Sustainable Alkylation of Unactivated Esters and Amides with Alcohols Enabled by Manganese Catalysis

Iron- and base-catalyzed C(α)-alkylation and one-pot sequential alkylation-hydroxylation of oxindoles with secondary alcohols

The utilization of economical and environmentally benign transition metals in crucial catalytic processes is pivotal for sustainable advancement in synthetic organic chemistry. Iron, as the most abundant transition metal in the Earth's crust, has gained significant attention for this purpose. A combination of FeCl2 (5 mol%) in the presence of phenanthroline (10 mol%) and NaOtBu (1.5 equivalent) proved effective for the C(α)-alkylation of oxindole, employing challenging secondary alcohol as a non-hazardous alkylating agent. The C(α)-alkylation of oxindole was optimized in green solvent or under neat conditions. The substrate scope encompasses a broad array of substituted oxindoles with various secondary alcohols. Further post-functionalization of the C(α)-alkylated oxindole products demonstrated the practical utility of this catalytic alkylation. One-pot C-H hydroxylation of alkylated oxindoles yielded 3-alkyl-3-hydroxy-2-oxindoles using air as the most sustainable oxidant. Low E-factors (3.61 to 4.19) and good Eco-scale scores (74 to 76) of these sustainable catalytic protocols for the alkylation and one-pot sequential alkylation-hydroxylation of oxindoles demonstrated minimum waste generation. Plausible catalytic paths are proposed on the basis of past reports and control experiments, which suggested that a borrowing hydrogen pathway is involved in this alkylation.

Copper-Catalyzed Borrowing Hydrogen Reaction for α-Alkylation of Amides with Alcohols

We report the first example of copper-catalyzed α-alkylation of acetamides with alcohols via a borrowing hydrogen strategy. Catalyzed by the in situ-generated copper particles, acetamides and various substituted benzyl or alkyl alcohols were transformed into functionalized amides in good yields with excellent selectivity. Compared with previous work, this process is simple using commercially available Cu(OAc)2 as a precatalyst, without an additional ligand or a metal complex, and easier. Mechanistic studies revealed that aldehyde and α,β-unsaturated amides were the intermediates of this reaction and also disclosed the role of copper in alcohol dehydrogenation and C═C bond hydrogenation.

Quinoline-derived NNP-manganese complex catalyzed α-alkylation of ketones with primary alcohols

An air-stable quinoline-derived NNP ligand chelated Mn catalyst was developed for the efficient α-alkylation of ketones with primary alcohols via a hydrogen auto-transfer methodology. The sole by-product formed is water, rendering the protocol atom efficient. A wide range of ketone and alcohol substrates were employed, providing the α-alkylated ketones with isolated yields up to 94%. This system was also efficient for the green synthesis of quinoline derivatives while using (2-aminophenyl)methanol as an alkylating reagent.

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.

Nickel-Catalyzed α-Alkylation of Arylacetonitriles with Challenging Secondary Alcohols

Nickel(II) complex 1 was utilized as a sustainable catalyst for α-alkylation of arylacetonitriles with challenging secondary alcohols. Arylacetonitriles with a wide range of functional groups were tolerated, and various cyclic and acyclic secondary alcohols were utilized to yield a large number of α-alkylated products. The plausible mechanism involves the base-promoted activation of precatalyst 1 to an active catalyst 2 (dehydrochlorinated product) which activates the O-H and C-H bonds of the secondary alcohol in a dehydrogenative pathway.

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.

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.

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.

Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones

Manganese has attracted significant recent attention due to its abundance, low toxicity, and versatility in catalysis. In the present study, a series of manganese (III) complexes supported by salan ligands have been synthesized and characterized, and their activity as catalysts in the hydrosilylation of carbonyl compounds was examined. While manganese (III) chloride complexes exhibited minimal catalytic efficacy without activation of silver perchlorate, manganese (III) azide complexes showed good activity in the hydrosilylation of carbonyl compounds. Under optimized reaction conditions, several types of aldehydes and ketones could be reduced with good yields and tolerance to a variety of functional groups. The possible mechanisms of silane activation and hydrosilylation were discussed in light of relevant experimental observations.

