Victor-Emmanuel Kassin

Diana V Silva Brenes

Thomas Bernard

Julien Legros

Jean-Christophe M Monbaliu
email: jc.monbaliu@uliege.be|www.citos.uliege

A Continuous Flow Generator of Organic Hypochlorites for the Neutralization of Chemical Warfare Agent Simulants with Frugal Resources

Herein is reported the development of a continuous flow generator that produces highly reactive organic hypochlorites for the chemical neutralization of sulfur-based chemical warfare agent (CWA) simulants. The generator relies on widely available, cheap and common chemicals that can be sourced from household products (such as bleach). This work relates to the development of cost-effective, low footprint and operator-friendly protocols for the oxidative neutralization of sulfurbased CWA "mustard gas" (aka Yperite or HD), a noxious compound belonging to the vesicant class of CWAs. A standard decontamination protocol relies on bleach (aqueous sodium hypochlorite) as a convenient oxidative neutralization measure, though the high lipophilicity of HD causes the formation of micelles and hence superficial and/or unselective detoxification.

This work illustrates how aqueous bleach can be rapidly upgraded to more lipophilic organic hypochlorites (MeOCl, EtOCl, iPrOCl and tBuOCl) with low environmental footprint resources and equipment. The latter are demonstrated to readily oxidize simulants of HD (i.e. 2-chloroethyl ethyl sulfide (CEES) and 2-chloroethyl phenyl sulfide (CEPS)) to the corresponding sulfoxides (CEESO and CEPSO, respectively). With a fine tuning of the reaction conditions, the oxidation is nearly instantaneous and prevents the formation of toxic overoxidized sulfones, though organic hypochlorites also contribute to the innocuous chlorination of the neutralized sulfoxide. The assets of continuous flow technology enable to merge both the upstream hypochlorite generator and the downstream neutralization setup to provide a safe, compact and mobile setup that relies only on stable, widely available chemicals. Some aspects of the reactivity of organic hypochlorites with model sulfur compounds are also discussed by means of computational chemistry (DFT).

Introduction

The wide diversity of chemical processes can be labelled as constructive processes and aim at increasing the molecular diversity and the added value of compounds, starting from widely available and affordable building blocks. For instance, the manufacture of pharmaceutical active ingredients clearly fits in such definition, where complex reaction sequences or costly catalysts and reagents are acceptable, as long as the added value of the final target leaves room for comfortable margins. [REF] Destructive processes are different in essence. Regardless of the end application, the latter aim at the physical or chemical destruction of molecular entities to annihilate safety concerns through a chemical modification. [REF] In the specific context of the chemical destruction of Chemical Warfare Agents (CWAs), added-value (in the sense of their production costs and not for their end application) molecular entities with an acute toxicity are neutralized through chemical modification of their backbone toward lower value, lower toxicity entities that can be next safely handled for disposal.

[REF] It comes without saying that such processes inherently come with a reverse economical scheme. An ideal destructive process would therefore rely not only on affordable reagents and catalysts, but also on robust and straightforward process conditions that are amenable to chemical threats of high societal impact. [REF] The design of affordable and robust conditions is not an easy task. [REF] The affordability of a process comes mostly from the chemicals (ideally off-the-convenience-store) and the operating conditions (ideally room temperature and low pressure), as well as from the process technology that is relied on. The robustness of a process results from the combination of a thorough understanding of the mechanisms at stakes with process technologies that enable steady process conditions. Besides, within the specific context of the neutralization of CWAs, additional important criteria include a potential mobility and high operational safety to make it deployable anywhere with minimal resources. [REF] The development of new neutralization protocols on CWAs simulants (as well as on actual CWAs with military clearance) attracts an increasing attention from the Chemical and Chemical Engineering communities, as witnessed by an increasing number of reviews and original articles over the past 10 years. [REF] This is somehow paradoxical, yet easily justifiable with the current background threat of terror attacks against civilians in the Middle East as well as in the Western countries. Indeed, the inception of the Chemical Weapons Convention (CWC) bans since 1993 the possession, manufacture and use of CWAs and all of their precursors. A CWA is defined as "a chemical (gas, liquid or solid) which through its chemical action on life processes can cause death, temporary incapacitation or permanent harm to humans or animals". [REF] CWAs are considered as Weapons of Mass Destruction (WMDs) along with biological agents and nuclear weapons (Figure 1). [REF] The CWC was ratified in 1997 by over 193 countries, although large inventories of CWAs are still stockpiled across the world. For instance, the USA aimed at the complete destruction of its reminiscent inventory by 2023. Besides the CWC, the European Union (EU) adopted coercive measures in 2018 against the use of chemical weapons in a strong response to the use of CWAs against civilians in the Middle-East. Just in Syria, there have been no less than 336 reported incidents involving CWAs at least credibly substantiated between 2012-2018 according to the Global Policy Public Institute (GPPI). [REF] Besides, the emergence of a smaller scale, targeted use of CWAs such as in Salisbury (UK, 2018) or more recently with the poisoning of Aleksej Navalnyj in 2021, justifies the investment of resources and the development of research programs aiming at the detection and/or the chemical neutralization of CWAs [REF]. Mustard gas (HD, 1-chloro-2-[(2-chloroethyl)sulfanyl] ethane, CAS 505-60-2) is a viscous liquid with vesicant (blistering) properties that causes severe, yet delayed, damages to the eyes, skin, and respiratory tract. Besides its vesicant properties, HD also damages cells as a strong alkylating agent and can also affect the nervous system upon acute exposure. While the most common destruction of HD is through incineration, 3 main chemical neutralization protocols are reported in the literature (oxidation, hydrolysis and dehydrohalogenation). Most labs involved in research programs aiming at neutralizing HD rely on simulants to mitigate the risk of exposure. Simulants share with the parent HD specific inherent features, i.e. the ability to form a strongly alkylating episulfonium species through the neighboring interaction of a donor nS lone pair and an acceptor *C-Cl (Figure 2). The closest simulant of HD is hemi mustard CEES (2-chloroethyl ethyl sulfide, CAS 693-07-2), which is commercially available. CEPS (2chloroethyl phenyl sulfide, CAS 5535-49-9) is also often relied on, despite not being strictly speaking a simulant of HD, yet it shares a common 2-chloroethyl limb and has the advantage of being UV active for LC monitoring. CEES and CEPS are not considered as CWAs, yet both are highly toxic and severe vesicants. Other molecular structures can be used as less noxious simulants, as we reported in a recent article. [REF] Among the neutralization methods, the oxidative neutralization of HD (or its simulants CEES or CEPS) toward the corresponding sulfoxide HDO (CEESO or CEPSO) is by far the most represented in the literature. Through a selective oxidation to a sulfoxide, the formation of the episulfonium species is disrupted, hence suppressing most of the toxicity profile of the corresponding structure. It is however critical to ensure a selective oxidation and to avoid an overoxidation to the corresponding sulfones (HDO2, CEESO2 or CEPSO2), the latter having an acute toxicity as precursors of potentially strong Michael acceptors (vinyl sulfone derivatives CEVSO2, EVSO2 or PVSO2) (Figure 2).

Fig. 2. Reactivity of HD and common simulants under oxidative neutralization conditions

Among oxidative neutralization protocols, photocatalytic processes have attracted a lot of attention (Figure 3a). [REF] A large array of homogeneous and heterogeneous photosensitizers was developed, among which Zeolites, MOFs and organic photosensitizers (methylene blue, rose Bengal, porphyrins, riboflavin tetraacetate or BODIPY to name a few), for the generation of reactive oxygen species (ROS, such as singlet oxygen) from oxygen that would selectively oxidize mustard gas and/or its simulants to the corresponding sulfoxides. [REF] The synergistic combination of continuous flow process technology with photocatalysis is now well established, and efficient photocatalytic continuous flow processes for the neutralization of CWAs were therefore reported. [REF] Using our expertise in such processes, our group reported in 2020 a low footprint continuous flow setup that integrates a singlet oxygen photogenerator and exploits its oxidative power to selectively neutralize CEES. [REF] Stoichiometric oxidizers are also reported in the literature (Figure 3a). Unlike ROS-based process, these processes inherently bear the disadvantage of not being catalytic, hence requiring a large supply of oxidizers and generating significant waste (Figure 3b). They, however, do not require the access to specific light sources and the complex setups that come along. The ideal case scenario for such processes relies on affordable, widely available and non-toxic oxidizers with a high atom economy. Among stoichiometric oxidizers, the most common decontamination protocol used by Emergency Responders or Armed Forces relies on aqueous solutions of hypochlorites (e.g. 3-5% commercial bleach as well as chlorinated lime). [REF] A military report from 1969 disclosed that the oxidation of HD with mineral hypochlorites in aqueous solutions leads to the formation of a mixture of the corresponding sulfoxide, sulfone, as well as a diversity of elimination and chlorination products mostly derived from the sulfoxide (HDOCl, CEESOCl or CEPSOCl). [REF] The aggressive nature of bleach solution also raises significant corrosion issues in the decontamination of metal-based hardware. The reaction of HD with bleach is also described as violently exothermic and very sensitive to concentration and pH. [REF] Above all these issues, the high lipophilicity of HD often leads to micelle formation and, hence, superficial oxidative neutralization of HD droplets. [REF] Other stoichiometric chlorine-based oxidants, including N,Ndichlorourethane and N,N-dichlorovaleramide, were reported by Mazumder for the sulfoxidation of HD in batch, yet they suffer from a poor atom-economy. [REF] The authors also reported the extensive formation of 2-chloro-1-2-dichloroethyl sulfoxide (HDOCl) through a competitive chlorination of HDO; HDOCl can therefore be also considered as a neutralized form of HD, as long as it does not lead to the formation of strong Michael acceptors. N,N-dichlorovaleramide was also reported as an effective oxidizer for the oxidative neutralization of VX and related simulants. [REF] In 2018, Smolkin and Colombus reported an electrophilic iodine reagent (N-iodosuccinimide, NIS) as a fast and selective oxidative neutralization cocktail for HD and simulants.

