The Flow’s the Thing…︁Or Is It? Assessing the Merits of Homogeneous Reactions in Flask and Flow

Synthesis of Natural and Unnatural Cyclooligomeric Depsipeptides Enabled by Flow Chemistry

Flow chemistry has been successfully integrated into the synthesis of a series of cyclooligomeric depsipeptides of three different ring sizes including the natural products beauvericin (1 a), bassianolide (2 b) and enniatin C (1 b). A reliable flow chemistry protocol was established for the coupling and macrocyclisation to form challenging N‐methylated amides. This flexible approach has allowed the rapid synthesis of both natural and unnatural depsipeptides in high yields, enabling further exploration of their promising biological activity.

A monolithic 5-(pyrrolidin-2-yl)tetrazole flow microreactor for the asymmetric aldol reaction in water–ethanol solvent

An organocatalytic polystyrene monolithic column produces chiral β-hydroxyketones under benign, continuous-flow conditions with high long-term stability.

Practical aspects of real-time reaction monitoring using multi-nuclear high resolution FlowNMR spectroscopy

FlowNMR spectroscopy is an excellent technique for non-invasive real-time reaction monitoring under relevant conditions that avoids many of the limitations that bedevil other reaction monitoring techniques.

Liquid phase oxidation chemistry in continuous-flow microreactors

This review gives an exhaustive overview of the engineering principles, safety aspects and chemistry associated with liquid phase oxidation in continuous-flow microreactors.

Catalysis by electrons and holes: formal potential scales and preparative organic electrochemistry

The present review surveys current chemical understanding of catalysis by addition and removal of an electron.

Development of a flow method for the hydroboration/oxidation of olefins

A method for the continuous preparation of alcohols by hydroboration/oxidation of olefins using flow techniques is described.

Porous double-layer polymer tubing for the potential use in heterogeneous continuous flow reactions

Functional polymer tubing with an OD of 1/16 or 1/8 in. was fabricated by a simple polymer coextrusion process. The tubing was made of an outer impervious polypropylene layer and an inner layer, consisting of a blend of a functional polymer, polyethylene-co-methacrylic acid, and a sacrificial polymer, polystyrene. After a simple solvent leaching step using common organic solvents, the polystyrene was removed, leaving behind a porous inner layer that contains functional carboxylic acid groups, which could then be used for the immobilization of target molecules. Solution-phase reactions using amines or isocyanates have proven successful for the immobilization of a series of small molecules and polymers. This flexible multilayered functional tubing can be easily cut to the desired length and connected via standard microfluidic fittings.

Self-assembled asymmetric catalyst engaged in a continuous-flow platform: an anti-selective catalytic asymmetric nitroaldol reaction

An anti-selective catalytic asymmetric nitroaldol reaction was manifested in a continuous-flow platform. The requisite Nd/Na heterogeneous catalyst was readily prepared by self-assembly of an amide-based chiral ligand, NdO1/5(O(i)Pr)13/5, NaHMDS, and a multiwalled carbon nanotube without covalent linkage. A stainless-steel column filled with the Nd/Na catalyst was incorporated in a flow system to promote the nitroaldol reaction with high stereoselectivity. The flow system with the heterogeneous catalyst obviated the quenching operation, and the cooling system was minimized.

Micrometer-scale mixing with Pickering emulsions: biphasic reactions without stirring

A general strategy that avoids stirring for organic/aqueous reactions involving solid catalysts is reported. The strategy involves converting a conventional biphasic system into a Pickering emulsion phase with micrometer-scale droplets ensuring good mixing. In test reactions, nitrotoluene reduction and epoxidation of allylic alcohols, the reaction efficiency is comparable to conventional stirrer-driven biphasic catalysis reaction systems. Short diffusion distances, arising from the compartmentalization of densely packed droplets, play an important role in boosting the reaction efficiency.

"Batch" kinetics in flow: online IR analysis and continuous control

Currently, kinetic data is either collected under steady-state conditions in flow or by generating time-series data in batch. Batch experiments are generally considered to be more suitable for the generation of kinetic data because of the ability to collect data from many time points in a single experiment. Now, a method that rapidly generates time-series reaction data from flow reactors by continuously manipulating the flow rate and reaction temperature has been developed. This approach makes use of inline IR analysis and an automated microreactor system, which allowed for rapid and tight control of the operating conditions. The conversion/residence time profiles at several temperatures were used to fit parameters to a kinetic model. This method requires significantly less time and a smaller amount of starting material compared to one-at-a-time flow experiments, and thus allows for the rapid generation of kinetic data.

Applying flow chemistry: methods, materials, and multistep synthesis

The synthesis of complex molecules requires control over both chemical reactivity and reaction conditions. While reactivity drives the majority of chemical discovery, advances in reaction condition control have accelerated method development/discovery. Recent tools include automated synthesizers and flow reactors. In this Synopsis, we describe how flow reactors have enabled chemical advances in our groups in the areas of single-stage reactions, materials synthesis, and multistep reactions. In each section, we detail the lessons learned and propose future directions.

Novel process windows for enabling, accelerating, and uplifting flow chemistry

Novel Process Windows make use of process conditions that are far from conventional practices. This involves the use of high temperatures, high pressures, high concentrations (solvent-free), new chemical transformations, explosive conditions, and process simplification and integration to boost synthetic chemistry on both the laboratory and production scale. Such harsh reaction conditions can be safely reached in microstructured reactors due to their excellent transport intensification properties. This Review discusses the different routes towards Novel Process Windows and provides several examples for each route grouped into different classes of chemical and process-design intensification.

A combined kinetic and thermodynamic approach for the interpretation of continuous-flow heterogeneous catalytic processes

The heterogeneous proline-catalyzed aldol reaction was investigated under continuous-flow conditions by means of a packed-bed microreactor. Reaction-progress kinetic analysis (RPKA) was used in combination with nonlinear chromatography for the interpretation, under synthetically relevant conditions, of important mechanistic aspects of the heterogeneous catalytic process at a molecular level. The information gathered by RPKA and nonlinear chromatography proved to be highly complementary and allowed for the assessment of optimal operating variables. In particular, the determination of the rate-determining step was pivotal for optimizing the feed composition. On the other hand, the competitive product inhibition was responsible for the unexpected decrease in the reaction yield following an apparently obvious variation in the feed composition. The study was facilitated by a suitable 2D instrumental arrangement for simultaneous flow reaction and online flow-injection analysis.

Photocatalytic synthesis of allylic trifluoromethyl substituted styrene derivatives in batch and flow

A cobalt-catalyzed photochemical synthesis of allylic trifluoromethanes from styrene derivatives using 2,2,2-trifluoroethyl iodide is described. The method complements existing approaches, providing an alternative bond construction strategy to access these compounds. The process may be conducted in continuous mode in a novel photochemical flow reactor, resulting in a notable productivity increase.