Manganese-Catalyzed Anti-Markovnikov Hydroamination of Allyl Alcohols via Hydrogen-Borrowing Catalysis

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.

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.

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.

Synthesis of anti-depressant molecules via metal-catalyzed reactions: a review

Depression is one of the most mutilating conditions in the world today. It has been difficult to make advancements toward better, more effective therapies since the introduction of antidepressant medicines in the late 1950s. One important field of medicinal chemistry is the synthesis of antidepressant molecules through metal-catalyzed procedures. The important role that different transition metals, including iron, nickel, ruthenium, and others, serve as catalysts in the synthesis of antidepressants is examined in this review. Key structural motifs included in antidepressant drugs such as tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), and others can be synthesized in a variety of effective ways using metal-catalyzed steps. This review examines current developments in the catalytic synthesis of antidepressants and their potential application over the previous thirteen years.

Catalytic acceptorless dehydrogenative borylation of styrenes enabled by a molecularly defined manganese complex

In this study, we employed a 3d metal complex as a catalyst to synthesize alkenyl boronate esters through the dehydrogenative coupling of styrenes and pinacolborane. The process generates hydrogen gas as the sole byproduct without requiring an acceptor, rendering it environmentally friendly and atom-efficient. This methodology demonstrated exceptional selectivity for dehydrogenative borylation over direct hydroboration. Additionally, it exhibited a preference for borylating aromatic alkenes over aliphatic ones. Notably, derivatives of natural products and bioactive molecules successfully underwent diversification using this approach. The alkenyl boronate esters served as precursors for the synthesis of various pharmaceuticals and potential anticancer agents. Our research involved comprehensive experimental and computational studies to elucidate the reaction pathway, highlighting the B-H bond cleavage as the rate-determining step. The catalyst's success was attributed to the hemilability and metal-ligand bifunctionality of the ligand backbone.

Biomass-Derived Core-Shell Carbon Dots with Embedded Tripodal Receptors for the Selective Recognition of Mefenamic Acid in Pharmaceutical Formulations and Urine

A tripodal amine (TPA) with -OH, N, and S donors is synthesized to functionalize a core-shell carbon dot composite (FCDs@SiO2-TPA) for sensing application. The TPA is characterized by spectroscopic and spectrometric techniques, and the composite is characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectra (EDS) techniques. The composite has the ability to recognize mefenamic acid (MFA) selectively even in the presence of other drugs like ibuprofen sodium, acetylsalicylic acid, naproxen sodium, diclofenac sodium, and ketoprofen. It can also be used for the quantification of MFA by recording the emission quenching response of the sample at λexc. = 350 nm and λems. = 460 nm (linear range = 1-8 μM and LOD = 197 nM). The density functional theory calculations and 1H NMR titration suggest quenching of the emission signal due to photoinduced electron transfer via hydrogen bonding between the probe and MFA. The composite FCDs@SiO2-TPA has been demonstrated as a reliable and cost-effective sensing probe for the detection of MFA in pharmaceutical formulations, water samples, and cow urine samples.

Palladium-Catalyzed C-H Allylation/Annulation Reaction of Amides and Allylic Alcohols: Regioselective Construction of Vinyl-Substituted 3,4-Dihydroisoquinolones

A palladium-catalyzed highly regioselective C-H allylation/annulation reaction of N-sulfonyl amides with secondary or tertiary allylic alcohols has been developed to construct 3,4-dihydroisoquinolones bearing a synthetically valuable vinyl substituent. This cascade cyclization approach of allylic alcohols involving C-H allylation has not been reported previously. The commercially available allylic alcohol substrates, the only by-product of water, and the used terminal oxidant of O₂ provide environmentally benign advantages.

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.

Zn(II)-Catalyzed Selective N-Alkylation of Amines with Alcohols Using Redox Noninnocent Azo-Aromatic Ligand as Electron and Hydrogen Reservoir

We report a sustainable and eco-friendly approach for selective N-alkylation of various amines by alcohols, catalyzed by a well-defined Zn(II)-catalyst, Zn(L^a)Cl₂ (1a), bearing a tridentate arylazo scaffold. A total of 57 N-alkylated amines were prepared in good to excellent yields, out of which 17 examples are new. The Zn(II)-catalyst shows wide functional group tolerance, is compatible with the synthesis of dialkylated amines via double N-alkylation of diamines, and produces the precursors in high yields for the marketed drugs tripelennamine and thonzonium bromide in gram-scale reactions. Control reactions and DFT studies indicate that electron transfer events occur at the azo-chromophore throughout the catalytic process, which shuttles between neutral azo, one-electron reduced azo-anion radical, and two-electron reduced hydrazo forms acting both as electron and hydrogen reservoir, enabling the Zn(II)-catalyst for N-alkylation reaction.

Homogeneous Manganese-Catalyzed Hydrofunctionalizations of Alkenes and Alkynes: Catalytic and Mechanistic Tendencies

In recent years, many manganese-based homogeneous catalytic precursors have been developed as powerful alternatives in organic synthesis. Among these, the hydrofunctionalizations of unsaturated C-C bonds correspond to outstanding ways to afford compounds with more versatile functional groups, which are commonly used as building blocks in the production of fine chemicals and feedstock for the industrial field. Herein, we present an account of the Mn-catalyzed homogeneous hydrofunctionalizations of alkenes and alkynes with the main objective of finding catalytic and mechanistic tendencies that could serve as a platform for the works to come.