[REF]

N-methyl-1,4diazabicyclo[2.2.2]octane fluoride (Me-DABCOF) was studied by Fridkin and Zafrani for the oxidative neutralization of HD and VX. [REF] Other more typical oxidizers were reported, in combination with innovative process technologies. For instance, Legros et al. reported a series of continuous flow processes using stoichiometric oxidizers under homogeneous or heterogeneous conditions (Figure 3b,c): (a) a first report in 2017 where the authors disclosed a continuous flow setup integrated with in-line low field NMR and relying on the hydrogen peroxide/urea complex (UHP) as oxidizer in methanol in the presence of methanesulfonic acid [REF] and (b) a more recent report in 2021 relying on Oxone (KHSO5) in a packed-bed flow setup. [REF] In both cases, the oxidation of CEES afforded selectively the corresponding sulfoxide CEESO with a complete conversion in less than 5 min. Despite its effectiveness, their procedure imposes the availability and transportation of explosive, corrosive or at least specialty reagents that can be difficult to source for rapid and large-scale neutralization. With the increase of restrictive safety and environmental guidelines, the use of unstable, toxic and explosive chemicals is now barely an option in either academic or industrial settings, at least using conventional batch processing. The latter indeed precludes large scale operations with such chemicals and is restricted to discrete amounts under strict safety protocols. Consequently, a range of highly reactive intermediates, yet very useful according to their reactivity profiles, becomes inaccessible to the Chemist. [REF] The emergence of continuous flow strategies in this context, and in particular the development of continuous flow Chemical Generators, has emerged over the last decade as a game changer when unstable chemicals are at stakes. The concept of Chemical Generator, which was extensively documented by Kappe and coworkers, [REF] is summarized in Figure 4a. A suitable chemical generator for the preparation of lower alkyl hypochlorous esters relies on stable, low toxicity and widely available chemicals that are reacted under continuous flow conditions to access dangerous chemicals of limited stability. As is, this generator does not mitigate the risk associated with the downstream handling of dangerous chemicals (here lower alkyl hypochlorous esters), since downstream collection would require batch processing, hence exposing the operator to high chemical hazards. Connecting the upstream generator to a downstream unit that immediately consumes organic hypochlorites (aka reaction concatenation/telescoping) contributes significantly to reducing operation and exposure hazards. Based on our recently published work on the electrophilic amination with chloro nitrosocycloalkyl derivatives, the preparation and use of tBuOCl and other organic hypochlorites was investigated. A solution was sought for addressing the oxidative neutralization of HD simulants, including CEES and CEPS, as well as other relevant thioethers, using frugal and widely available resources with a compact and safe setup. The high lipophilicity of CEES and CEPS was circumvented through the use of lipophilic alkyl hypochlorites (MeOCl, EtOCl, iPrOCl and tBuOCl). Given the instability of the latter, the opportunity to exploit the assets of continuous flow process technology was investigated for the design of an upstream hypochlorite generator. The hypochlorite generator is fed with widely available, stable and low toxicity chemicals, including common lower alcohols (MeOH, EtOH, iPrOH, tBuOH), acetic acid and an aqueous solution of sodium hypochlorite. The corresponding lipophilic alkyl hypochlorites are then subjected to in-line extraction and liquid-liquid membrane separation, providing a neat organic stream (MTBE) of hypochlorites directly usable for the sulfoxidation of HD simulants and other thioethers. The upstream generator is directly concatenated with the downstream oxidation, also featuring in-line NMR analysis. Besides, this work documents a thorough mechanistic investigation, combining experiments and computational chemistry, as well as an advanced optimization of the process conditions. Once validated at the microfluidic scale, the process is transposed to a mesofluidic commercial setup to validate its utilization at larger scales. This work discloses a simple, yet elegant and robust protocol that provides consistent neutralization with high selectivity yet relying on simple, nontoxic, cheap and widely available chemicals. The setup is engineered to be functional with frugal technology requirements and has a low footprint, which makes it an ideal solution for rapid and efficient deployment.

Experimental section

General information

Conversion, selectivity and yield were determined by Gas Chromatography coupled to Flame Ionization Detection (GC-FID) or Mass Spectrometry (GC-MS) or by High Performance Liquid Chromatography coupled to Diode-Array Detection (HPLC-DAD) or coupled with Mass Spectrometry (LC-MS). GC and HPLC conversions and yields were determined using external calibration curves established with commercial standards (2-chloroethylethyl sulfide, 2-chloroethylethyl sulfone, ethyl vinyl sulfide, ethyl vinyl sulfone, methyl vinyl sulfone, methyl phenyl sulfide, xxxxxxxxx) or with synthesized reference samples following reported procedures (see Supporting Information, Section XXXX for experimental procedures). The selectivity toward the sulfoxide is defined as the ratio between the area% of the sulfoxide and the sum of the area% of all oxidation products. The neutralization selectivity is defined as the ratio between the sum of the area% of all sulfoxide derivatives (including the chlorinated sulfoxides) and sum of the area% of all oxidation products. The neutralization selectivity is defined only for the most favorable cases. Structural identity was confirmed by 1 H and 13 C NMR spectroscopy (400 MHz Bruker Avance spectrometer), by LC-MS or GC-MS (Supporting Information, Section XXXX). Thioanisole (1a), methyl phenyl sulfoxide (2a), chloromethyl phenyl sulfoxide (3a), methyl phenyl sulfone (5a), chloromethyl phenyl sulfone (6a), diphenyl sulfide (1b [b,d]thiophene 5,5-dioxide (5c), dipropyl sulfide (1d), dipropyl sulfoxide (2d), dipropyl sulfone (5d), methanol, ethanol, 2-propanol, tertbutanol, MTBE, acetic acid, sodium hypochlorite pentahydrate were purchased from commercial sources and used without additional purification. CEESO, CEPSO and CEPSOCl were prepared according by adapting protocols from the literature. CAUTION: 1-chloro-2-(ethylsulfinyl)ethane (CEES) and 2chloroethyl phenyl sulfide (CEPS) are highly toxic and severe vesicants. Organic hypochlorites are heat, light and shock sensitive materials. Methyl hypochlorite spontaneously and vigorously decomposes at room temperature. The isolation of MeOCl, EtOCl and iPrOCl should not be attempted. tBuOCl can be stored over CaCl2 at 4 °C in a brown glass container for several days. Reactor setups involving organic hypochlorites were covered in aluminum foil to prevent light exposure.

), diphenyl sulfoxide (2b), diphenyl sulfone (5b), dibenzo[b,d]thiophene (1c), dibenzo[b,d]thiophene 5-oxide (2c), dibenzo

Computations

Computations were performed at the B3LYP/6-311+G** level of theory with empirical dispersion (gd3bj) using the Gaussian 09 package of programs (Revision D.01) with implicit solvation (SMD, methanol). Stationary points were optimized with gradient techniques (tight optimization convergence). Transition states were localized using the Newton-Raphson algorithm, and the nature of the stationary points was determined by analysis of the Hessian matrix. Intrinsic reaction coordinate (IRC) calculations were performed on representative transition states. Cartesian coordinates for representative stationary points are available in the Supporting Information (section XXXX).

Experimental setup

Typical run Microfluidic setups. Microfluidic setups were constructed from PFA tubing (1.58 mm outer diameter, 750 µm internal diameter) equipped with PEEK/ETFE connectors and ferrules (IDEX/Upchurch Scientific). Feed and collection lines consisted of PFA (1.58 mm outer diameter, 750 μm internal diameter) equipped with PEEK/ETFE connectors and ferrules (IDEX/Upchurch Scientific). Liquid feeds were handled with Chemyx Fusion 6000 syringe pumps (SS syringes equipped with Dupont Kalrez O-rings) or with HPLC pumps (Knauer Asurea equipped with a ceramic head). The temperature was regulated with a Heidolph MR Hei-Tec equipped with a Pt-1000 temperature sensor. Downstream pressure was regulated with back pressure regulators from Zaiput Flow Technologies (BPR-10) or from IDEX/Upchurch Scientific (BPR 75 psi). Liquid-liquid extraction was carried out with a Zaiput Flow Technologies membrane separator (SEP-10, equipped with a 1 µm pore hydrophobic membrane). See Supporting Information, section XXXX for details of the microfluidic setups.

Mesofluidic setup -lab scale. Lab scale mesofluidic experiments were carried out in a Corning® Advanced-Flow™ Lab Reactor (2.5 mL internal volume glass fluidic modules). Feed and collection lines consisted of PFA tubing (1/8" o.d.) with PFA or SS Swagelok connectors and ferrules. Liquid feeds were handled with Syrris Asia pumps. The process temperature was regulated with a LAUDA Integral XT 280 thermostat. A Zaiput Flow Technologies membrane separator (SEP-10, equipped with a 1 µm pore hydrophobic membrane) was utilized for downstream extraction and liquid-liquid separation. Downstream pressure was regulated with a backpressure regulator from Zaiput Flow Technologies (BPR-10). In-line analysis was performed on an inline benchtop NMR (43 MHz Spinsolve™ 1H NMR spectrometer from Magritek® equipped with a flow-through cell). See Supporting Information, section XXXX for details of the mesofluidic setup.

Results and Discussion

Various (in)organic hypochlorites were investigated for the oxidation of thioethers [REF] and sodium hypochlorite pentahydrate (NaOCl•5H2O) was recently featured for large scale oxidations on model thioethers and other model sulfurbased compounds under homogeneous conditions (CH3CN/H2O) or as a slurry. [REF] Under these conditions, the formation of the sulfone was reported to a certain extent, as well as -chlorinated sulfoxides, the occurrence of which was strongly solvent-, pH-and time-dependent. [REF ] Organic lower alkyl hypochlorous esters, such as tBuOCl were also reported for accessing sulfoxides. The advantages of using such hypochlorous esters include the selective formation of the corresponding sulfoxide with barely trace amounts of the sulfone, their lipophilicity as well as their convenient preparation from simple and common lower alcohols.

MeOCl, EtOCl, iPrOCl and tBuOCl are the smallest organic hypochlorites, the synthesis of which typically starts from the corresponding widely available lower alcohols. The first report by Sandmeyer on MeOCl dates back to 1885. [REF] MeOCl was prepared through the chlorination of a mixture of methanol and sodium hydroxide. A common feature from these lower esters of hypochlorous acid (HClO) is their relative insolubility in water that leads to spontaneous phase separation. Preliminary reports in the literature by Chattarway and Backeberg emphasized that primary and secondary hypochlorites, such as MeOCl, EtOCl, as well as the hypochlorous esters of all isomers of propyl alcohol were much less stable than tertiary hypochlorites such as tBuOCl. [REF] Sandmayer original's protocol involved passing Cl2 gas through a saturated solution of chalk or limestone, and then adding a lower alcohol to the corresponding filtrated solution. Further processing of the hypochlorous ester includes extraction with an organic solvent and sodium carbonate, as well as storage over CaCl2 in the dark at low temperature. [REF] Several reports indicate that spontaneous and vigorous decomposition occurs upon standing at room temperature for MeOCl and EtOCl, yielding the corresponding aldehydes. [REF] Such instability led to several explosions in research laboratories. It is also a significant safety concern at larger scales, where the oxidation of thioether compounds into their corresponding sulfoxides is implemented as a standard protocol for the treatment of waste effluents. An industrial accident that killed three people at the Midland Resource Recovery facility in West Virginia in 2017 was investigated by the US Chemical Safety Board. [REF] The explosion very likely involved the unexpected formation of MeOCl with sodium hypochlorite in conjunction with trace amounts of MeOH in a downstream odor neutralization step. Secondary hypochlorous esters are described as more stable. Tertiary hypochlorous esters such as tBuOCl are the most stable of the series, yet decomposition occurs upon exposure to intense day light or at higher temperature. Purification of tBuOCl can be carried out through careful vacuum distillation, although this is not recommended for safety reasons. Consequently, the relative stability and reactivity profile of tBuOCl has led to a diversity of applications in preparative organic chemistry. [REF] All organic hypochlorites share a pungent smell.

