Homogeneous Transition Metal Catalysis of Acceptorless Dehydrogenative Alcohol Oxidation: Applications in Hydrogen Storage and to Heterocycle Synthesis

Ni-Catalyzed α-Alkylation of Unactivated Amides and Esters with Alcohols by Hydrogen Auto-Transfer Strategy

A transition-metal-catalyzed borrowing hydrogen/hydrogen auto-transfer strategy allows the utilization of feedstock alcohols as an alkylating partner, which avoids the formation of stoichiometric salt waste and enables a direct and benign approach for the construction of C-N and C-C bonds. In this study, a nickel-catalyzed α-alkylation of unactivated amides and ester (tert-butyl acetate) is carried out by using primary alcohols under mild conditions. This C-C bond-forming reaction is catalyzed by a new, molecularly defined nickel(II) NNN-pincer complex (0.1-1 mol %) and proceeds through hydrogen auto-transfer, thereby releasing water as the sole byproduct. In addition, N-alkylation of cyclic amides under Ni-catalytic conditions is demonstrated.

Simple ruthenium-catalyzed reductive amination enables the synthesis of a broad range of primary amines

The production of primary benzylic and aliphatic amines, which represent essential feedstocks and key intermediates for valuable chemicals, life science molecules and materials, is of central importance. Here, we report the synthesis of this class of amines starting from carbonyl compounds and ammonia by Ru-catalyzed reductive amination using H2. Key to success for this synthesis is the use of a simple RuCl2(PPh3)3 catalyst that empowers the synthesis of >90 various linear and branched benzylic, heterocyclic, and aliphatic amines under industrially viable and scalable conditions. Applying this catalyst, -NH2 moiety has been introduced in functionalized and structurally diverse compounds, steroid derivatives and pharmaceuticals. Noteworthy, the synthetic utility of this Ru-catalyzed amination protocol has been demonstrated by upscaling the reactions up to 10 gram-scale syntheses. Furthermore, in situ NMR studies were performed for the identification of active catalytic species. Based on these studies a mechanism for Ru-catalyzed reductive amination is proposed.

Transition metal-catalyzed dehydrogenation of amines

This review focuses on the use of homogeneous transition metal complexes for the catalytic dehydrogenation of amines for synthetic purposes, and for hydrogen storage applications. The catalytic dehydrogenation of primary, secondary and cyclic amines is reviewed looking at reaction conditions, different catalysts and common side reactions. Recent developments in this active field of research showcase how cooperative ligands and photocatalysts can overcome the need for noble metals or harsh reaction conditions.

Cp*CoIII -Catalyzed Efficient Dehydrogenation of Secondary Alcohols

A novel, well-defined molecular Cp*Co^III complex was isolated and structurally characterized for the first time. The efficiency of this cobalt catalyst was demonstrated in the alcohol dehydrogenation and dehydrative coupling of secondary alcohols under mild conditions into ketones and ethers, respectively.

Dehydrogenative Transformation of Alcoholic Substrates in Aqueous Media Catalyzed by an Iridium Complex Having a Functional Ligand with α-Hydroxypyridine and 4,5-Dihydro-1H-imidazol-2-yl Moieties

A new catalytic system that employs water as an environmentally friendly solvent for the dehydrogenative oxidation of alcohols and lactonization of diols has been developed. In this catalytic system, a water-soluble dicationic iridium complex having a functional ligand that comprises α-hydroxypyridine and 4,5-dihydro-1H-imidazol-2-yl moieties exhibits high catalytic performance. For example, the catalytic dehydrogenative oxidation of 1-phenylethanol in the presence of 0.25 mol % of the iridium catalyst and base under reflux in water proceeded to give acetophenone in 92% yield. Additionally, under similar reaction conditions, the iridium-catalyzed dehydrogenative lactonization of 1,2-benzenedimethanol gave phthalide in 98% yield.

Direct C-OH/P(O)-H dehydration coupling forming phosphine oxides

A t-BuONa-mediated C-OH/P(O)-H cross dehydration coupling to produce alkylphosphine oxides is developed. This reaction employed readily available alcohols and P(O)-H compounds as the starting materials, providing an efficient alternative method for constructing sp3 C-P bonds. A reasonable reaction path involving dehydration and subsequent regio-selective hydrophosphorylation of the resulting alkenes was proposed.

Cobalt-Catalyzed Acceptorless Dehydrogenative Coupling of Primary Alcohols to Esters

A novel catalytic system with a tripodal cobalt complex is developed for efficiently converting primary alcohols to esters. KO ^tBu is found essential to the transformation. A preliminary mechanistic study suggests a plausible reaction route that involves an initial Co-catalyzed dehydrogenation of alcohol to aldehyde, followed by a Tishchenko-type pathway to ester mediated by KO ^tBu.

