Acceptorless Dehydrogenative Coupling Reactions by Manganese Pincer Complexes

Divergent Synthesis of Pyrazoles via Manganese Pincer Complex Catalyzed Acceptorless Dehydrogenative Coupling Reactions

This report detailed the synthesis of multi-substituted pyrazoles through the acceptorless dehydrogenative coupling (ADC) reaction catalyzed by a well-defined manganese(I)-pincer complex. Symmetrically substituted pyrazoles were synthesized by reacting 1,3-diols with hydrazines. Unsymmetrically substituted pyrazoles were selectively made via the ADC of primary alcohols with methyl hydrazones. Water and hydrogen are liberated as the green byproducts. The endurance of these methodologies has been presented by producing 30 substrates with varied functionalities. Model reactions were scaled up to demonstrate practicability. The reaction rate and order were measured to transparent the involvement of the reagents during catalysis. Control experiments elucidated the plausible reaction mechanisms.

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.

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.

Unlocking the potential of metal ligand cooperation for enantioselective transformations

Metal-ligand cooperation, in which both the metal and the ligand of a transition metal complex actively participate in chemical transformations leading to enhanced reactivity or selectivity in chemical reactions, has emerged as a powerful and versatile concept in catalysis. This Viewpoint discusses the development trajectory of transition metal-based complexes as catalysts in (de)hydrogenative processes, in particular those cases where metal-ligand cooperation has been invoked to rationalise the observed high reactivities and excellent selectivities. The historical context, mechanistic aspects and current applications are discussed with the suggestion to explore the potential of the MLC mode of action of such catalysts in enantioselective transformations beyond (de)hydrogenative processes.

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.

Dual Role of TiO(acac)2 as a Reagent and an Activator/Catalyst: A Study on the Solvent Dependent Product Formation

The dual operation of a chemical species in synthetic chemistry is an intriguing and relatively unexplored phenomenon. The application of such a species is expected to reduce the use of multiple reaction partners and catalysts/activators. Herein, we report a simple and easy-to-use protocol for the twin application of TiO(acac)2 , as a reagent and an activator to synthesize β-enamino ketones with amines in acetonitrile. The same early transition metal precursor when employed in N,N-dimethylformamide with the amines, resulted in the formation of the substituted amides. Both reactions were explored with various substrates to check the viability of the present protocol. Moreover, experimental studies were conducted to understand the mechanism of both reactions.

Catalytic Acceptorless Dehydrogenation (CAD) of Secondary Benzylic Alcohols into Value-Added Ketones Using Pd(II)-NHC Complexes

For the creation of adaptable carbonyl compounds in organic synthesis, the oxidation of alcohols is a crucial step. As a sustainable alternative to the harmful traditional oxidation processes, transition-metal catalysts have recently attracted a lot of interest in acceptorless dehydrogenation reactions of alcohols. Here, using well-defined, air-stable palladium(II)-NHC catalysts (A-F), we demonstrate an effective method for the catalytic acceptorless dehydrogenation (CAD) reaction of secondary benzylic alcohols to produce the corresponding ketones and molecular hydrogen (H2). Catalytic acceptorless dehydrogenation (CAD) has been successfully used to convert a variety of alcohols, including electron-rich/electron-poor aromatic secondary alcohols, heteroaromatic secondary alcohols, and aliphatic cyclic alcohols, into their corresponding value-added ketones while only releasing molecular hydrogen as a byproduct.

Chemoselective α-Alkylation of Nitriles with Primary Alcohols by Manganese(I)-Catalysis

A sustainable and easy-to-use protocol for the alkylation of aryl nitriles with the earth-abundant manganese(I) catalyst is presented. The alkylation reaction employs readily available nitriles and naturally abundant alcohols as the coupling partners. The reaction proceeds chemoselectively and encompasses a broad substrate scope with good to excellent yields. The catalytic reaction yields selectively α-branched nitriles and water as the sole byproduct. Experimental studies were executed to understand the mechanism of the catalytic reaction.

Cyclotrimetaphosphate-assisted ruthenium catalyst for the hydration of nitriles and oxidation of primary amines to amides under aerobic conditions in water

Amide bonds are ubiquitous and regarded as an essential constituent of many biologically active drug molecules and fine chemicals. We report a practical and operationally simple ruthenium-based catalytic system for the hydration of nitriles and aerobic oxidation of primary amines to the corresponding amides. Both reactions proceed without any external oxidant in water under aerobic conditions and exhibit a broad substrate scope. The mechanistic investigation was executed with the aid of control experiments and kinetic and spectroscopic studies of the reaction mixture.