A Bench-stable 8-Aminoquinoline Derived Phosphine-free Manganese (I)-Catalyst for Environmentally Benign C(α)-Alkylation of Oxindoles with Secondary and Primary Alcohols

Herein, we report a new air-stable phosphine-free 8-AQ (8-aminoquinoline) based Mn(I) carbonyl complex as the catalyst for the C(α)-alkylation of oxindoles with alcohols. The Mn complex [(8-AQ)Mn(CO)₃ Br] works effectively as a catalyst for the α-alkylation of oxindoles by both secondary as well as primary alcohols. The procedure has been used for the synthesis of pharmaceutically important recently developed oxindoles such as 3-(4-methoxybenzyl)indolin-2-one, 3-(4-(dimethylamino)benzyl)indolin-2-one, 3-(4-(dimethylamino)phenyl)-5-fluoroindolin-2-one and 3-(benzo[d][1,3]dioxol-5-ylmethyl)indolin-2-one, which are found to be effective in preventing specific types of cell death in neurodegenerative disorders. Control experiments have been carried out to investigate the reaction mechanism and the crucial role of metal-ligand cooperation via -NH₂ moiety during catalysis.

Divergent upgrading pathways of sulfones with primary alcohols: nickel-catalyzed α-alkylation under N2 and metal-free promoted β-olefination in open air

We report here our findings on the diverse reaction results of sulfones and alcohols. In the presence of NiCl₂/P(t-Bu)₃ and under a N₂ atmosphere, α-C-alkylation of sulfones with alcohols occurs through a borrowing-hydrogen mechanism; when the reaction was carried out in the open air without nickel, the product was not the predicted α,β-unsaturated sulfone, but the β-alkenyl sulfone, which is a useful building block in organic synthesis.

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 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.

Evaluation of Manganese Catalysts for the Hydrogenative Deconstruction of Commercial and End-of-Life Polyurethane Samples

Polyurethane (PU) is a thermoset plastic that is found in everyday objects, such as mattresses and shoes, but also in more sophisticated materials, including windmills and airplanes, and as insulation materials in refrigerators and buildings. Because of extensive inter-cross linkages in PU, current recycling methods are somewhat lacking. In this work, the effective catalytic hydrogenation of PU materials is carried out by applying a catalyst based on the earth-abundant metal manganese, to give amine and polyol fractions, which represent the original monomeric composition. In particular, Mn-^Ph MACHO is found to catalytically deconstruct flexible foam, molded foams, insulation, and end-of-life materials at 1 wt.% catalyst loading by applying a reaction temperature of 180 °C, 50 bar of H₂ , and 0.9 wt.% of KOH in isopropyl alcohol. The protocol is showcased in the catalytic deconstruction of 2 g of mattress foam using only 0.13 wt.% catalyst, resulting in 90 % weight recovery and a turnover number of 905.

Hydrogenative Depolymerization of Silicon-modified Polyureas

Silicon-modified polyureas were depolymerized by hydrogenation in the presence of Ru and Mn catalysts. Yields of up to 84% of the aliphatic diamine and 81% of silicon-containing diamine were achieved...

A Journey from June 2018 to October 2021 with N,N-Dimethylformamide and N,N-Dimethylacetamide as Reactants

A rich array of reactions occur using N,N-dimethylformamide (DMF) or N,N-dimethylacetamide (DMAc) as reactants, these two amides being able to deliver their own H, C, N, and O atoms for the synthesis of a variety of compounds. This account highlights the literature published since June 2018, completing previous reviews by the author.

Manganese catalyzed switchable C-alkylation/alkenylation of fluorenes and indene with alcohols

The usage of earth-abundant, nontoxic transition metals in place of rare noble metals is a central goal in catalysis. This would be especially interesting when the reactivity and selectivity patterns can be tuned. Herein, we introduced the first Mn-catalyzed selective C-alkylation and olefination of fluorene, and indene with alcohols. Various substrates including benzylic, heteroaromatic, and aliphatic primary and secondary alcohols are employed as alkylating agents. Mechanistic investigations and a kinetic study underpin the involvement of the olefinated intermediate to furnish the alkylated product.

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.

