Synthesis of Prilocaine Hydrochloride in Continuous Flow Systems

Abstract

Herein, we showcase how continuous flow technology enables the development of time-efficient, high-yielding synthetic routes for the anesthetic drug prilocaine. Starting from toluene nitration, prilocaine was synthesized in 74% (high-performance liquid chromatography (HPLC)) overall yield with a total residence time of just 13.6 min. Continuous flow technology markedly enhanced the nitration step’s selectivity, increasing it from 59% in batch to 79%. The toluidine alkylation step benefited significantly from superheating at 150 °C and enhanced mixing in the flow system, reducing the reaction time from 48 h in batch to merely 4.4 min, affording 98% yield. Additionally, we present streamlined continuous flow reaction, telescoping, and inline workup strategies.

1. Introduction

Local anesthetics have a remarkable history of efficacy and safety in medical and dental practice. These drugs function by disrupting neural conduction through the inhibition of the influx of sodium ions through channels within neural membranes. They are utilized for the prevention of pain during procedures as well as the management of postoperative pain. The application of local anesthesia results in a temporary loss of sensation or pain in a specific area of the body without alteration of the patient’s level of consciousness. With regard to dentistry, two distinct types of local anesthetics exist, which are based on chemical structure: the ester and the amide local anesthetics. The amide-based anesthetics are frequently used due to lower toxicity compared to the ester-based anesthetics. Local anesthetics procaine 1 and benzocaine 2 (Figure a) are usually an alternative for patients who show sensitivity to the amide derivatives. Examples of common amide local anesthetic drugs used in dentistry include prilocaine hydrochloride 3, mepivacaine 4, lidocaine 5, etidocaine 6, and bupivacaine 7 (Figure b).

1.

Representative local anesthetic drugs: (a) ester group and (b) amide group.

Prilocaine hydrochloride 3, commercially known as Citanest, is an FDA-approved amide-based local anesthetic widely employed in dentistry for its rapid onset and efficacy. Compared to lidocaine 5, prilocaine facilitates faster dental anesthesia and earlier tooth extraction, establishing it as the most frequently used amide anesthetic in dental procedures. Prilocaine hydrochloride 3 is administered in two injectable formulations: 4% prilocaine plain and 4% prilocaine with 1:200,000 epinephrine, with a maximum adult dosage of 400 mg for both. The addition of epinephrine prolongs the anesthetic effect, reduces systemic absorption, and minimizes local bleeding. , Additionally, prilocaine is available as a eutectic mixture with lidocaine (2.5% prilocaine/2.5% lidocaine), marketed as EMLA, which is applied topically to numb skin prior to procedures or injections.

The clinical importance of prilocaine, coupled with the growing demand in the pharmaceutical industry to deliver high-quality, cost-effective medicines in a timely and sustainable manner, has driven the pharmaceutical production community to pursue the development of efficient, scalable, and environmentally friendly synthetic routes. To the best of our knowledge, all of the reported prilocaine synthetic procedures are based on conventional batch technology and accompanied by inherent typical limitations such as long reaction times, low yields, low selectivity, and low safety profile of reactions. Several reported methods utilize ortho-toluidine 12 as a starting material, achieving good yields but often requiring prolonged reaction times. − These reported routes utilize ortho-toluidine 12 as the starting material, with the processes characterized by good yields but long reaction times. , A notable approach is the batch synthesis reported by Regla and Demare, which employs a four-step telescoped sequence (no isolation of intermediates) starting from toluene 8 (Figure ). The initial step involves nitration of toluene to yield a mixture of mononitrotoluenes (MNTs): ortho-nitrotoluene 9, meta-nitrotoluene 10, and para-nitrotoluene 11 in a 58:3.5:38.5 ratio, respectively. This step is critical as the selectivity for ortho-nitrotoluene 9 dictates the yield of the desired ortho-toluidine 12 following reduction.

2.

Synthetic route toward prilocaine hydrochloride 3.

A key innovation in this synthesis is the selective isolation of ortho-toluidine 12. After reduction of the MNT mixture to a toluidine mixture, selective acetylation of para-toluidine 14 is achieved using acetic anhydride (1 w.r.t. para-toluidine). The resulting mixture is treated with 10% aqueous HCl, partitioning ortho-toluidine HCl 15 into the aqueous phase, while para-methylacetanilide 16 and meta-methylacetanilide 13 remained in the organic phase. Ortho-toluidine 12 is then acylated with 2-chloropropionyl chloride to form chloroamide 17, achieving a 30% overall yield from toluene in 0.5 h. The final step involves nucleophilic halogen displacement on chloroamide 17 to yield prilocaine 3a.

