Borrowing Hydrogen for Organic Synthesis

Functional group tolerant hydrogen borrowing C-alkylation

Hydrogen borrowing is an attractive and sustainable strategy for carbon-carbon bond formation that enables alcohols to be used as alkylating reagents in place of alkyl halides. However, despite intensive efforts, limited functional group tolerance is observed in this methodology, which we hypothesize is due to the high temperatures and harsh basic conditions often employed. Here we demonstrate that room temperature and functional group tolerant hydrogen borrowing can be achieved with a simple iridium catalyst in the presence of substoichiometric base without an excess of reagents. Achieving high yields necessitates the application of anaerobic conditions to counteract the oxygen sensitivity of the catalytic iridium hydride intermediate, which otherwise leads to catalyst degradation. Substrates containing heteroatoms capable of complexing the catalyst exhibit limited room temperature reactivity, but the application of moderately higher temperatures enables extension to a broad range of medicinally relevant nitrogen rich heterocycles. These newly developed conditions allow alcohols possessing functional groups that were previously incompatible with hydrogen borrowing reactions to be employed.

Development of ruthenium complexes with S-donor ligands for application in synthesis, catalytic acceptorless alcohol dehydrogenation and crossed-aldol condensation

The reaction of [Ru(dmso)4Cl2] with a potassium salt of four xanthate (RO-C(S)S-; R = Me, Et, iPr and tBu) ligands (depicted as Ln; n = 1-4) in hot methanol afforded a group of mixed-ligand complexes of type [Ru(Ln)2(dmso)2]. The crystal structures of all the four complexes have been determined, which show that the xanthate ligands are bound to the metal center forming four-membered chelates and dmso is coordinated through sulfur and they are mutually cis. The relative thermodynamic stability of this cis and the other possible trans-isomers of these complexes has been assessed with the help of DFT calculations, which have revealed that the cis-isomer is the more stable isomer. The coordinated dmso in the [Ru(Ln)2(dmso)2] complexes could be easily displaced by chelating bidentate ligands (depicted as L') to furnish complexes of type [Ru(Ln)2(L')], as demonstrated through isolation of two such complexes, viz. [Ru(L3)2(bpy)] and [Ru(L2)2(phen)] (bpy = 2,2'-bipyridine and phen = 1,10-phenanthroline). The crystal structure of [Ru(L3)2(bpy)] has been determined and the structure of [Ru(L2)2(phen)] has been optimized by the DFT method. The electronic spectra of the four [Ru(Ln)2(dmso)2] complexes and the two derivatives ([Ru(Ln)2(L')]; n = 3, L' = bpy; n = 2, L' = phen), recorded in dichloromethane solutions, show intense absorptions spanning the visible and ultraviolet regions, which have been analyzed by the TDDFT method. The [Ru(Ln)2(dmso)2] complexes are found to serve as efficient catalyst precursors for the acceptorless dehydrogenation of 2-propanol followed by crossed-aldol condensation with substituted benzaldehydes (and related aldehydes), using tert-butoxide as the co-catalyst, producing dibenzylideneacetone derivatives in good yields.

Ruthenium Complexes with NNN-Pincer Ligands for N-Methylation of Amines Using Methanol

A series of ruthenium complexes (Ru1-Ru4) bearing new NNN-pincer ligands were synthesized in 58-78% yields. All of the complexes are air and moisture stable and were characterized by IR, NMR, and high-resolution mass spectra (HRMS). In addition, the structures of Ru1-Ru3 were confirmed by X-ray crystallographic analysis. These Ru(II) complexes exhibited high catalytic efficiency and broad functional group tolerance in the N-methylation reaction of amines using CH3OH as both the C1 source and solvent. Experimental results indicated that the electronic effect of the substituents on the ligands considerably affects the catalytic reactivity of the complexes in which Ru3 bearing an electron-donating OMe group showed the highest activity. Deuterium labeling and control experiments suggested that the dehydrogenation of methanol to generate ruthenium hydride species was the rate-determining step in the reaction. Furthermore, this protocol also provided a ready approach to versatile trideuterated N-methylamines under mild conditions using CD3OD as a deuterated methylating agent.

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.

Ruthenium-Catalyzed Selective Mono N-Ethylation of Arylamines and Tandem Reduction/N-Ethylation of Nitroarenes Using Triethylamine and Formic Acid

The mono N-alkylation of arylamines using alkylamines as alkyl group donors has been scarcely investigated. In this work, we report the mono N-alkylation of several arylamines (52-95%) catalyzed by the complex ruthenium-triphos in the presence of Al(OTf)3. Moreover, the highly reductant ability of the catalyst system allows the tandem reduction/N-alkylation of nitrobenzenes in good yields (up to 80%). In addition, the catalyst can be recycled after three reaction cycles without loss of catalyst activity.

