydrazine, and for the synthesis of mixed alkenes by the reaction of alcohols with hydrazones. The reactions are catalyzed by a manganese pincer complex and proceed in absence of added base or hydrogen acceptors, liberating dihydrogen, dinitrogen, and water as the only byproducts. The proposed mechanism, based on preparation of proposed intermediates and control experiments, suggests that the transformation occurs through metal–ligand cooperative N−H activation of a hydrazone intermediate. [ABSTRACT FROM AUTHOR]

Production of carboxylic acids from alcohols is an important process for both industry and laboratory. Traditional methods usually require environmentally unfriendly oxidants and generate stoichiometric waste. Recently, methods using O2 as oxidant, oxidation processes applying stoichiometric hydrogen acceptors and acceptorless dehydrogenative coupling reactions to generate carboxylic acids/carboxylic acid salts have been developed. This chapter reviews the reported results on the generation of carboxylic acids/carboxylates by acceptorless dehydrogenative coupling of alcohols and water. The chapter is according to the types of catalysts used; reaction conditions, product yields, and mechanisms are also discussed. [ABSTRACT FROM AUTHOR]

Abstract: An unprecedented one‐step synthesis of N‐substituted hydrazones by coupling of alcohols with hydrazine is reported. This partial hydrogen‐borrowing reaction is catalyzed by a new manganese pincer complex under mild reaction conditions, thus liberating water and dihydrogen as the only byproducts. Mechanistic insight, based on the observation of intermediates, is provided. [ABSTRACT FROM AUTHOR]

The first example of base-metal-catalysed synthesis of amides from the coupling of primary amines with either alcohols or esters is reported. The reactions are catalysed by a new manganese pincer complex and generate hydrogen gas as the sole byproduct, thus making the overall process atom-economical and sustainable. [ABSTRACT FROM AUTHOR]

The first example of a base metal (manganese) catalyzed acceptorless dehydrogenative coupling of methanol and amines to form formamides is reported herein. The novel pincer complex ( iPr-PNHP)Mn(H)(CO)2 catalyzes the reaction under mild conditions in the absence of any additives, bases, or hydrogen acceptors. Mechanistic insight based on the observation of an intermediate and DFT calculations is also provided. [ABSTRACT FROM AUTHOR]

Herein, the first example of base-metal-catalyzed dehydrogenative coupling of diols and amines to selectively form functionalized 1,2,5-substituted pyrroles liberating water and hydrogen gas as the sole by-products is presented. The reaction is catalyzed by pincer complexes of earth-abundant cobalt. [ABSTRACT FROM AUTHOR]

A novel and simple hydrogen storage system was developed, based on the dehydrogenative coupling of inexpensive ethylenediamine with ethanol to form diacetylethylenediamine. The system is rechargeable and utilizes the same ruthenium pincer catalyst for both hydrogen loading and unloading procedures. It is efficient and uses a low catalyst loading. Repetitive reversal reactions without addition of new catalyst result in excellent conversions in both the dehydrogenation and hydrogenation procedures in three cycles. [ABSTRACT FROM AUTHOR]

Metal-ligand cooperation (MLC) plays an important role in catalysis. Systems reported so far are generally based on a single mode of MLC. We report here a system with potential for MLC by both amine-amide and aromatization-dearomatization ligand transformations, based on a new class of phosphino-pyridyl ruthenium pincer complexes, bearing sec-amine coordination. These pincer complexes are effective catalysts under unprecedented mild conditions for acceptorless dehydrogenative coupling of alcohols to esters at 35 °C and hydrogenation of esters at room temperature and 5 atm H2. The likely actual catalyst, a novel, crystallographically characterized monoanionic de-aromatized enamido-RuII complex, was obtained by deprotonation of both the NH and the methylene proton of the N-arm of the pincer ligand. [ABSTRACT FROM AUTHOR]

Dehydrogenative coupling of primary alcohols with secondary amines to form tertiary amides and dihydrogen (H2) is efficiently catalyzed by bipyridyl-based ruthenium pincer complexes (0.2-1 mol%) under neutral conditions (in case of the dearomatized complexes), or with added catalytic amount of base. The reaction is sensitive to steric hindrance; in the case of amidation of bulky secondary amines a less sterically hindered complex is more efficient. Selective acylation of primary amines in the presence of secondary amines was also demonstrated. [ABSTRACT FROM AUTHOR]

The synthesis and reactivity of ruthenium complexes containing the tetradentate phenanthroline-based phosphine ligand 2,9-bis((di- tert-butylphosphino)methyl)-1,10-phenanthroline (PPhenP) is described. The hydrido chloro complex [RuHCl(PPhenP)] ( 2) undergoes facile dearomatization upon deprotonation of the benzylic position, to give [RuH(PPhenP- H)] ( 4). Addition of dihydrogen to 4 causes rearomatization of the phenanthroline moiety to trans-[Ru(H)2(PPhenP)] ( 5), followed by hydrogenation of an aromatic heterocycle in the ligand backbone, to give a new dearomatized and deconjugated complex [RuH(PPhenP*- H)] ( 6). These aromatization/deconjugation steps of the coordinated ligand were demonstrated to be reversible and operative in the dehydrogenation of primary alcohols without the need for a hydrogen acceptor. This aromatization/deconjugation sequence constitutes an unprecedented mode of a stepwise cooperation between the metal center and the coordinated ligand. [ABSTRACT FROM AUTHOR]