This study focuses on the synthesis of prilocaine hydrochloride 3 using continuous flow technology, employing a modified route originally developed by Regla and Demare with the goal of enhancing process efficiency, ensuring safety, and increasing yield. Continuous flow technology offers transformative advantages for pharmaceutical manufacturing by enabling safer, more efficient, and sustainable production of active pharmaceutical ingredients (APIs). − Unlike traditional batch processes, continuous flow systems offer enhanced control over hazardous reactions, such as nitrations, hydrogenations, and azide chemistry. This is achieved by introducing reactants and removing products continuously, which prevents the accumulation of unstable intermediates and mitigates the risks of exothermic events, pressure build-ups, or thermal runaway. − The enhanced heat and mass transfers in microreactors allow for rapid reaction and efficient mixing, leading to higher selectivity, improved yields, and greater reproducibility. − Additionally, flow chemistry facilitates access to novel process windows, including solvent-free or superheated conditions, which are difficult to achieve in batch setups. − The ability to telescope multiple reaction steps in a single continuous stream reduces the need for intermediate isolation, enhancing both safety and productivity. − Scale-up of a proven reaction in microreactors is easily achieved with little or no process improvement work. ,− Owing to these benefits, flow chemistry has gained recognition from regulatory bodies, including the U.S. FDA, which has endorsed its application in API manufacturing for its capacity to enhance drug quality and ensure consistent production. Leading pharmaceutical companies have increasingly adopted flow chemistry, as evidenced by the rising number of approved drugs produced using continuous flow technology. This trend highlights its expanding significance in modernizing and optimizing drug-manufacturing processes. − Notably, this study will investigate the influence of continuous flow technology on the selectivity of toluene nitration, with the specific objective of enhancing the formation of the desired isomer, ortho-nitrotoluene 9, in comparison to batch processes. Additionally, the research will evaluate the broader impact of flow conditions on reaction efficiency across all synthetic steps for the anesthetic drug prilocaine hydrochloride 3 from a commercially available starting material, toluene. To the best of our knowledge, no previous studies have documented the synthesis of prilocaine hydrochloride 3 under continuous flow conditions. Consequently, this research presents novel continuous flow methodologies for the production of this pharmaceutical compound.

2. Results and Discussion

2.1. Batch Synthesis of Prilocaine Hydrochloride

Our work started with validating Regla and Demare’s batch synthetic procedure for prilocaine hydrochloride 3 starting from toluene 8 (Figure ). Toluene 8 was nitrated using HNO₃/H₂SO₄ to afford MNTs mixture in the ratio 59:3:38 ortho 9 -/meta 10 -/para 11; as confirmed by high-performance liquid chromatography (HPLC) using respective known nitrotoluenes standards. After the reaction workup, the MNTs mixture was used in the next step without separating the isomers. The MNTs mixture was subjected to nitro-reduction using hydrazine hydrate as a hydrogen donor in the presence of Fe­(acac)₃ in ethanol under reflux for 2 h. Reaction completion was confirmed by TLC and followed by performing the proper workup procedure to obtain the toluidine isomeric mixture.

The resultant toluidine mixture was acetylated with acetic anhydride (1 equiv. w.r.t. para-toluidine 14) for 0.5 h to selectively acetylate para-toluidine 14 to para-methylacetanilide 16 by exploiting its lesser steric hindrance relative to the ortho-toluidine 12 isomer. This method presented an interesting way of easily separating toluidine isomers without requiring cumbersome fractional distillation. Upon reaction completion, the reaction mixture was extracted with an aqueous HCl (10%) solution. Thus, our preferred isomer 12 was taken into the aqueous phase as ortho-toluidine HCl 15, while para-methylacetanilide 16 and traces of acetylated meta-toluidine 13 were in the organic phase. Further workup, isolation, and purification procedures to isolate our target product, ortho-toluidine 12. Thereafter, acylation of ortho-toluidine 12 with 2-chloropropionyl chloride at room temperature for 1.5 h in the presence of triethylamine to afford chloroamide 17 in 89% isolated yield. Subsequent treatment of chloroamide 17 with an excess of propylamine at room temperature afforded prilocaine 3a in a 78% isolated yield after 48 h. Lastly, prilocaine hydrochloride 3 was achieved in 76% isolated yield upon treatment of prilocaine 3a with HCl gas generated in situ from reacting H₂SO₄ with NaCl. The overall yield for the synthesis of prilocaine hydrochloride 3 was 30%, which is consistent with the yields reported in existing literature.

Due to sluggish chloroamide 17 to prilocaine 3a reaction kinetics in batch using conventional heating, we investigated the effect of microwave heating in batch (Table ). Prilocaine 3a synthesis from chloroamide 17 and propylamine under microwave conditions started by heating the reaction mixture at 90 °C for 15 min to afford prilocaine 3a in 67% conversion (Table , Entry 1). Doubling the reaction time afforded 79% conversion (Table , entries 1 and 2). Increasing the reaction temperature to 100 °C while reducing the reaction time to 15 min afforded prilocaine 3a in 100% conversion (Table , Entry 3). Further increasing the temperature to 130 °C and 10 min afforded prilocaine 3a in 1 conversion (Table , Entry 4). Furthermore, full conversion to prilocaine 3a was achieved with a reduced reaction time of 5 min with 81% isolated yield (Table , Entry 5). Delightfully, we successfully synthesized prilocaine 3a in high yield under microwave heating at 130 °C in a short reaction time of 5 min compared to 48 h using conventional heating in a batch. Notably, such a significant reduction in reaction time demonstrates the capability of microwave heating to accelerate reaction rate, making it attractive in organic synthetic chemistry. ,

1. Preparation of Prilocaine under Microwave Irradiation .

entry	temp. (°C)	reaction time (min)	conversion (%)
1	90	15	67
2	90	30	79
3	100	15	100
4	130	10	100
5	130	5	100 (81)

^aStandard conditions: chloroamide 17 (0.5 g, 2.5 mmol) dissolved in propylamine (2.8 g, 47.7 mmol). Propylamine is used as both a reagent and a solvent.