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 sustainable strategic approach for N-alkylation of amines with activation of alcohols triggered via a hydrogen auto-transfer reaction using a Pd(II) complex: evidence for metal-ligand cooperativity

This work describes a new well-defined, air-stable, phosphine free palladium(II) [Pd(L)Cl] (1) catalyst. This catalyst was utilized for N-alkylation of amines and indole synthesis where H2O was found to be the by-product. A broad range of aromatic amines were alkylated using this homogeneous catalyst with a catalyst loading of 0.1 mol%. Greener aromatic and aliphatic primary alcohols were utilized and a hydrogen auto-transfer strategy via a metal-ligand cooperative approach was investigated. The precursor of the antihistamine-containing drug molecule tripelennamine was synthesized on a gram scale for large-scale applicability of the current synthetic methodology. A number of control experiments were performed to investigate the possible reaction pathway and the outcomes of these experiments indicated the azo-chromophore as a hydrogen reservoir during the catalytic cycle.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

A Guide for Mono-Selective N-Methylation, N-Ethylation, and N-n-Propylation of Primary Amines, Amides, and Sulfonamides and Their Applicability in Late-Stage Modification

This review provides a comprehensive overview of mono-alkylation methodologies targeting crucial nitrogen moieties - amines, amides, and sulfonamides - found in organic building blocks and pharmaceuticals. Emphasizing the intersection of chemical precision with drug discovery, the central challenge addressed is achieving one-pot mono-selective short-chain N-alkylations (methylations, ethylations, and n-propylations), preventing undesired overalkylation. Additionally, sustainable, safe, and benign alternatives to traditional alkylating agents, including alcohols, carbon dioxide, carboxylic acids, nitriles, alkyl phosphates, quaternary ammonium salts, and alkyl carbonates, are explored. This review, categorized by the nature of the alkylating agent, aids researchers in selecting suitable methods for mono-selective N-alkylation.

Cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles

This Feature Article discusses recent advances in the development of cascade ring expansion reactions for the synthesis of medium-sized rings and macrocycles. Cascade ring expansion reactions have much potential for use in the synthesis of biologically important medium-sized rings and macrocycles, most notably as they don't require high dilution conditions, which are commonly used in established end-to-end macrocyclisation methods. Operation by cascade ring expansion method can allow large ring products to be accessed via rearrangements that proceed exclusively by normal-sized ring cyclisation steps. Ensuring that there is adequate thermodynamic driving force for ring expansion is a key challenge when designing such methods, especially for the expansion of normal-sized rings into medium-sized rings. This Article is predominantly focused on methods developed in our own laboratory, with selected works by other groups also discussed. Thermodynamic considerations, mechanism, reaction design, route planning and future perspective for this field are all covered.

Harnessing alcohols as sustainable reagents for late-stage functionalisation: synthesis of drugs and bio-inspired compounds

Alcohol is ubiquitous with unparalleled structural diversity and thus has wide applications as a native functional group in organic synthesis. It is highly prevalent among biomolecules and offers promising opportunities for the development of chemical libraries. Over the last decade, alcohol has been extensively used as an environmentally friendly chemical for numerous organic transformations. In this review, we collectively discuss the utilisation of alcohol from 2015 to 2023 in various organic transformations and their application toward intermediates of drugs, drug derivatives and natural product-like molecules. Notable features discussed are as follows: (i) sustainable approaches for C-X alkylation (X = C, N, or O) including O-phosphorylation of alcohols, (ii) newer strategies using methanol as a methylating reagent, (iii) allylation of alkenes and alkynes including allylic trifluoromethylations, (iv) alkenylation of N-heterocycles, ketones, sulfones, and ylides towards the synthesis of drug-like molecules, (v) cyclisation and annulation to pharmaceutically active molecules, and (vi) coupling of alcohols with aryl halides or triflates, aryl cyanide and olefins to access drug-like molecules. We summarise the synthesis of over 100 drugs via several approaches, where alcohol was used as one of the potential coupling partners. Additionally, a library of molecules consisting over 60 fatty acids or steroid motifs is documented for late-stage functionalisation including the challenges and opportunities for harnessing alcohols as renewable resources.

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.

Recent advances in cross-coupling of alcohols via borrowing hydrogen catalysis

Application of the borrowing hydrogen strategy facilitates utilization of abundantly available alcohols for linear or branched long-chain alcohols. Selective synthesis of such alcohols is highly challenging and involves the utilization of transition metal catalysts towards the desired cross-coupled product. Herein, we have highlighted recent advances (from 2015 to 2023) towards the synthesis of higher alcohols. Major focus has been given to the development of ligands, including transition metal catalysts. Judicious catalyst design plays a key role in the alkylation process and is summarised in this review.