Iridium-based hydride transfer catalysts: from hydrogen storage to fine chemicals

Selective hydrogen transfer remains a central research focus in catalysis: hydrogenation and dehydrogenation have central roles, both historical and contemporary, in all aspects of fuel, agricultural, pharmaceutical, and fine chemical synthesis. Our lab has been involved in this area by designing homogeneous catalysts for dehydrogenation and hydrogen transfer that fill needs ranging from on-demand hydrogen storage to fine chemical synthesis. A keen eye toward mechanism has enabled us to develop systems with excellent selectivity and longevity and demonstrate these in a diversity of high-value applications. Here we describe recent work from our lab in these areas that are linked by a central mechanistic trichotomy of catalyst initiation pathways that lead highly analogous precursors to a diversity of useful applications.

Hydrogenation of carbonyl compounds of relevance to hydrogen storage in alcohols

Alcohols are a promising source for the sustainable production of hydrogen that may also serve as rechargeable liquid organic hydrogen carriers (LOHCs). Metal-catalyzed acceptorless dehydrogenation of alcohols produces carbonyl derivatives as H2-depleted by-products, which by means of a hydrogenation reaction can be reconverted to the initial alcohols. Hence, reversible H2-storage systems based on pairs of secondary alcohols/ketones and primary alcohols/carboxylic acid derivatives may be envisaged. In this contribution, the hydrogenation of carbonyl derivatives, including ketones, esters, amides and carboxylic acids, is reviewed from the perspective of the hydrogen storage in alcohols.

Divergent Coupling of Anilines and Enones by Integration of C-H Activation and Transfer Hydrogenation

Cp*Rh^III /Ir^III complexes are known to play important roles in both C-H activation and transfer hydrogenation (TH). However, these two areas evolved separately. They have been integrated in redox- and chemodivergent coupling reactions of N-pyridylanilines with enones. The iridium-catalyzed coupling with enones leads to the efficient synthesis of tetrahydroquinolines through TH from ⁱ PrOH. Counterintuitively, ⁱ PrOH does not serve as the sole hydride source, and the major reaction pathway involves disproportionation of a dihydroquinoline intermediate, followed by the convergent and iterative reduction of quinolinium species.

Application of Silicon-Initiated Water Splitting for the Reduction of Organic Substrates

The use of water as a donor for hydrogen suitable for the reduction of several important classes of organic compounds is described. It is found that the reductive water splitting can be promoted by several metalloids among which silicon shows the best efficiency. The developed methodologies were applied for the reduction of nitro compounds, N-oxides, sulfoxides, alkenes, alkynes, hydrodehalogenation as well as for the gram-scale synthesis of several substrates of industrial importance.

cis-Oxoruthenium complexes supported by chiral tetradentate amine (N4) ligands for hydrocarbon oxidations

We report the first examples of ruthenium complexes cis-[(N₄)Ru^IIICl₂]⁺ and cis-[(N₄)Ru^II(OH₂)₂]²⁺ supported by chiral tetradentate amine ligands (N₄), together with a high-valent cis-dioxo complex cis-[(N₄)Ru^VI(O)₂]²⁺ supported by the chiral N₄ ligand mcp (mcp = N,N'-dimethyl-N,N'-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine). The X-ray crystal structures of cis-[(mcp)Ru^IIICl₂](ClO₄) (1a), cis-[(Me₂mcp)Ru^IIICl₂]ClO₄ (2a) and cis-[(pdp)Ru^IIICl₂](ClO₄) (3a) (Me₂mcp = N,N'-dimethyl-N,N'-bis((6-methylpyridin-2-yl)methyl)cyclohexane-1,2-diamine, pdp = 1,1'-bis(pyridin-2-ylmethyl)-2,2'-bipyrrolidine)) show that the ligands coordinate to the ruthenium centre in a cis-α configuration. In aqueous solutions, proton-coupled electron-transfer redox couples were observed for cis-[(mcp)Ru^III(O₂CCF₃)₂]ClO₄ (1b) and cis-[(pdp)Ru^III(O₃SCF₃)₂]CF₃SO₃ (3c'). Electrochemical analyses showed that the chemically/electrochemically generated cis-[(mcp)Ru^VI(O)₂]²⁺ and cis-[(pdp)Ru^VI(O)₂]²⁺ complexes are strong oxidants with E° = 1.11-1.13 V vs. SCE (at pH 1) and strong H-atom abstractors with D_O-H = 90.1-90.8 kcal mol^-1. The reaction of 1b or its (R,R)-mcp counterpart with excess (NH₄)₂[Ce^IV(NO₃)₆] (CAN) in aqueous medium afforded cis-[(mcp)Ru^VI(O)₂](ClO₄)₂ (1e) or cis-[((R,R)-mcp)Ru^VI(O)₂](ClO₄)₂ (1e*), respectively, a strong oxidant with E(Ru^VI/V) = 0.78 V (vs. Ag/AgNO₃) in acetonitrile solution. Complex 1e oxidized various hydrocarbons, including cyclohexane, in acetonitrile at room temperature, affording alcohols and/or ketones in up to 66% yield. Stoichiometric oxidations of alkenes by 1e or 1e* in ^t BuOH/H₂O (5 : 1 v/v) afforded diols and aldehydes in combined yields of up to 98%, with moderate enantioselectivity obtained for the reaction using 1e*. The cis-[(pdp)Ru^II(OH₂)₂]²⁺ (3c)-catalysed oxidation of saturated C-H bonds, including those of ethane and propane, with CAN as terminal oxidant was also demonstrated.