Manganese(I) Catalyzed α-Alkenylation of Amides Using Alcohols with Liberation of Hydrogen and Water

Herein, unprecedented manganese-catalyzed direct α-alkenylation of amides using alcohols is reported. Aryl amides are reacted with diverse primary alcohols, which provided the α,β-unsaturated amides in moderate to good yields with excellent selectivity. Mechanistic studies indicate that Mn(I) catalyst oxidizes the alcohols to their corresponding aldehydes and also plays an important role in efficient C═C bond formation through aldol condensation. This selective olefination is facilitated by metal-ligand cooperation by the aromatization-dearomatization process operating in the catalytic system. Biorenewable alcohols are used as alkenylation reagents for the challenging α-alkenylation of amides with the highly abundant base metal manganese as a catalyst, which results in water and dihydrogen as the only byproduct, making this catalytic transformation attractive, sustainable, and environmentally benign.

Borrowing Hydrogen for Organic Synthesis

Borrowing hydrogen is a process that is used to diversify the synthetic utility of commodity alcohols. A catalyst first oxidizes an alcohol by removing hydrogen to form a reactive carbonyl compound. This intermediate can undergo a diverse range of subsequent transformations before the catalyst returns the "borrowed" hydrogen to liberate the product and regenerate the catalyst. In this way, alcohols may be used as alkylating agents whereby the sole byproduct of this one-pot reaction is water. In recent decades, significant advances have been made in this area, demonstrating many effective methods to access valuable products. This outlook highlights the diversity of metal and biocatalysts that are available for this approach, as well as the various transformations that can be performed, focusing on a selection of the most significant and recent advances. By succinctly describing and conveying the versatility of borrowing hydrogen chemistry, we anticipate its uptake will increase across a wider scientific audience, expanding opportunities for further development.

Chemoselective Hydrogenation of Nitroarenes Using an Air-Stable Base-Metal Catalyst

The reduction of nitroarenes to anilines as well as azobenzenes to hydrazobenzenes using a single base-metal catalyst is reported. The hydrogenation reactions are performed with an air-and moisture-stable manganese catalyst and proceed under relatively mild reaction conditions. The transformation tolerates a broad range of functional groups, affording aniline derivatives and hydrazobenzenes in high yields. Mechanistic studies suggest that the reaction proceeds via a bifunctional activation involving metal–ligand cooperative catalysis.

Transition Metal-Free Oxidative Cross-Coupling Reaction of Activated Olefins with N-Alkyl Amides

The K₂S₂O₈-mediated transition metal-free oxidative cross-coupling reaction of activated olefins with N-alkyl amides was developed, and the reaction gave N-allylic amides in moderate to good yield. This reaction protocol was suitable for different kinds of activated olefins.

Tungsten-Catalyzed Direct N-Alkylation of Anilines with Alcohols

The implementation of non-noble metals mediated chemistry is a major goal in homogeneous catalysis. Borrowing hydrogen/hydrogen autotransfer (BH/HA) reaction, as a straightforward and sustainable synthetic method, has attracted considerable attention in the development of non-noble metal catalysts. Herein, we report a tungsten-catalyzed N-alkylation reaction of anilines with primary alcohols via BH/HA. This phosphine-free W(phen)(CO)₄ (phen=1,10-phenthroline) system was demonstrated as a practical and easily accessible in-situ catalysis for a broad range of amines and alcohols (up to 49 examples, including 16 previously undisclosed products). Notably, this tungsten system can tolerate numerous functional groups, especially the challenging substrates with sterically hindered substituents, or heteroatoms. Mechanistic insights based on experimental and computational studies are also provided.

Enantioselective direct, base-free hydrogenation of ketones by a manganese amido complex of a homochiral, unsymmetrical P–N–P′ ligand

Base-free direct hydrogenation of ketones using a Mn(PNP′)(CO)₂ complex is more enantioselective than that of a related base-activated iron complex.

Nickel-catalyzed C3-alkylation of indoles with alcohols via a borrowing hydrogen strategy

An efficient Ni-catalyzed C3-alkylation of indoles with alcohols via a borrowing hydrogen pathway was achieved utilizing an N,O-donor coordinated nickel complex as the precatalyst.

Nickel-catalyzed C-alkylation of thioamide, amides and esters by primary alcohols through a hydrogen autotransfer strategy

A simple catalyst of Ni(OAc)2 and P(t-Bu)3 enables selective C-alkylation of thioacetamides and primary acetamides with alcohols for the first time. Monoalkylation of thioamides, amides and t-butyl esters occurs in excellent yields (>95%). Mechanistic studies reveal that the reaction proceeds via a hydrogen autotransfer pathway.