Manganese Complex Catalyzed Sequential Multi-component Reaction: Enroute to a Quinoline-Derived Azafluorenes

The development of innovative synthetic strategies for constructing complex molecular structures is the heart of organic chemistry. This significance of novel reactions or reaction sequences would further enhance if they permitted the synthesis of new classes of structural motifs, which have not been previously created. The research on the synthesis of heterocyclic compounds is one of the most active topics in organic chemistry due to the widespread application of N-heterocycles in life and material science. The development of a new catalytic process that employs first-row transition metals to produce a range of heterocycles from renewable raw materials is considered highly sustainable approach. This would be more advantageous if done in an eco-friendly and atom-efficient manner. Herein we introduce, the synthesis of various new quinoline based azafluorenes via sequential dehydrogenative multicomponent reaction (MCR) followed by C(sp3)-H hydroxylation and annulation. Our newly developed, Mn-complexes have the ability to direct the reaction in order to achieve a high amount of desired functionalized heterocycles while minimizing the possibility of multiple side reactions. We also performed a series of control experiments, hydride trapping experiments, reaction kinetics, catalytic intermediate and DFT studies to comprehend the detailed reaction route and the catalyst's function in the MCR sequence.

Lewis Acid-Driven Inverse Hydride Shuttle Catalysis

Inverse hydride shuttle catalysis provides a multicomponent platform for the highly efficient synthesis of alkaloid frameworks with exquisite diastereoselectivity. However, a number of limitations hinder this method, primarily the strict requirement for highly electron-deficient acceptors. Herein, we present a general Lewis acid-driven approach to address this constraint, and have developed two broad strategies enabling the modular synthesis of complex azabicycles that were entirely unattainable using the previous method. The enhanced synthetic flexibility facilitates a streamlined asymmetric cyclization, leading to a concise total synthesis of the alkaloid (-)-tashiromine.

Biocatalytic Hydrogen-Borrowing Cascade in Organic Synthesis

Biocatalytic hydrogen borrowing represents an environmentally friendly and highly efficient synthetic method. This innovative approach involves converting various substrates into high-value-added products, typically via a one-pot, two/three-step sequence encompassing dehydrogenation (intermediate transformation) and hydrogenation processes employing the hydride shuffling between NAD(P)+ and NAD(P)H. Represented key transformations in hydrogen borrowing include stereoisomer conversion within alcohols, conversion between alcohols and amines, conversion of allylic alcohols to saturated carbonyl counterparts, and α,β-unsaturated aldehydes to saturated carboxylic acids, etc. The direct transformation methodology and environmentally benign characteristics of hydrogen borrowing have contributed to its advancements in fine chemical synthesis or drug developments. Over the past decades, the hydrogen borrowing strategy in biocatalysis has led to the creation of diverse catalytic systems, demonstrating substantial potential for straightforward synthesis as well as asymmetric transformations. This perspective serves as a detailed exposition of the recent advancements in biocatalytic reactions employing the hydrogen borrowing strategy. It provides insights into the potential of this approach for future development, shedding light on its promising prospects in the field of biocatalysis.

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.

Preparation of a novel cadmium-containing coordination polymer and catalytic application in the synthesis of N-alkylated aminoquinoline derivatives via the borrowing hydrogen approach

Herein, we report an efficient and straightforward approach for the synthesis of N-alkylated aminoquinoline derivatives by recyclable Cd-containing coordination polymer-catalyzed reactions of aminoquinolines with primary alcohols via the borrowing hydrogen strategy. In this work, a new type of coordination polymer [Cd(CIA)(phen)2(H2O)]n was successfully designed and fabricated. The molecular structure was corroborated by single-crystal X-ray diffraction and fully characterized by PXRD, FT-IR, TGA, and XPS. Importantly, this polymer revealed high catalytic activity for the N-alkylation reaction of 2-aminoquinoline and 8-aminoquinoline with inexpensive and low-toxicity alcohols as alkylating agents in excellent yields up to 95%. Interestingly, the present synthetic protocol was successfully applied for the gram-level synthesis of several biologically active compounds. In addition, several control reactions were carried out to investigate the possible mechanisms of this transformation. Finally, recycling experiments indicated that the cadmium coordination polymer showed good recovery performance for borrowing hydrogen reactions.

Manganese-catalyzed Efficient Synthesis of N-heterocycles and Aminoketones Using Glycerol as a C3 Synthon

Glycerol is one of the important biomass-derived feedstocks and the high-value utilizations of glycerol have attracted much attentions in recent years. Herein, we report a manganese catalyzed dehydrogenative coupling of glycerol with amines for the synthesis of substituted 2-methylquinoxalines, 2-ethylbenzimidazoles, and α-aminoketones without any external oxidant. In these reactions, NHC-based pincer manganese complex featuring a pyridine backbone displayed high catalytic activity and selectivity, in which hydrogen and water were produced as the only by-products using glycerol as a C3 synthon.

Micro-flow heteroatom alkylation via TfOH-mediated rapid in situ generation of carbocations and subsequent nucleophile addition

A rapid nucleophilic substitution reaction was developed using carbocations generated from diarylmethanol and trifluoromethanesulfonic acid. Undesired reactions caused by the carbocations were suppressed, presumably due to the rapid and uniform generation of carbocations and the subsequent rapid and uniform distribution of nucleophiles by the micro-flow technology.