The original protocol from Mintz and Walling was adapted for the preparation of a small library of hypochlorous esters. This process involves the reaction of a lower (MeOH, EtOH, iPrOH and tBuOH) in the presence of sodium hypochlorite and acetic acid in water (Figure 5). [REF] Preliminary trials in batch confirmed the near instantaneous reaction of hypochlorous acid with MeOH, EtOH, iPrOH and tBuOH, yielding a yellowish lipophilic phase of lower density than the aqueous reaction mixture. The extraction efficiency can be improved with an organic solvent (typically MTBE). The reaction was carried out at 0 °C for 30 minutes in the dark to avoid degradation of the corresponding hypochlorites (Supporting Information, section XXXX). A suitable organic solvent was then added; the resulting organic phase was washed with 10% Na2CO3 (1 x) and brine (1 x) and finally dried over CaCl2. The protocol was first optimized under batch conditions, and rapidly transposed to flow conditions after witnessing the spontaneous and vigorous decomposition of MeOCl and EtOCl upon extraction. NMR analysis on the crude hypochlorites revealed that the formation of tBuOCl was very selective, while the formation of MeOCl, EtOCl and iPrOCl came with a variety of side products (Supporting Information, section XXXX).

The absence of solids or issues related to an increased viscosity were also important preliminary observations that foresaw a straightforward implementation under continuous flow conditions. The hypochlorite generator was constructed from PFA coils and HPLC-type connectors (Supporting Information, section XXXX) under strict thermoregulation (Figure 5). In a typical procedure, a feed solution of NaOCl in water (1.5 M) and a feed solution of alcohol (1 M) and AcOH (1.5 M) in water were injected at 0.1 mL min -1 through a static high performer arrowhead micromixer and reacted for 1 min at 0 °C in a PFA coil (I.D. = 750 µm). To avoid the collection of neat hypochlorites, the reactor effluent was connected to an additional T-mixer for a downstream extraction with an organic solvent. The flow rate of the extraction medium was adjusted to reach the desired concentration (typ. 0.5 M). The resulting segmented reaction mixture was next redirected to a membrane separator (hydrophobic PTFE membrane, 1 µm pore size); the organic stream was collected, and the aqueous stream was discarded. Extraction with chloroform enabled off-line 1 H NMR quantification, as well as qualitative in-line NMR monitoring (Supporting Information, section XXXX), yet the most reproducible results were obtained with MTBE and off-line back-titration with potassium iodide. MTBE was selected as a preferential extraction medium for further experiments since it does not react with hypochlorites and, as a common fuel additive, might be considered as a widely accessible chemical. The results of the iodide back titration emphasized the much lower stability of MeOCl compared to the 3 other organic hypochlorites. With theoretical concentrations of 1 M in MTBE, the titrations of MeOCl, EtOCl, iPrOCl and tBuOCl in MTBE gave 0.11, 0.96, 0.86 and 0.98 M, respectively (Supporting Information, section XXXX). With the proof of concept of an upstream generator of organic hypochlorites in hands, the oxidation of model thioether compounds was next investigated. The selection of model thioethers included thioanisole (1a), diphenyl sulfide (1b), dibenzothiophene (1c) and dipropyl sulfide (1d). Compounds 1a-d were used to calibrate the protocol and set boundaries, as well as to identify potential competitive reactions and side products. Compounds 1b,c are known to be quite reluctant to well-described photocatalytic oxidations with singlet oxygen, while compounds 1a and 1d were validated as reliable, low toxicity simulants of HD in a recent publication.

[REF] There is, however, an additional structural feature shared by compound 1a and 1d, which is lacking from 1b,c, namely, a pseudo-enolizable position in  of the corresponding sulfoxides. Such pseudo-enolizable position opens up a very competitive path, that is, an α-electrophilic chlorination of the sulfoxide. With compound 1a, the electrophilic chlorination can potentially yield the corresponding -chlorinated, ,-bischlorinated and ,,-tris-chlorinated sulfoxides 3a, 4a and 5a, respectively (Scheme 1). [REF] Since tBuOCl was the most stable of the series, a small inventory of 10 mL was prepared and stored over CaCl2 in a brown glass bottle for preliminary trials in batch (Supporting Information, section XXXX). A fresh 1 M solution in MTBE was prepared and used to investigate the reaction with model compound 1a (Scheme 1) under various scenarios. For the first set of reactions, the experiments involved reacting a 0.5 M solution of 1a and a 0.5 M solution of tBuOCl both in MeOH (Schema 1). However, the stock solution of tBuOCl in MeOH was not stable overtime. Iodide back titration after 30, 60, 90 and 120 min gave XX, XX, XX and XX M, respectively. An additional NMR study clearly showed the hypochlorite exchange between tBuOCl and MeOH, hence leading to the near-instantaneous of MeOCl that further decomposed (Figure 6) (Supporting Information, section XXXX). Preliminary trials in batch were therefore attempted in MTBE. Compounds 1-3a and 5-7a were purchased from commercial sources either as starting material or as reference, while compound 4a was prepared according to an adapted protocol from the literature (Supporting Information, section XXXX). [REF] A preliminary test reaction involving the batch oxidation of thioanisole (1a, in MTBE) under biphasic conditions with aqueous NaOCl5H2O at 0 °C led to a conversion of 58% and 87% selectivity to sulfoxide 2a along with the formation of significant amount of sulfone 6a (10%) after 1 min of reaction. Next, the oxidation under homogeneous conditions with tBuOCl was investigated (Scheme 1). Adding dropwise neat tBuOCl (over 10 s, 1.1 equiv.) to a 15 mL solution of 1a in MTBE (0.5 M) gave a near instantaneous conversion at 0 °C that plateaued at 69% after 4 min with a selectivity of 69% toward sulfoxide 2a (Scheme 1a). Besides trace amounts of α-monochlorinated sulfoxide 3a (4%), α,α-bis-chlorinated sulfoxide 4a (1%) and α,α,α-tris-chlorinated sulfoxide 5a (2%) and sulfone 6a (1%), a large fraction of unidentified products (14%) was detected as well. The reaction was monitored over additional 40 min: the selectivity significantly decreased to 49%. The decrease in selectivity was mostly associated with a competitive reaction that further consumed 2a (31% after 40 min vs 47% after 4 min) and led to the formation of unidentified decomposition products (up to 25%). The amounts of mono and di-chlorinated sulfoxides 3,4a remained mostly unaffected, while 5a increased up to 3% (Supporting Information, section XXXX). The next set of experiments aimed at identifying the effect of MeOH on the oxidation reaction; a solution of model substrate 1a (0.5 M, 15 mL) was prepared in MTBE with 10% MeOH and treated likewise with neat tBuOCl at 0 °C (Scheme 1b). The reaction was again monitored over 40 min and the results differed quite significantly from the previous batch experiment. After complete addition of tBuOCl, the conversion plateaued at 77% with a selectivity of 88% toward sulfoxide 2a with small amounts of 3a (8%) and traces of 4a (<1%) and of sulfone 6a (<1%); compound 5a was not detected. The reaction was monitored over 40 min and this time both conversion and selectivity remained constant. This preliminary set of observations thus suggested a very quick and selective oxidation of substrate 1a with tBuOCl in the presence of MeOH and, most importantly, the formation of only trace amounts of sulfone 6a. Besides, in conjunction with the rapid hypochlorite exchange reaction observed upon studying a solution of tBuOCl in MeOH (see above), the higher conversion and cleaner reaction profile for the oxidation of 1a in MTBE/MeOH strongly incriminated MeOCl as the terminal oxidizer. These observations were supported with the computed activation barriers (see below). An additional set of experiments was scheduled to gather more information on the mechanism and the species involved in the oxidation of 1a (Supporting Information, section XXXX).

The formation of mono-chlorinated sulfoxides such as 3a under similar conditions was documented in the literature, [REF] yet it involved the presence of a base (Scheme 1c). However, when the reaction was performed in the presence of pyridine (1.1 equiv.) with 1a in MTBE/MeOH (0.5 M, 10% MeOH), the conversion reached 80% with a 51% selectivity towards 2a after 4 min (36% after 40 min) with the significant formation of unidentified products (32% after 3min, 42% after 40 min); surprisingly, the formation of α-chlorinated sulfoxides 3,4a did not increase significantly (4 and <1%, respectively) in the presence of pyridine and remained stable over time; the formation of tris-chlorinated sulfoxide 5a increased slightly over time (up to 4% after 40 min). [REF] The likelihood of a homolytic mechanism [REF] was investigated by performing the oxidation of 1a (0.5 M in MTBE with 10% MeOH) with tBuOCl in the presence of TEMPO, a well-described radical-scavenger (Scheme 1d) (Supporting Information, section XXXX). Under these conditions, the conversion toward 2a plateaued at 80% with a selectivity of which remained steady over 40 min for reaction monitoring. Interestingly, the formation of all chlorinated sulfoxides 3-5a was suppressed, while the overoxidation sulfone 6a became the main side-product (8%).

Neither decomposition products from a potential sulfohaloform [REF] reaction nor from the reaction [REF] were detected in these preliminary experiences. These results suggest that the formation of chlorinated sulfoxides 3-5a most likely form according to a competitive radical-based mechanism. In the presence of TEMPO, the suppression of such competitive path leaves a slight excess of tBuOCl, which triggers the overoxidation of 1a toward 6a to a minor extent. An additional "competition" experiment with the oxidation of an equimolar mixture of 1a, 2a and 6a further confirmed these hypotheses (Supporting Information, section XXXX).

While many aspects of the reaction of tBuOCl and other related hypochlorous esters with organosulfur derivatives lead to conclude they behave as electrophilic Cl + donors and proceed through the formation of an intermediate chlorosulfonium cation, [REF] very much alike N-chlorosuccinimide (NCS) or 1chlorobenzotriazole, [REF] the mechanism actually involves the formation of an oxysulfonium cationic intermediate (Figure 7).

[REF] The mechanism of sulfoxide formation was thoroughly discussed by Ruff in 2011, including the exchange of hypochlorites with water and alcohols. [REF] Computations were performed at the B3LYP/6-31+G** level of theory with the SMD model for the implicit inclusion of solvent for the reaction of 1a with MeOCl, EtOCl, iPrOCl and tBuOCl (Supporting Information, section XXXX). The direct addition of the sulfur atom of 1a onto the supposedly electrophilic Cl atom of the hypochlorites only gave an increasing enthalpy barrier without a transition state, while the addition on the O atom gave a transition state leading to the formation of an oxysulfonium cation intermediate oxy-1a(a-d) (Figure 7). The activation barrier towards oxy-1a increased with the steric hindrance of the starting hypochlorous esters: oxy-1a(a) (R = Me) G  15.1 kcal mol -1 < oxy-1a(b) (R = Et) 16.5 kcal mol -1 < oxy-1a(c) (R = iPr) 20.3 kcal mol -1 < oxy-1a(d) (R = tBu) 23.3 kcal mol -1 . The first step appeared exergonic (-18.4 kcal mol -1 < G° < -21.2 kcal mol - 1 ).

