The application of modern reactions in large-scale synthesis

From Plastic Waste to Green Hydrogen and Valuable Chemicals Using Sunlight and Water

Over 79 % of 6.3 billion tonnes of plastics produced from 1950 to 2015 have been disposed in landfills or found their way to the oceans, where they will reside for up to hundreds of years before being decomposed bringing upon significant dangers to our health and ecosystems. Plastic photoreforming offers an appealing alternative by using solar energy and water to transform plastic waste into value-added chemical commodities, while simultaneously producing green hydrogen via the hydrogen evolution reaction. This review aims to provide an overview of the underlying principles of emerging plastic photoreforming technologies, highlight the challenges associated with experimental protocols and performance assessments, discuss recent global breakthroughs on the photoreforming of plastics, and propose perspectives for future research. A critical assessment of current plastic photoreforming studies shows a lack of standardised conditions, hindering comparison amongst photocatalyst performance. Guidelines to establish a more accurate evaluation of materials and systems are proposed, with the aim to facilitate the translation of promising fundamental discovery in photocatalysts design.

Bis(pinacolato)diboron-Enabled Ni-Catalyzed Reductive Arylation/Vinylation of Alkyl Electrophiles

Herein, the use of economically and environmentally friendly bis(pinacolato)diboron (B2Pin2) is described as a non-metallic reductant in mediating Ni-catalyzed C(sp3)-C(sp2) reductive cross-coupling of alkyl electrophiles with aryl/vinyl halides. This method exhibits excellent suitability for heteroaryl halides and alkyl halides/Katritzky salts. The present study is compatible with an in situ halogenation of alcohol method, allowing for selective mono-functionalization of diols and bio-relevant alcohols (e.g., carbohydrates). The use of B2Pin2 shows potential for easy scalability without introducing additional metal impurities into the products. It is observed for the first time in the realm of cross-electrophile coupling chemistry that B2Pin2 can sever as a reductant to reduce NiII to Ni0. This mechanistic insight may inspire the development of new reductive bond-forming methodologies that can otherwise be difficult to achieve with a metal reductant.

Process Development and Scale-Up of a Novel Atypical DAT Inhibitor (S)-CE-123

Large-scale syntheses of small molecules and kilo laboratories are crucial steps in drug development, especially in advanced stages. (S)-5-((Benzhydrylsulfinyl)methyl)thiazole, (S)-CE-123, a potent, selective, and novel atypical DAT inhibitor, has undergone iterative testing as part of the preclinical evaluation step. This required the process transfer, scale-up, and synthesis of a 1 kg preclinical batch. The Kagan protocol for asymmetric sulfide to sulfoxide oxidation was successfully applied within a four-step synthetic process for the successful upscaling of (S)-CE-123. During the scale-up of the last step, several changes were made to the original synthetic procedure, as with every increase in batch size, new problems had to be overcome. These include, among others, the workup optimization of the last step, the simplification of chromatographic purification, elution modification to improve the purity of the product and saving of workup time. Two washing steps were added to the original procedure to enhance both the yield and the enantiomeric excess value of the final product. The modifications introduced allowed access to a 1 kg (S)-CE-123 batch with a purity >99% and an enantiomeric excess value of 95%.

Fluoride-triggered phase transition of metallogels for on-demand in situ containment of fluids

Sol-gel transition regulates mass transport in fluidic systems. We designed pre-gelators that react with fluoride anions to form a metallogel barrier. A combination of spectroscopic, rheological, and X-ray spectroscopic studies elucidated the mechanism of gelation involving desilylation followed by metal coordination-driven self-assembly, the kinetics of which can be finely controlled by the chemical structure of the silyl substituents. Protonation-induced degelation restores flow, allowing the metallogel to function as a reversible chemical valve.

