Homogeneous Catalysis Reactions Research Articles

ReNeGate: A Reaction Network Graph-Theoretical Tool for Automated Mechanistic Studies in Computational Homogeneous Catalysis.

Exploration of the chemical reaction space of chemical transformations in multicomponent mixtures is one of the main challenges in contemporary computational chemistry. To remove expert bias from mechanistic studies and to discover new chemistries, an automated graph-theoretical methodology is proposed, which puts forward a network formalism of homogeneous catalysis reactions and utilizes a network analysis tool for mechanistic studies. The method can be used for analyzing trajectories with single and multiple catalytic species and can provide unique conformers of catalysts including multinuclear catalyst clusters along with other catalytic mixture components. The presented three-step approach has the integrated ability to handle multicomponent catalytic systems of arbitrary complexity (mixtures of reactants, catalyst precursors, ligands, additives, and solvents). It is not limited to predefined chemical rules, does not require prealignment of reaction mixture components consistent with a reaction coordinate, and is not agnostic to the chemical nature of transformations. Conformer exploration, reactive event identification, and reaction network analysis are the main steps taken for identifying the pathways in catalytic systems given the starting precatalytic reaction mixture as the input. Such a methodology allows us to efficiently explore catalytic systems in realistic conditions for either previously observed or completely unknown reactive events in the context of a network representing different intermediates. Our workflow for the catalytic reaction space exploration exclusively focuses on the identification of thermodynamically feasible conversion channels, representative of the (secondary) catalyst deactivation or inhibition paths, which are usually most difficult to anticipate based solely on expert chemical knowledge. Thus, the expert bias is sought to be removed at all steps, and the chemical intuition is limited to the choice of the thermodynamic constraint imposed by the applicable experimental conditions in terms of threshold energy values for allowed transformations. The capabilities of the proposed methodology have been tested by exploring the reactivity of Mn complexes relevant for catalytic hydrogenation chemistry to verify previously postulated activation mechanisms and unravel unexpected reaction channels relevant to rare deactivation events.

Recent progress of cobalt catalysts for homogeneous catalysis (de)hydrogenation

<p indent="0mm">In recent years, the catalytic systems of iron, cobalt, nickel and manganese complexes have made a series of progress in the field of base-metal catalysis. One of the most impressive achievements is the catalysis by cobalt complexes, which has become increasingly attractive in homogeneous catalysis reactions because of its unique characteristics and outstanding catalytic performance. These reactions involving (de)hydrogenation play an important role in academic research and the chemical industry. In this paper, we review homogeneous cobalt-catalyzed (de)hydrogenation reactions according to the classified reaction types including hydrogenation, transfer hydrogenation, acceptorless dehydrogenation, dehydrogenative coupling, hydrogen borrowing, and provide a comprehensive overview of the reactivity of cobalt catalysts, their catalytic applications, and reaction mechanisms.

An intensified approach for transesterification of biodiesel from Annona squamosa seed oil using ultrasound-assisted homogeneous catalysis reaction and its process optimization

In this work, Annona squamosa seed oil (ASSO) was extracted using ultrasound-assisted extraction (UAE) and converted into fatty acid methyl esters (FAME) by ultrasound-assisted transesterification (UAT) using methanol and KOH. The multivariable process of UAE and UAT was optimized using a response surface methodology (RSM). The experimental data were analyzed by Pareto analysis of variance (ANOVA) and quadratic polynomial models developed using non-linear regression analysis. A maximum ASSO yield of 31.91% was achieved by the model with the optimum conditions for UAE: liquid-to-solid ratio, 8.8 mL g−1; extraction temperature, 45 °C; sonication time, 19.5 min; and sonication amplitude, 88%. The maximum conversion of ASSO into biodiesel (97.6%) was achieved with the optimum condition of the model: oil-to-methanol ratio, 5.04; catalyst concentration, 1.12%; temperature, 57 °C; and sonication time, 113 min. These models were found to be significant at the 95% confidence level. The transesterified ASSO FAME was characterized using Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), gas chromatography mass spectrometry (GC–MS), 1H-nuclear magnetic resonance (1H NMR), and fluorescence spectroscopy methods. Various physicochemical properties of ASSO and ASSO FAME were characterized according to ASTM standards. The ultrasound-assisted extraction and transesterification process was found to be rapid and simple without any change in the product quality. Importantly, ASSO was found to be a possible new source for potential biodiesel production.