Fig. 7 Computational study for the oxidation and overoxidation of thioether with organic hypochlorites. (a) Computed reaction and activation enthalpies for selected model reactions (B3LYP/6-311+G**). Values are given in kcal mol - 1 and computed in MeOH (n.c. = not computed). The oxidation toward sulfoxide 2a and sulfone 6a proceed through a two-step mechanism: the preliminary formation of oxysulfonium cationic intermediates (oxy-1a,2a) is followed with a SN2-type displacement with a chloride anion. (Supporting Information, section XXXX). (b) Snapshots of the transitions states corresponding to steps 1, 2 (oxidation to sulfoxide 2a) and to steps 1', 2' (overoxidation to sulfone 6a). [REF] The second step involved the nucleophilic displacement of the electrophilic alkyl moiety of oxy-1a(a-d) with the chloride anion that was previously expelled. While a SN2 mechanism is foreseen for oxy-1a(a,b), a competition may occur for oxy-1a(c) and a SN1 process is expected for oxy-1a(d). It goes without saying the corresponding enthalpy barriers are expected to follow the same order, the smallest one being associated with the SN2 reaction on oxy-1a(a) (14.0 kcal mol -1 ), yielding sulfoxide 2a and methyl chloride (Figure 7). Step 2 appeared mostly isoenergetic to step 1 regarding the activation barriers. The overoxidation toward the formation of sulfone 6a is expected to follow the same pattern, with the intermediate formation of an oxysulfonium intermediate oxy-2a, followed with a SN2 displacement. [REF] The overoxidation path was computed likewise for 2a with MeOCl only. The formation of oxy-2a from sulfoxide 2a was associated with an activation barrier of G  = 21.6 kcal mol -1 and a comparable exergonicity as for the formation of oxy-1a (G° = -20.1 kcal mol -1 ). The second step, namely, the SN2 displacement with chloride anion was associated with a much smaller activation barrier (G  = 9.3 kcal mol -1 ) and a strong exergonicity (G° = -22.7 kcal mol -1 ) that both reflect the much higher electrophilicity of oxy-2a vs oxy-1a and its better leaving group ability (Figure 7). These results emphasize that it is unclear whether step 1 or step 2 are rate determining for the formation of the sulfoxide, while for the overoxidation toward 6a, it is clear that step 1' to oxysulfonium intermediate oxy-2a(a) is rate-determining. The computations however emphasized that step 1' leading to the overoxidation intermediate oxy-2a(a) comes with a much higher activation barrier than step 1 leading to oxy-1a(a) (G  = 6.5 kcal mol -1 ) (Figure 7).

Regarding the sulfoxidation step, the much lower activation barrier for the rate-determining step is obtained with MeOCl (15.1 kcal mol -1 ), hence pointing toward its selection of as a potent oxidizer. The observed instability for MeOCl, however, precludes its generation upstream despite its potential as a sulfoxidation reagent. The computed activation barriers agree with the hypothetic formation of much more reactive MeOCl upon mixing a stock solution of 1a in MeOH and of tBuOCl in MTBE (see above). This opportunity was therefore envisioned as a robust and straightforward compromise between the stability combined with a lower sulfoxidation activity for tBuOCl and the instability of MeOCl with its superior ability to produce sulfoxides. The computations, however, clearly indicate the likelihood of overoxidation towards 6a should be minimized under these conditions.

With the combined insights from the preliminary batch trials and the computations, the oxidation of model 1a was next attempted under continuous conditions (Figure 8): (a) a highperformance micromixer was selected to reach rapid homogenization and hence potentially control the formation of competitive radical-based chlorination mechanisms and (b) tBuOCl was selected as a primary hypochlorous ester and was solubilized in MTBE to avoid decomposition in the feed, in conjunction with substrate 1a in MeOH for taking advantage of the in situ formation of more reactive MeOCl. The flow rates were set to have a 1:1.1 1a/tBuOCl ratio. The crude reactor effluents were collected, quenched by dilution in acetonitrile and then analyzed by HPLC (200 nm). The conditions were further optimized to assess the effects of the residence time, the mixing efficiency, the temperature, the excess tBuOCl, the concentration and the nature of the alcohol additive (MeOH, EtOH, iPrOH and tBuOH) (Supporting Information, section XXXX).

The temperature was the first parameter evaluated, starting with standard operating conditions with 60 s residence time and a 1:1.1 1a/tBuOCl. At -78 °C, the conversion dropped significantly to 45% (98% selectivity); rising progressively the temperature to 20 °C increased the conversion to 80% with a selectivity of 98%, hence pointing 0 °C as the best compromise ensuring short residence time with high conversion (93%) and selectivity (99%) toward 2a. The effects of both the excess tBuOCl and the concentration were next addressed. As expected, a substoichiometric amount (0.75 equiv.) of the oxidizer led to a much lower conversion (62%) while maintaining an excellent selectivity (98%); by contrast, a larger excess (2.1 equiv.) of oxidizer gave a quantitative conversion but a very low selectivity (30%) with the emergence of sulfone 6a, the α-chlorinated sulfoxide 3a and α,α-bis-chlorinated sulfoxide 4a in significant amounts (6, 25 and 38%, respectively). Despite a lower selectivity toward 2a, the neutralization selectivity, which includes all sulfoxide products, was still excellent (94%). The concentration had also a significant impact. With 0.5 M for both feeds, a conversion of 99% was reached within 60 s a 0 °C (91% selectivity, 99% neutralization selectivity). Increasing progressively the concentration to 2 M for both feeds led to a decreased conversion (81%) and selectivity (49%) with the formation of significant amounts of sulfone 6a, α-chlorinated and α,αbischlorinated sulfoxides 3,4a in significant amounts (2, 15 and 15%, respectively). Despite a lower output, the concentration was therefore set at 0.5 M for the subsequent optimizations.

The next experiments addressed the effect of the residence and the mixing efficiency. The effect of the residence time was assessed at constant flow rates for both feeds by adjusting the internal volume of the coil reactor, and 3 residence times (10, 30 and 60 s) were considered at 0 °C. The shortest residence time provided a conversion of 93% with a 99% selectivity towards sulfoxide 2a with trace amounts of sulfone 6a (<1%) and of the α-chlorinated sulfoxide 3a (1%) Increasing the residence to 30 s pushed the conversion to >99%, while the selectivity toward 2a slightly decreased to 91%. In the latter, sulfone 6a remained barely detectable (<1%), while the amount of 3a increased significantly (8%). For the longest residence time (60 s), both conversion and selectivity remained unaffected. These preliminary microfluidic results emphasized once more the near-instantaneous and selective oxidation of 1a toward 2a and the control of the residence appeared as an important parameter to minimize the competitive radical chlorination of 2a and the overoxidation to 6a. In the next set of experiments, the impact of the mixing efficiency was evaluated through experiments carried out in the same PFA coil with decreasing flow rates for both feeds with a constant 1:1.1 1a/tBuOCl ratio at 0 °C. Residence times of 10, 30 and 60 s were achieved with decreasing mixing efficiencies, i.e. keeping the same internal reactor volume and decreasing the overall flow rate. With the reference reaction at 10 s of residence time in mind (93% conversion, >99% selectivity), progressively degrading the mixing efficiency had a deleterious impact on both the conversion and the selectivity: 82% conversion and 98% selectivity were achieved within 30 s of residence time, while the experiment at 60 s gave 72% conversion and 84% selectivity (99% neutralization selectivity). In conjunction with the preliminary batch trials, these results emphasize that poor mixing efficiency affects the output of the oxidation reaction; it is expected that longer residence times (not tested) associated with a poor mixing efficiency would lead to increasing impurities associated with a local excess of tBuOCl (presumably chlorinated derivatives sulfoxides 3-5a and sulfone 6a). The last parameter that required some further preliminary trials concerned the selection of the alcohol used as solvent for substrate 1a. Though the main idea was to combined both the stability of tBuOCl with the high reactivity of MeOCl through an in situ generation of the latter from MeOH and the former, 3 additional experiments were designed with incremental lower alcohols as solvents for 1a. Under comparable conditions as for the MeOH/MTBE system (1.1 equiv. tBuOCl, 0 °C, 60 s), a lower conversion of 63% (39% selectivity) was obtained with EtOH as solvent for 1a. With isopropanol, the selectivity went back to the upper 80s (87%) with 74% selectivity (84% neutralization selectivity). Lastly, the least system with tBuOH led to 65% conversion and 62% selectivity (83% neutralization selectivity). In all 3 cases, sulfone 6a was formed in about 5%, 3,4a ranged from 10 to 27% and from 1 to 6%, respectively. Compound 5a was detected in the last experiment with tBuOH (1%). To conclude this section, the best compromise to balance conversion and selectivity, while forming minimal amounts of sulfoxide 6a involves reacting M feed solutions (thioanisole in MeOH and tBuOCl in MTBE), with a 1:1.1 stoichiometric ratio at 0 °C within 60 s of residence time with a high-performance arrow-head micromixer to reach complete conversion and very high selectivity (91%) and excellent neutralization selectivity (99%). Under these conditions, α-chlorinated sulfoxide 3a becomes the major side product (7%) and sulfone 6a remains barely detectable (1%). Several of these experiments were run in triplicate to validate the reproducibility of the protocol (Supporting Information, section XXXX). Extrapolating these data for a potential application to neutralize HD simulants foresees a selective sulfoxidation along with the formation of α-chlorinated sulfoxide, which still is considered as a neutralized form of the parent simulant, as a minor contaminant and only trace amounts of the corresponding sulfone.

The feed of thioether was next changed to diphenylsulfide (1b) and dibenzothiophene (1c), both in solution in MeOH (Figure 8). tBuOCl was generated upstream as a 0.5 M solution in MTBE. The crude reactions were collected and diluted in acetonitrile, and then analyzed by HPLC at a 200 nm. With substrates 1b,c, which lack a pseudo-enolizable position, the formation of chlorinated sulfoxides does not compete anymore with the oxidation or the overoxidation. With 1b, total conversion was obtained at under the same conditions. The corresponding sulfone 6b was barely detected (<1%). With 1b, the concentration had a limited impact on the reaction, since complete conversion was maintained with 1 M concentration for each feed solution (99% selectivity). Decreasing the residence time to 30 s had a minor effect on the conversion (97%), while the selectivity remained unchanged (99%). Dibenzothiophene (1c) was poorly soluble in MeOH, and the corresponding feed solution was prepared in a 50:50 MeOH/CH2Cl2 blend (0.25 M). 1c is notoriously difficult to oxidize; it was therefore not surprising that a larger excess of tBuOCl was required to reach complete conversion. A slight increase of the excess oxidant (1.2 equiv.) gave 99% conversion with a 98% selectivity.

