A Catalyst-Free Amination of Functional Organolithium Reagents by Flow Chemistry

Quantitative reaction monitoring using parahydrogen-enhanced benchtop NMR spectroscopy

The parahydrogen-induced polarisation (PHIP) NMR signal enhancement technique is used to study H2 addition to Vaska's complex (trans-[IrCl(CO)(PPh3)2]) with both standard high-field (9.4 T) NMR and benchtop (1 T) NMR detection. Accurate and repeatable rate constants of (0.84 ± 0.03) dm3 mol-1 s-1 and (0.89 ± 0.03) dm3 mol-1 s-1 were obtained for this model system using standard high-field and benchtop NMR, respectively. The high-field NMR approach is shown to be susceptible to systematic errors associated with interference from non-hyperpolarised signals, which can be overcome through a multiple-quantum filtered acquisition scheme. This challenge is avoided when using benchtop NMR detection because the non-hyperpolarised signals are much weaker due to the lower magnetic field, enabling the use of a simpler and more efficient single RF pulse detection scheme. Method validation against several experimental parameters (NMR relaxation, %pH2 enrichment and temperature) demonstrates the robustness of the benchtop NMR approach but also highlights the need for sample temperature control throughout reaction monitoring. A simple temperature equilibration protocol, coupled with use of an insulated sample holder while manipulating the sample outside the spectrometer, is found to provide sufficient temperature stabilisation to ensure that accurate and repeatable rate constants are obtained. Finally, the benchtop NMR reaction monitoring protocol is applied to the analysis of a complex mixture, where multiple reaction products form simultaneously. H2 addition to a mixture of three Vaska's complex derivatives was monitored, revealing the presence of competitive reaction pathways within the mixture.

Preparation of Aromatic and Heterocyclic Amines by the Electrophilic Amination of Functionalized Diorganozincs with Polyfunctional O-2,6-Dichlorobenzoyl Hydroxylamines

We report a catalyst-free electrophilic amination, which enables the synthesis of aromatic and heterocyclic amines. By subjecting diarylzinc or diheteroarylzinc compounds to readily accessible O-2,6-dichlorobenzoyl hydroxylamines in the presence of MgCl2 in dioxane at a temperature of 60 °C (8-16 h). This new electrophilic amination allowed an expedited synthesis of two pharmaceutically significant compounds: vortioxetine is a key intermediate of delamanid. This approach offers opportunities for the streamlined synthesis of amine-based molecules in the pharmaceutical industry.

In Situ Quench Reactions of Enantioenriched Secondary Alkyllithium Reagents in Batch and Continuous Flow Using an I/Li-Exchange

We report a practical in situ quench (ISQ) procedure involving the generation of chiral secondary alkyllithiums from secondary alkyl iodides (including functionalized iodides bearing an ester or a nitrile) in the presence of various electrophiles such as aldehydes, ketones, Weinreb amides, isocyanates, sulfides, or boronates. This ISQ-reaction allowed the preparation of a broad range of optically enriched ketones, alcohols, amides, sulfides and boronic acid esters in typically 90-98 % ee. Remarkably, these reactions were performed at -78 °C or -40 °C in batch. A continuous flow set-up permitted reaction temperatures between -20 °C and 0 °C and allowed a scale-up up to a 40-fold without further optimization.

Scalable Subsecond Synthesis of Drug Scaffolds via Aryllithium Intermediates by Numbered-up 3D-Printed Metal Microreactors

Continuous-flow microreactors enable ultrafast chemistry; however, their small capacity restricts industrial-level productivity of pharmaceutical compounds. In this work, scale-up subsecond synthesis of drug scaffolds was achieved via a 16 numbered-up printed metal microreactor (16N-PMR) assembly to render high productivity up to 20 g for 10 min operation. Initially, ultrafast synthetic chemistry of unstable lithiated intermediates in the halogen-lithium exchange reactions of three aryl halides and subsequent reactions with diverse electrophiles were carried out using a single microreactor (SMR). Larger production of the ultrafast synthesis was achieved by devising a monolithic module of 4 numbered-up 3D-printed metal microreactor (4N-PMR) that was integrated by laminating four SMRs and four bifurcation flow distributors in a compact manner. Eventually, the 16N-PMR system for the scalable subsecond synthesis of three drug scaffolds was assembled by stacking four monolithic modules of 4N-PMRs.

Flash chemistry enables high productivity metalation-substitution of 5-alkyltetrazoles

Tetrazoles play a prominent role in medicinal chemistry due to their role as carboxylate bioisosteres but have largely been overlooked as C-H functionalisation substrates. We herein report the development of a high-yielding and general procedure for the heterobenzylic C-H functionalisation of 5-alkyltetrazoles in up to 97% yield under batch conditions using a metalation/electrophilic trapping strategy. Through the use of thermal imaging to identify potentially unsafe exotherms, a continuous flow procedure using a flash chemistry strategy has also been developed, allowing products to be accessed in up to 95% yield. This enabled an extremely high productivity rate of 141 g h-1 to be achieved on an entry-level flow system.