Cross-dehydrogenative coupling of primary alcohols with secondary alcohols to obtain mixed esters with the liberation of molecular hydrogen is achieved in high yield and good selectivity under neutral conditions, using a bipyridyl-based PNN ruthenium(II) pincer catalyst. [ABSTRACT FROM AUTHOR]

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Examines the cyclometalated phosphine-based pincer complexer. Formation of functional nanoscale assemblies; Generation of an efficient dehydrogenation; Enhancement of metal complex reactivity, stability, reaction, and selectivity.

A novel, one-step synthesis of substituted pyridine- and quinoline-derivatives was achieved by acceptorless dehydrogenative coupling of γ-aminoalcohols with secondary alcohols. The reaction involves consecutive C–N and C–C bond formation, catalyzed by a bipyridyl-based ruthenium pincer complex with a base. [ABSTRACT FROM AUTHOR]

Review: 58 refs. [ABSTRACT FROM AUTHOR]

The article presents the chemical steps involved in the applications of acceptorless dehydrogenation in chemical synthesis.

Primary alcohol dehydrogenation by a PNP-Ru(II) catalyst was probed by low-temperature NMR experiments. Facile dehydrogenation occurred at -30 °C, but the resulting aldehydes were not found in solution, as they were trapped by the catalyst through a new mode of metal-ligand cooperation involving Ru-O coordination and an unusual, highly reversible C-C coupling with the PNP pincer ligand. [ABSTRACT FROM AUTHOR]

The first example of base-metal-catalyzed dehydrogenative coupling of diols and amines to form cyclic imides is reported. The reaction is catalyzed by a pincer complex of the earth abundant manganese and forms hydrogen gas as the sole byproduct, making the overall process atom economical and environmentally benign. [ABSTRACT FROM AUTHOR]

A ferraquinone-ferrahydroquinone organometallic redox couple was prepared and characterized. Intricate cooperativity of the metal was observed with different positions on the ligand. This allowed cooperative activation of small molecules like molecular hydrogen, oxygen, and bromine. Likewise, dehydrogenation of alcohols was achieved through 1,6 metal-ligand cooperation. [ABSTRACT FROM AUTHOR]

A simple and higlhy efficient method for the dehydrogenative conversion of amino alcohols to structurally diverse amino acids is developed using sodium or potassium hydroxide as base and a ruthenium pincer complex as catalyst. [ABSTRACT FROM AUTHOR]

The catalytic dehydrogenative coupling of alcohols and amines toform aldimines represents an environmentally benign methodology in organic chemistry. This has been accomplished in recent years mainly with precious-metal-based catalysts. We present the dehydrogenative coupling of alcohols and amines toform imines and H2 that is catalyzed, for the first time, by a complex of the earth-abundant Mn. Detailed mechanistic study was carried out with the aid of NMR spectroscopy, intermediate isolation, and X-ray analysis. [ABSTRACT FROM AUTHOR]

The article presents chemical equations related to the article "Oxidant-Free Conversion of Cyclic Amines to Lactams and H2 Using Water as the Oxygen Atom Source" by J. R. Khusnutdinova and Y. Ben-David, published in the "Journal of the American Chemical Society " in 2014.

The PNS pincer-type ligand 1and the novelRu(PNS)complexes 2–8were synthesized andcharacterized. The (PNS)RuH(Cl)CO complex 2was preparedby reaction of ligand 1with RuH(Cl)CO(PPh3)3. 2reacted with KHMDS (potassium bis(trimethylsilyl)amide)to form the symmetrical dimeric complex 4via the intermediacyof the dearomatized complex (PNS*)Ru(H)CO 3, in whichdeprotonation of the benzylic-S “arm” took place. Reactionof 2with excess NaH gave the dimeric 4,by a formal intermolecular attack of the benzylic “arm”on a second ruthenium center. Complex 4underwent spontaneoustransformation in solution to the dinuclear complex 5via C–S bond cleavage, resulting in the loss of a S-bound tBu group. Treatment of 2with KHMDS in the presenceof PEt3resulted in the trapping of intermediate 3in the form of the dearomatized complex 8.Reaction of 2with LiHBEt3gave the trans-dihydride complex 6, which reacted withCO2to give the formato complex 7, in whichthe formato ligand is located transto the hydride.Complexes 2, 4, and 5werealso investigated as catalysts for the dehydrogenative coupling ofalcohols with amines. [ABSTRACT FROM AUTHOR]

The article reports on system with potential dual modes of metal-ligand Cooperation.

The direct amidation of primary alcohols with challenging secondary amines is achieved in the presence of small amounts of Ru-catalysts. [ABSTRACT FROM AUTHOR]

It is crucial to use an excess of the secondary alcohols to achieve the high yields of the esters. [ABSTRACT FROM AUTHOR]