Scientific Deep Machine Learning Concepts for the Prediction of Concentration Profiles and Chemical Reaction Kinetics: Consideration of Reaction Conditions

Emerging concepts from scientific deep machine learning such as physics-informed neural networks (PINNs) enable a data-driven approach for the study of complex kinetic problems. We present an extended framework that combines the advantages of PINNs with the detailed consideration of experimental parameter variations for the simulation and prediction of chemical reaction kinetics. The approach is based on truncated Taylor series expansions for the underlying fundamental equations, whereby the external variations can be interpreted as perturbations of the kinetic parameters. Accordingly, our method allows for an efficient consideration of experimental parameter settings and their influence on the concentration profiles and reaction kinetics. A particular advantage of our approach, in addition to the consideration of univariate and multivariate parameter variations, is the robust model-based exploration of the parameter space to determine optimal reaction conditions in combination with advanced reaction insights. The benefits of this concept are demonstrated for higher-order chemical reactions including catalytic and oscillatory systems in combination with small amounts of training data. All predicted values show a high level of accuracy, demonstrating the broad applicability and flexibility of our approach.

Monolithic Catalysts Supported by Emulsion-Templated Porous Polydivinylbenzene for Continuous Reduction of 4-Nitrophenol

A monolithic catalyst was fabricated through an emulsion-templating method, postpolymerization modification, and in situ loading of active constituents. To achieve a high specific surface area, divinylbenzene (DVB) was solely employed as the monomer, while the porous structure was adjusted with the porogen content and the types of initiators. Then, anchor points were introduced on the pore wall through nitration and amination of the polymeric scaffold. Using a controlled "silver mirror reaction", monolithic catalysts were obtained after loading of silver nanoparticles (Ag NPs), which was verified from morphological and crystallinity characteristics. The catalytic performance of the resultant monolithic catalyst was determined with the model reduction of 4-nitrophenol (4-NP). In static catalysis, the monolithic catalyst was proved to have a reactively high apparent rate constant and a good reusability. Furthermore, a flow reactor was fabricated with the monolithic catalyst, showing a high efficiency and long-term durability for the continuous reduction of 4-NP. This work broadened the adjustment of porous structures and the subsequent application for emulsion-templated monoliths.

Scale-up of Sodium Persulfate Mediated, Nitroxide Catalyzed Oxidative Functionalization Reactions

BACKGROUND

Oxidation is a valuable tool in preparative organic chemistry. Oxoammonium salts and nitroxides have proven valuable as reagents and catalysts in this endeavor.

OBJECTIVE

The objective of this study is to scale up the oxidative amidation, ester formation, and nitrile formation using nitroxide as an organocatalyst.

METHODS

Oxidative functionalization reactions were scaled from the 1 mmol to the 1 mole level. Sodium persulfate was used as the primary oxidant, and a nitroxide was employed as a catalyst. The products of the reactions were isolated in analytically pure form by extraction with no need for column chromatography.

RESULTS

The oxidative amidation and esterification of aldehydes can be scaled up from 1 mmol to 1 mole effectively, with comparable product yields being obtained at each increment. This work shows that conditions developed on a small scale can be transferred to a larger scale without reoptimization. The oxidative functionalization of aldehydes to prepare nitriles is not amenable to direct scale-up due to the concomitant formation of significant quantities of the corresponding carboxylic acid, thereby compromising the product yield.

CONCLUSION

Two of the three oxidative transformations studied here can be scaled up successfully from the 1 mmol to the 1 mole level.

Preparative Scale Applications of C-H Activation in Medicinal Chemistry

C-H activation is an attractive methodology to increase molecular complexity without requiring substrate prefunctionalization. In contrast to well-established cross-coupling methods, C-H activation is less explored on large scales and its use in the production of pharmaceuticals faces substantial hurdles. However, the inherent advantages, such as shorter synthetic routes and simpler starting materials, motivate medicinal chemists and process chemists to overcome these challenges, and exploit C-H activation steps for the synthesis of pharmaceutically relevant compounds. In this review, we will cover examples of drugs/drug candidates where C-H activation has been implemented on a preparative synthetic scale (range between 355 mg and 130 kg). The optimization processes will be described, and each example will be examined in terms of its advantages and disadvantages, providing the reader with an in-depth understanding of the challenges and potential of C-H activation methodologies in the production of pharmaceuticals.

A field guide to flow chemistry for synthetic organic chemists

Flow chemistry has unlocked a world of possibilities for the synthetic community, but the idea that it is a mysterious "black box" needs to go. In this review, we show that several of the benefits of microreactor technology can be exploited to push the boundaries in organic synthesis and to unleash unique reactivity and selectivity. By "lifting the veil" on some of the governing principles behind the observed trends, we hope that this review will serve as a useful field guide for those interested in diving into flow chemistry.

