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The hydroformylation of conjugated dienes is a highly interesting reaction, giving access to bifunctional aldehyde products. However, due to low activity, caused by the formation of stable η3-allylic intermediates, it has received much less academic attention, both experimentally and theoretically, than classical monoalkene hydroformylation. We herein present a comprehensive computational mechanistic study of the hydroformylation of isoprene as the simplest example of an asymmetrically substituted conjugated diene, via a Rh/dppe catalyst. A benchmarking study assessing the accuracy of Rh complex structures and energies with various methods was carried out, from which a protocol combining dispersion-corrected density functional theory (DFT) for geometries and vibrational corrections and DLPNO–CCSD(T) for reaction energies was derived. The predicted reaction mechanism of isoprene hydroformylation leading to the formation of 3-methyl-3-pentenal is derived from the classic Wilkinson cycle for monoene hydroformylation with a few deviations. Among them is the prediction of a η3/η3-allylic rearrangement following a Berry pseudorotation mechanism that has rarely been discussed in the literature. The prediction of the relevant intermediates and transition states is highly sensitive to the computational protocol, with the implicit solvation model introducing the greatest uncertainty. A TOF of 47 h–1was predicted to be in good agreement with experimental results. An investigation of subsequent product isomerization revealed a high hydrogen transfer barrier, making the isomerization via β-hydride elimination unfavorable.

Hydrogenation reactions of carbon–carbon unsaturated bonds are central in synthetic chemistry. Efficient catalysis of these reactions classically recourses to heterogeneous or homogeneous transition-metal species. Whether thermal or electrochemical, C–C multiple bond catalytic hydrogenations commonly involve metal hydrides as key intermediates. Here, we report that the electrocatalytic alkyne semihydrogenation by molecular Ni bipyridine complexes proceeds without the mediation of a hydride intermediate. Through a combined experimental and theoretical investigation, we disclose a mechanism that primarily involves a nickelacyclopropene resting state upon alkyne binding to a low-valent Ni(0) species. A following sequence of protonation and electron transfer steps viaNi(II) and Ni(I) vinyl intermediates then leads to olefin release in an overall ECEC-type pattern as the most favored pathway. Our results also evidence that pathways involving hydride intermediates are strongly disfavored, which in turn promotes high semihydrogenation selectivity by avoiding competing hydrogen evolution. While bypassing catalytically competent hydrides, this type of mechanism still retains inner-metal-sphere characteristics with the formation of organometallic intermediates, often essential to control regio- or stereoselectivity. We think that this approach to electrocatalytic reductions of unsaturated organic groups can open new paradigms for hydrogenation or hydroelementation reactions.

Electrocatalysis enables the formation or cleavage of chemical bonds by a genuine use of electrons or holes from an electrical energy input. As such, electrocatalysis offers resource-economical alternative pathways that bypass sacrificial, waste-generating reagents often required in classical thermal redox reactions. In this Perspective, we showcase the exploitation of molecular electrocatalysts for electrosynthesis, in particular for reductive conversion of organic substrates. Selected case studies illustrate that efficient molecular electrocatalysts not only are appropriate redox shuttles but also embrace the features of organometallic catalysis to facilitate and control chemical steps. From these examples, guidelines are proposed for the design of molecular electrocatalysts suited to the reduction of organic substrates. We finally expose opportunities brought by catalyzed electrosynthesis to functionalize organic backbones, namely using sustainable building blocks.

Electrifying the production of base and fine chemicals calls for the development of electrocatalytic methodologies for these transformations. We show here that the semihydrogenation of alkynes, an important transformation in organic synthesis, is electrocatalyzed at room temperature by a simple complex of earth-abundant nickel, [Ni(bpy)3]2+. The approach operates under mild conditions and is selective toward the semihydrogenated olefins with good to very good Zisomer stereoselectivity. (Spectro)electrochemistry supports that the electrocatalytic cycle is initiated in an atypical manner with a nickelacyclopropene complex, which upon further protonation is converted into a putative cationic Ni(II)–vinyl intermediate that produces the olefin after electron–proton uptake. This work establishes a proof of concept for homogeneous electrocatalysis applied to alkyne semihydrogenation, with opportunities to improve the yields and stereoselectivity.