Ruthenium carbonyl complexes with pyridine-alkoxide ligands: synthesis, characterization and catalytic application in dehydrogenative oxidation of alcohols

Dehydrogenative oxidation of secondary alcohols catalyzed by several new trinuclear ruthenium carbonyl complexes under mild conditions was proposed.

Exploring the mechanisms of aqueous methanol dehydrogenation catalyzed by defined PNP Mn and Re pincer complexes under base-free as well as strong base conditions

The mechanisms of aqueous methanol dehydrogenation reaction catalyzed by defined Mn and Re pincer complexes under different conditions were computed.

Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

This review is aimed at introducing the remarkable progress made in the last three years in the development of base metal catalysts for hydrogenations and dehydrogenative transformations.

Unsupported shaped cobalt nanoparticles as efficient and recyclable catalysts for the solvent-free acceptorless dehydrogenation of alcohols

Unsupported cobalt nanoparticles are active and chemoselective catalysts for the solvent-free acceptorless dehydrogenation of alcohols.

Methanol as hydrogen source: transfer hydrogenation of aromatic aldehydes with a rhodacycle

A rhodacycle catalyses efficient hydrogenation of aldehydes, deriving the hydrogen from methanol.

A nickel catalyzed acceptorless dehydrogenative approach to quinolines

A general, efficient and environmentally benign, one-step synthesis of substituted quinoline derivatives was achieved by acceptorless dehydrogenative coupling of o-aminobenzylalcohols with ketones and secondary alcohols catalyzed by a cheap, earth abundant and easy to prepare macrocyclic nickel catalyst [Ni(MeTAA)].

Phosphine free Mn-complex catalysed dehydrogenative C–C and C–heteroatom bond formation: a sustainable approach to synthesize quinoxaline, pyrazine, benzothiazole and quinoline derivatives

Sustainable synthesis of quinoxalines, pyrazines, benzothiazoles and quinolines catalysed by a phosphine free Mn(i) complex.

Unveiling the role of ancillary ligands in acceptorless benzyl alcohol dehydrogenation and etherification mediated by mesoionic carbene iridium complexes

We synthesized a set of triazolylidene iridium(iii) complexes [IrCp*(C^N)L]ⁿ⁺ (Cp* = pentamethylcyclopentadienyl, C^N = C,N-bidentate coordinating pyridyl-triazolylidene) containing different neutral or anionic ancillary ligands L and evaluated their impact on the catalytic activity in alcohol conversion. We demonstrate that these ancillary ligands have a strong influence on the catalytic selectivity and direct whether the iridium center preferentially catalyzes either the dehydrogenation or the dehydration of benzyl alcohol. Ligand exchange experiments provide a direct correlation of ligand lability with catalytic activity and selectivity. These results underline the relevance of ancillary ligands and provide a rational approach to tailor the catalytic activity of the iridium center towards aldehyde formation (loss of H₂) or etherification (elimination of H₂O).

Cellulose nanocrystal-derived hollow mesoporous carbon spheres and their application as a metal-free catalyst

In this contribution, we demonstrate the fabrication of hollow mesoporous carbon spheres (HCSs) derived from cellulose nanocrystals (CNCs). The HCSs were prepared by templating CNCs onto sacrificial silica spheres followed by heat treatment. Mesoporous carbon spheres result from the removal of the silica spheres by etching. The walls of the HCSs are approximately 4 nm thick and are composed of amorphous and graphitic carbon. The catalytic activity of the HCSs was investigated for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by sodium borohydride (NaBH₄). The present investigation reveals the outstanding catalytic activity of these particles. The reaction rate followed pseudo-first order kinetics with k value of 4.72 × 10^-3 s^-1 and activity parameter of 52.2 s^-1 g^-1, which showed superior performance compared to that of metal nanoparticle and metal nanoparticle-carbon hybrid based catalysts.

Redox-Tag Processes: Intramolecular Electron Transfer and Its Broad Relationship to Redox Reactions in General

Explosive growth in the use of open shell reactivity, including neutral radicals and radical ions, in the field of synthetic organic chemistry has been observed in the past decade, particularly since the advent of ruthenium complexes in 2008. These complexes generally induce single-electron transfer (SET) processes via visible-light absorption. Additionally, recent significant advancements in organic electrochemistry involving SET processes to provide open shell reactivity offer a complementary method to traditional polarity-driven reactions described by two-electron transfer processes. In this Review, we highlight the importance of intramolecular SET processes in the field of synthetic organic chemistry, which seem to be more elusive than the intermolecular versions, since they are net redox-neutral and thus cannot simply be regarded as oxidations or reductions. Such intramolecular SET processes can rationally be understood in combination with concomitant bond formations and/or cleavages, and are regulated by a structural motif that we call a "redox tag." In order to describe modern radical-driven reactions involving SET processes, we focus on a classical formalism in which electrons are treated as particles rather than waves, which offers a practical yet powerful approach to explain and/or predict synthetic outcomes.