A Brief Introduction to Chemical Reaction Optimization

From the start of a synthetic chemist's training, experiments are conducted based on recipes from textbooks and manuscripts that achieve clean reaction outcomes, allowing the scientist to develop practical skills and some chemical intuition. This procedure is often kept long into a researcher's career, as new recipes are developed based on similar reaction protocols, and intuition-guided deviations are conducted through learning from failed experiments. However, when attempting to understand chemical systems of interest, it has been shown that model-based, algorithm-based, and miniaturized high-throughput techniques outperform human chemical intuition and achieve reaction optimization in a much more time- and material-efficient manner; this is covered in detail in this paper. As many synthetic chemists are not exposed to these techniques in undergraduate teaching, this leads to a disproportionate number of scientists that wish to optimize their reactions but are unable to use these methodologies or are simply unaware of their existence. This review highlights the basics, and the cutting-edge, of modern chemical reaction optimization as well as its relation to process scale-up and can thereby serve as a reference for inspired scientists for each of these techniques, detailing several of their respective applications.

Poly(heptazine imide) ligand exchange enables remarkable low catalyst loadings in heterogeneous metallaphotocatalysis

The development of heterogeneous metallaphotocatalysis is of great interest for sustainable organic synthesis. The rational design and controllable preparation of well-defined (site-isolated) metal/photo bifunctional solid catalysts to meet such goal remains a critical challenge. Herein, we demonstrate the incorporation of privileged homogeneous bipyridyl-based Ni-catalysts into highly ordered and crystalline potassium poly(heptazine imide) (K-PHI). A variety of PHI-supported cationic bipyridyl-based Ni-catalysts (L_nNi-PHI) have been prepared and fully characterized by various techniques including NMR, ICP-OES, XPS, HAADF-STEM and XAS. The L_nNi-PHI catalysts exhibit exceptional chemical stability and recyclability in diverse C-P, C-S, C-O and C-N cross-coupling reactions. The proximity and cooperativity effects in L_nNi-PHI significantly enhances the photo/Ni dual catalytic activity, thus resulting in low catalyst loadings and high turnover numbers.

Will the next generation of chemical plants be in miniaturized flow reactors?

For decades, a production paradigm based on centralized, stepwise, large scale processes has dominated the chemical industry horizon. While effective to meet an ever increasing demand for high value-added chemicals, the so-called macroscopic batch reactors are also associated with inherent weaknesses and threats; some of the most obvious ones were tragically illustrated over the past decades with major industrial disasters and impactful disruptions of advanced chemical supplies. The COVID pandemic has further emphasized that a change in paradigm was necessary to sustain chemical production with an increased safety, reliable supply chains and adaptable productivities. More than a decade of research and technology development has led to alternative and effective chemical processes relying on miniaturised flow reactors (a.k.a. micro and mesofluidic reactors). Such miniaturised reactors bear the potential to solve safety concerns and to improve the reliability of chemical supply chains. Will they initiate a new paradigm for a more localized, safe and reliable chemical production?

Red light-driven electron sacrificial agents-free photoreduction of inert aryl halides via triplet-triplet annihilation

Selective photoactivation of inert aryl halides is a fundamental challenge in organic synthesis. Specially, the long-wavelength red light is more desirable than the widely-applied blue light as the excitation source for photoredox catalysis, due to its superior penetration depth. However, the long-wavelength red light-driven photoactivation of inert aryl halides remains a challenge, mainly because of the low energy of the single long-wavelength red photon. Herein, we report the photoreduction of aryl bromides/chlorides with 656 nm LED via triplet-triplet annihilation (TTA) strategy. This method is based on our discovery that the commonly used chromophore of perylene can serve as an efficient and metal-free photocatalyst to enable the photoreduction of inert aryl halides without the conventional need for electronic sacrificial agents. By introducing a red light-absorbing photosensitizer to this perylene system, we accomplish the long-wavelength red light-driven photoreduction of aryl halides via sensitized TTA mechanism. Moreover, the performance of such a TTA-mediated photoreduction can be significantly enhanced when restricting the rotation freedom of phenyl moiety for perylene derivatives to suppress their triplet nonradiative transition, in both small and large-scale reaction settings.