Photo-optical probes with automated image analysis are among the most promising measurement techniques for gas–liquid–liquid systems, but the literature on the application in lean water-in-oil dispersions is scarce. Taking water-in-1-octene as a model system, adhering droplets that severely cloud the images are successfully prevented using a polymeric disc made of polycarbonate or fluorinated ethylene propylene. Sharp bright-field imaging is enabled using a neither fully diffuse nor specular reflection pane. Various 3D-printed stirrers are characterized by their energy input, including gas-inducing modifications. Gas induction impairs the energy input of the stirrer and the imaging of droplets. Nevertheless, the measurement technique reliably provides droplet size distributions that exclude gas bubbles. Axial stirrers are preferable if one position at high stirring speeds is measured, as the homogeneity of droplet sizes along the reactor height is promoted. Radial stirrers are preferable if the stirring speed is varied, as the same trend of the droplets sizes is measurable along the reactor height. For radial stirring, the most beneficial position of the probe is close or slightly above the stirrer. The use of borescopy is thus found to be feasible if adhering droplets can be prevented and vision on droplets is not obstructed by too many bubbles.

The production of alcohols from olefin-enriched Fischer–Tropsch products represents a promising route for CO2-neutral bio-synthetic fuels. Tandem-catalytic systems as alternatives for the conventional two-step production of long-chain alcohols are attractive in terms of energy and resource efficiency. Herein, we present the first rhodium-based catalytic system capable of direct conversion of olefins to alcohols in a biphasic liquid/liquid system. After optimizing the reaction conditions for the biphasic operation, an alcohol selectivity of up to 64% was achieved, while aldehydes and olefin isomers were observed as main by-products. By employing water-soluble alkanolamines, the catalyst is immobilized in a water phase and can be easily separated from the product containing an organic phase with a rhodium loss of as low as 0.1%. After investigation of various reaction parameters, a TON of 128 in batch operation was achieved. Furthermore, the developed catalyst recycling strategy was implemented in a continuously operated miniplant, reaching a TTON for alcohol production of 1236 after 50 hours.

Coordination compounds of earth-abundant 3d transition metals are among the most effective catalysts for the electrochemical reduction of carbon dioxide (CO2). While the properties of the metal center are crucial for the ability of the complexes to electrochemically activate CO2, systematic variations of the metal within an identical, redox-innocent ligand backbone remain insufficiently investigated. Here, we report on the synthesis, structural and spectroscopic characterization, and electrochemical investigation of a series of 3d transition-metal complexes [M = Mn(I), Fe(II), Co(II), Ni(II), Cu(I), and Zn(II)] coordinated by a new redox-innocent PNP pincer ligand system. Only the Fe, Co, and Ni complexes reveal distinct metal-centered electrochemical reductions from M(II) down to M(0) and show indications for interaction with CO2in their reduced states. The Ni(0) d10species associates with CO2to form a putative Aresta-type Ni-η2-CO2complex, where electron transfer to CO2through back-bonding is insufficient to enable electrocatalytic activity. By contrast, the Co(0) d9intermediate binding CO2can undergo additional electron uptake into a formal cobalt(I) metallacarboxylate complex able to promote turnover. Our data, together with the few literature precedents, single out that an unsaturated coordination sphere (coordination number = 4 or 5) and a d7-to-d9configuration in the reduced low oxidation state (+I or 0) are characteristics that foster electrochemical CO2activation for complexes based on redox-innocent ligands.

The selective hydrosilylation of carbon dioxide (CO2) to either the formic acid, formaldehyde, or methanol level using a molecular cobalt(II) triazine complex can be controlled based on reaction parameters such as temperature, CO2pressure, and concentration. Here, we rationalize the catalytic mechanism that enables the selective arrival at each product platform. Key reactive intermediates were prepared and spectroscopically characterized, while the catalytic mechanism and the energy profile were analyzed with density functional theory (DFT) methods and microkinetic modeling. It transpired that the stepwise reduction of CO2involves three consecutive catalytic cycles, including the same cobalt(I) triazine hydride complex as the active species. The increasing kinetic barriers associated with each reduction step and the competing hydride transfer steps in the three cycles corroborate the strong influence of the catalyst environment on the product selectivity. The fundamental mechanistic insights provide a consistent description of the catalytic system and rationalize, in particular, the experimentally verified opportunity to steer the reaction toward the formaldehyde product as the chemically most challenging reduction level.

