Aerobic Oxidations in Continuous Flow

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.

Ligand-Promoted Palladium-Catalyzed Aerobic Oxidation Reactions

Palladium-catalyzed aerobic oxidation reactions have been the focus of industrial application and extensive research efforts for nearly 60 years. A significant transition occurred in this field approximately 20 years ago, with the introduction of catalysts supported by ancillary ligands. The ligands play crucial roles in the reactions, including promotion of direct oxidation of palladium(0) by O₂, bypassing the typical requirement for Cu salts or related redox cocatalysts to facilitate oxidation of the reduced Pd catalyst; facilitation of key bond-breaking and bond-forming steps during substrate oxidation; and modulation of chemo-, regio-, or stereoselectivity of a reaction. The use of ligands has contributed to significant expansion of the scope of accessible aerobic oxidation reactions. Increased understanding of the role of ancillary ligands should promote the development of new synthetic transformations, enable improved control over the reaction selectivity, and improve catalyst activity and stability. This review surveys the different ligands that have been used to support palladium-catalyzed aerobic oxidation reactions and, where possible, describes mechanistic insights into the role played by the ancillary ligand.

Continuous Photo-Oxidation in a Vortex Reactor: Efficient Operations Using Air Drawn from the Laboratory

We report the construction and use of a vortex reactor which uses a rapidly rotating cylinder to generate Taylor vortices for continuous flow thermal and photochemical reactions. The reactor is designed to operate under conditions required for vortex generation. The flow pattern of the vortices has been represented using computational fluid dynamics, and the presence of the vortices can be easily visualized by observing streams of bubbles within the reactor. This approach presents certain advantages for reactions with added gases. For reactions with oxygen, the reactor offers an alternative to traditional setups as it efficiently draws in air from the lab without the need specifically to pressurize with oxygen. The rapid mixing generated by the vortices enables rapid mass transfer between the gas and the liquid phases allowing for a high efficiency dissolution of gases. The reactor has been applied to several photochemical reactions involving singlet oxygen (1O2) including the photo-oxidations of α-terpinene and furfuryl alcohol and the photodeborylation of phenyl boronic acid. The rotation speed of the cylinder proved to be key for reaction efficiency, and in the operation we found that the uptake of air was highest at 4000 rpm. The reactor has also been successfully applied to the synthesis of artemisinin, a potent antimalarial compound; and this three-step synthesis involving a Schenk-ene reaction with 1O2, Hock cleavage with H+, and an oxidative cyclization cascade with triplet oxygen (3O2), from dihydroartemisinic acid was carried out as a single process in the vortex reactor.

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.

Continuous flow synthesis of indoles by Pd-catalyzed deoxygenation of 2-nitrostilbenes with carbon monoxide

The palladium-catalyzed cyclization of o-vinylnitrobenzenes employing carbon monoxide as terminal reductant is investigated. The reaction proceeds with 1 to 2 mol% of Pd(OAc)₂ and generates carbon dioxide as the only stoichiometric side-product.

Batch and Flow Synthesis of Disulfides by Visible-Light-Induced TiO2 Photocatalysis

A mild and practical method for the preparation of disulfides through visible-light-induced photocatalytic aerobic oxidation of thiols has been developed. The method involves the use of TiO2 as a heterogeneous photocatalyst. The catalyst's high stability and recyclability makes this method highly practical. The reaction can be substantially accelerated in a continuous-flow packed-bed reactor, which enables a safe and reliable scale-up of the reaction conditions. The batch and flow protocol described herein can be applied to a diverse set of thiol substrates for the preparation of homo- and hetero-dimerized disulfides. Furthermore, biocompatible reaction conditions (i.e., room temperature, visible light, neutral buffer solution, and no additional base) have been developed, which permits the rapid and chemoselective modification of densely functionalized peptide substrates without recourse to complex purification steps.

Batch- and Continuous-Flow Aerobic Oxidation of 14-Hydroxy Opioids to 1,3-Oxazolidines-A Concise Synthesis of Noroxymorphone

14-Hydroxymorphinone is converted to noroxymorphone, the immediate precursor of important opioid antagonists, such as naltrexone and naloxone, in a three-step reaction sequence. The initial oxidation of the N-methyl group in 14-hydroxymorphinone with in situ generated colloidal palladium(0) as the catalyst and molecular oxygen as the terminal oxidant constitutes the key transformation in this new route. This oxidation results in the formation of an unexpected oxazolidine ring structure. Subsequent hydrolysis of the oxazolidine under reduced pressure followed by hydrogenation in a packed-bed flow reactor using palladium(0) as the catalyst provides noroxymorphone in high purity and good overall yield. To overcome challenges associated with gas-liquid reactions with molecular oxygen, the key oxidation reaction was translated to a continuous-flow process.

Applications of Continuous-Flow Photochemistry in Organic Synthesis, Material Science, and Water Treatment

Continuous-flow photochemistry in microreactors receives a lot of attention from researchers in academia and industry as this technology provides reduced reaction times, higher selectivities, straightforward scalability, and the possibility to safely use hazardous intermediates and gaseous reactants. In this review, an up-to-date overview is given of photochemical transformations in continuous-flow reactors, including applications in organic synthesis, material science, and water treatment. In addition, the advantages of continuous-flow photochemistry are pointed out and a thorough comparison with batch processing is presented.

A convenient numbering-up strategy for the scale-up of gas–liquid photoredox catalysis in flow

An operationally simple numbering-up strategy for the scale-up of gas–liquid photocatalytic reactions was developed, which provides an excellent flow distribution (SD_w < 10%).