Copper/Ruthenium Relay Catalysis for Stereodivergent Access to δ-Hydroxy α-Amino Acids and Small Peptides

An atom- and step-economical and redox-neutral cascade reaction enabled by asymmetric bimetallic relay catalysis by merging a ruthenium-catalyzed asymmetric borrowing-hydrogen reaction with copper-catalyzed asymmetric Michael addition has been realized. A variety of highly functionalized 2-amino-5-hydroxyvaleric acid esters or peptides bearing 1,4-non-adjacent stereogenic centers have been prepared in high yields with excellent enantio- and diastereoselectivity. Judicious selection and rational modification of the Ru catalysts with careful tuning of the reaction conditions played a pivotal role in stereoselectivity control as well as attenuating undesired α-epimerization, thus enabling a full complement of all four stereoisomers that were otherwise inaccessible in previous work. Concise asymmetric stereodivergent synthesis of the key intermediates for biologically important chiral molecules further showcases the synthetic utility of this methodology.

Air-Stable Coordinatively Unsaturated Ruthenium(II) Complex for Ligand Binding and Catalytic Transfer Hydrogenation of Ketones from Ethanol

Coordinatively unsaturated complexes are interesting from a fundamental level for their formally empty coordination site and, in particular, from a catalytic perspective as they provide opportunities for substrate binding and transformation. Here, we describe the synthesis of a novel underligated ruthenium complex [Ru(cym)(N,N')]+, 3, featuring an amide-functionalized pyridylidene amide (PYA) as the N,N'-bidentate coordinating ligand. In contrast to previously investigated underligated complexes, complex 3 offers potential for dynamic modifications, thanks to the flexible donor properties of the PYA ligand. Specifically, they allow both for stabilizing the formally underligated metal center in complex 3 through nitrogen π-donation and for facilitating through π-acidic bonding properties the coordination of a further ligand L to the ruthenium center to yield the formal 18 e- complexes [Ru(cym)(N,N')(L)]+ (4: L = P(OMe)3; 5: L = PPh3; 6: L = N-methylimidazole; 7: L = pyridine) and neutral complex [RuCl(cym)(N,N')] 8. Analysis by 1H NMR and UV-vis spectroscopies reveals an increasing Ru-L bond strength along the sequence pyridine <1-methylimidazole < PPh3 < P(OMe)3 with binding constants varying over 3 orders of magnitude with log(Keq) values between 2.8 and 5.7. The flexibility of the Ru(PYA) unit and the ensuing accessibility of saturated and unsaturated species with one and the same ligand are attractive from a fundamental point of view and also for catalytic applications, as catalytic transformations rely on the availability of transiently vacant coordination sites. Thus, while complex 3 does not form stable adducts with O-donors such as ketones or alcohols, it transiently binds these species, as evidenced by the considerable catalytic activity in the transfer hydrogenation of ketones. Notably, and as one of only a few catalysts, complex 3 is compatible with EtOH as a hydrogen source. Complex 3 shows excellent performance in the transfer hydrogenation of pyridyl-containing substrates, in agreement with the poor coordination strength of this functional group to the ruthenium center in 3.

NNN manganese complex-catalyzed α-alkylation of methyl ketones using alcohols: an experimental and computational study

We present here a phosphine-free, quinoline-based pincer Mn catalyst for α-alkylation of methyl ketones using primary alcohols as alkyl surrogates. The C-C bond formation reaction proceeds via a hydrogen auto-transfer methodology. The sole by-product formed is water, rendering the protocol atom efficient. Electronic structure theory studies corroborated the proposed mechanism.

Heteroditopic NHC Ligand Supported Manganese(I) Complexes: Synthesis, Characterization, and Activity as Non-bifunctional Phosphine-Free Catalyst for the α-Alkylation of Nitriles

In the present work, several manganese(I) complexes of chelating heteroditopic ligands Mn1-3, featuring ImNHC (imidazol-2-ylidene) connected to a 1,2,3-triazole-N or tzNHC (1,2,3-triazol-5-ylidene) donors via a methylene spacer, with possible modifications at the triazole backbone have been synthesized and completely characterized. Notably, the CO stretching frequencies, electrochemical analysis, and frontier orbital analysis certainly suggest that the chelating ImNHC-tzNHC ligands have stronger donation capabilities than the related ImNHC-Ntz ligand in the synthesized complexes. Moreover, these well-defined phosphine-free Mn(I)-NHC complexes have been found to be effective non-bifunctional catalysts for the α-alkylation of nitriles using alcohols and importantly, the catalyst Mn1 containing ImNHC connected to a weaker triazole-N donor displayed higher activity compared to Mn2/Mn3 containing an unsymmetrical bis-carbene donors (ImNHC and tzNHC). A wide range of aryl nitriles were coupled with diverse (hetero)aromatic as well as aliphatic alcohols to get the corresponding products in good to excellent yields (32 examples, up to 95 % yield). The detailed mechanistic studies including deuterium labelling experiments reveal that the reaction follows a Borrowing Hydrogen pathway.

