Switching between P-acylation and O-acylation of H-phosphonates with chloroformates by changing acyl pyridinium and acyl ammonium ions in a microflow reactor

We report the first switchable acylation of H-phosphonate with chloroformate. The acylation site (P vs. O) in H-phosphonate was switched by changing the acyl pyridinium/ammonium ions. Unexpected phosphite formation was observed during the O-acylation of H-phosphonate. Twenty-six structurally diverse phosphotriesters and phosphonoformate esters were synthesized in microflow reactors.

Effect of Brønsted Acids on the Activation of Mixed Anhydride/Mixed Carbonic Anhydride and C-Terminal-Free N-Methylated Peptide Synthesis in a Micro-Flow Reactor

Amidations employing mixed (carbonic) anhydrides have long been favoured in peptide synthesis because of their cost-effectiveness and less waste generation. Despite their long history, no study has compared the effects of additives on the activation of mixed anhydrides and carbonic anhydrides. In this study, we investigated the amidation of mixed (carbonic) anhydride in the presence of a base and/or Brønsted acids. The use of NMI⋅HCl significantly improved the conversion of the mixed carbonic anhydride, while expediting nucleophilic attacks on the desired carbonyl group. In contrast, in the case of mixed anhydrides, neither the conversion nor the desired nucleophilic attack improved significantly. We developed a C-terminus-free N-methylated peptide synthesis method using mixed carbonic anhydrides in a micro-flow reactor. Fourteen N-alkylated peptides were synthesized in moderate to high yields (55-99 %) without severe racemization (<1 %). Additionally, a significant enhancement in the amidation between mixed carbonic anhydrides and bis-TMS-protected N-methyl amino acids with the inclusion of NMI⋅HCl was observed for the first time. In addition, we observed unexpected C-terminal epimerization of the C-terminus-free N-methyl peptides.

Convergent approach for direct cross-coupling enabled by flash irreversible generation of cationic and anionic species

In biosynthesis multiple kinds of reactive intermediates are generated, transported, and reacted across different parts of organisms, enabling highly sophisticated synthetic reactions. Herein we report a convergent synthetic approach, which utilizes dual intermediates of cationic and carbanionic species in a single step, hinted at by the ideal reaction conditions. By reactions of unsaturated precursors, such as enamines, with a superacid in a flow microreactor, cationic species, such as iminium ions, are generated rapidly and irreversibly, and before decomposition, they are transported to react with rapidly and independently generated carbanions, enabling direct C-C bond formation. Taking advantage of the reactivity of these double reactive intermediates, the reaction take place within a few seconds, enabling synthetic reactions which are not applicable in conventional reactions.

Rapid in situ generation of 2-(halomethyl)-5-phenylfuran and nucleophilic addition in a microflow reactor

2,5-Disubstituted furans are frequently found in pharmaceuticals and bioactive natural products. Nucleophilic substitution reactions on the carbon atom adjacent to the furan ring are useful for producing various furan derivatives. However, the formation of 5-substituted 2-halomethylfuran and the subsequent nucleophilic substitution reactions are often limited by severe undesired reactions caused by the highly reactive halomethylfurans. This paper reports the successful rapid synthesis of various 2,5-disubstituted furans using microflow technology, which suppresses undesired reactions including dimerization and ring opening of the furans. We observed that Brønsted acids had a significant effect on the nucleophilic substitution reaction and the use of HBr and HI gave the best results. A plausible mechanism of the Brønsted acid-mediated nucleophilic substitutions in the developed approach was proposed.

Microflow, Sequential Coupling and Cyclization Approach for Synthesis of Cyclic Phosphotriesters from PCl3

An approach for the synthesis of cyclic phosphotriesters with various ring sizes (5- to 8-membered rings) from phosphorus trichloride and diols was developed. The major challenge in developing this approach is the suppression of the undesired reactions caused by substrates containing multiple highly reactive sites. These undesired reactions were successfully suppressed by microflow technology, which can precisely control the reaction time and temperature. Two optimal conditions were developed, depending on the speed of cyclization. Fifteen cyclic phosphotriesters and their analogs were synthesized. A plausible mechanism for suppressing undesired reactions is proposed.

One-Flow Synthesis of Substituted Indoles via Sequential 1,2-Addition/Nucleophilic Substitution of Indolyl-3-Carbaldehydes

Substituted indoles are important as drugs. A number of valuable indoles have been synthesized via nucleophilic substitution at the 3'-position of indoles. However, the preparation of an indolylmethyl electrophile containing a tertiary carbon at the 3'-position and its subsequent nucleophilic substitution are challenging owing to the instability of the electrophile. Herein, we demonstrated the rapid one-flow synthesis of indoles via sequential 1,2-addition/nucleophilic substitution of indolyl-3-carbaldehydes. The use of a microflow technology helped in suppressing the undesired reactions caused by the unstable intermediates, resulting in significantly higher yields and reproducibility compared to those under batch conditions. A crown ether was effective when 1-alkylindole-3-carboxaldehyde was used as a substrate. However, the crown ether exerted a detrimental effect when 1H-indole-3-carboxaldehyde was used. A total of 15 structurally diverse indole derivatives were obtained in generally acceptable to good yields.