Computational modelling of Pd-catalysed alkoxycarbonylation of alkenes and alkynes.

This perspective highlights the computational modelling of alkene and alkyne alkoxycarbonylation at palladium catalysts. We cover studies on Pd-catalysed alkoxycarbonylation of alkenes with bidentate diphosphine ligands, which reveal a hydride pathway is operating with an intermolecular alcoholysis step, where explicit solvation is mandatory to estimate the overall barriers correctly and model alcoholysis/copolymerisation selectivities. Subsequently, we discuss Pd-catalysed alkyne alkoxycarbonylation with P,N-chelating ligands, where an in situ base mechanism is operating involving ketene-type intermediates. We also discuss catalyst poisoning due to allene and designing a potential new catalyst tolerant towards allene poisoning.

Enantioselective Rhodium-Catalyzed Desymmetric Hydrosilylation of Cyclopropenes

Catalytic hydrosilylation of alkenes ranks as one of the most important academic and industrial synthetic reactions in homogeneous catalysis and organosilicon chemistry. In this work, a highly enan...

Palladium Acetate Trimer: Understanding Its Ligand-Induced Dissociation Thermochemistry Using Isothermal Titration Calorimetry, X-ray Absorption Fine Structure, and 31P Nuclear Magnetic Resonance

Palladium acetate trimer, Pd3(OAc)6, is a common precursor for Pd nanoparticle synthesis and an important precatalyst for homogeneous catalysis reactions. Added ligands (L, e.g., phosphines) are well-known to dissociate the Pd3(OAc)6 trimer into Pd(OAc)2 monomers and form Pd(OAc)2(L)2 complexes. Despite the importance of the trimer dissociation and ligand–monomer binding thermodynamics on the properties of the resulting Pd complex, little is known about either reaction. Using a combination of isothermal titration calorimetry (ITC), X-ray absorption fine structure (XAFS), and 31P nuclear magnetic resonance (31P NMR) on trimer samples containing monomer impurities, we developed a methodology and a trimer–monomer ITC model to obtain the Gibbs free energy, enthalpy, and entropy for both reactions in toluene and quantify the monomer content. The results provide the following, previously inaccessible, quantitative insights on the reactions. Trioctylphosphine (TOP)-monomer binding reaction is enthalpically driven (ΔH = −187 ± 8 kJ/mol), but the entropy loss due to binding (−TΔS = 84 ± 8 kJ/mol) was significant. However, while the trimer dissociation is enthalpically uphill (ΔH = 323 ± 35 kJ/mol), the increase in entropy due to the formation of monomers was very large (−TΔS = −266 ± 35 kJ/mol). The entropy gains and losses for both reactions have major contributions to each reaction Gibbs free energy and almost compensate for each other, making the overall reaction (i.e., Pd3(OAc)6 + 6 TOP ↔ 3 Pd(OAc)2(TOP)2) enthalpically driven. The results show the importance of determining the relative enthalpy and entropy contributions to the free energies for the complete thermochemical cycle of Pd acetate trimer dissociation and reaction with TOP. The methodology can be generalized to other ligands and metal or nonmetal homopolymers to obtain detailed thermochemistry of thermodynamically or kinetically unfavorable reactions.

Ionic Liquid Droplet Microreactor for Catalysis Reactions Not at Equilibrium.

We develop a novel strategy to more effectively and controllably process continuous enzymatic or homogeneous catalysis reactions based on nonaqueous Pickering emulsions. A key element of this strategy is "bottom-up" construction of a macroscale continuous flow reaction system through packing catalyst-containing micron-sized ionic liquid (IL) droplet in oil in a column reactor. Due to the continuous influx of reactants into the droplet microreactors and the continuous release of products from the droplet microreactors, catalysis reactions in such a system can take place without limitations arising from establishment of the reaction equilibrium and catalyst separation, inherent in conventional batch reactions. As proof of the concept, enzymatic enantioselective trans-esterification and CuI-catalyzed cycloaddition reactions using this IL droplet-based flow system both exhibit 8 to 25-fold enhancement in catalysis efficiency compared to their batch counterparts, and a durability of at least 4000 h for the enantioselective trans-esterification of 1-phenylethyl alcohol, otherwise unattainable in their batch counterparts. We further establish a theoretical model for such a catalysis system working under nonequilibrium conditions, which not only supports the experimental results but also helps to predict reaction progress at a microscale level. Being operationally simple, efficient, and adaptive, this strategy provides an unprecedented platform for practical applications of enzymes and homogeneous catalysts even at a controllable level.