Synthesis of Macrolactams from Macrolactones Using Ru-/Ir-Catalytic System under Neutral Conditions

Herein, we report the Ru-/Ir-catalyzed synthesis of valuable macrolactams from macrolactones and esters. The ring-opening of the macrolactones was efficaciously facilitated by the Ru catalyst to generate 32 amides in the first step. In the second step, intramolecular N-alkylative ring closure of amides with alcohols was succeeded by Ir catalyst to provide a series of 22 macrolactams and gave water as a byproduct. Moreover, this approach proceeded under neutral conditions and avoided the use of external additives.

Titanium-Catalyzed Selective N-Alkylation of Amines with Alcohols via Borrowing Hydrogen Methodology

The N-alkylation of amines with alcohols using earth-abundant and nonprecious metal catalysts has gained considerable attention in the pharmaceutical industry. We described titanium-catalyzed synthetic protocol for N-alkylation of amines with alcohols via borrowing hydrogen or hydrogen autotransfer reactions. The methodology enables the selective monoalkylation of various substituted (hetero)aromatic amines in good to excellent yields (up to 97% yield). The importance of the protocol was further demonstrated by the applicability of earth-abundant metal catalysis and the synthesis of 32 N-alkylated amines. The work allows the utilization of titanium-based catalysts for various reactions to expand the nature blueprint in catalysis.

Ruthenium-Catalyzed Self-Coupling of Secondary Alcohols

A simple catalytic method for self-coupling of secondary alcohols leading to the synthesis of β-branched ketones under mild conditions is reported. Well-defined ruthenium pincer complex catalyzed the reactions. Optimization studies revealed that sodium tert-butoxide is an appropriate base for this transformation. Functionalized aryl methanols, heteroaryl methanols, and linear and branched aliphatic secondary alcohols underwent facile catalytic self-coupling reactions. Mechanistic studies revealed that both catalyst and base are crucial to achieve dehydrogenation of secondary alcohols to ketones, their subsequent controlled aldol condensation, and further hydrogenation of α,β-unsaturated intermediates, leading to the selective formation of β-branched ketone products. Notably, the noninnocent PNP ligand which displays amine-amide metal-ligand cooperation operative in a catalyst played a key role in facilitating this catalytic self-coupling of secondary alcohols. Liberated molecular hydrogen and water are the only byproducts.

Advances in Group VI Metal-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions

Transition metal-catalyzed homogeneous hydrogenation and dehydrogenation reactions for attaining plethora of organic scaffolds have evolved as a key domain of research in academia and industry. These protocols are atom-economic, greener, in line with the goal of sustainability, eventually pave the way for numerous novel environmentally benign methodologies. Appealing progress has been achieved in the realm of homogeneous catalysis utilizing noble metals. Owing to their high cost, less abundance along with toxicity issues led the scientific community to search for sustainable alternatives. In this context, earth- abundant base metals have gained substantial attention culminating enormous progress in recent years, predominantly with pincer-type complexes of nickel, cobalt, iron, and manganese. In this regard, group VI chromium, molybdenum and tungsten complexes have been overlooked and remain underdeveloped despite their earth-abundance and bio-compatibility. This review delineates a comprehensive overview in the arena of homogeneously catalysed (de)hydrogenation reactions using group VI base metals chromium, molybdenum, and tungsten till date. Various reactions have been described; hydrogenation, transfer hydrogenation, dehydrogenation, acceptorless dehydrogenative coupling, hydrogen auto transfer, along with their scope and brief mechanistic insights.

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.

Oxygen Dependent Switchable Selectivity during Ruthenium Catalyzed Selective Synthesis of C3-Alkylated Indoles and Bis(indolyl)methanes

Herein, we report a ligand-centered redox-controlled oxygen-dependent switchable selectivity during ruthenium-catalyzed selective synthesis of C3-alkylated indoles and bis(indolyl)methanes (BIMs). A wide variety of C3-alkylated indoles and BIMs were prepared selectively in moderate to good isolated yields by coupling a wide variety of indoles and alcohols, catalyzed by a well-defined, air-stable, and easy-to-prepare Ru(II)-catalyst (1a) bearing a redox-active tridentate pincer (L1a). Catalyst 1a efficiently catalyzed the C3-alkylation of indoles under an argon atmosphere while, under an oxygen environment, exclusively producing the BIMs. A few drug molecules containing BIMs were also synthesized efficiently. 1a exhibited excellent chemoselectivity with alcohols containing internal carbon-carbon double bonds. Mechanistic investigation revealed that the coordinated azo-aromatic ligand actively participates during the catalysis. During the dehydrogenation of alcohols, the azo-moiety of the ligand stores the hydrogen removed from the alcohols and subsequently transfers the hydrogen to the alkylideneindolenine intermediate, forming the C3-alkylated indoles. While under an oxygen environment, the transfer of hydrogen from the ligand scaffold to the molecular oxygen generates H2O2, leaving no scope for hydrogenation of the alkylideneindolenine intermediate, rather than it undergoing 1,4-Michael-type addition forming the BIMs.