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.

In Situ-Generated CuI Catalytic System for Oxidative N-Formylation of N-Heterocycles and Acyclic Amines with Methanol

The development of a sustainable and simple catalytic system for N-formylation of N-heterocycles with methanol by direct coupling remains a challenge, owing to many competing side reactions, given the sensitivity of N-heterocycles to many catalytic oxidation or dehydrogenation systems. This work concerns the development of an in situ-generated Cu^I catalytic system for oxidative N-formylation of N-heterocycles with methanol that is based on the case study of a more typical 1,2,3,4-tetrahydroquinoline as substrate. Aside from N-heterocycles, some acyclic amines are also transformed into the corresponding N-formamides in moderate yields. Furthermore, a probable reaction mechanism and reaction pathway are proposed and extension of work based on some findings leads to a demonstration that the formed ⋅O₂ ^- and ⋅OOH radicals in the catalytic system is related to the formation of undesired tar-like products.

Catalytic and Mechanistic Approach to the Metal-Free N-Alkylation of 2-Aminopyridines with Diketones

N-alkylation of amines is an important catalytic reaction in synthetic chemistry. Herein, we report a simple strategy for the N-alkylation of 2-aminopyridines with 1,2-diketones using BF₃·OEt₂ as a catalyst. The reaction proceeds under aerobic conditions, leading to the formation of a diverse range of substituted secondary amines in good to excellent yields. A close inspection of the mechanistic pathway using various spectroscopic techniques and the computational study revealed that the reaction proceeds through the formation of an iminium-keto intermediate with the liberation of CO₂.

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.

Transamidation of aromatic amines with formamides using cyclic dihydrogen tetrametaphosphate

Amide fragments are found to be one of the key constituents in a wide range of natural products and pharmacologically active compounds. Herein, we report a simple and efficient procedure for transamidation with a cyclic dihydrogen tetrametaphosphate. The protocol is simple, does not require any additives, and encompasses a broad substrate scope. To comprehend the mechanism of the present methodology, detailed spectroscopic and kinetic studies were undertaken.

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.

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.

Ruthenium-catalyzed acceptorless dehydrogenative coupling of amino alcohols and ynones to access 3-acylpyrroles

Herein, a new strategy for the direct synthesis of functionalized pyrroles from β-amino alcohols and ynones via ruthenium-catalyzed acceptorless dehydrogenative coupling has been demonstrated. This developed methodology proceeds in an atom- and step-economic fashion together with the merits of broad substrate scope, operational simplicity, and water and hydrogen gas as the sole by-products, which provides an alternative and sustainable way to access functionalized pyrroles. Further, this method was applied to the rapid synthesis of the COX-1/COX-2 inhibitor and boron dipyrromethene derivative successfully.

Guerbet-type β-alkylation of secondary alcohols catalyzed by chromium chloride and its corresponding NNN pincer complex

β-Alkylation of alcohols has been efficiently accomplished using readily available 3d metal Cr under microwave conditions in air. Well-defined molecular Cr is involved with a KIE of 7.33 and insertion of α-alkylated ketone into Cr–H bond as the RDS.

A phosphine-free Mn(I)-NNS catalyst for asymmetric transfer hydrogenation of acetophenone: a theoretical prediction

The density functional theory (DFT) method was employed to investigate the reaction mechanism of the hydrogen activation and asymmetric transfer hydrogenation (ATH) of acetophenone catalyzed by a well-defined phosphine-free Mn(I)-NNS complex. The calculation results indicate that the Mn-NNS complex has potential high catalytic hydrogenation activity. Meanwhile, the hydrogen transfer step of this reaction is proposed to be a concerted but asynchronous process, and the hydride transfer precedes proton transfer. This work also pointed out that the stereoselectivity of ATH catalyzed by the Mn(I)-NNS complex mainly originates from the noncovalent interaction between the substrate and the catalyst. Additionally, the catalytic activities of Mn-NNS complexes with different X ligands (X = CO, Cl, H, OMe, NCMe, CCMe, and CHCHMe) were compared, and the calculated total reaction energy barriers were all viable, which indicates that these Mn-NNS complexes show higher CO bond hydrogenation activity under mild conditions. This theoretical study predicts that the reactions catalyzed by complexes with H and NCMe ligands exhibit high stereoselectivity with enantiomeric excess (ee) values of 97% and 93%, respectively.