A micro-flow rapid dual activation approach for urethane-protected α-amino acid N-carboxyanhydride synthesis

This study demonstrated the rapid dual activation (10 s, 20 °C) of a combination of an α-amino acid N-carboxyanhydride and alkyl chloroformate in the synthesis of a urethane-protected α-amino acid N-carboxyanhydride in a micro-flow reactor. The key to success was the combined use of two amines that activated both substrates with proper timing. Three amines, i-Pr₂NEt, Me₂NBn, or N-ethylmorpholine, were used with pyridine in accordance with the steric bulkiness of a side chain in the α-amino acid N-carboxyanhydride. A variety of 16 urethane-protected α-amino acid N-carboxyanhydrides were synthesized in high yields. The role of amines was investigated based on the measurement of the time-dependent (0.5 to 10 s) decrease of α-amino acid N-carboxyanhydrides and alkyl chloroformates in the presence of amines via flash mixing technology using a micro-flow reactor. It was suggested that the in situ generated acylpyridinium cation was highly active and less prone to causing undesired decomposition compared with the acylammonium cation examined in this study. Thus, even at a very low concentration, the acylpyridinium cation facilitated the desired coupling reaction.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

A colorimetric technique to characterize mass transfer during liquid-liquid slug flow in circular capillaries

Continuous slug flow in microreactors are featured by alternative presence of regulate segments of immiscible phases in microchannel or capillaries with lateral dimensions below 1 mm. Due to the high interfacial area and short diffusive distance therein, such microreactors have been widely applied in chemical engineering processes that are sensitive to mass transfer. Therefore, mass transfer rates in microreactors have long been broadly investigated via either typical offline or online methods. Compared to these conventional methods, the colorimetric technique based on the oxidation of resazurin with oxygen enables direct determination of physical mass transfer rates. However, this technique was currently applied only to the gas-liquid system in microreactors, and mostly in rectangular channels due to the simplicity in image processing. Based on this, the current paper showed a demo where the colorimetric technique using resazurin was adapted to a liquid-liquid system for the mass transfer study of flowing droplets within a slug flow capillary. Experimental tips and tricks were summarized, and a sliced color-concentration calibration strategy was proposed to balance analyzing efficiency and accuracy.

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

Due to their intrinsic physical properties, which includes being able to perform as volatile liquids at room and biological temperatures, fragrance ingredients/intermediates make ideal candidates for continuous-flow manufacturing. This review highlights the potential crossover between a multibillion dollar industry and the flourishing sub-field of flow chemistry evolving within the discipline of organic synthesis. This is illustrated through selected examples of industrially important transformations specific to the fragrances and flavours industry and by highlighting the advantages of conducting these transformations by using a flow approach. This review is designed to be a compendium of techniques and apparatus already published in the chemical and engineering literature which would constitute a known solution or inspiration for commonly encountered procedures in the manufacture of fragrance and flavour chemicals.

Rapid and Mild Lactamization Using Highly Electrophilic Triphosgene in a Microflow Reactor

Lactams are cyclic amides that are indispensable as drugs and as drug candidates. Conventional lactamization includes acid-mediated and coupling-agent-mediated approaches that suffer from narrow substrate scope, much waste, and/or high cost. Inexpensive, less-wasteful approaches mediated by highly electrophilic reagents are attractive, but there is an imminent risk of side reactions. Herein, a methods using highly electrophilic triphosgene in a microflow reactor that accomplishes rapid (0.5-10 s), mild, inexpensive, and less-wasteful lactamization are described. Methods A and B, which use N-methylmorpholine and N-methylimidazole, respectively, were developed. Various lactams and a cyclic peptide containing acid- and/or heat-labile functional groups were synthesized in good to high yields without the need for tedious purification. Undesired reactions were successfully suppressed, and the risk of handling triphosgene was minimized by the use of microflow technology.

Membrane Microreactors for the On-Demand Generation, Separation, and Reaction of Gases

The use of gases as reagents in organic synthesis can be very challenging, particularly at a laboratory scale. This Concept takes into account recent studies to make the case that gases can indeed be efficiently and safely formed from relatively inexpensive commercially available reagents for use in a wide range of organic transformations. In particular, we argue that the exploitation of continuous flow membrane reactors enables the effective separation of the chemistry necessary for gas formation from the chemistry for gas consumption, with these two stages often containing incompatible chemistry. The approach outlined eliminates the need to store and transport excessive amounts of potentially toxic, reactive or explosive gases. The on‐demand generation, separation and reaction of a number of gases, including carbon monoxide, diazomethane, trifluoromethyl diazomethane, hydrogen cyanide, ammonia and formaldehyde, is discussed.