Diphenyl Ditelluride: An Unconventional Reducing Agent in the Sulfonylative Cascade of Alkynyl Cyclohexadienones

Herein, we report a reductive hydrazo-sulfonylative difunctionalization cascade of alkynyl cyclohexadienones employing PhTeTePh as an uncommon reducing agent. Diphenyl ditelluride is a commercially available solid with a good solubility profile in most organic solvents, and this is the first report illustrating it as a reducing agent. The protocol afforded a variety of difunctionalized dihydrochromenones and dihydrobenzofuranones in good yields under relatively mild conditions. The reactions were scalable, and mechanistic studies were conducted to probe the reaction mechanism. Additionally, photophysical studies of the products were carried out, which revealed that they had significant absorption (400-450 nm) and emission (520-570 nm) in the visible region.

Unravelling the Effect of Water Addition in Consecutive Photocatalysis with Naphthalene Diimide

We describe the effect of water addition in the catalyst performance for a C-H functionalization of benzene. Improved yields and selectivity were achieved in a consecutive photoredox catalysis in contrast to the reaction without water. Mechanistic analysis demonstrated this is due to a better catalyst stability and faster kinetic rather than a change in the different steps of the mechanism. The addition of water constitutes a convenient approach to improve catalyst performance, and it was also observed with other catalysts.

Zinc-Free, Scalable Reductive Cross-Electrophile Coupling Driven by Electrochemistry in an Undivided Cell

Nickel-catalyzed reductive cross-electrophile coupling reactions are becoming increasingly important in organic synthesis, but application at scale is limited by three interconnected challenges: a reliance on amide solvents (complicated workup, regulated), the generation of stoichiometric Zn salts (complicated isolation, waste disposal issue), and mixing/activation challenges of zinc powder. We show here an electrochemical approach that addresses these three issues: the reaction works in acetonitrile with diisopropylethylamine as the terminal reductant in a simple undivided cell (graphite(+)/nickel foam(-)). The reaction utilizes a combination of two ligands, 4,4'-di-tert-butyl-2,2'-bipyridine and 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine. Studies show that, alone, the bipyridine nickel catalyst predominantly forms protodehalogenated aryl and aryl dimer, whereas the terpyridine nickel catalyst predominantly forms bialkyl and product. By combining these two unselective catalysts, a tunable, general system results because excess radical formed by the terpyridine catalyst can be converted to product by the bipyridine catalyst. As the aryl bromide becomes more electron rich, the optimal ratio shifts to have more of the bipyridine nickel catalyst. Lastly, examination of a variety of flow-cell configurations establishes that batch recirculation can achieve higher productivity (mmol product/time/electrode area) than single-pass, that high flow rates are essential to maximizing current, and that two flow cells in parallel can nearly halve the reaction time. The resulting reaction is demonstrated on gram scale and should be scalable to kilogram scale.

High-Productivity Single-Pass Electrochemical Birch Reduction of Naphthalenes in a Continuous Flow Electrochemical Taylor Vortex Reactor

We report the development of a single-pass electrochemical Birch reduction carried out in a small footprint electrochemical Taylor vortex reactor with projected productivities of >80 g day-1 (based on 32.2 mmol h-1), using a modified version of our previously reported reactor [Org. Process Res. Dev. 2021, 25, 7, 1619-1627], consisting of a static outer electrode and a rapidly rotating cylindrical inner electrode. In this study, we used an aluminum tube as the sacrificial outer electrode and stainless steel as the rotating inner electrode. We have established the viability of using a sacrificial aluminum anode for the electrochemical reduction of naphthalene, and by varying the current, we can switch between high selectivity (>90%) for either the single ring reduction or double ring reduction with >80 g day-1 projected productivity for either product. The concentration of LiBr in solution changes the fluid dynamics of the reaction mixture investigated by computational fluid dynamics, and this affects equilibration time, monitored using Fourier transform infrared spectroscopy. We show that the concentrations of electrolyte (LiBr) and proton source (dimethylurea) can be reduced while maintaining high reaction efficiency. We also report the reduction of 1-aminonaphthalene, which has been used as a precursor to the API Ropinirole. We find that our methodology produces the corresponding dihydronaphthalene with excellent selectivity and 88% isolated yield in an uninterrupted run of >8 h with a projected productivity of >100 g day-1.

