Continuous, Fast, and Safe Aerobic Oxidation of 2-Ethylhexanal: Pushing the Limits of the Simple Tube Reactor for a Gas/Liquid Reaction

Development of Lab-Scale Continuous Stirred-Tank Reactor as Flow Process Tool for Oxidation Reactions Using Molecular Oxygen

The use of sustainable oxidants is of great interest to the chemical industry, considering the importance of oxidation reactions for the manufacturing of chemicals and society's growing awareness of its environmental impact. Molecular oxygen (O2), with an almost optimal atom efficiency in oxidation reactions, presents one of the most attractive alternatives to common reagents that are not only toxic in most cases but produce stoichiometric amounts of waste that must be treated. However, fire and explosion safety concerns, especially when used in combination with organic solvents, restrict its easy use. Here, we use state-of-the-art 3D printing and experimental feedback to develop a miniature continuous stirred-tank reactor (mini-CSTR) that enables efficient use of O2 as an oxidant in organic chemistry. Outstanding heat dissipation properties, achieved through integrated jacket cooling and a high surface-to-volume ratio, allow for a safe operation of the exothermic oxidation of 2-ethylhexanal, surpassing previously reported product selectivity. Moving well beyond the proof-of-concept stage, we characterize and illustrate the reactor's potential in the gas-liquid-solid triphasic synthesis of an endoperoxide precursor of antileishmanial agents. The custom-designed magnetic overhead stirring unit provides improved stirring efficiency, facilitating the handling of suspensions and, in combination with the borosilicate gas dispersion plate, leading to an optimized gas-liquid interface. These results underscore the immense potential that lies within the use of mini-CSTR in sustainable chemistry.

Efficient Synthesis of 2-Ethylhexanoic Acid via N-Hydroxyphthalimide Catalyzed Oxidation of 2-Ethylhexanal with Oxygen

An efficient method for the synthesis of 2-ethylhexanoic acid has been reported. The method involves the 2-ethylhexanal oxidation using oxygen or air in the presence of N-hydroxyphthalimide in isobutanol as a solvent under mild conditions. A high selectivity of >99% for 2-ethylhexanoic acid was achieved. The influence of catalyst amount, solvent type and quantity, temperature, and reaction time on the product composition was studied. The developed method is in line with the global trends aimed at developing green oxidation processes as well as having potential for implementation in industry due to its high selectivity, cost-effective oxidizing agent, and mild reaction conditions. The use of isobutanol as a solvent is of crucial importance providing an opportunity for potential producers of 2-EHAL from butanal to employ the less valuable alcohol.

Solvent Effect on Product Distribution in the Aerobic Autoxidation of 2-Ethylhexanal: Critical Role of Polarity

In the aerobic oxidation of aldehydes to acids, how the solvent affect the reaction remains unclear. Herein, the solvent effect in the oxidation of 2-ethylhexanal (2-ETH) to 2-ethylhexanoic acid (2-ETA) was systematically investigated. The vastly different product distributions were observed which could be ascribed to the dominant intermolecular forces. Though strong intermolecular forces in protic solvents limit the oxidation, the optimal 2-ETA yield (96%) was obtained in ipropanol via gradually evaporating the solvent to remove the interactions. Theoretical calculations further revealed that the hydrogen bonds between reactant and protic solvent increase the C-H bond energy (-CHO in 2-ETH). Meanwhile, the hydrogen bonds may improve 2-ETA selectivity by promoting H transfer in the oxidation rearrangement step. Our work discloses the critical role of polarity in determining the reactivity and selectivity of 2-ETH oxidation, and could guide the rational design of more desirable reaction processes with solvent effect.

Telescoped Continuous Flow Synthesis of Optically Active γ-Nitrobutyric Acids as Key Intermediates of Baclofen, Phenibut, and Fluorophenibut

The two-step flow asymmetric synthesis of chiral γ-nitrobutyric acids as key intermediates of the GABA analogues baclofen, phenibut, and fluorophenibut is reported on a multigram scale. The telescoped process comprises an enantioselective Michael-type addition facilitated by a polystyrene-supported heterogeneous organocatalyst under neat conditions followed by in situ-generated performic acid-mediated aldehyde oxidation. Simple access to valuable optically active substances is provided with key advances in terms of productivity and sustainability compared to those of previous batch approaches.

Reinvestigation of the Organocatalyzed Aerobic Oxidation of Aldehydes to Acids

The organocatalyzed aerobic oxidation of aldehydes to acids was reproduced from the original report. In- and ex-situ analysis of the reaction mixture as the function of time reveals that, unlike the claim in the publication, the aerobic oxidation of aromatic and aliphatic aldehydes leads predominantly to the formation of peracids. The latter are transformed into the corresponding carboxylic acids during the workup procedure. The buildup of peracids in solution poses safety problems that should not be overlooked. This finding has also an influence on the way new catalysts are investigated to improve this reaction as well as on aerobic aldehyde-mediated co-oxidation.

Continuous flow aerobic alcohol oxidation using a heterogeneous Ru0 catalyst

Stable Ru⁰ on γ-Al₂O₃ has been used for the selective aerobic oxidation of benzyl alcohol in flow.