Alcohol-assisted hydrogenation of carbon monoxide (CO) to methanol was achieved using homogeneous molecular complexes. The molecular manganese complex [Mn(CO)2Br[HN(C2H4PiPr2)2]] ([HN(C2H4PiPr2)2] = MACHO-iPr) revealed the best performance, reaching up to turnover number = 4023 and turnover frequency 857 h–1in EtOH/toluene as solvent under optimized conditions (T= 150 °C, p(CO/H2) = 5/50 bar, t= 8–12 h). Control experiments affirmed that the reaction proceeds via formate ester as the intermediate, whereby a catalytic amount of base was found to be sufficient to mediate its formation from CO and the alcohol in situ. Selectivity for methanol formation reached >99% with no accumulation of the formate ester. The reaction was demonstrated to work with methanol as the alcohol component, resulting in a reactive system that allows catalytic “breeding” of methanol without any coreagents.

With the advent of renewable carbon resources, multifunctional catalysts are becoming essential to hydrogenate selectively biomass-derived substrates and intermediates. However, the development of adaptive catalytic systems, that is, with reversibly adjustable reactivity, able to cope with the intermittence of renewable resources remains a challenge. Here, we report the preparation of a catalytic system designed to respond adaptively to feed gas composition in hydrogenation reactions. Ruthenium nanoparticles immobilized on amine-functionalized polymer-grafted silica act as active and stable catalysts for the hydrogenation of biomass-derived furfural acetone and related substrates. Hydrogenation of the carbonyl group is selectively switched on or off if pure H2or a H2/CO2mixture is used, respectively. The formation of alkylammonium formate species by the catalytic reaction of CO2and H2at the amine-functionalized support has been identified as the most likely molecular trigger for the selectivity switch. As this reaction is fully reversible, the catalyst performance responds almost in real time to the feed gas composition.

Phosphine sulfonamido Pd(II) complexes bearing p-anisyl ([(P,N)Pd(4-OMe-C6H4)L]; L = C5H5N, 5-py; L = OS(CH3)2, 5-dmso) or carboxylato ([(P,N)Pd(OOCR)], 1-X: R = 4-OMe-C6H4, 1-pAn; R = tBu, 1-Piv; R = C6H5, 1-Ph) ligands were synthesized and their structures and dynamic behaviors were characterized. The compound 5-dmsocan effectively be converted to 1-pAnby migratory insertion of carbon dioxide into the Pd–aryl bond with precoordination of the CO2molecule. Additionally, carboxylato-substituted compounds 1-Xcan serve as catalysts for protodecarboxylation reactions of bis-o-OMe-substituted carboxylic acids at room temperature without the need for an external proton source. Stoichiometric decarboxylative coupling of these acids with styrene selectively yields 1,1-diarylated olefins.

A Cp*Ir(III) complex (1) bearing a proton-responsive hydroxy unit on an annulated imidazo[1,2-a][1,8]naphthyridine based mesoionic carbene scaffold was synthesized by two different synthetic routes. The molecular structure of 1revealed an anionic lactam form of the ligand. The acid–base equilibrium between the lactam-lactim tautomers on the ligand scaffold was examined by 1H NMR and UV–vis spectra. The pKaof the appendage −OH group in the lactim form of 1was estimated to assess the proton transfer property of the catalyst. The catalytic efficacy of 1for reductive amination of aldehyde was evaluated by utilizing three different hydrogen sources: molecular H2, iPrOH/KOtBu combination, and HCOOH/Et3N (5:2) azeotropic mixture. The HCOOH/Et3N (5:2) azeotropic mixture protocol was found to be the best among the three different hydrogenation methods. Catalyst 1hydrogenates imines chemoselectively over carbonyls under the reaction conditions. A range of aldehydes was reductively aminated to the corresponding secondary amines using the HCOOH/Et3N (5:2) azeotropic mixture. Further, catalyst 1showed high efficiency for the reduction of a wide variety of N-heterocyclic imine derivatives. The lactam-lactim tautomerization of the ligand system is proposed for direct hydrogenation, whereas only the lactam form operates in the strongly basic medium (iPrOH/KOtBu). Under HCOOH/Et3N (5:2) conditions, the lactam scaffold is not protonated; rather, an outer-sphere hydride transfer from formate to the Ir is proposed, which is supported by 1H NMR and DFT calculations. Finally, ligand-promoted hydride transfer from metal-hydride to the protonated imine affords the corresponding amine. A close agreement between the experimentally estimated and computed thermodynamic/kinetic parameters gives credence to the metal-ligand cooperative mechanism for the imine hydrogenation reaction using the HCOOH/Et3N (5:2) azeotropic mixture.