The optimized conditions were next transposed to dipropyl sulfide (1d) (Figures 8 and9). The lack of chromophores on 1d imposed a GC-MS monitoring for the oxidation reaction (Supporting Information, section XXXX). Transposition on 1d gave 98% conversion with 96% selectivity toward 2d (97% neutralization selectivity), with small amount of the corresponding sulfone (6d, 2%), traces of the corresponding αchlorinated sulfone 7d (<1%) and of the α-chlororinated sulfoxide 3d (1%). 1,2-Dipropyldisulfane (8) was also detected in traces (<1%). A tentative mechanism of formation for 8, based on the study of Barnwell et al, [REF] is proposed in Figure 9b. Slightly increasing the excess of tBuOCl to 1.2 equiv. slightly improved the conversion (98%), yet it produced more sulfone 6d (5%) and its chlorinated analog 7d (1%) with an overall neutralization selectivity of 94%. With the data gathered on model substrates and a refined understanding of the mechanism and competitive reactions, the hypochlorite oxidation was attempted on CEES and CEPS (Figure 8). The conditions with 1.1 equiv. tBuOCl with CEES provided excellent conversion of 94% with a selectivity of 95% toward CEESO (neutralization selectivity of 97%) after 60 s of residence at 0 °C. Sulfone CEESO2 and its chlorinated derivatives were barely detected (<1%). Other minor impurities included αchlorinated sulfoxide CEESOCl (4%) and diethylsulfinate 9 (<1%). [REF] A tentative mechanism of formation for 9, based on the study of Barnwell et al, [REF] is proposed in Figure 9b. Increasing the excess of tBuOCl to 1.2 equiv. led to 98% conversion with 92% selectivity toward CEESO (neutralization selectivity of 99%). Despite the slight excess of tBuOCl in the last trial, both CEESO2 and its chlorinated analog CEESO2Cl remained barely detected, while diethylsulfinate 9 slightly increased (1%). With CEPS, 93% conversion was achieved with 90% selectivity towards CEPSO (98% neutralization selectivity). Sulfone CEPSO2 was not detected. Increasing the excess tBuOCl to 2 equiv. pushed the conversion to 98% but the selectivity towards CEPSO dropped to 16%, with major side-products including the mono-and bis-chlorinated sulfoxides CEPSOCl and CEPSOCl2 in 73 and 10%, respectively. Sulfone CEPSO2 remained undetected, despite the larger excess tBuOCl.

The conditions for the neutralization of CEES and CEPS were next transposed to a commercial mesofluidic glass reactor setup (Corning® Advanced-Flow™ LF/G1 skid Reactor equipped with 2 glass fluidic modules connected in series (2.7 mL internal volume each, Figure 10a). For simplifying the setup, the upstream generator was disconnected from the downstream oxidation module. A fresh solution of tBuOCl, the molarity of which was controlled by back-titration, was used for the trials on CEES and CEPS. Full concatenation of a similar process was demonstrated elsewhere. [REF] With a 60 s residence time at 0 °C, the oxidation of CEES reached 99% conversion with a 96% selectivity toward CEESO. The major impurity became monochlorinated sulfoxide CEESOCl, while sulfone CEESO2 remained undetected, hence providing a neutralization selectivity of 99%. With CEPS, the conversion with 1.2 equiv. tBuOCl reached 97% with 97% selectivity % toward CEPSO; increasing the excess tBuOCl to 1.4 equiv. pushed the conversion up to 98% with 79% selectivity toward CEPSO, the major impurity becoming its monochlorinated sulfoxide CEPSOCl (up to 20%), yet the overall neutralization selectivity was excellent (>99%).

Lastly, the oxidation of HD with MeOCl was computed and compared to the computational results associated with CEES to further validate in silico the potential transposition of this oxidative neutralization protocol to an actual CWA (Figure 10b). Both the simulant and the actual HD came up with very close reaction profiles and activation barriers for the various steps leading to the corresponding sulfoxides and sulfones, hence suggesting that the experimental results for the neutralization of CEES are representative of the behavior of HD under similar experimental oxidative conditions (Supporting Information, section XXXX).

Conclusion

This work reports on a scalable and robust continuous flow process for the oxidative neutralization of organosulfur CWA simulants. The process proposes an original solution to both produce and mitigate the high reactivity of lower alkyl hypochlorites from aqueous solutions of sodium hypochlorite. An upstream generator of organic hypochlorites produces a homogenous stream of organic hypochlorites in MTBE that is further used downstream for the selective oxidation of thioethers. Despite the higher computed reactivity of MeOCl for the oxidation of thioethers in the corresponding sulfoxides, its inherent instability precludes its formation in the upstream generator, extraction in MTBE and direct use for oxidative neutralization. Instead, the upstream generation of tBuOCl, which is much more stable than MeOCl, but also much less reactive, is advantageously exploited to feed the downstream oxidation module. The latter is fed with the thioether substrate in MeOH, which provokes a near-instantaneous hypochlorite exchange with MeOH hence producing "on-the-spot" MeOCl. Such strategy exploits the higher reactivity of MeOCl without the safety and reproducibility issues related to its instability. It provides convenient and fast oxidative conditions for the transformation of thioethers into their corresponding sulfoxides. There are, however, several competitive reactions: (a) a radical-based chlorination of the sulfoxide, typically leading to their corresponding α-chlorinated derivatives (sometimes multiple chlorinations are observed); (b) the formation of disulfides or sulfinates through an intermediate sulfenyl chloride intermediates and (c) an to the corresponding sulfone. While competitive paths (a) and (b) are not concerning when it comes to efficiency of the oxidative neutralization (since they may be considered as neutralized species), the emergence of a completive overoxidation is more concerning. Fortunately, in all trials executed in this study, the formation of the overoxidation sulfone is barely detected and is usually <1%. The use of organic hypochlorites alleviates the formation of micelles and superficial chemical neutralization of sulfur mustards with aqueous hypochlorites. The preliminary transformation of aqueous sodium hypochlorite into corresponding lower alkyl hypochlorous esters remains accessible according to a straightforward protocol that accommodates either batch (for the most stable hypochlorites such as tBuOCl) or flow procedures. The conditions were optimized on model thioethers and then adapted for the neutralization of CEES and CEPS, both under microfluidic and mesofluidic conditions, with excellent conversion and neutralization selectivity. Preliminary results indicated that "off-the-convenience-store" chemicals can be used for the preparation of organic hypochlorites and used likewise. The process relies on widely accessible and affordable chemicals and is amenable to larger scales for the chemical neutralization of large inventories of sulfur mustard vesicants to low-toxicity sulfoxides, which can thereafter be safely transported to incineration facilities. Besides the pragmatic aspects of this process, the assets of computational chemistry are further illustrated here as a convenient protocol for the validation of neutralization protocols developed on simulants, hence providing indication of transposability toward actual sulfur CWAs without military clearance and without extremely restrictive safety protocols.

Fig. 1 .

 1 Fig. 1. Representative examples of CWAs. [REF]

Fig. 3 .

 3 Fig. 3. Typical oxidative continuous flow protocols for the chemical neutralization of HD and stimulants. (a) Photocatalytic oxidation with singlet oxygen. (b) Stoichiometric oxidizer under homogeneous conditions. (c) Stoichiometric oxidizer under heterogeneous conditions.

Fig. 4 .

 4 Fig. 4. (a) General concept of chemical generator under continuous flow conditions. (b) This work: development of an upstream chemical generator for the safe preparation of discrete amounts of lower alkyl hypochlorous esters.

Fig. 5

 5 Fig. 5 Continuous flow generator of hypochlorous esters at the microfluidic scale. (Supporting Information, section XXXX). [REF]

Fig. 6

 6 Fig. 6 Exchange of hypochlorites. (a) 1 H NMR monitoring of the hypochlorite exchange between tBuOCl and MeOH (in CDCl3). The first spectrum is recorded 5 min after the addition of tBuOCl and shows the formation of MeOCl and tBuOH; after 10 min, MeOCl has almost completely decomposed and the amount of tBuOCl has decreased significantly; (b) Tentative transition state for the hypochlorite exchange between tBuOCl and MeOH with a small cluster of MeOH (B3LYP/6-311+G**) (Supporting Information, section XXXX).

Scheme 1 .

 1 Scheme 1. Preliminary optimization in batch for the oxidation of thioanisole (1a) as a model compound (Supporting Information, section XXXX) with the distribution of products from the direct oxidation of 1a with tBuOCl in MTBE or MTBE with MeOH (10%). The reaction was monitored by LC analysis at 200 nm; values are expressed in %area for after 4 min of reaction (values in parentheses are for data collected after 40 min of reaction). n.d. = not detected. Conditions: (a) addition of neat tBuOCl (1.1 equiv.) over 10 s to 0.5 M solution of 1a (in MTBE), 0 °C with reaction monitoring over 40 min, after which up to 25% of unidentified products were detected by LC; (b) addition of neat tBuOCl (1.1 equiv.) over 10 s to 0.5 M solution of 1a (in MTBE with 10% MeOH), 0 °C with reaction monitoring over 40 min; (c) addition of neat tBuOCl over 10 s to 0.5 M solution of 1a (in MTBE with 10% MeOH) and pyridine (1.1 equiv.), 0 °C with reaction monitoring over 40 min, after which up to 42% of unidentified products were detected by LC; (d) addition of neat tBuOCl over 10 s to 0.5 M solution of 1a (in MTBE with 10% MeOH) and TEMPO (2 equiv.), 0 °C with reaction monitoring over 40 min, after which up to 16% of unidentified products were detected by LC.

Fig. 8

 8 Fig. 8 Microfluidic setup for the hypochlorite-mediated oxidation of model thioethers 1-4a, CEES and CEPS with the upstream generator of tBuOCl. Data from LC analysis (200 nm, for the oxidation on compounds 1a-c and CEPS) or from GC-MS analysis (oxidation on compounds 1d and CEES) on the crude reactor effluents using optimized conditions: 60 s of residence time, 0 °C with a 1:1.1 1a/tBuOCl unless otherwise stated in the main text. Feed solutions of 1a-c, CEES and CEPS were prepared in MeOH (0.5 M). Feed solution of 1d was prepared in 50:50 MeOH/CH2Cl2 (0.25 M).