Homogeneous Organic Electron Donors in Nickel-Catalyzed Reductive Transformations

Many contemporary organic transformations, such as Ni-catalyzed cross-electrophile coupling (XEC), require a reductant. Typically, heterogeneous reductants, such as Zn0 or Mn0, are used as the electron source in these reactions. Although heterogeneous reductants are highly practical for preparative-scale batch reactions, they can lead to complications in performing reactions on process scale and are not easily compatible with modern applications, such as flow chemistry. In principle, homogeneous organic reductants can address some of the challenges associated with heterogeneous reductants and also provide greater control of the reductant strength, which can lead to new reactivity. Nevertheless, homogeneous organic reductants have rarely been used in XEC. In this Perspective, we summarize recent progress in the use of homogeneous organic electron donors in Ni-catalyzed XEC and related reactions, discuss potential synthetic and mechanistic benefits, describe the limitations that inhibit their implementation, and outline challenges that need to be solved in order for homogeneous organic reductants to be widely utilized in synthetic chemistry. Although our focus is on XEC, our discussion of the strengths and weaknesses of different methods for introducing electrons is general to other reductive transformations.

Doubly stereoconvergent crystallization enabled by asymmetric catalysis

Synthetic methods that enable simultaneous control over multiple stereogenic centers are desirable for the efficient preparation of pharmaceutical compounds. Herein, we report the discovery and development of a catalyst-mediated asymmetric Michael addition/crystallization-induced diastereomer transformation of broad scope. The sequence controls three stereogenic centers, two of which are stereochemically labile. The configurational instability of 1,3-dicarbonyls and nitroalkanes, typically considered a liability in stereoselective synthesis, is productively leveraged by merging enantioselective Brønsted base organocatalysis and thermodynamic stereocontrol using a single convergent crystallization. The synthesis of useful γ-nitro β-keto amides containing three contiguous stereogenic centers is thus achieved from Michael acceptors containing two prochiral centers.

Evolving New Chemistry: Biocatalysis for the Synthesis of Amine-Containing Pharmaceuticals

Biocatalysis has become an attractive tool in modern synthetic chemistry both in academic and industrial settings, offering access to enantiopure molecules. In industry, biocatalysis found use in small molecule pharmaceutical development. For several amine-containing drugs, biotransformations were applied in the process routes, improving the original syntheses employing classical chemical methods. This review illustrates how and why biocatalysis has been applied to create safer, more efficient and less costly processes for the manufacture of chiral amine-containing pharmaceuticals and alkaloids. Several enzyme classes have been applied to syntheses of natural products, pharmaceutical products and their intermediates, including transaminases, imine reductases, monoamine oxidases and Pictet-Spenglerases. The routes with and without application of biocatalysis are compared, and the potential of these enzyme classes in redesigned synthetic routes to natural products, alkaloids and high-value chemicals is evaluated, using syntheses of sitagliptin, suvorexant, PF-04449913, MK-7246, vernakalant, GSK-2879552, boceprevir and (−)-strictosidine as examples. Application of biocatalysis in the synthesis of amine-containing pharmaceuticals constitutes a greener alternative to transition metal-catalysed routes, facilitates installation of chiral amine functionalities at a late stage of the synthesis and provides exquisite stereocontrol. Opportunities and challenges of biocatalysis for the synthesis of chiral amines are reviewed with respect to use in drug discovery and development.