K2S2O8 promoted metal-free direct C-alkylation of acetophenones with alcohols

Herein, we report a metal-free synthetic methodology for the C-alkylation of acetophenones following a hydrogen borrowing-like pathway using the commercially available inorganic oxidant K2S2O8 in conjunction with KOtBu. This study articulates the potential of K2S2O8 in fast initiation of the oxidation of benzyl alcohols to develop an atom-economical, easy, and more efficient methodology for the C-alkylation of various acetophenones and synthesis of a variety of substituted quinolines. Experimental data from control experiments, literature and characterization of intermediates through spectroscopic techniques support the proposed plausible mechanism.

C3-Functionalization of indoles with α-heteroaryl-substituted methyl alcohols

The transition metal-free Cs2CO3/Oxone®-mediated C3-alkylation of indoles proceeds in moderate to high yields with a variety of C4-C7 functionalized indoles and is applicable to 2-, 3- and 4-hydroxymethyl pyridines and related electron-deficient heterocycles, permitting novel late-stage drug functionalizations. Preliminary mechanistic studies support a hydrogen autotransfer-type chain process starting with an initial oxidation of the alcohol to the corresponding aldehyde, followed by a subsequent condensation onto indole and reduction/hydride delivery from another equivalent of the primary alcohol.

Metal-organic framework-derived CoNx nanoparticles on N-doped carbon for selective N-alkylation of aniline

N-alkylation of anilines by alcohols can be used as an efficient strategy to synthesise a wide range of secondary amines. In this respect, a hydrogen borrowing methodology has been explored using precious metal-based catalysts. However, the utilisation of cheap and readily available transition metal based catalysts is required for large-scale applications. In this work, we have reported metal-organic framework-derived CoNx@NC catalysts for the selective N-alkylation of anilines with different types of alcohols. The Co-N coordination in CoNx@NC was found to be extremely important to improve the conversion efficiency and yield of the product. As a result, CoNx@NC produced 99% yield of the desired amines, which is far better than that of Co@C (yield = 65%). In addition, CoNx@NC showed remarkable recyclability for six cycles with a minimum drop in the yield of the desired product.

Catalytic Amination of Alcohols Using Diazo Compounds under Manganese Catalysis Through Hydrogenative N-Alkylation Reaction

Sustainable chemical production requires fundamentally new types of catalysts and catalytic technologies. The development of coherent and robust catalytic systems based on earth-abundant transition metals is essential, but highly challenging. Herein, we systematically explored a general hydrogenative cleavage/N-alkylation tandem of cyclic and acyclic diazo (N=N) compounds to value-added amines under manganese catalysis. The reaction is catalyzed by a single-site molecular manganese complex and proceeds via tandem dehydrogenation, transfer hydrogenation, and borrowing hydrogenation strategies. Interestingly, the reaction involves abundantly available renewable feedstocks, such as alcohols, that can act as (transfer)hydrogenating and alkylating agents. The synthetic application of our approach in large-scale pharmaceutical synthesis and easy access to highly demanding N-CH3 /CD3 derivatives are also demonstrated. Kinetic studies show that the reaction rate depends on the concentration of alcohol and Mn-catalyst and follows fractional orders. Several selective bond activation/formation reactions occur sequentially via amine-amide metal-ligand cooperation.

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Ruthenium-Catalyzed Selective α-Alkylation of β-Naphthols using Primary Alcohols: Elucidating the Influence of Base and Water

Functionalized arenes and arenols have diverse applications in chemical synthesis and material chemistry. Selective functionalization of arenols is a topic of prime interest. In particular, direct alkylation of arenols using alcohols is a challenging task. In this report, a ruthenium pincer catalyzed direct α-alkylation of β-naphthol using primary alcohols as alkylating reagents is reported. Notably, aryl and heteroaryl methanols and linear and branched aliphatic alcohols underwent selective alkylation reactions, in which water is the only byproduct. Notably, catalytically derived α-alkyl-β-naphthol products displayed high absorbance, emissive properties, and quantum yields (up to 93.2 %). Dearomative bromination on α-alkyl-β-naphthol is demonstrated as a synthetic application. Mechanistic studies indicate that the reaction involves an aldehyde intermediate. DFT studies support this finding and further reveal that a stoichiometric amount of base is required to make the aldol condensation as well as elementary steps required for regeneration of catalytically active species. In situ-generated water molecule from the aldol condensation reaction plays an important role in the regeneration of an active catalyst.