Fig. 9

 9 Fig. 9 (a) Typical GC chromatogram for the oxidation of 1d (dipropyl sulfide), featuring the distribution of side-products. The attribution of picks was carried out with the injection of commercial or synthesized reference samples (1d, 2d and 6d) or by GC-MS with NIST identification (3d, 7d and 8). (b) Tentative mechanism for the formation of impurities 8 and 9. [REF]

Fig. 10

 10 Fig. 10 (a) Mesofluidic reactor setup for the scalability trials featuring a Corning® Advanced-Flow™ LF/G1 skid Reactor equipped with 2 glass fluidic modules connected in series. Data from GC-MS (CEES) or LC-DAD/MS (CEPS); (b) Transitions state structures (B3LYP/6-311+G**) and activation energies (in kcal mol -1 ) for the in silico oxidation of HD. Values in parentheses are calculated for CEES (Supporting Information, section XXXX). See also Figure 7 for details on the various steps.

Acknowledgments

This work was supported by the F.R.S.-FNRS. Computational resources were provided by the "Consortium des Équipements de Calcul Intensif" (CÉCI), funded by the "Fonds de la Recherche Scientifique de Belgique" (F.R.S.-FNRS) under Grant No. 2.5020.11. VEK holds a F.R.S.-FNRS FRIA PhD fellowship (grant No. XXXXX). The authors thank Dr. Morgan Hans for the access to the GC-MS facility. Dr. Guillaume Gauron and Alessandra Vizza (Corning SAS) are acknowledged for the loan of the AFR reactors, technical and scientific support for the scalability trials.

Conflicts of interest

There are no conflicts to declare.

Notes and references


Maria Ivanova

Julien Legros
email: julien.legros@univ-rouen.fr

Thomas Poisson
email: thomas.poisson@insa-rouen.fr

Philippe Jubault
email: philippe.jubault@insa-rouen.fr

Continuous Flow Synthesis of Celecoxib

Keywords: organofluorine chemistry, active pharmaceutical ingredients, organolithium chemistry, oxidation, NSAID

published or not. The documents may come

The current situation with the COVID-19 pandemic has revealed the significant limitations of several countries in providing large quantities of active ingredients for the treatment of the pandemic. Indeed, considerable drug shortages were cruelly experienced, jeopardizing the capacity of the public health systems to treat their fellow citizens. Over the last twenty years, the production of intermediates and active pharmaceutical ingredients (API) has been stopped in numerous countries for economic reasons, which is intrinsically linked to the batch method used to produce these active pharmaceutical ingredients. Indeed, the use of batch reactors means that, in order to produce more, their capacity must be increased, leading to high investments. However, over the last several decades new promising technologies have been integrated in organic synthesis, enabling to address some of the challenges and limitations faced by organic practitioners. Namely, continuous flow synthesis can have a number of advantages, including the ability to handle hazardous compounds safely, enhanced heat and mass transfers, improved selectivity of reactions, higher purity of compounds, as well as an accurate control over the residence time and stoichiometry, and even the performance of « batch-hopeless » reactions. 1

In the field of API, pharmaceutical industry expresses a strong interest in pyrazole scaffold compounds, since they are linchpins for the treatment of cancer, diabetes, central nervous system and metabolic diseases. 2 Among them, Celecoxib (Celebrex™), a NSAID used to treat inflammation and pain in various type of arthritis developed by G. D Searle & company and marketed by Pfizer, is considered as a blockbuster and widely prescribed ($1.129 billion retail sales in 2018). 3 Pyrazoles are usually obtained via the cyclo-condensation of hydrazines with 1,3dicarbonyl compounds. Hence, Penning, Talley and co-workers described the first batch synthesis of Celecoxib along with other 1,5-diarylpyrazoles in 1997. 4 The authors noticed the exclusive formation of 1,5-regioisomers, when carrying out the condensation of the hydrochloride salt of the phenylhydrazine with 1,3-dicarbonyl compounds in either refluxing ethanol or MTBE. Subsequently, the group of Ley reported a continuous flow synthesis of celecoxib in a modest yield (48%) via the vitamin C-assisted reduction of a diazonium salt followed by hydrolysis and cyclo-condensation at 140 °C (Fig. 1). 5 Afterwards, a very few Celecoxib continuous flow syntheses were depicted, namely a process implying one-step system platform with in-line extraction and separation, based on cyclo-condensation of diketone and hydrazine hydrochloride in an i-PrOH/water mixture. 6 In 2020, Sandford described a batch synthesis of the same target through batch deprotonation of 2,3,3,3-tetrafluoropropene. 7 This year, a stepwise flow synthesis of Celecoxib was reported, starting from a Claisen condensation to access 4,4,4-trifluoro-1-(4-methyl-phenyl)-butane-1,3-dione followed by a cyclo-condensation reaction with-sulfamidophenylhydrazine hydrochloride. 8 Along these lines, we now describe a simple, safe and time-saving three steps procedure for the preparation of Celecoxib. Starting from the commercially available 2-bromo-3,3,3trifluoropropene 1, the transient and difficult-to-handle gaseous 3,3,3-trifluoromethylpropyne was formed and converted into the corresponding lithium alkynyl to trap the appropriate aldehyde, furnishing an alcohol, which was successfully oxidized to a corresponding ketone.

The latter was then reacted with 4-sulfamidophenylhydrazine via 1,4-conjugate addition/cyclization reaction with the formation of Celecoxib (Fig. 1). For all steps in flow, we used the commercially available flow synthesis platform from Vapourtec (R2 and R4 combination). This system incorporates a pumping module (2 pumps) and a reagent module (4 reactors). The proposed synthetic route to the trifluoromethyl butynone is shown in Scheme 1. In this first step of the sequence, we generated the volatile Thus, BTP 1 (1 equiv. 0.13 M in THF) and LDA (2 equiv. 0.26 M in THF) were loaded through inlet 1 and 2, respectively, and mixed in a simple T-mixer at -78° C and the combined output was directed into a cooled 1 mL reactor at a total flow rate of 0.8 mL . min -1 (0.4 mL .

min -1 /inlet; PFA tubing with ID = 1.0 mm) to form the lithiated alkyne. The latter was reacted in a second T-mixing piece at -78 °C with the stream of the p-tolualdehyde solution (1 equiv.

0.13 M in THF), which was introduced at the same flow rate of 0.4 mL . min -1 , using a portable SF-10 Vapourtec pump. The resulting solution passed through a cooled 5 mL reactor and the outlet was quenched with a NH 4 Cl aqueous solution into an Erlenmeyer flask. After extraction, alcohol 2 was obtained in good 62% yield and a fairly decent purity to pursue the second step without further purification. Notably, the total residence time of this transformation comes up to only 5 min 25 sec. whereas the completion of a similar transformation in batch takes at least 2 hours. 8 Moreover, flow systems are particularly well suited for such reactions involving significant amounts of pyrophoric lithiated compounds and gaseous intermediates, whereas a batch process would be much more problematic and even dangerous to handle when scaling up. Interestingly, THF could be potentially exchanged for the industrially preferred MTBE, (62 % yield in both cases).

Then, we designed the oxidation reaction of the secondary alcohol 2 into the targeted ketone 3 under continuous flow. Even though such transformation is a well-described textbook transformation, those dealing with fluorinated acetylenic alcohols are still underexplored and appeared tricky. 9 Initially, we tested the most common approaches, namely manganese(IV) oxide, IBX, as well as Oppenauer, Swern, Ley-Griffith oxidations, etc…, which were unfortunately inefficient in our hand. We found that the flow reaction with a solution of Dess-Martin periodinane (DMP) with the outlet connected to a short plug of silica allowed in-line removal of the DMP byproducts and gave a purification-free access to the pure ketone 3 (100% 1 H NMR yield). Nevertheless, the cost and instability of the DMP reagent hamper its use in large scale and process. Therefore, we continued our investigations, looking for a more suitable alternative. Eventually, we found out that passing a solution of alcohol 2 in DCM (0.2 M) through a packed bed reactor containing a mixture of barium permanganate (15 equiv.) and silica (1 g total) constituted an effective, simple and practical set up for the oxidation reaction (Fig. 3). Delightfully, with an optimized heating at 50 °C (and 6 bar BPR) with a flow rate of 0.1 mL . min -1 , a residence time of 10 min gave full conversion of 2 into 3. The collection of the clean product at the reactor outlet was then easily accomplished, followed by a simple evaporation of the solvent (87% yield, no purification).

Alternatively, the reaction could be carried out in acetone to circumvent the use of dichloromethane, without significant loss of the reaction efficiency. The final step was the reaction between the solution of ketone 3 (0.1 M in EtOH) and the 4-sulfamidophenylhydrazine 4 (0.1 M in EtOH/H 2 O, 3 :1); the latter was easily prepared in batch via diazotization of amine, followed by the reduction of the N-N triple bond with tin(II) chloride. 10 The condensation of these two reagents 3 and 4 was performed in a classical flow system equipped with a simple T-mixing piece, a 10 mL tubular reactor (flow rate = 0.1 mL . min -1 per inlet) and 6 bar BPR, to allow the heating of the solvent at 100 °C (Fig. 4). Under these conditions, the desired Celecoxib was obtained. After the evaporation of the solvents, ethanol was added and Celecoxib was fully solubilized, while insoluble byproducts were easily removed by simple filtration. The Celecoxib was then obtained in 90% yield as a pure product without any further purification.

In summary, we have developed the synthesis of the anti-inflammatory COX-2 inhibitor Celecoxib under continuous flow in three steps, starting from 2-bromo-3,3,3trifluoropropene in a fair 50% overall yield. This novel synthetic pathway goes through the easy generation of trifluoropropynyl lithium and its subsequent trapping with an aldehyde in a single flow set-up, followed by an oxidation in a packed bed reactor (set-up 2) and the final condensation between a ketone and a hydrazine (set-up 3). The sum of these three residence times is close to 1 h and no tedious purification was required at any stage, with an appreciable time saving. This process therefore opens avenue toward its implementation on a larger scale and an economically viable production of Celecoxib. Moreover, from a methodological standpoint, the flow chemistry of trifluoropropyne could be further developed and could even offer an access to other CF 3 -containing APIs.

Figure 1 .

 1 Figure 1. Selected synthetic pathways to Celecoxib.

  trifluoromethylpropyne (bp = 48 °C) from the deprotonation of the commercially available 2-bromo 3,3,3-trifluoropropene (BTP, 1) with lithium diisopropylamide (LDA). A second deprotonation occurred to generate the nucleophilic lithiated alkyne that could react with an appropriate electrophile introduced through a third inlet. Unfortunately, the formation of the ketone 3 by trapping an ester, acyl chloride or Weinreb amide was not observed.However, the trifluoromethyl propynyl lithium reacted with p-tolualdehyde to afford the secondary alcohol 2 (Fig2).

Figure 2 .

 2 Figure 2. Flow synthesis of aryl trifluoropropynyl alcohol 2.

Figure 3 .

 3 Figure 3. Oxidation of alcohol 2 into ketone 3 through the BaMnO 4 packed bed reactor.

Figure 4 .