Interfacing single-atom catalysis with continuous-flow organic electrosynthesis

The global warming crisis has sparked a series of environmentally cautious trends in chemistry, allowing us to rethink the way we conduct our synthesis, and to incorporate more earth-abundant materials in our catalyst design. "Single-atom catalysis" has recently appeared on the catalytic spectrum, and has truly merged the benefits that homogeneous and heterogeneous analogues have to offer. Further still, the possibility to activate these catalysts by means of a suitable electric potential could pave the way for a true integration of diverse synthetic methodologies and renewable electricity. Despite their esteemed benefits, single-atom electrocatalysts are still limited to the energy sector (hydrogen evolution reaction, oxygen reduction, etc.) and numerous examples in the literature still invoke the use of precious metals (Pd, Pt, Ir, etc.). Additionally, batch electroreactors are employed, which limit the intensification of such processes. It is of paramount importance that the field continues to grow in a more sustainable direction, seeking new ventures into the space of organic electrosynthesis and flow electroreactor technologies. In this piece, we discuss some of the progress being made with earth abundant homogeneous and heterogeneous electrocatalysts and flow electrochemistry, within the context of organic electrosynthesis, and highlight the prospects of alternatively utilizing single-atom catalysts for such applications.

Exploring metallic and plastic 3D printed photochemical reactors for customizing chemical synthesis

Visible light photocatalysis is a rapidly developing branch of chemical synthesis with outstanding sustainable potential and improved reaction design. However, the challenge is that many particular chemical reactions may require dedicated tuned photoreactors to achieve maximal efficiency. This is a critical stumbling block unless the possibility for reactor design becomes available directly in the laboratories. In this work, customized laboratory photoreactors were developed with temperature stabilization and the ability to adapt different LED light sources of various wavelengths. We explore two important concepts for the design of photoreactors: reactors for performing multiple parallel experiments and reactors suitable for scale-up synthesis, allowing a rapid increase in the product amount. Reactors of the first type were efficiently made of metal using metal laser sintering, and reactors of the second type were successfully manufactured from plastic using fused filament fabrication. Practical evaluation has shown good accuracy of the temperature stabilization in the range typically required for organic synthesis for both types of reactors. Synthetic application of 3D printed reactors has shown good utility in test reactions-furan C-H arylation and thiol-yne coupling. The critical effect of temperature stabilization was established for the furan arylation reaction: heating of the reaction mixture may lead to the total vanishing of photochemical effect.

Accelerated and Scalable C(sp3)-H Amination via Decatungstate Photocatalysis Using a Flow Photoreactor Equipped with High-Intensity LEDs

Carbon-nitrogen bonds are ubiquitous in biologically active compounds, prompting synthetic chemists to design various methodologies for their preparation. Arguably, the ideal synthetic approach is to be able to directly convert omnipresent C-H bonds in organic molecules, enabling even late-stage functionalization of complex organic scaffolds. While this approach has been thoroughly investigated for C(sp2)-H bonds, only few examples have been reported for the direct amination of aliphatic C(sp3)-H bonds. Herein, we report the use of a newly developed flow photoreactor equipped with high intensity chip-on-board LED technology (144 W optical power) to trigger the regioselective and scalable C(sp3)-H amination via decatungstate photocatalysis. This high-intensity reactor platform enables simultaneously fast results gathering and scalability in a single device, thus bridging the gap between academic discovery (mmol scale) and industrial production (>2 kg/day productivity). The photocatalytic transformation is amenable to the conversion of both activated and nonactivated hydrocarbons, leading to protected hydrazine products by reaction with azodicarboxylates. We further validated the robustness of our manifold by designing telescoped flow approaches for the synthesis of pyrazoles, phthalazinones and free amines.

The assembly of integrated continuous flow platform for on-demand rosiglitazone and pioglitazone synthesis

Manufacturing thiazolidinediones in a batch process is often carried out at different locations, where each successive batch collects a certain amount of intermediate followed by its transportation to another location.

Enhanced flow synthesis of small molecules by in-line integration of sequential catalysis and benchtop twin-column continuous chromatography

In-line integration of sequential catalysis and continuous multi-column purification. Adapted for small compound amounts (hit-to-lead). Suitable for large-scale purification (process chemistry).

Evolution towards green radical generation in total synthesis

The use of radicals as intermediates in total synthesis has evolved since their initial use in the latter half of the twentieth century. Radical generation from metal hydride methodologies has shifted to "greener" techniques including catalytic metal-mediated systems, electrochemical and photoredox-mediated processes. This review will focus on these classical and contemporary methods for radical generation and their applications in recent total syntheses.