^bConversion: determined by the HPLC method B following the disappearance of chloroamide 17.

^cNumber in parentheses is the isolated yield of prilocaine 3a.

Having successfully achieved the modified batch protocol to prepare prilocaine 3 and its intermediates, we realized that toluene 8 nitration is the key transformation of this protocol. With the understanding of the high impact of microreactors on influencing yield, selectivity, mass transfer, and offering enhanced safety when conducting highly exothermic reactions, we investigated the synthesis of prilocaine 3a under continuous flow conditions. ,,−

2.2. Toluene Nitration in Continuous Flow

Nitration is considered one of the most hazardous processes because it is highly exothermic. , It is associated with issues such as selectivity, mass transfer (in case of biphasic), and safety risks due to handling of strong and highly corrosive acids, thus also making it difficult to handle and scale up. Flow chemistry technology has proven to offer attractive benefits in nitration processes such as efficient heat and mass transfer, short residence times, enhanced safety, rapid responsiveness, and high process productivity. ,,− The most key part of the toluene 8 nitration toward prilocaine 3a synthesis is the control of the reaction selectivity toward the preferred ortho-isomer. Building on the flow benefits, numerous toluene 8 nitration in microreactors have been reported, particularly liquid–liquid phase. Reported work on nitration of toluene 8 in flow has reported ortho-isomer being achieved in the range 48–61% selectivity. ,− An improvement in selectivity to 75% toward ortho-isomer was achieved in our recent work with the use of HNO₃/H₂SO₄ as a nitrating mixture. In our investigation, we noted that the reaction’s biphasic nature makes it mass transfer dependent. As such, the use of microreactor mixing structures was necessary for efficient mixing of the reaction mass, resulting in improved reaction performance. Efficient mixing greatly influenced the reaction selectivity and reaction time.

Guided by our previous continuous flow xylenes nitration work, we utilized a 0.2 mL chip mixer reactor with static mixing structures for continuous flow nitration of toluene 8 herein (Figure and Table ).

3.

Continuous flow synthesis of ortho-nitrotoluene 9.

2. Continuous Flow Synthesis of ortho-Nitrotoluene 9 .

entry	res. time (min)	toluene conversion (%)	nitrotoluene isomers selectivity 9:10:11
1	0.5	61	79:1:20
2	1	84	79:1:20
3	2	100	79:1:20
4	3	100	79:1:20

^aStandard conditions: toluene 8 (neat), nitrating mixture: 50:50 (% v/v) H₂SO₄/HNO₃ neat, room temperature.

^bConversion was determined by the HPLC method A following the disappearance of toluene 8.

^cSelectivity was determined by the HPLC method A, observing the isomer’s peak percentage area.

We started by treating toluene 8 with a 50:50 (% v/v) HNO₃/H₂SO₄ nitrating mixture in a 0.2 mL chip mixer reactor at room temperature and 0.5 min of residence time (Table , Entry 1). The reaction afforded 61% conversion of toluene 8 with ortho-nitrotoluene 9 and para-nitrotoluene 11 constituting 79 and 20% selectivity as determined by HPLC. Traces of the meta-nitrotoluene 63 and other minor impurities accounted for 1%. Doubling the reaction residence time resulted in an increase in conversion of toluene 8 to 84% while selectivity toward our preferred nitro isomer 9 was unchanged (Table , Entries 1 and 2). Further increasing the reaction time to 2 min achieved full conversion of toluene 8 with 79% selectivity toward our preferred isomer 9 (Table , Entries 2 and 3). Although the reaction conversion changed under different reaction conditions, the selectivity was constant (79%). The optimum conditions of the reaction were found to be 2 min residence time using 50:50 (% v/v) HNO₃/H₂SO₄ and neat toluene at room temperature to afford full conversion and 79% selectivity toward the preferred ortho-nitrotoluene 9, translating to a throughput of 22 mmol/h. Due to the poor miscibility of toluene 8 in the acid phase, intense mixing was important in maximizing the interfacial area of the two phases and mass transfer of the reactants. Pleasingly, we developed a time-economic process that offers high safety and selectivity toward ortho-nitrotoluene 9 under mild reaction conditions. This protocol affords a high selectivity of 79% toward our preferred nitro isomer 9 compared to 59% obtained in batch and reported toluene protocols under flow conditions. ,− Clearly, continuous flow technology enhanced the nitration reaction mass and heat transfer.

2.3. Continuous Flow Nitro-Reduction of ortho-Nitrotoluene

Previously, we successfully nitrated toluene 8, affording a mixture of MNTs, which we require to subject under reduction to afford the respective toluidines. To gain an understanding of the reaction progress, we began by carrying out a thorough study on nitro-reduction using ortho-nitrotoluene 9 as substrate to afford ortho-toluidine 12. Thereafter, we performed a two-step synthesis of toluidines by combining the nitration and reduction steps together.