C-H functionalization reactions under flow conditions

C-H functionalization technologies have progressed enormously in the last decade as testified by the great number of publications that have appeared in the literature, which are receiving great attention from researchers from different areas of expertise. While most of the protocols reported realize the C-H functionalization processes under batch conditions, there is a growing interest in the development of continuous-flow procedures aiming at increasing the performances of established methodologies or the definition of otherwise unfeasible transformations. This review summarizes the application of flow technologies for the realization of C-H functionalization reactions. According to the type of flow reactors necessary, two main general approaches are possible for the application of flow techniques, namely the use of homogeneous or heterogeneous conditions. Each example is discussed and accompanied by the description of the main features and benefits of the use of flow compared to batch conditions.

Mesoscale triphasic flow reactors for metal catalyzed gas–liquid reactions

Design and operation of a mesoscale triphasic reactor for flow hydrogenations, capable of delivering kg per day productivity from a single channel.

The Use of Molecular Oxygen for Liquid Phase Aerobic Oxidations in Continuous Flow

Molecular oxygen (O₂) is the ultimate “green” oxidant for organic synthesis. There has been recent intensive research within the synthetic community to develop new selective liquid phase aerobic oxidation methodologies as a response to the necessity to reduce the environmental impact of chemical synthesis and manufacture. Green and sustainable chemical processes rely not only on effective chemistry but also on the implementation of reactor technologies that enhance reaction performance and overall safety. Continuous flow reactors have facilitated safer and more efficient utilization of O₂, whilst enabling protocols to be scalable. In this article, we discuss recent advancements in the utilization of continuous processing for aerobic oxidations. The translation of aerobic oxidation from batch protocols to continuous flow processes, including process intensification (high T/p), is examined. The use of “synthetic air”, typically consisting of less than 10% O₂ in N₂, is compared to pure O₂ (100% O₂) as an oxidant source in terms of process efficiency and safety. Examples of homogeneous catalysis and heterogeneous (packed bed) catalysis are provided. The application of flow photoreactors for the in situ formation of singlet oxygen (¹O₂) for use in organic reactions, as well as the implementation of membrane technologies, green solvents and recent reactor solutions for handling O₂ are covered.

Continuous flow oxidation of benzylic and aliphatic alcohols using bleach: process improvement by precise pH adjustment in flow with CO2

Commercial bleach neutralization using CO₂ enhanced the oxidation rate of benzylic and aliphatic alcohols to their corresponding aldehydes and ketones.

The Hitchhiker's Guide to Flow Chemistry ∥

Flow chemistry involves the use of channels or tubing to conduct a reaction in a continuous stream rather than in a flask. Flow equipment provides chemists with unique control over reaction parameters enhancing reactivity or in some cases enabling new reactions. This relatively young technology has received a remarkable amount of attention in the past decade with many reports on what can be done in flow. Until recently, however, the question, "Should we do this in flow?" has merely been an afterthought. This review introduces readers to the basic principles and fundamentals of flow chemistry and critically discusses recent flow chemistry accounts.

The Use of Molecular Oxygen in Pharmaceutical Manufacturing: Is Flow the Way to Go?

Molecular oxygen is arguably the greenest reagent available to the organic chemist. Most commonly, a diluted form of oxygen gas, consisting of less than 10 % O₂ in N₂ ("synthetic air"), is used in pharmaceutical and fine chemical batch manufacturing to effectively address safety concerns when handling molecular oxygen. Concentrations of O₂ in N₂ below 10 % are generally required to prevent the risk of combustions in the presence of flammable organic solvents ("limiting oxygen concentration"). Nonetheless, the use of pure oxygen is more efficient than using O₂ diluted with N₂ and can often provide enhanced reaction rates, resulting in significant improvements in product quality and process efficiency. This Concept takes into account recent studies to make the argument that, for liquid-phase aerobic oxidations, pure oxygen can indeed be handled safely on large scale by employing continuous-flow reactors, while also providing highly convincing synthetic and manufacturing benefits.

A flow reactor setup for photochemistry of biphasic gas/liquid reactions

A home-built microreactor system for light-mediated biphasic gas/liquid reactions was assembled from simple commercial components. This paper describes in full detail the nature and function of the required building elements, the assembly of parts, and the tuning and interdependencies of the most important reactor and reaction parameters. Unlike many commercial thin-film and microchannel reactors, the described set-up operates residence times of up to 30 min which cover the typical rates of many organic reactions. The tubular microreactor was successfully applied to the photooxygenation of hydrocarbons (Schenck ene reaction). Major emphasis was laid on the realization of a constant and highly reproducible gas/liquid slug flow and the effective illumination by an appropriate light source. The optimized set of conditions enabled the shortening of reaction times by more than 99% with equal chemoselectivities. The modular home-made flow reactor can serve as a prototype model for the continuous operation of various other reactions at light/liquid/gas interfaces in student, research, and industrial laboratories.

Epoxidation using molecular oxygen in flow: facts and questions on the mechanism of the Mukaiyama epoxidation

Mukaiyama reaction was performed G/L continuous-flow microreactor. In less than 5 minutes at room temperature, cyclooctene was efficiently transformed to the corresponding epoxide using O₂ as oxidant and aldehyde as co-reductant.

Aerobic oxidations in flow: opportunities for the fine chemicals and pharmaceuticals industries

This collaborative review (between teams of chemists and chemical engineers) describes the current scientific and operational hurdles that prevent the utilisation of aerobic oxidation reactions for the production of speciality chemicals and active pharmaceutical ingredients (APIs).