 4 Figure 4. Access to celecoxib by condensation between ketone 3 and hydrazine 4.

Acknowledgements

This work is part of the API program supported by the European Community through the operational program FEDER/FSE 2014-2020, and has been partially supported by University of Rouen Normandy, INSA Rouen Normandy, the Centre National de la Recherche Scientifique (CNRS), European Regional Development Fund (ERDF), Labex SynOrg (ANR-11-LABX-0029), Carnot Institute I2C, the graduate school for research XL-Chem (ANR-18-EURE-0020 XL CHEM), and by Région Normandie. T. P. thanks the Institut Universitaire de France (IUF) for support.

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest


Maria Ivanova

Julien Legros
email: julien.legros@univ-rouen.fr

Thomas Poisson
email: thomas.poisson@insa-rouen.fr

Philippe Jubault
email: philippe.jubault@insa-rouen.fr

A multi-step continuous flow synthesis of pomalidomide

The immunomodulatory drugs (IMiDs) thalidomide, pomalidomide, and lenalidomide are widely used to treat multiple myeloma. The permanent need of IMiDs analogues in clinical screening and practice has created the demand to find a reliable and robust method for their preparation. To reach this goal, the use of flow chemistry is quite appealing, allowing a safe operation, an excellent reproducibility and an efficient process.

Herein, we describe a continuous 3-4 step flow approach for the synthesis of the pomalidomide (38-47% overall yield) and a few analogues.

Keywords: active pharmaceutical ingredients, IMiDs, thalidomide, glutamine, reduction, Thalidomide (2-(2,6-dioxo-3-piperidyl)isoindoline-1,3-dione) was developed by Chemie Grunenthal in 1956 (Figure 1). [START_REF] Lenz | A Short History of Thalidomide Embryopathy[END_REF][START_REF] Eriksson | Clinical Pharmacology of Thalidomide[END_REF] Initially, it was put in application as a sedative and had much softer side effects than barbiturates; it was therefore prescribed to pregnant women for the treatment of morning nausea. A few years later, the raise of the cases of phocomelia and malformations of the newborns were detected, caused by the use of thalidomide during pregnancy. [START_REF] Mcbride | Thalidomide and Congenital Abnormalities[END_REF][START_REF] Speirs | Thalidomide and Congenital Abnormalities[END_REF] As a result, the drug was withdrawn from the marketplace. After further study, it was discovered that thalidomide is efficient against various diseases, namely, a number of cancers, graft-versus-host disease, and a number of skin conditions. For that reason, it was reapproved to be marketed.

The sedative effect is linked to (R)-isomer, whereas the teratogenic effects are associated with the (S)-isomer. At physiological conditions the asymmetric tertiary carbon-center of thalidomide is epimerized, hence the clinical trials of a single isomer are not possible and the drug is marketed as a racemic mixture. [START_REF] Wnendt | Enantioselective Inhibition of TNF-α Release by Thalidomide and Thalidomide-Analogues[END_REF][START_REF] Eriksson | Stereospecific Determination, Chiral Inversion in Vitro and Pharmacokinetics in Humans of the Enantiomers of Thalidomide[END_REF] The immunomodulatory thalidomide analogues (IMiDs) are constantly being searched for and tested, to boost some properties for the treatment of autoimmune and inflammatory diseases and also reduce the harmfulness of side effects. One of the best thalidomide analogues with direct antitumor effect, ability to inhibit angiogenesis, and immunomodulatory role is pomalidomide, the 4-amino substituted analog of thalidomide (Figure 1). It was proved to be a potent inhibitor of tumor necrosis factor (TNF-α) release as assessed in cultured human peripheral blood mononuclear cells challenged to lipopolysaccharide (LPS), a major component of the outer membrane of gramnegative bacteria that is used as a pharmacological tool to induce TNF-α synthesis and release. [START_REF] Muller | Amino-Substituted Thalidomide Analogs: Potent Inhibitors of TNF-α Production[END_REF] Besides, continuous flow chemistry has been gaining interest in recent years because of improvements in efficiency and safety as the scale of production increases. [START_REF] Wiles | Continuous Flow Reactors: A Perspective[END_REF][START_REF] Malet-Sanz | Continuous Flow Synthesis. A Pharma Perspective[END_REF] Therefore, we conjectured that the development of a continuous flow process to access pomalidomide would be beneficial to the subsequent development of an industrially viable flow route.

Moreover, such a process would offer an interesting synthetic manifold to pave the way of the development of libraries in the context of a drug discovery program. [START_REF] De Angelis | Development of a Continuous Flow Synthesis of Propranolol: Tackling a Competitive Side Reaction[END_REF][START_REF] Bloemendal | Continuous One-Flow Multi-Step Synthesis of Active Pharmaceutical Ingredients[END_REF][START_REF] Baumann | A Perspective on Continuous Flow Chemistry in the Pharmaceutical Industry[END_REF][START_REF] Mougeot | Continuous Flow Synthesis of Propofol[END_REF][START_REF] Ivanova | Continuous Flow Synthesis of Celecoxib from 2-Bromo-3,3,3-Trifluoropropene[END_REF] Hence, we report herein a continuous flow synthesis of pomalidomide and analogues, as a proof of concept.

Most of the synthetic pathways to build up pomalidomide are based on the cyclisation of glutamine and the subsequent condensation with phthalic anhydride. Previous reports described the stepwise deprotection and condensation reactions, [START_REF] Heim | De-Novo Design of Cereblon (CRBN) Effectors Guided by Natural Hydrolysis Products of Thalidomide Derivatives[END_REF] as an alternative, the condensation could be followed by cyclisation. [START_REF] Muller | Amino-Substituted Thalidomide Analogs: Potent Inhibitors of TNF-α Production[END_REF] Here, we chose a recent method, which combined the thermal Boc deprotection with the condensation step. [START_REF] Burslem | Efficient Synthesis of Immunomodulatory Drug Analogues Enables Exploration of Structure-Degradation Relationships[END_REF] Our synthesis of pomalidomide 11 begins by the treatment of the commercially available Boc-L-glutamine 1 with N-hydroxysuccinimide 2 and N,N′-Diisopropylcarbodiimide 3 in DMF, [START_REF] Fox | Synthesis, and Preliminary Pharmacological Evaluation of N-Acyl-3-Aminoglutarimides as Broad-Spectrum Chemokine Inhibitors in Vitro and Anti-Inflammatory Agents in Vivo[END_REF] in which racemization of the product occurs. The resultant N-Boc glutarimide ring 4 is either deprotected by under acidic conditions to afford to the glutarimide moiety 5 as hydrochloride salt, or directly involved in the step of the condensation with 4-nitrophthalic anhydride 6, affording the 4-nitro-substituted thalidomide derivative 7. A final reduction of the nitro group proceeded smoothly to yield the 4-amino-substituted analogue 11 (Figure 2). The mixture of Boc-L-Glutamine 1 and N-hydroxysuccinimide 2 was dissolved in DMF (0.25 M), this solution was then combined at 0.1 mL/min flow rate with the solution of DIC 3 in DMF (0.25 M) within a T-mixer at the same flow rate. Then, the blended solution passed through a 22 mL PTFE reactor (1.6 mm ID) at 80 °C placed into a water bath, the residence time amounted to 1 h 50 min, whereas in batch this reaction takes 6 hours. All attempts to downsize the reactor, or an increase the flow rate, or the concentration, led to inferior conversions. We checked as well several carbodiimides and solvents (namely DCC in DCM, DCC in DMF, EDC in DMF, DIC in DMF) and in terms of the best solubility, DIC in DMF was chosen (For more details see SI, Table SI1). The product 4 was then obtained in 71% yield on 1 mmol scale (Figure 3), (productivity: 0.011 kg/h.L).

Then, the deprotection of the amino group on the obtained cyclic amide 4 was achieved. To do so, it should be diluted in dioxane. A 0.2 M solution of 4 was the most convenient to handle, a more concentrated solution tended to precipitate. The solution was then combined with the second one 4 M HCl in dioxane, flowed through a 10 mL tubular reactor at 50°C at 1.0 mL/min total flow rate. The residence time of this transformation was 10 min. After the solvent was evaporated, the pure product 5 was delivered in quantitative yield (productivity: 0.097 kg/h . L) (Figure 4) With regard to the next step, we tested various reaction conditions in order to carry out the condensation step between the commercially available phthalic anhydride 6 and the N-Bocdeprotected cyclic L-Glutamine 5, namely: AcOH/Ac 2 O, MeOH/H 2 O, 1 M NaOH, propionic acid/butyric acid, Et 3 N/ACN, AcOH/H 2 O/dioxane, etc (For more details see SI). We found that the best conditions to perform this reaction in flow made use of the following solvent mixture:

AcOH/H 2 O/ 1,4-dioxane (2 :1 :1), 0.1 M at 130 °C at a flow rate of 0.1 mL/min in a 10 mL PTFE coil reactor. The residence time amounted to 1.66 h. The yield of this transformation was 71% calculated on 1 mmol scale (productivity: 0.013 kg/h . L), (Figure 5).

Nevertheless, the purity of the product was not fully satisfactory and it was decided to move on with the direct use of N-Boc-protected cyclic L-Glutamine, allowing us to skip the deprotection step and to shorten the synthetic route. With this in mind, we evaluated a few conditions and the following turned out to be the best : the commercially available phthalic anhydride 6 and the N-Boc-protected cyclic L-Glutamine 4 were mixed in a trifluoroethanol/ toluene mixture (2:1) at the concentration of 0.1 M and then passed through a 10 mL high temperature tube reactor made of stainless steel, at 150 °C, at 0.1 mL/min. The flow system was equipped with a 250 psi BPR at the outlet of the reactor. Regarding the purity and conversion, these conditions were the most favorable. Indeed, attempts with HFIP/toluene, trifluoroethanol/1,4-dioxane mixtures at various proportions and temperatures were less efficient in terms of yields and purity. By these means, we produced a greyish solid 7 in 68% yield, based on a 0.9 mmol scale (productivity: 124 mg/h). Besides, it was recently reported by Hashimoto and co-workers that thalidomide tetrafluorophthalimides and tetrachlorophthalimides are possible therapeutic agents, which enhance the TNF-R production in PMA-treated HL-60 cells. [START_REF] Fujimoto | Effects of Immunomodulatory Derivatives of Thalidomide (IMiDs) and Their Analogs on Cell-Differentiation, Cyclooxygenase Activity and Angiogenesis[END_REF][START_REF] Niwayama | Potent Inhibition of Tumor Necrosis Factor-α Production by Tetrafluorothalidomide and Tetrafluorophthalimides[END_REF] In view of this, we applied the above-mentioned conditions to the synthesis of these promising bioactive molecules (Figure 6). Using our developed conditions, the chloro, perchloro and perbromo analogues were synthesized in moderate to good yield under the developed standard conditions.