Kappe and co-workers reported a protocol for continuous flow reduction of nitro-arenes using a combination of both homogeneous and heterogeneous nanocatalysis. The protocol involved the in situ generation of highly reactive colloidal Fe₃O₄ nanocrystals from the reaction of Fe­(acac)₃ together with hydrazine hydrate. Following this disclosure and our previous work with slight modifications, we performed continuous flow nitro-reduction of ortho-nitrotoluene 9 at 170 °C to afford ortho-toluidine 12 at 170 °C. This was performed in a 1.7 mL chip reactor with a 10 bar back pressure regulator fitted downstream (Figure ). In the preliminary experiment, we reacted ortho-nitrotoluene 9 (0.1 M) with hydrazine hydrate (3 equiv) in the presence of 25 mol % Fe­(acac)₃ in ethanol at 170 °C for 10 min residence time under continuous flow conditions to afford 71% conversion. With this observation, we went to perform optimization studies for ortho-toluidine 12 synthesis. The reaction parameters investigated include the effect of residence time and molar equiv of hydrazine hydrate and catalyst loading.

4.

Continuous flow system for the synthesis of ortho-toluidine 12.

We started by investigating the effect of residence time and hydrazine hydrate molar equiv. on conversion of ortho-nitrotoluene 9 toward ortho-toluidine 12 (Figure ). The reaction parameters of ortho-nitrotoluene 9 (0.1 M) in ethanol, 170 °C temperature, and 25 mol % Fe­(acac)₃ were kept constant, while residence time and hydrazine hydrate molar equiv. were varied.

5.

Effect of residence time and hydrazine hydrate molar equiv. conversion of ortho-nitrotoluene 9 toward ortho-toluidine 12.

Evidently, the reduction of ortho-nitrotoluene 9 is influenced by both residence time and hydrazine hydrate molar equiv. At 2.5 min residence time, conversion increased from 23 to 100% using 3 and 5 equiv of hydrazine hydrate, respectively. We could not further increase the reaction temperature due to the decomposition temperature of hydrazine hydrate, reported to be in the range 180–183 °C. As such, optimum conditions for the reduction of ortho-nitrotoluene 9 were determined to be 5 equiv of hydrazine hydrate and 2.5 min residence time, affording 100% conversion at 170 °C.

After determining the optimum hydrazine hydrate equivalents, reaction temperature, and residence time, we investigated if an Fe­(acac)₃ amount (mol %) less than 25 mol % could be used for ortho-nitrotoluene 9 to ortho-toluidine 12 (Figure ). At the optimum conditions, the conversion of ortho-nitrotoluene 9 increased with an increase in the amount of Fe­(acac)₃; the optimum Fe₃O₄ nanocrystals amount was determined to be 20 mol %. Thus, the final optimum conditions for ortho-nitrotoluene 9 (0.1 M) were determined to be 5 equiv. hydrazine hydrate, 170 °C, 2.5 min, and 20 mol % Fe­(acac)₃, affording 100% conversion and 4 mmol/h output to ortho-toluidine 12.

6.

Effect of catalyst amount (mol %) on conversion of ortho-nitrotoluene 9 toward ortho-toluidine 12.

Of note, the reaction stream remains homogeneous, making it suitable for microreactor processing in all experiments carried out. We successfully developed an efficient, scalable, and time-economic protocol for the reduction of ortho-nitrotoluene 9. This method eliminates the use of expensive precious metal catalysts and flammable molecular hydrogen. In our method, hydrazine hydrate is utilized as a source of hydrogen in combination with in situ-generated Fe₃O₄ nanocrystals, which offer high catalytic activity. The nanocrystals can be easily removed by simple filtration or a magnet and reused as reported by Kappe and co-workers, though we did not explore it in our work. The use of a small amount (20 mol %) of a cheap iron source makes this method very attractive for application in both research laboratories and the industry.

2.4. Two-Step Continuous Flow Synthesis of ortho-Toluidine

Following successful optimization of the first two steps, nitration and subsequent nitro-reduction under continuous flow, we investigated the multistep synthesis of ortho-toluidine 12 by combining the two steps into a single-step flow sequence (Figure ). Guided by the predetermined optimum conditions of the individual steps, slight adjustments were made to enable the two steps to telescope seamlessly. The first reactor (0.2 mL chip mixer reactor) was dedicated to nitrate toluene 8 (neat, 9.4 M) with 50:50 (% v/v) HNO₃/H₂SO₄ for a 2 min residence time at room temperature. The effluent from the first reactor containing the MNTs mixture was neutralized in-line with 40% NaOH at a T-mixer held at 0–5 °C using an ice bath before subsequent in-line liquid/liquid separation of the organic and aqueous phases. Thereafter, the eluent organic phase containing MNTs (2.35 M, 1 equiv) was treated with hydrazine hydrate (11.75 M, 5 equiv) and Fe­(acac)₃ (0.47 M, 0.2 equiv) in the second chip reactor held at 170 °C for a 4.25 min residence time, affording respective toluidines. A 10 bar back pressure regulator was fitted downstream to allow homogeneous flow. For the telescoped process, 100% conversion to toluidines and 79% selectivity with 22 mmol/h output toward the preferred isomer, ortho-toluidine 12 in 6.25 min total residence time. The system was run continuously for 10 min with product being collected continuously; the ratio of the toluidine isomers was confirmed to be consistent throughout, ortho-/meta-/para- 79:1:20. After collection, workup procedures were done to obtain the toluidine crude mixture, which was used in the next reaction without further purification.