Microfluidics for Two-Dimensional Nanosheets: A Mini Review

Since the discovery of graphene, there has been increasing interest in two-dimensional (2D) materials. To realize practical applications of 2D materials, it is essential to isolate mono- or few-layered 2D nanosheets from unexfoliated counterparts. Liquid phase exfoliation (LPE) is the most common technique to produce atomically thin-layered 2D nanosheets. However, low production yield and prolonged process time remain key challenges. Recently, novel exfoliation processes based on microfluidics have been developed to achieve rapid and high yield production of few-layer 2D nanosheets. We review the primary types of microfluidic-based exfoliation techniques in terms of the underlying process mechanisms and the applications of the 2D nanosheets thus produced. The key challenges and future directions are discussed in the above context to delineate future research directions in this exciting area of materials processing.

N-Methylated Peptide Synthesis via Generation of an Acyl N-Methylimidazolium Cation Accelerated by a Brønsted Acid

The development of a robust amide-bond formation remains a critical aspect of N-methylated peptide synthesis. In this study, we synthesized a variety of dipeptides in high yields, without severe racemization, from equivalent amounts of amino acids. Highly reactive N-methylimidazolium cation species were generated in situ to accelerate the amidation. The key to success was the addition of a strong Brønsted acid. The developed amidation enabled the synthesis of a bulky peptide with a higher yield in a shorter amount of time compared with the results of conventional amidation. In addition, the amidation can be performed by using either a microflow reactor or a conventional flask. The first total synthesis of naturally occurring bulky N-methylated peptides, pterulamides I-IV, was achieved. Based on experimental results and theoretical calculations, we speculated that a Brønsted acid would accelerate the rate-limiting generation of acyl imidazolium cations from mixed carbonic anhydrides.

Micro-flow synthesis of β-amino acid derivatives via a rapid dual activation approach

Rapid dual activation (≤3.3 s) of both β-amino acid N-carboxy anhydride and alkyl chloroformate for the synthesis of a β-amino acid-derived scaffold was demonstrated. The key to success was the use of rapid mixing enabled by using a micro-flow reactor. The one-flow synthesis of 22 β-amino acid derivatives was achieved.

Compact reaction-module on a pad for scalable flow-production of organophosphates as drug scaffolds

Continuous pharmaceutical manufacturing receives intense attention as an alternative way to meet flexible market needs with the assurance of higher safety and quality control. Here, we report a compact reaction-module on a pad (CRP, 170 × 170 × 1.2 mm) for scale-up production of drug precursors in a continuous-flow. The CRP system was devised by stacking 9 films of the patterned polyimide to integrate micro-flow circuits, combining the functions of the even distribution of feeds, being completely mixed in less than a few milliseconds. A methodology of using a highly reactive species for the single-step synthesis of α-phosphonyloxy ketone, a drug scaffold, required the synthesis time of a few seconds in microfluidics. The fast reaction in the single CRP was capable of producing 19.2 g h-1 drug precursor, which indicates a solid step toward kilogram-scale pharmaceutical manufacturing in small footage.

Single-Step, Rapid, and Mild Synthesis of β-Amino Acid N-Carboxy Anhydrides Using Micro-Flow Technology

β-Amino acid N-carboxy anhydrides (β-NCAs) are rarely used in the synthesis of β-peptides, which is due mainly to the poor availability of these potentially useful substrates. Herein, we describe the heretofore challenging synthesis of β-NCAs via a single-step, rapid, and mild formation using pH flash switching and flash dilution, which are aspects of micro-flow technology. We synthesized 15 β-NCAs in good to excellent yields that included acid-labile β-NCAs that cannot be readily synthesized using the conventional Leuchs approach. Scaled-up synthesis using this process can be readily achieved via continuous operation.

Homogeneous synthesis of hydroxylamine hydrochloride via acid-catalyzed hydrolysis of nitromethane

A continuous homogeneous synthesis of NH₂OH·HCl was achieved and well described with a segmented semi-empirical kinetics model.

Ultrafast synthesis of 2-(benzhydrylthio)benzo[d]oxazole, an antimalarial drug, via an unstable lithium thiolate intermediate in a capillary microreactor

We present an ultrafast approach for the synthesis of 2-(benzhydrylthio)benzo[d]oxazole, an antimalarial drug, in 75% yield from benzo[d]oxazole-2-thiol and benzhydryl bromide via an unstable lithium thiolate intermediate in the presence of n-BuLi.