To complete the synthesis of the pomalidomide a final reduction of the nitro function is needed. We used the H-cube TM with 30 mm CatCart® and tested various catalysts: Raney nickel, platinum (IV) oxide, Pd/C (various %). We discovered that a single cycle of 3 min (1 mL/min) with CatCart® 10% Pd/C at 45 °C and 0.01 M EtOH with full mode of hydrogenation was sufficient to reduce the nitro group. We obtained a bright yellow oil in 78% yield, starting from 0.5 mmol of 7 (productivity: 0.043 kg/h . L) (Figure 7).

The reduction could also be carried out inside of a Vapourtec column reactor heated at 80 °C. In order to do so, the Omnifit column should be packed with a mixture of 20 equiv of Fe and silica gel, and the solution of the nitro-compound 7 in AcOH (0.02 M) passed through it at a flow rate of 0.2 mL/min. The reduced product 11 was thus obtained in 95% yield (productivity: 0.062 g/h) (For more details see SI).

In summary, we have demonstrated the 3-/4-steps flow-based synthetic approaches to pomalidomide, which was applied to the synthesis of other immunomodulatory thalidomide analogues (IMiDs). Starting from the commercially available N-Boc-L-Glutamine, after the cyclisation, the N-Boc glutarimide ring is either deprotected or directly involved in the step of the condensation with 4-nitrophthalic anhydride. The final step of the smooth reduction of the nitro group delivered pomalidomide. We believe that this first synthesis in flow of such important scaffolds would pave the way to further development toward the implementation of continuous flow for the synthesis of libraries of bioactive molecules or towards the development of industrial continuous flow processes with higher efficiency. a n t nstit te the ad ate s h esea h he ---

) and y i n andie. T.P. thanks the Institut Universitaire de France (IUF) for support.

  program 2020-2023 ((N)15179). This work has been partially supported by University of Rouen Normandy, INSA Rouen Normandy, the Centre National de la Recher he ienti i

Acknowledgements

This work is part of the API program supported by the European Union through the operational program FEDER/FSE 2014-2020 (no. 20E04976) and FNADT-DRACCARE

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest


Titouan Desrues

Julien Legros

Philippe Jubault

Thomas Poisson

Flow synthesis of an α-amino boronic ester as key precursor of Bortezomib drug

The flow synthesis of the optically active α-amino boronate precursor of the Bortezomib drug is described, including a key a diasteroselective Matteson rearrangement. This reaction sequence provides an efficient, prompt and valuable strategy for the preparation of Bortezomib (<30 min total residence time over 3 steps for the flow synthesis of the protected L-boroleucine).

The demand for flexible and sustainable production of active pharmaceutical ingredients (API), drastically increased since the arise of the COVID-19 pandemic. Hence, the design of straightforward synthetic routes, a longstanding challenge for organic chemists, to provide fast and secured access to key intermediates or APIs became mandatory to secure national production and pharmaceutical independence. In that context, continuous flow synthesis could play a major role to support these goals, 1-3 since it can provide some decisive benefits such as a firm grip on sensitive reactions [4][5][6] and an easier scale-up. 7,8 In this frame, Bortezomib 1 (Velcade TM , Cytomib TM ), is a blockbuster drug (1,39 $ billion in retail sales in 2020) 9 prescribed for the treatment of myeloma and cell lymphoma. 10- 16 The structure of Bortezomib is derived from pyrazinoic acid, L-phenylalanine, and L-boroleucine moieties, which are connected through amide bonds. To the best of our knowledge, six diastereoselective total synthesis have been reported in the literature (all under batch conditions) and four of them relies on the diastereoselective preparation of the key enantiopure αaminoboronic acid 2 (scheme 1). [17][18][19][20][21][22] This fragment plays a major role in the biological activity of Bortezomib. 13 In this context, we now report a synthesis of the key enantiopure synthon 2a under continuous flow conditions (Scheme 1).

Scheme 1. Synthetic path to Bortezomib and key precursors. This flow synthesis was therefore planned in three steps as follow: 1) esterification of i-BuB(OH)2 with (+)-pinanediol, 2) formation of the enantiopure α-chloro-boronic ester 4 from a diastereoselective Matteson rearrangement and 3) the final nucleophilic substitution to install the amino group.

Thus, a solution of isobutyl boronic acid in THF (1.1 equiv.) was easily reacted with a THF solution of (+)-pinanediol (1 equiv.) in a flow reactor at 80 °C to afford the boronic ester 3 within only t R = 2.5 min (88 % yield, Scheme 2a). The next critical step involves the formation and reaction of the highly reactive carbenoid intermediate (LiCHCl2) to add onto the boronate for subsequent Matteson rearrangement. The carbenoid generated by deprotonation of dichloromethane is only stable under cryogenic conditions, which severely hampers the use of this transformation on an industrial scale. 23,24 However, flow technology has been shown to be an unvaluable asset to control such unstable organolithium species in virtue of excellent mixing and fine tuning of the reaction time. 5,[25][26][27] In this line, Sedelmeier reported a breakthrough in halogenomethyl lithium chemistry/Matteson rearrangement by the deprotonation of the inexpensive CH2Cl2, 28 which was then applied by Schuster in the large scale synthesis of a Vaborbactam intermediate. 29 More recently, Kirschning extended the methodology to the metal-halogen exchange of ClCH2Br with a specific static mixer, 30 whereas Luisi described the synthesis of chloroaldehydes from the unstable ClCH(Li)I and ketones. 31 In our case, it was decided to perform the transformation of ibutyl boronate 3 into target compound 4 with common and accessible equipment (simple T-shaped mixer notably) and with widely available CH2Cl2 as reagent for easier reproducibility. Thus, after optimization (see ESI for details), we implemented the formation of the LiCHCl2 and its subsequent trapping with the chiral boronate 3 as follow: a precooled commercial solution of n-BuLi (1.6 equiv, 2.33 M in hexane) was mixed with a precooled diluted solution of CH2Cl2 in THF (2.5 equiv. 0.3 M) to form the CHCl2Li with a flow rate of 20.5 mL/min and a very short residence time (t R = 2.9 s, Scheme 2b). A third flow inlet was then connected to the flow system: a solution of boronate 3 (1 equiv.) with a flow rate of 2.2 mL/min was then injected to perform the Matteson step that occurred in a second reactor (t R = 13 s), the outlet being collected into a flask containing a commercial solution of ZnCl2 (2 equiv., 1 M in Et2O) at -50 °C and let rise to room temperature after collection (scheme 2b). The reaction was monitored by 1 H NMR and we were very pleased to observe a full conversion into the desired diastereomer after 5 min in the flask. This sequence was highly effective and provided the chiral α-chloro-boronic ester 4, with a space-time yield of 7.2 kg/h/L (94% yield, Scheme 2b) and with an excellent purity (> 97%) after a simple aqueous work-up. The target compound 4 was thus directly used for the next step without further tedious purification.

We then focused on the possibility to perform a fully telescoped flow process from isobutyl boronic acid to the stereospecific 1,2-metallate rearrangement through the introduction of the commercially available ZnCl2 solution via an additional inlet. Thus, solutions of (+)-pinanediol (1 equiv.) and isobutyl boronic acid (1 equiv.) were injected through two separate inlets and reacted in a tubing reactor at 35 °C to form 3 (t R = 5 min, Scheme 3a). The next step involving pyrophoric and sensitive organolithium compounds, the water formed during the formation of boronate 3, was removed inline by using a packed-bed reactor filled with 4 Å molecular sieves (t R = 7 min, Scheme 3a). This flow system was then connected to the set-up for the Matteson rearrangement including a supplementary inlet with a solution of ZnCl2. The reaction between LiCHCl2 and 3 occurred was performed at -50 °C (t R = 11 s) before being heated at 35 °C in a further reactor (t R = 33 s) and finally quenched by connecting the outlet to a saturated aqueous solution of NH4Cl (Scheme 3a). This telescoped sequence allows to afford the target 4 in very good yield (85% over two steps). Therefore, this last optimisation led to the implementation of a fully concatenated flow synthesis of a key intermediate α-chloro-boronic ester from available starting chemicals, starting from commercially available materials in a single operation, without isolation of intermediates nor tedious purification, apart from a final aqueous work-up.

Last, we moved to the development of the final step to access the target amino boronate 2a, i.e. the installation of the amino residue with the use of LiHMDS (Scheme 3b). A solution of the chloroboronic ester 4 (1 equiv) was successfully reacted with a solution of LiHMDS (2.8 equiv) in a flow reactor cooled at -50 °C (t R = 2.5 min) before being warmed to 35 °C in a second reactor (t R = 15 min) to ensure full conversion. The reaction was carried out on a 3.69 mmol scale (20.5 min of collection) to afford the key precursor 2a in 75% yield after purification (Scheme 3b). The completion of the synthesis of Bortezomib 1 was finally achieved from this key intermediate under classical batch reaction conditions, as already depicted in the literature (Scheme 3b). 20 In summary, we have achieved the flow synthesis of the strategic α-amino boronic ester 2a, key precursor of the drug Bortezomib 1. This protocol involved telescoped reactions and classical equipment for flow chemistry. The total residence time for the flow steps is around 30 min, which represents a significant productivity gain with regards to the batch operations. Moreover, a single final purification is needed to ensure a high purity level. The protocol delineated within this study offers thus a fast and practical access to this key intermediate, and therefore drastically shortens the access to Bortezomib. Moreover, this protocol would also be further applied to other APIs, containing -aminoboronic acid derivative. [32][33][34] There are no conflicts to declare.

Notes and references

1 R. Gérardy, N. Emmanuel, T. Toupy, V.-E. Kassin, N. N.

Scheme 2 .

 2 Scheme 2. Flow set-ups for (a) the formation of boronate 3 (T-mixer with ID = 1 mm, Reactor tubing with ID = 1 mm, L = 12.7 m); and (b) the Matteson reaction to provide 4 (T-mixer with ID = 1 mm, Reactor tubing with ID = 1.59 mm, L1 = 50 cm, L2 = 250 cm).

Scheme 3 .

 3 Scheme 3. (a) Flow synthesis of α-chloro-boronic ester 4 from (+)-pinanediol (see ESI for details regarding the set-up); (b) Flow synthesis of the α-amino boronic ester 2a (T-mixer with ID = 1 mm, Reactor tubing with ID = 1.59 mm, L1 = 600 cm, L2 = 250 cm) and batch conversion into Bortezomib 1.

  . INSA Rouen, CNRS, UNIROUEN, COBRA, Normandie Univ, 76600 Rouen, France b. Institut Universitaire de France, 1 Rue Descartes, 75231 Paris, France c. Corresponding authors : d. Julien Legros, julien.legros@univ-rouen.fr e. Philippe Jubault, philippe.jubault@insa-rouen.fr

af. Thomas Poisson, thomas.poisson@insa-rouen.fr † Footnotes relating to the title and/or authors should appear here. Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x