7.

Two-step synthesis of ortho-toluidine 12 via biphasic nitration under continuous flow.

2.5. Continuous Flow Selective Acetylation of para-Toluidine for Isolation of ortho-Toluidine

Acetylation is an important transformation in organic synthesis because acetyl groups can be conveniently utilized to protect various functional groups such as amines, thiols, and alcohols. Here, selective acetylation of para-toluidine 14 was used to separate the toluidine isomers by taking advantage of the difference in steric hindrance of the isomers.

We conducted selective acetylation of para-toluidine 14 in a 0.2 mL chip mixer reactor utilizing acetic anhydride as the acetylating agent (Figure ). The toluidine crude mixture obtained from the previous step was used as the starting material. Guided by the toluidine isomer ratio 79:1:20 ortho-/meta-/para-, the amount of para-toluidine 14 was determined as 20% of the total amount of the toluidine mixture. As such, it was critical that only acetic anhydride (1 equiv) w.r.t. para-toluidine 14 amount is used to prevent acetylation of ortho-toluidine 12.

8.

Continuous flow selective acetylation of para-toluidine 14 toward para-methylacetanilide 16.

Toluidine mixture (0.2 M) from a previous step in DCM and acetic anhydride (1 equiv. w.r.t. para-toluidine 14) were reacted in a chip reactor at room temperature (Figure and Table ). Allowing the reagents to react for 1.5 min of residence time, para-methylacetanilide 16 was afforded in full conversion (Table , Entry 1). Full conversion was still achieved when the residence time was reduced to 1 min (Table , entries 1 and 2). However, with further reduction of reaction time to 0.5 min, the conversion reduced to 87% conversion (Table , Entry 3). The optimum residence time for continuous flow acetylation of para-toluidine 14 was determined to be 1 min, affording 100% conversion and 1.3 mmol/h throughput. Offline, the crude product mixture was treated with HCl (aq) to get the desired ortho-toluidine HCl salt from the unreacted ortho-toluidine 14 in the aqueous phase, while the other acetylated isomers were in the organic phase. This allowed for the isolation of ortho-toluidine 14 from the toluidine isomers mixture after the necessary workup procedures.

3. Continuous Flow Selective Acetylation of para-Toluidine 14 .

entry	temp. (°C)	res. time (min)	conversion (%)
1	rt	1.5	100
2	rt	1	100
3	rt	0.5	87

^aStandard conditions: toluidine mixture (0.2 M), 1 equiv. Ac₂O w.r.t. toluidine 14, room temperature, DCM solvent.

^bConversion: determined by the HPLC method A, observing the disappearance of toluidine 14.

2.6. Continuous Flow Synthesis of 2-Chloro-N-(o-Tolyl)­Propenamide

The synthesis of chloroamide 17 in batch mode was characterized by a highly exothermic reaction that necessitates effective heat dissipation to manage the associated thermal hazards and ensure process safety. We envisage that using high surface area microreactors will allow for rapid heat dissipation. To enhance the safe and efficient handling of this reaction, we initiated an investigation by transitioning the batch process directly into a continuous flow system. The batch process done in THF is characterized by the formation of triethylammonium chloride salt precipitation, formed from the quenching of HCl byproduct with triethylamine, which could potentially cause reactor blockage in continuous flow mode. To avoid blockages, we replaced triethylamine with diisopropylethylamine (DIPEA), which produces a more soluble HCl salt in THF.

In the preliminary study, ortho-toluidine 12 (0.4 M) and DIPEA (0.4 M, 1 equiv) in THF were treated with 2-chloropropionyl chloride (0.4 M, 1 equiv) in THF in a 0.2 mL LTF-MS micro mixer at room temperature for a 0.5 min residence time, resulting in 60% conversion (Figure ). With this information, we further performed optimization studies under continuous flow conditions (Table ).

9.

Continuous flow synthesis of 2-chloro-N-(o-tolyl) propenamide 17.

4. Continuous Flow Synthesis of 2-Chloro-N-(o-Tolyl)­Propenamide .

entry	equiv. of DIPEA	temp (°C)	res. time (min)	conversion (%)
1	1	rt	1	74
2	1	rt	1.5	88
3	1	rt	2	100 (94)
4	1	rt	2	100
5	1	rt	2	100

^aStandard conditions: ortho-toluidine 12 (0.4 M), 2-chloropropionyl chloride (1 equiv), base (1 equiv), room temperature, THF solvent.

^bConversion: determined by HPLC method B.

^cNumber in parentheses is the yield determined by HPLC.

^dDBU was used as an alternative base to DIPEA.

^eTBA was used as an alternative base to DIPEA.

Ortho-toluidine 12 (0.4 M) was first treated with 2-chloropropionyl chloride (1 equiv) at room temperature for 1 min to achieve 74% conversion (Table , Entry 1). Conversion increased to 88% when the residence time was increased to 1.5 min (Table , entries 1 and 2). Full conversion was afforded when the residence time was increased to 2 min (Table , Entry 4). Interestingly, either tributylamine (TBA) or 1,8-diazabicyclo[5.4.0]­undec-7-ene (DBU) could be used as DIPEA alternatives without challenges (Table , entries 4 and 5). Therefore, the optimum conditions were determined to be a 2 min residence time affording full conversion, 94% yield with 1 mmol/h output of chloroamide 17 at room temperature.