Synchronous Proton-Hydride Transfer by a Pyrazole-Functionalized Protic Mn(I) Complex in Catalytic Alcohol Dehydrogenative Coupling

A series of Mn(I) complexes Mn(L1 )(CO)3 Br, Mn(L2 )(CO)3 Br, Mn(L1 )(CO)3 (OAc) and Mn(L3 )(CO)3 Br [L1 =2-(5-tert-butyl-1H-pyrazol-3-yl)-1,8-naphthyridine, L2 =2-(5-tert-butyl-1H-pyrazol-3-yl)pyridine, L3 =2-(5-tert-butyl-1-methyl-1H-pyrazol-3-yl)-1,8-naphthyridine] were synthesized and fully characterized. The acid-base equilibrium between the pyrazole and the pyrazolato forms of Mn(L1 )(CO)3 Br was studied by 1 H NMR and UV-vis spectra. These complexes are screened as catalysts for acceptorless dehydrogenative coupling (ADC) of primary alcohols and aromatic diamines for the synthesis of benzimidazole and quinoline derivatives with the release of H2 and H2 O as byproducts. The protic complex Mn(L1 )(CO)3 Br shows the highest catalytic activity for the synthesis of 2-substituted benzimidazole derivatives with broad substrate scope, whereas a related complex [Mn(L3 )(CO)3 Br], which is devoid of the proton responsive β-NH unit, shows significantly reduced catalytic efficiency validating the crucial role of the β-NH functionality for the alcohol dehydrogenation reactions. Control experiments, kinetic and deuterated studies, and density functional theory (DFT) calculations reveal a synchronous hydride-proton transfer by the metal-ligand construct in the alcohol dehydrogenation step.

Direct Synthesis of Aldoximes: Ruthenium-Catalyzed Coupling of Alcohols and Hydroxylamine Hydrochloride

A catalytic method for the direct synthesis of oximes from alcohols and hydroxyl amine hydrochloride salt is reported. The reaction is catalyzed by a ruthenium pincer catalyst, which oxidizes alcohols involving amine-amide metal-ligand cooperation, and the in situ formed aldehydes condense with hydroxyl amine to deliver the oximes. Notably, the reaction requires only a catalyst and base; water and liberated hydrogen are the only byproducts, making this protocol attractive and environmentally benign.

meta-Selective C-H Functionalization of Pyridines

The pyridine moiety is an important core structure for a variety of drugs, agrochemicals, catalysts, and functional materials. Direct functionalization of C-H bonds in pyridines is a straightforward approach to access valuable substituted pyridines. Compared to the direct ortho- and para-functionalization, meta-selective pyridine C-H functionalization is far more challenging due to the inherent electronic properties of the pyridine entity. This review summarizes currently available methods for pyridine meta-C-H functionalization using a directing group, non-directed metalation, and temporary dearomatization strategies. Recent advances in ligand control and temporary dearomatization are highlighted. We analyze the advantages as well as limitations of current techniques and hope to inspire further developments in this important area.

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.

Enantioconvergent transformations of secondary alcohols through borrowing hydrogen catalysis

Direct substitution of readily available alcohols is recognized as a key research area in green chemical synthesis. Starting from simple racemic secondary alcohols, the achievement of catalytic enantioconvergent transformations of the substrates will be highly desirable for efficient access to valuable enantiopure compounds. To accomplish such attractive yet challenging transformations, the strategy of the enantioconvergent borrowing hydrogen methodology has proven to be uniquely effective and versatile. This review aims to provide an overview of the impressive progress made on this topic of research that has only thrived in the past decade. In particular, the conversion of racemic secondary alcohols to enantioenriched chiral amines, N-heterocycles, higher-order alcohols and ketones will be discussed in detail.

S-Alkylation of dithiocarbamates via a hydrogen borrowing reaction strategy using alcohols as alkylating agents

Herein, we report an operationally simple, environmentally benign and scalable approach towards the synthesis of S-benzyl/alkyl dithiocarbamates via a hydrogen borrowing reaction between alcohols and dithiocarbamate anions catalyzed using a hydroxyapatite-supported copper nano-catalyst. This strategy has a broad substrate scope and offers high yields of products using inexpensive and readily available alcohols as starting materials. The catalyst was prepared by easy and straightforward methods and analyzed by several analytical techniques, e.g., FESEM, HR-TEM, BET, XRD, EDS, and XPS, demonstrating the anchoring of Cu nanoparticles on hydroxyapatite in the zero oxidation state.

Silver Supported Nanoparticles on [Mg4 Al-LDH] as an Efficient Catalyst for the α-Alkylation of Nitriles, Oxindoles and Other Carboxylic Acid Derivatives with Alcohols

An efficient heterogeneous silver-catalyzed α-alkylation of nitriles and oxindoles using alcohols via borrowing hydrogen strategy has been developed for the first time. The active nanostructured material, namely [Ag/Mg4 Al-LDH], composed by silver nanoparticles (3-4 nm average particle size) homogeneously stabilized onto a [Mg4 Al-LDH] support with suitable Brønsted basic properties, constitutes a stable catalyst for the sustainable building of novel C-C bonds from alcohols and C-nucleophiles. By applying this catalyst, a broad range of α-functionalized nitriles and oxindoles has been accessed with good to excellent isolated yields and without the addition of external bases. Moreover, the novel silver nanocatalyst has also demonstrated its successful application to the cyclization of N-[2-(hydroxymethyl)phenyl]-2-phenylacetamides to afford 3-arylquinolin-2(1H)-ones, through a one-pot dehydrogenation and intramolecular α-alkylation. Control experiments, kinetic studies, and characterization data of a variety of [Ag/LDH]-type materials confirmed the silver role in the dehydrogenation and hydrogenation steps, while [Mg4 Al-LDH] matrix is able to catalyze condensation. Interestingly, these studies suggest as key point for the successful activity of [Ag/Mg4 Al-LDH], in comparison with other [Ag/LDH]-type nanocatalysts, the suitable acid-base properties of this material.