Regioselective Synthesis of α-Functional Stilbenes via Precise Control of Rapid cis-trans Isomerization in Flow

The rapid cis-trans isomerization of α-anionic stilbene was regioselectively controlled by using flow microreactors, and its reaction with various electrophiles was conducted. The reaction time was precisely controlled within milliseconds to seconds at -50 °C to selectively give the cis- or trans-isomer in high yields. This synthetic method in flow was well-applied to synthesize precursors of commercial drug compound, (E)- and (Z)-tamoxifen with high regioselectivity and productivity.

Multi-Step Continuous-Flow Organic Synthesis: Opportunities and Challenges

Continuous-flow multi-step synthesis takes the advantages of microchannel flow chemistry and may transform the conventional multi-step organic synthesis by using integrated synthetic systems. To realize the goal, however, innovative chemical methods and techniques are urgently required to meet the significant remaining challenges. In the past few years, by using green reactions, telescoped chemical design, and/or novel in-line separation techniques, major and rapid advancement has been made in this direction. This minireview summarizes the most recent reports (2017-2020) on continuous-flow synthesis of functional molecules. Notably, several complex active pharmaceutical ingredients (APIs) have been prepared by the continuous-flow approach. Key technologies to the successes and remaining challenges are discussed. These results exemplified the feasibility of using modern continuous-flow chemistry for complex synthetic targets, and bode well for the future development of integrated, automated artificial synthetic systems.

Continuous Flow Preparation of (Hetero)benzylic Lithiums via Iodine-Lithium Exchange Reaction under Barbier Conditions

Herein we report the generation of benzylic lithiums via an iodine-lithium exchange reaction on benzylic iodides performed in continuous flow using tBuLi as the exchange reagent. The resulting benzylic lithium species are trapped in situ by carbonyl electrophiles under Barbier conditions, resulting in benzylic secondary and tertiary alcohols. This flow procedure further allows the generation of highly reactive heterobenzylic lithium compounds, which are difficult to generate under batch conditions. A general scale-up was possible without further optimization.

Enhanced Controllability of Fries Rearrangements Using High-Resolution 3D-Printed Metal Microreactor with Circular Channel

High-resolution 3D-printed stainless steel metal microreactors (3D-PMRs) with different cross-sectional geometry are fabricated to control ultrafast intramolecular rearrangement reactions in a comparative manner. The 3D-PMR with circular channel demonstrates the improved controllability in rapid Fries-type rearrangement reactions, because of the superior mixing efficiency to rectangular cross-section channels (250 µm × 125 µm) which is confirmed based on the computational flow dynamics simulation. Even in case of very rapid intramolecular rearrangement of sterically small acetyl group occurring in 333 µs of reaction time, the desired intermolecular reaction can outpace to the undesired intramolecular rearrangement using 3D-PMR to result in high conversion and yield.

Flow Technology for the Genesis and Use of (Highly) Reactive Organometallic Reagents

In the field of organic synthesis, the advent of flow chemistry and flow microreactor technology represented a tremendous novelty in the way of thinking and performing chemical reactions, opening the doors to poorly explored or even impossible transformations using batch methods. In this Concept article, we would like to highlight the impact of flow chemistry for exploiting highly reactive organometallic reagents, and how, alongside the well-known advantages concerning safety, scalability, and productivity, flow chemistry makes possible processes that are impossible to control by using the traditional batch approach.

Progress in low-field benchtop NMR spectroscopy in chemical and biochemical analysis

The employment of spectroscopically-resolved NMR techniques as analytical probes have previously been both prohibitively expensive and logistically challenging in view of the large sizes of high-field facilities. However, with recent advances in the miniaturisation of magnetic resonance technology, low-field, cryogen-free "benchtop" NMR instruments are seeing wider use. Indeed, these miniaturised spectrometers are utilised in areas ranging from food and agricultural analyses, through to human biofluid assays and disease monitoring. Therefore, it is both intrinsically timely and important to highlight current applications of this analytical strategy, and also provide an outlook for the future, where this approach may be applied to a wider range of analytical problems, both qualitatively and quantitatively.

Sodiation of Arenes and Heteroarenes in Continuous Flow

The first sodiations of (hetero)arenes in continuous flow using NaDA (sodium diisopropylamide) in Me₂ EtN are reported. This flow procedure enables sodiation of functionalized arenes and heteroarenes that decompose under batch-sodiation conditions. The resulting sodiated (hetero)arenes react instantly with various electrophiles, such as ketones, aldehydes, isocyanates, alkyl bromides, and disulfides, affording polyfunctionalized (hetero)arenes in high yields. Scale-up is possible without further optimization.

Continuous Visible-Light Photoflow Approach for a Manganese-Catalyzed (Het)Arene C-H Arylation

Manganese photocatalysts enabled versatile room-temperature C-H arylation reactions by means of continuous visible-light photoflow, thus allowing for efficient C-H arylations in 30 minutes with ample scope. The robustness of the manganese-catalyzed photoflow strategy was shown by visible light-induced gram-scale synthesis, clearly outperforming the batch performance.