2.7. Continuous Flow Synthesis of Prilocaine and Salt Formation

Guided by our investigations on prilocaine 3a synthesis under microwave conditions, we directly transferred the reaction into continuous flow. In the preliminary experiment, chloroamide 17 (0.3 M) in propylamine (it acts as both reagent and solvent) was reacted in a chip mixer reactor held at 100 °C for 15 min of residence time to afford prilocaine 3a in 100% conversion (Figure ).

10.

Continuous flow synthesis of prilocaine 3a.

With the preliminary results in hand, we went on to investigate the effect of residence time and temperature on the conversion of chloroamide 17 to prilocaine 3a. We observed that conversion is greatly influenced by temperature, as conversion increased with an increase in temperature (Figure ). Full conversion of the starting material was achieved at 150 °C in 4.4 min residence time. As such, optimum conditions were determined to be a 4.4 min residence time at 150 °C, affording prilocaine 3a in full conversion with a throughput of 1.4 mmol/h. Despite the reaction being extremely slow in batch mode (48 h), continuous flow technology significantly reduced the reaction time to 4.4 min through superheating under pressure.

11.

Effect of residence time and temperature of synthesis of prilocaine 3a in continuous flow.

2.8. Two-Step Continuous Flow Synthesis of Prilocaine

After successful optimization of chloroamide 17 and prilocaine 3a synthesis in flow, we went on to telescope the two steps. Guided by optimum conditions of the two individual steps, a 0.2 mL chip mixer reactor was connected in series with a 1.1 mL glass reactor under 10 bar back pressure (Figure ). In the first reactor, ortho-toluidine 12 (0.4 M, 1 equiv) and DIPEA (0.4 M, 1 equiv) in THF were treated with 2-chloropropionyl chloride (0.4 M, 1 equiv) at room temperature for 2 min to allow in situ generation of chloroamide 17. Subsequently, chloroamide 17 was treated with propylamine in the second reactor held at 150 °C for a 4.4 min residence time to afford prilocaine 3a in 98% yield with 0.5 mmol/h throughput over two steps.

12.

Two-step continuous flow synthesis of prilocaine 3a.

2.9. Offline Prilocaine Hydrochloride Salt Formation

Finally, conversion of prilocaine 3a free base to prilocaine hydrochloride 3 was performed in batch by treating prilocaine 3a with HCl gas generated in situ from reacting H₂SO₄ with NaCl (Figure ). Preliminary prilocaine hydrochloride 3 formation investigations began by bubbling HCl gas into an EtOAc solution of free base 3a to afford the salt 3 in 45% isolated yield (Table , Entry 1). To improve the solubility of HCl gas in the prilocaine 3a solution and salt formation performance, prilocaine 3a in MeOH:EtOAc was utilized to improve the isolated yield to 59% (Table , Entry 2). Replacing EtOAc with Et₂O improved the isolated yield of salt 3 to 67% (Table , Entry 3). Higher isolated yield of prilocaine hydrochloride 3 in Et₂O than in EtOAc was mainly because of the relatively lower solubility of prilocaine hydrochloride 3 in Et₂O than in EtOAc. Reaction scale was increased to 2.5 g, and prilocaine hydrochloride 3 was achieved in a 76% isolated yield (Table , Entry 4). Thus, the best isolated yield of prilocaine hydrochloride 3 was found to be 76% at a 2.5 g scale using MeOH:Et₂O (9:15 mL), H₂SO₄ (7.5 mL), and NaCl (5 g), and a 0.5 h reaction time.

13.

Preparation of prilocaine hydrochloride 3.

5. Batch Preparation of Prilocaine Hydrochloride 3 .

entry	prilocaine 3a (g)	solvent system	solvent volume (mL)	prilocaine HCl 3 (g)	isolated yield of 3 (%)
1	1	EtOAc	8	0.53	45
2	1	MeOH:EtOAc	3:5	0.68	59
3	1	MeOH:Et2O	3:5	0.75	67
4	2.5	MeOH:Et2O	9:15	2.2	76

^aStandard conditions: prilocaine 3a in solvent, hydrogen chloride gas bubbling in situ from H₂SO₄ + NaCl for 0.5 h.

^bH₂SO₄ (5 mL) and NaCl (2.5 g).

^cH₂SO₄ (7.5 mL) and NaCl (5 g).

3. Conclusions

This study presents a groundbreaking continuous flow synthesis of prilocaine, achieving 74% overall yield in just 13.6 min, far surpassing batch methods. The telescoped process combines biphasic toluene nitration with inline workup, enhancing selectivity from 59% (batch) to 79% (flow) through optimized micro mixing, while safely handling this hazardous reaction. Superheating in flow dramatically reduced alkylation time from 48 h (batch) to 4.4 min with 98% yield. The hydrochloride salt was obtained offline in 76% yield (0.5 h) using in situ-generated HCl. This work demonstrates continuous flow’s ability to safely intensify challenging reactions while improving efficiency and sustainability. The approach sets a precedent for synthesizing other pharmaceuticals involving hazardous or exothermic steps. Future directions include optimizing the reactor design, exploring greener reagents, and integrating salt formation for complete end-to-end continuous production. These advances establish flow chemistry as a transformative platform for scalable, sustainable, and cost-effective pharmaceutical manufacturing.