A simple dinuclear monohydrido bridged ruthenium complex [{(η6-p-cymene)RuCl}2(μ-H–μ-Cl)] acts as an efficient and selective catalyst for the hydrogenation of various heteroarenes and arenes. The nature of the catalytically active species was investigated using a combination of techniques including in situreaction monitoring, kinetic studies, quantitative poisoning experiments and electron microscopy, evidencing a dual reactivity. The results suggest that the hydrogenation of heteroarenes proceeds viamolecular catalysis. In particular, monitoring the reaction progress by NMR spectroscopy indicates that [{(η6-p-cymene)RuCl}2(μ-H–μ-Cl)] is transformed into monomeric ruthenium intermediates, which upon subsequent activation of dihydrogen and hydride transfer accomplish the hydrogenation of heteroarenes under homogeneous conditions. In contrast, carbocyclic aryl motifs are hydrogenated viaa heterogeneous pathway, by in situgenerated ruthenium nanoparticles. Remarkably, these hydrogenation reactions can be performed using molecular hydrogen under solvent-free conditions or with 1,4-dioxane, and thus give access to a broad range of saturated heterocycles and carbocycles while generating no waste.

The production of fluorinated cyclohexane derivatives is accomplished through the selective hydrogenation of readily available fluorinated arenes using Rh nanoparticles on molecularly modified silica supports (Rh@Si–R) as highly effective and recyclable catalysts. The catalyst preparation comprises grafting non-polar molecular entities on the SiO2surface generating a hydrophobic environment for controlled deposition of well-defined rhodium particles from a simple organometallic precursor. A broad range of fluorinated cyclohexane derivatives was shown to be accessible with excellent efficacy (0.05–0.5 mol% Rh, 10–55 bar H2, 80–100 °C, 1–2 h), including industrially relevant building blocks. Addition of CaO as scavenger for trace amounts of HF greatly improves the recyclability of the catalytic system and prevents the risks associated to the presence of HF, without compromising the activity and selectivity of the reaction.

The chemical industry can now seize the opportunity to improve the sustainability of its processes by replacing fossil carbon sources with renewable alternatives such as CO2, biomass, and plastics, thereby thinking ahead and having a look into the future. For their conversion to intermediate and final products, different types of catalysts─microbial, enzymatic, and organometallic─can be applied. The first part of this review shows how these catalysts can work separately in parallel, each route with unique requirements and advantages. While the different types of catalysts are often seen as competitive approaches, an increasing number of examples highlight, how combinations and concatenations of catalysts of the complete spectrum can open new roads to new products. Therefore, the second part focuses on the different catalysts either in one-step, one-pot transformations or in reaction cascades. In the former, the reaction conditions must be conflated but purification steps are minimized. In the latter, each catalyst can work under optimal conditions and the “hand-over points” should be chosen according to defined criteria like minimal energy usage during separation procedures. The examples are discussed in the context of the contributions of catalysis to the envisaged (bio)economy.

An adaptive catalytic system for selective hydrogenation was developed exploiting the H2+ CO2⇔HCOOH equilibrium for reversible, rapid, and robust on/off switch of the ketone hydrogenation activity of ruthenium nanoparticles (Ru NPs). The catalyst design was based on mechanistic studies and DFT calculations demonstrating that adsorption of formic acid to Ru NPs on silica results in surface formate species that prevent C═O hydrogenation. Ru NPs were immobilized on readily accessible silica supports modified with guanidinium-based ionic liquid phases (Ru@SILPGB) to generate in situ sufficient amounts of HCOOH when CO2was introduced into the H2feed gas for switching off ketone hydrogenation while maintaining the activity for hydrogenation of olefinic and aromatic C═C bonds. Upon shutting down the CO2supply, the C═O hydrogenation activity was restored in real time due to the rapid decarboxylation of the surface formate species without the need for any changes in the reaction conditions. Thus, the newly developed Ru@SILPGBcatalysts allow controlled and alternating production of either saturated alcohols or ketones from unsaturated substrates depending on the use of H2or H2/CO2as feed gas. The major prerequisite for design of adaptive catalytic systems based on CO2as trigger is the ability to shift the H2+ CO2⇔HCOOH equilibrium sufficiently to exploit competing adsorption of surface formate and targeted functional groups. Thus, the concept can be expected to be more generally applicable beyond ruthenium as the active metal, paving the way for next-generation adaptive catalytic systems in hydrogenation reactions more broadly.