Cooperative Strategies for Catalytic Hydrogenation of Unsaturated Hydrocarbons

This Perspective discusses catalytic hydrogenation reactions of alkenes, alkynes, and other unsaturated C-C substrates in which the catalyst design involves cooperative strategies that enable bifunctional H2 activation and delivery. These approaches complement the more traditional use of single-site precious metal systems for such hydrogenation reactions in homogeneous catalysis. Strategies included in this Perspective are cooperation between (a) a catalytic metal site and a basic ligand residue, (b) a catalytic metal site and an acidic ligand residue, (c) frustrated acid/base pairs, and (d) two co-catalytic metal sites. Unique reactivity and selectivity patterns with non-precious elements that have emerged from successful implementation of these strategies in catalytic transformations are emphasized.

ChemInform Abstract: Copper(II)‐Catalyzed Nitroaldol (Henry) Reactions: Recent Developments

AbstractReview: [32 refs.

High yield synthesis of 5-hydroxymethylfurfural from cellulose using FePO4 as the catalyst

It is of great significance to explore and develop a cost-effective and environmentally friendly way to covert cellulosic materials into 5-hydroxymethylfurfural (5-HMF). An inexpensive FePO4 catalyst, which is insoluble at room temperature but can be partially dissolved and behaves as homogeneous catalyst at elevated temperatures, was used to directly convert cellulose into 5-HMF. The effect of various reaction conditions including reaction temperature, reaction time, cellulose concentration and catalyst amount on the conversion of cellulose and 5-HMF yield was investigated. A 5-HMF yield up to 49.5mol% was achieved at 160°C for 60min with the FePO4 catalyst, which is comparable to the highest yield of 5-HMF obtained in ionic liquid or homogeneous catalysis reaction systems. Moreover, this unique catalyst is readily separated from the aqueous solution via precipitation after cooling to room temperature since it is insoluble at room temperature. The catalyst can be used over five successive cycles although the amorphous FePO4 underwent the structural transformation from an amorphous phase into a new crystalline form of FePO4, evidenced by XRD, IR, and visible Raman spectroscopic results. The soluble iron species, such as Fe3+ species, act as Lewis acid sites catalyzing isomerization of glucose to fructose followed by the dehydration of fructose into 5-HMF catalyzed by H+ ions.

Copper(II)-Catalyzed Nitroaldol (Henry) Reactions: Recent Developments.

Self-assembled copper(II) complexes are described as effective catalysts for nitroaldol (Henry) reactions on water. The protocol involves a heterogeneous process and the catalysts can be recovered and recycled without loss of activity. Further, C2-symmetric N,N'-substituted chiral copper(II) salan complexes are found to be more effective catalysts than chiral copper(II) salen complexes for reactions in homogeneous catalysis, with high enantioselectivities. The reactions involve bifunctional catalysis, bearing the properties of a Brønsted base, as well as a Lewis acid, to effect the reaction in the absence of external additives.

ChemInform Abstract: Kinetic Models of Multiroute Reactions in Homogeneous Catalysis with Metal Complexes

AbstractReview: 207 refs.

Kinetic models of multi-route reactions in homogeneous catalysis with metal complexes (A Review)

The state-of-the-art in the theory of routes for multi-route reactions in the homogeneous catalysis with metal complexes is reviewed. Key problems and the possible approaches to their solution based on the Horiuti-Temkin theory of steady-state reactions, which are useful in constructing the kinetic algebraic models of multi-route processes, are discussed. A classification of the mechanisms of real processes involved in the homogeneous catalysis with metal complexes is presented. The chain mechanisms of catalytic processes, conjugated and associated reactions, and critical phenomena in the dynamics of homogeneous reactions are considered.

Organometallics: transition metals in organic synthesis

Homogeneous catalysis reactions centring on the elaboration of simple unfunctionalised compounds have attracted significant attention in 2008.