4. Experimental Procedures

4.1. General Information

All of the chemicals used were purchased from Merck, Sigma-Aldrich, Spellbound, or Industrial Analytical. They were used as received without further purification. Nuclear magnetic resonance (NMR) spectra were recorded at room temperature as solutions in deuterated dimethyl sulfoxide (DMSO-d ₆) or deuterium oxide (D₂O). A Bruker Avance-400 spectrometer (400 MHz) was used to record the spectra, and the chemical shifts are reported in parts per million (ppm) with coupling constants in Hertz (Hz). Infrared spectra were recorded from 4000 to 500 cm^–1 using a Bruker spectrometer and peaks (V _max) reported in wavenumbers (cm^–1). Melting points of the synthesized compounds were determined using the Stuart Melting Point - SMP10 digital apparatus. High-performance liquid chromatography (HPLC) data were obtained using an Agilent 1200 Infinity LC instrument equipped with a diode array detector (DAD) fitted with an Agilent Eclipse Plus C18 reverse-phase column.

4.2. Procedure 1: Toluene Nitration in Flow

Toluene 8 and the nitrating mixture 50:50 (% v/v) 70% HNO₃/98% H₂SO₄ were delivered into a temperature-controlled 0.2 mL chip mixer reactor at varying flow rates. A product sample was collected in a glass vial with a known amount of ice-cold water, and the organic layer was drawn using a syringe, followed by dilution with an equal volume of methanol prior to analysis by HPLC. The product peak was confirmed by HPLC using a known standard of the nitro compound. An investigation into the effect of the residence time on conversion was done.

4.3. Procedure 2: Continuous Flow Synthesis of ortho-Toluidine

A solution of ortho-nitrotoluene 9, hydrazine hydrate, and Fe­(acac)₃ prepared in EtOH was delivered into a temperature-controlled 1.7 mL glass reactor fitted with a back pressure regulator. Prior to analysis by HPLC to confirm the target product, ortho-toluidine 9, product samples were collected into a vial and filtered to remove iron particles formed using a syringe filter. Investigation on other parameters, such as the effect of residence time, the effect of reaction temperature, the molar equiv of hydrazine hydrate, and the amount of catalyst loading, was conducted. The prepared batch of ortho-toluidine 12 and the characterized standard were used to confirm ortho-toluidine 12 from the continuous flow stream.

Yellow oil, 1.8 g, 78% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.27 (t, J = 7.1 Hz, 2H), 6.95 (t, J = 7.4 Hz, 1H), 6.83 (d, J = 8.5 Hz, 1H), 3.70 (d, J = 1.6 Hz, 2H), 2.35 (s, 3H). ¹³C­{¹H} NMR (101 MHz, CDCl₃) δ: 144.94, 130.67, 127.20, 122.50, 118.72, 115.17, 17.71. IR (KBr, cm^–1): 3361, 3020, 2913, 1619, 1583, 1495, 1467, 1441, 1380, 1267, 1143, 1034, 985, 927, 845, 745, 713, 535, 438.

4.4. Procedure 3: Two-Step Continuous Flow Synthesis of Toluidine Incorporating Liquid/Liquid Separator

Toluene 8 and 50:50 (% v/v) 70% HNO₃/98% H₂SO₄ were separately pumped through the 0.2 mL LTF micro mixer held at room temperature. The reaction stream is then fed into a T-mixer cooled at −5 °C in an ice bath, whereby another syringe pump delivers a 40% NaOH solution to neutralize the mixture before entering the separator. The inorganic phase is removed from the system by the separator, and a solution of hydrazine hydrate and Fe­(acac)₃ was pumped into the second T-mixer to allow premixing before being delivered into the glass reactor held at 170 °C. Product samples were analyzed by HPLC. A workup procedure similar to the batch experiment was done to obtain a crude mixture containing toluidine. HPLC was used to determine the isomer ratio.

4.5. Procedure 4: Selective Acetylation of para-Toluidine in Flow

Toluidine mixture (0.2 M) in DCM and acetic anhydride (1 equiv. w.r.t. para-toluidine 14) in DCM were pumped into the reactor. The amount of para-toluidine 14 was calculated based on the isomer ratio ortho-/meta-/para- 79:1:20. Samples were collected and analyzed by HPLC to observe the conversion of para-toluidine 14 toward para-methylacetanilide 16. The prepared batch para-acetanilide 16 and characterized standard were used to confirm para-acetanilide 16 from the continuous flow stream.

White crystals, 2.1 g, 81% yield. ¹H NMR (400 MHz, DMSO) δ 9.80 (s, 1H), 7.47 (d, J = 8.5 Hz, 2H), 7.06 (d, J = 8.5 Hz, 2H), 2.24 (s, 3H), 2.02 (s, 3H). ¹³C­{¹H} NMR (101 MHz, DMSO) δ: 168.48, 137.33, 132.29, 129.44, 119.54, 119.22, 24.35, 20.82. IR (KBr, cm^–1): 3656, 3286, 3122, 2980, 2888, 1659, 1600, 1548, 1507, 1454, 1399, 1378, 1363, 1318, 1262, 1152, 1073, 1039, 1013, 955, 817, 748, 618, 604, 505.