A method for the synthesis of substituted cycloalkanes was developed using diols and secondary alcohols or ketones via a cascade hydrogen borrowing sequence. A non-noble and air-stable manganese catalyst (2 mol %) was used to perform this transformation. Various substituted 1,5-pentanediols (3–4 equiv) and substituted secondary alcohols (1 equiv) were investigated to prepare a collection of substituted cyclohexanes in a diastereoselective fashion. Similarly, cyclopentane, cyclohexane, and cycloheptane rings were constructed from substituted 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol, and sterically hindered ketones following a (4 + 1), (5 + 1), and (6 + 1) strategy, respectively. This reaction provides an atom economic methodology to construct two C–C bonds at a single carbon center generating high-value cycloalkanes from readily available alcohols as feedstock using an earth-abundant metal catalyst.

Manganese(i) complexes bearing triazole ligands are reported as catalysts for the hydrosilylation of carbonyl and carboxyl compounds. The desired reaction proceeds readily at 80 °C within 3 hours at catalyst loadings as low as 0.25 to 1 mol%. Hence, good to excellent yields of alcohols could be obtained for a wide range of substrates including ketones, esters, and carboxylic acids illustrating the versatility of the metal/ligand combination.

A ruthenium(II) complex bearing a naphthyridine-functionalized pyrazole ligand catalyzes oxidant-free and acceptorless selective double dehydrogenation of primary amines to nitriles at moderate temperature. The role of the proton-responsive entity on the ligand scaffold is demonstrated by control experiments, including the use of a N-methylated pyrazole analogue. DFT calculations reveal intricate hydride and proton transfers to achieve the overall elimination of 2 equiv of H2.

Cleavage and (re)formation of oxymethylene moieties are essential steps during the build-up of polyoxymethylene (POM), a technically relevant high-performance polymer. To reveal how the catalyst accomplishes the cleavage of oxymethylene moieties, the kinetics of the ring-opening of trioxane was studied. Thereby insights into the initial phase of the growth of oxymethylene chains were obtained. The chain length of short oligomers was controlled with acetic anhydride as transfer agent. With a high ratio of acetic anhydride to trioxane, the first homologues of the series, trioxymethylene diacetate, dioxymethylene diacetate and monooxymethylene diacetate were obtained in excellent yield. The homologues show distinct features in the NMR and IR spectra that were related to their reactivity during chain propagation. Formation of intermediate hemi-acetal oxonium moieties is suggested to be a key step in the reaction pathway. Such molecular level insight may be a crucial factor for further tailoring production and properties of polyoxymethylene as well as introducing oligomeric oxymethylene diacetates to new application fields.

A process comprising a continuous-flow hydrogenation reaction integrated with selective water-organic solvent biphasic extraction using CO2as molecular switch to control partitioning was devised for the synthesis of arylpiperidines from arylpyridines. The selective hydrogenation of 4-phenylpyridine using heterogeneous carbon-supported metal catalysts was chosen as model reaction. A design-of-experiment approach was used for the identification of suitable reaction conditions under continuous-flow operation. A maximum selectivity for 4-phenylpiperidine of 96% was achieved at 87% conversion suppressing the deep hydrogenation to 4-cyclohexylpiperidine almost completely (≤5%). The higher basicity of piperidines over pyridines was exploited for selective and reversible protonation of the product upon pressurization with CO2separating it quantitatively from the remaining starting material in a water-EtOAc biphasic system. This concept enabled a fully integrated and a salt-free synthetic process using a standard Pd/C catalyst for the hydrogenation coupled with the CO2-triggered isolation of the desired product 4-phenylpiperidine in 81% yield and 98% purity.

Valuable substituted phenols are accessible via the selective decarboxylation of hydroxybenzoic acid derivatives using multifunctional catalysts composed of bimetallic iron–ruthenium nanoparticles immobilized on an amine-functionalized supported ionic liquid phase (Fe25Ru75@SILP+IL-NEt2). The individual components of the catalytic system are assembled using a molecular approach to bring metal and amine sites into close contact on the support material, providing high stability and high decarboxylation activity. Operating under a hydrogen atmosphere was found to be essential to achieve high selectivity and yields. As the catalyst materials enable also the selective hydrogenation and hydrodeoxygenation of various additional functional groups (i.e., formyl, acyl, and nitro substituents), direct access to the corresponding phenols can be achieved via integrated tandem reactions. The approach opens versatile synthetic pathways for the production of valuable phenols from a wide range of readily available substrates, including compounds derived from lignocellulosic biomass.