Chemoselective Hydrogenation of Alkynes to (Z)-Alkenes Using an Air-Stable Base Metal Catalyst

A highly selective hydrogenation of alkynes using an air-stable and readily available manganese catalyst has been achieved. The reaction proceeds under mild reaction conditions and tolerates various functional groups, resulting in (Z)-alkenes and allylic alcohols in high yields. Mechanistic experiments suggest that the reaction proceeds via a bifunctional activation involving metal-ligand cooperativity.

Radical condensation between benzylic alcohols and acetamides to form 3-arylpropanamides

A new radical condensation reaction is developed where benzylic alcohols and acetamides are coupled to generate 3-arylpropanamides with water as the only byproduct. The transformation is performed with potassium tert-butoxide as the only additive and gives rise to a variety of 3-arylpropanamides in good yields. The mechanism has been investigated experimentally with labelled substrates, trapping experiments and spectroscopic measurements. The findings indicate a radical pathway where potassium tert-butoxide is believed to serve a dual role as both base and radical initiator. The radical anion of the benzylic alcohol is proposed as the key intermediate, which undergoes coupling with the enolate of the amide to form the new C–C bond. Subsequent elimination to the corresponding cinnamamide and olefin reduction then affords the 3-arylpropanamides.

Benzylic alcohols and acetamides are coupled into 3-arylpropanamides by a new radical condensation through the radical anion of the alcohol.

Sustainable and Selective Alkylation of Deactivated Secondary Alcohols to Ketones by Non-bifunctional Pincer N-heterocyclic Carbene Manganese

A sustainable and green route to access diverse functionalized ketones via dehydrogenative-dehydrative cross-coupling of primary and secondary alcohols is demonstrated. This borrowing hydrogen approach employing a pincer N-heterocyclic carbene Mn complex displays high activity and selectivity. A variety of primary and secondary alcohols are well tolerant and result in satisfactory isolated yields. Mechanistic studies suggest that this reaction proceeds via a direct outer-sphere mechanism and the dehydrogenation of the secondary alcohol substrates plays a vital role in the rate-limiting step.

Manganese(I)-Catalyzed β-Methylation of Alcohols Using Methanol as C1 Source

Highly selective β-methylation of alcohols was achieved using an earth-abundant first row transition metal in the air stable molecular manganese complex [Mn(CO)2 Br[HN(C2 H4 Pi Pr2 )2 ]] 1 ([HN(C2 H4 Pi Pr2 )2 ]=MACHO-i Pr). The reaction requires only low loadings of 1 (0.5 mol %), methanolate as base and MeOH as methylation reagent as well as solvent. Various alcohols were β-methylated with very good selectivity (>99 %) and excellent yield (up to 94 %). Biomass derived aliphatic alcohols and diols were also selectively methylated on the β-position, opening a pathway to "biohybrid" molecules constructed entirely from non-fossil carbon. Mechanistic studies indicate that the reaction proceeds through a borrowing hydrogen pathway involving metal-ligand cooperation at the Mn-pincer complex. This transformation provides a convenient, economical, and environmentally benign pathway for the selective C-C bond formation with potential applications for the preparation of advanced biofuels, fine chemicals, and biologically active molecules.

Manganese complex-catalysed α-alkylation of ketones with secondary alcohols enables the synthesis of β-branched carbonyl compounds

Diverse β-functionalised carbonyl compounds were synthesized via a manganese(i) complex-catalysed α-alkylation of ketones with secondary alcohols.

Nonbifunctional Outer-Sphere Strategy Achieved Highly Active α-Alkylation of Ketones with Alcohols by N-Heterocyclic Carbene Manganese (NHC-Mn)

The unusual nonbifunctional outer-sphere strategy was successfully utilized in developing an easily accessible N-heterocyclic carbene manganese (NHC-Mn) system for highly active α-alkylation of ketones with alcohols. This system was efficient for a wide range of ketones and alcohols under mild reaction conditions, and also for the green synthesis of quinoline derivatives. The direct outer-sphere mechanism and the high activity of the present system demonstrate the potential of nonbifunctional outer-sphere strategy in catalyst design for acceptorless dehydrogenative transformations.