4.6. Procedure 5: Continuous Flow Synthesis of 2-Chloro-N-(o-Tolyl)­Propenamide

Ortho-toluidine (0.4 M) 12 and DIPEA (0.4 M) in THF were pumped together with 2-chloropropionyl chloride (0.4 M) in THF into a 0.2 mL chip mixer reactor, affording amide 17. Samples were collected and analyzed with HPLC to confirm our target product, chloroamide 17. The prepared batch of chloroamide 17 and the characterized standard were used to confirm chloroamide 17 from the continuous flow stream.

White solid, 3.3 g, 89% yield. ¹H NMR (400 MHz, DMSO) δ 9.58 (s, 1H), 7.28 (d, J = 9.4 Hz, 1H), 7.16–7.06 (m, 2H), 7.04 (d, J = 7.4 Hz, 1H), 4.69 (q, J = 6.7 Hz, 1H), 2.12 (s, 3H), 1.55 (d, J = 6.8 Hz, 3H). ¹³C­{¹H} NMR (101 MHz, DMSO) δ: 168.10, 135.95, 132.82, 130.86, 126.52, 126.31, 125.74, 54.99, 21.78, 18.07. IR (KBr, cm^–1): 3054, 3027, 2977, 2928, 1660, 1609, 1587, 1537, 1486, 1456, 1370, 1285, 1246, 1196, 1115, 1075, 1044, 993, 946, 926, 856, 783, 750, 709, 683, 587, 543, 444.

4.7. Procedure 6: Continuous Flow Synthesis of Prilocaine and Salt Formation

The single stream of reagents, chloroamide 17 (0.3 M) in propylamine, was directly delivered into a temperature-controlled 1.1 mL glass reactor fitted with a 10 bar BPR downstream. Samples were collected and analyzed by HPLC to confirm our target product, prilocaine 3a. The prepared batch of prilocaine 3a and the characterized standard were used to confirm prilocaine 3a from the continuous flow stream.

Pale-yellow oil, 1.7 g, 78% yield. ¹H NMR (400 MHz, DMSO) δ 9.52 (s, 1H), 7.77 (d, J = 9.5 Hz, 1H), 7.14 (d, J = 6.5 Hz, 1H), 7.11 (d, J = 7.8 Hz, 1H), 6.98 (t, J = 6.7 Hz, 1H), 3.16 (q, J = 6.9 Hz, 1H), 2.90 (s, 1H), 2.46 (q, J = 4.0 Hz, 2H), 2.18 (s, 3H), 1.49–1.37 (m, 2H), 1.24 (d, J = 7.0 Hz, 3H), 0.86 (t, J = 7.4 Hz, 3H). ¹³C­{¹H} NMR (101 MHz, DMSO) δ: 173.87, 136.74, 130.64, 129.33, 126.63, 124.62, 122.54, 58.81, 50.38, 23.43, 19.54, 17.81, 12.16. IR (KBr, cm^–1): 3295, 2960, 2928, 2873, 1740, 1681, 1586, 1518, 1451, 1368, 1291, 1216, 1368, 1291, 1216, 1133, 1069, 1046, 991, 914, 750, 713, 497, 442.

In a subsequent offline salt formation step, the free base prilocaine 3a was converted into the desired product, prilocaine hydrochloride 3, with HCl gas generated in situ.

White solid, 2.2 g, 76% yield. ¹H NMR (400 MHz, D₂O) δ 7.35 (d, J = 9.0 Hz, 1H), 7.32–7.22 (m, 3H), 4.21 (q, J = 7.9 Hz, 1H), 3.14–2.96 (m, 2H), 2.21 (s, 3H), 1.79–1.71 (m, 2H), 1.68 (d, J = 9.0 Hz, 3H), 0.97 (t, J = 7.5 Hz, 3H). ¹³C­{¹H} NMR (101 MHz, D₂O) δ: 169.38, 134.52, 133.34, 131.00, 128.17, 126.84, 126.62, 56.29, 48.14, 19.35, 16.93, 15.97, 10.27. IR (KBr, cm^–1): 3202, 2977, 2715, 1705, 1583, 1538, 1460, 1436, 1300, 1253, 1043, 936, 763, 723, 668, 486, 444.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c04742.

• Batch synthesis of compounds 3a, 3, 12, 16, and 17; continuous flow setups for the synthesis of compounds 3a, 3, 12, 16, and 17; copies of 1H and 13C­{1H} NMR spectra for all compounds (3a, 3, 12, 16, and 17); and copies of FTIR spectra of compounds 3a, 3, 12, 16, and 17 (PDF)

†.

Chemical Sciences Department, Midlands State University, Gweru 9055, Zimbabwe

‡.

Pharmaceutical Technologies, Council for Scientific and Industrial Research, Pretoria 0001, South Africa.

M.B.S.: conceptualization, methodology, investigation, and writing original draft. C.R.S.: conceptualization and writing original draftreview and editing. P.W.: writingreview and editing and supervision

The authors declare no competing financial interest.