Recent advances in C/N-alkylation with alcohols through hydride transfer strategies

This review highlights the most recent reports in three powerful and ever-growing fields of borrowing hydrogen, acceptorless dehydrogenative coupling, and base-mediated hydride transfer strategies; which pave the way for generating reactive intermediates via shuttling hydrogen (or hydride) between starting materials without any need for an external hydrogen source to easily construct more complex structures. There is a thorough focus on diversifying the utility of alcohols for C/N-alkylation leading to the synthesis of branched ketones, alcohols, amines, indols, and 6-membered nitrogen-containing heterocycles such as pyridines and pyrimidines, various transformations with the focus on C-C and C-N bond-forming reactions via metal-based catalysis or metal-free approaches in this context to give a global overview in this area.

Manganese-Catalyzed Asymmetric Formal Hydroamination of Allylic Alcohols: A Remarkable Macrocyclic Ligand Effect

A unique family of chiral peraza N₆ -macrocyclic ligands, which are conformationally rigid and have a tunable saddle-shaped cavity, is described. Utilizing their manganese(I) complexes, the first example of earth-abundant transition metal-catalyzed asymmetric formal anti-Markovnikov hydroamination of allylic alcohols was realized, providing a practical access to synthetically important chiral γ-amino alcohols in excellent yields and enantioselectivities (up to 99 % yield and 98 % ee). The single-crystal structure of a Mn^I complex indicates that the manganese atom coordinates with the chiral dialkylamine moiety in a bidentate fashion. Further DFT calculations revealed that five of the six nitrogen atoms in the ligand were engaged in multiple noncovalent interactions with Mn, an isopropanol molecule, and a β-amino ketone intermediate via coordination, hydrogen bonding, and/or CH⋅⋅⋅π interactions in the transition state, showing a remarkable role of the macrocyclic framework.

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.

Ruthenium-Catalyzed Dehydrogenative Functionalization of Alcohols to Pyrroles: A Comparison between Metal-Ligand Cooperative and Non-cooperative Approaches

Herein, we report the synthesis and characterization of two ruthenium-based pincer-type catalysts, [1]X (X = Cl, PF₆) and 2, containing two different tridentate pincer ligands, 2-pyrazolyl-(1,10-phenanthroline) (L¹) and 2-arylazo-(1,10-phenanthroline) (L^2a/2b, L^2a = 2-(phenyldiazenyl)-1,10-phenanthroline; L^2b = 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline), and their application in the synthesis of substituted pyrroles via dehydrogenative alcohol functionalization reactions. In catalyst [1]X (X = Cl, PF₆), the tridentate scaffold 2-pyrazolyl-(1,10-phenanthroline) (L¹) is apparently redox innocent, and all the redox events occur at the metal center, and the coordinated ligands remain as spectators. In contrast, in catalysts 2a and 2b, the coordinated azo-aromatic scaffolds are highly redox-active and known to participate actively during the dehydrogenation of alcohols. A comparison between the catalytic activities of these two catalysts was made, starting from the simple dehydrogenation of alcohols to further dehydrogenative functionalization of alcohols to various substituted pyrroles to understand the advantages/disadvantages of the metal-ligand cooperative approach. Various substituted pyrroles were prepared via dehydrogenative coupling of secondary alcohols and amino alcohols, and the N-substituted pyrroles were synthesized via dehydrogenative coupling of aromatic amines with cis-2-butene-1,4-diol and 2-butyne-1,4-diol, respectively. Several control reactions and spectroscopic experiments were performed to characterize the catalysts and establish the reaction mechanism.

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.

Fac-to-mer isomerization triggers hydride transfer from Mn(I) complex fac-[(dppm)Mn(CO)3H]

Low-temperature IR and NMR studies combined with DFT calculations revealed the mechanistic complexity of apparently simple reactions between Mn(I) complex fac-[(dppm)Mn(CO)₃H] and Lewis acids (LA = Ph₃C⁺, B(C₆F₅)₃) involving the formation of so-far elusive meridional hydride species mer-[(dppm)Mn(CO)₃H⋯LA] and unusual dearomatization of the Ph₃C⁺ cation upon hydride transfer.

Well-Defined NNS-Mn Complex Catalyzed Selective Synthesis of C-3 Alkylated Indoles and Bisindolylmethanes Using Alcohols

Herein, we demonstrated Mn-catalyzed selective C-3 functionalization of indoles with alcohols. The developed catalyst can also furnish bis(indolyl)methanes from the same set of substrates under slightly modified reaction conditions. Mechanistic studies reveal that the C-3 functionalization of indoles is going via a borrowing hydrogen pathway. To highlight the practical utility, a diverse range of substrates including nine structurally important drug molecules are synthesized. Furthermore, we also introduced a one-pot cascade strategy for synthesizing C-3 functionalized indoles directly from 2-aminophenyl ethanol and alcohol.

Nickel-Catalyzed Hydroarylation Reaction: A Useful Tool in Organic Synthesis

This article describes the recent advances in the field of nickel-catalyzed hydroarylation reaction of alkenes, alkynes, and arenes. All reactions proceeded either through internal hydride transfer or in presence of...

Manganese-catalyzed transfer semihydrogenation of internal alkynes to E-alkenes with iPrOH as hydrogen source

The Mn-catalyzed transfer semihydrogenation of internal alkynes to E-alkenes is reported herein, along with Mn-catalyzed hydration of α-keto alkynes. Mechanistic studies displayed an asymmetrical Mn-hydride species performing the catalytic turnover.

Redox-active ligand based Mn(I)-catalyst for hydrosilylative ester reduction

Herein a Mn(I) catalyst bearing a redox-active phenalenyl (PLY) based ligand is reported for the efficient hydrosilylation of esters to alcohols using the inexpensive silane source polymethylhydrosiloxane (PMHS) under mild conditions. Mechanistic investigations suggest a strong ligand-metal cooperation where a ligand-based single electron transfer (SET) process initiates the reaction through Si-H bond activation.

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.