Nickel-Catalyzed Alkylation of Oxindoles with Secondary Alcohols

Herein, we have demonstrated a simple nickel-catalyzed C-3-selective alkylation of 2-oxindoles using a wide variety of secondary alkyl alcohols. As a special highlight, functionalization of the cholesterol derivative was reported. Control experiments, initial mechanistic studies, and deuterium-labeling experiments were performed for the alkylation process.

Ir-Catalyzed Synthesis of Functionalized Pyrrolidines and Piperidines Using the Borrowing Hydrogen Methodology

The Ir(III)-catalyzed synthesis of 3-pyrrolidinols and 4-piperidinols combining 1,2,4-butanetriol or 1,3,5-pentanetriol with primary amines was carried out. This borrowing hydrogen methodology was further extended to the sequential diamination of triols leading to amino-pyrrolidines and amino-piperidines.

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes

Protic pyrazoles (N-unsubstituted pyrazoles) have been versatile ligands in various fields, such as materials chemistry and homogeneous catalysis, owing to their proton-responsive nature. This review provides an overview of the reactivities of protic pyrazole complexes. The coordination chemistry of pincer-type 2,6-bis(1H-pyrazol-3-yl)pyridines is first surveyed as a class of compounds for which significant advances have made in the last decade. The stoichiometric reactivities of protic pyrazole complexes with inorganic nitrogenous compounds are then described, which possibly relates to the inorganic nitrogen cycle in nature. The last part of this article is devoted to outlining the catalytic application of protic pyrazole complexes, emphasizing the mechanistic aspect. The role of the NH group in the protic pyrazole ligand and resulting metal-ligand cooperation in these transformations are discussed.

N-Alkylation of Amines by C1-C10 Aliphatic Alcohols Using A Well-Defined Ru(II)-Catalyst. A Metal-Ligand Cooperative Approach

A Ru(II)-catalyzed efficient and selective N-alkylation of amines by C1-C10 aliphatic alcohols is reported. The catalyst [Ru(L^1a)(PPh₃)Cl₂] (1a) bearing a tridentate redox-active azo-aromatic pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (L^1a) is air-stable, easy to prepare, and showed wide functional group tolerance requiring only 1.0 mol % (for N-methylation and N-ethylation) and 0.1 mol % of catalyst loading for N-alkylation with C3-C10 alcohols. A wide array of N-methylated, N-ethylated, and N-alkylated amines were prepared in moderate to good yields via direct coupling of amines and alcohols. 1a efficiently catalyzes the N-alkylation of diamines selectively. It is even suitable for synthesizing N-alkylated diamines using (aliphatic) diols producing the tumor-active drug molecule MSX-122 in moderate yield. 1a showed excellent chemo-selectivity during the N-alkylation using oleyl alcohol and monoterpenoid β-citronellol. Control experiments and mechanistic investigations revealed that the 1a-catalyzed N-alkylation reactions proceed via a borrowing hydrogen transfer pathway where the hydrogen removed from the alcohol during the dehydrogenation step is stored in the ligand backbone of 1a, which in the subsequent steps transferred to the in situ formed imine intermediate to produce the N-alkylated amines.

Iridium-Catalyzed Enantioconvergent Borrowing Hydrogen Annulation of Racemic 1,4-Diols with Amines

We present an enantioconvergent access to chiral N-heterocycles directly from simple racemic diols and primary amines, through a highly economical borrowing hydrogen annulation. The identification of a chiral amine-derived iridacycle catalyst was the key for achieving high efficiency and enantioselectivity in the one-step construction of two C-N bonds. This catalytic method enabled a rapid access to a wide range of diversely substituted enantioenriched pyrrolidines including key precursors to valuable drugs such as aticaprant and MSC 2530818.

C-Alkylation by Phenalenyl-Based Molecule via a Borrowing Hydrogen Pathway

The present study demonstrates the first transition-metal-free catalytic C-alkylation via a borrowing hydrogen pathway for the α-alkylation of ketone, synthesis of substituted quinoline, and 9-monoalkylation of fluorene. With applications on diversification of biologically active molecules and gram-scale synthesis, a preliminary investigation of the reaction mechanism has been carried out, suggesting a radical-mediated borrowing hydrogen pathway.