Results and Discussion

Experimental setup

The 3D-printed flow reactors used to carry out the organic syntheses were designed by using a 3D CAD software package (Autodesk123D®), which is freely distributed and produces files that can be converted to the correct format read by the 3DTouchTM printer. This 3D printer heats a thermopolymer through the extruder, depositing the material in a layer-by-layer fashion, converting the design into the desired 3D reactionware.

The thermoplastic employed to fabricate the devices presented herein is PP, selected to print robust, inexpensive and chemically inert devices. Comparing PP with other common and accessible thermoplastics, which have been used in 3D printing before, such as polylactic acid (PLA) and polyacrylates, in PP we can find the required characteristics to perform a chemical reaction: thermostability up to 150 °C, high chemical inertia, and low cost. PLA is widely used in medicinal chemistry because of its biocompatibility; however, from a chemical point of view its use is limited to a few solvents and organic compounds, and to preserve its integrity it can only be used up to temperatures of 60–66 °C [19]. Polyacrylates consist of a vast group of polymers with different physical and chemical properties; however their chemical compatibility is low. In fact they are not generally recommended for exposure to alcohol, glycols, alkalis, brake fluids, or to chlorinated or aromatic hydrocarbons [20]. Therefore, PP was the plastic of choice for the device fabrication.

The shape of the 3D-printed reactionware devices used herein (Figure 1) was chosen in order to combine a short design and print time with the robustness required for a flow system.

Figure 1: Schematic representation of the 3D-printed reactionware devices employed in this work showing the internal channels. Both have two inputs (A and B) and one output (C). The main difference consists in the length of the inlets/outlets: the dimension of the inlets/outlets in R1 is 3 mm and in R2 it is 6 mm where the latter is designed to match the size of standard check-valves. Figure 1: Schematic representation of the 3D-printed reactionware devices employed in this work showing the i... Jump to Figure 1

Each device has two inlets, followed by a mixing point, a length of reactor to ensure a controlled residence time (which is given by dividing the reactor volume by the total flow rate), and one outlet. The approximate volume of the first reactor (R1, see Figure 1, left) is ca. 0.4 mL and was employed in the imine syntheses, while the second reactor (R2, see Figure 1, right) has a volume of ca. 0.35 mL and was employed connected to another R2 for the imine reduction processes. All the characteristics of the devices are summarized in Table 1.

Table 1: 3D-printed reactionware device characteristics. Entry Characteristics R1 R2 1 printing time (min) 248 367 2 PP mass (g) 24.01 33.74 3 dimensions (mm) 30 × 80.2 × 10 70 × 30 × 15 4 internal diameter (mm) 1.5 1.5 5 theoretical volumea (mL) 0.54 0.51 6 reactor volume 0.4 0.35

aThe theoretical internal volumes of the devices are higher than the measured volumes. This is due to the printing process, where the internal channel diameter is always slightly smaller than the designed one.

The 3D-printed devices were integrated in the flow systems using 1.58 mm outer diameter (OD) polytetrafluoroethylene (PTFE) tubing, with an internal diameter of 0.5 mm and standard connectors made of polyfluoroelastomer (FPM) and polyether ether ketone (PEEK). PEEK is a harder plastic than PP and, thus, allowed the screwing of the standard connectors into the softer PP inlets/outlets of the devices, resulting in a tight seal to the device. The screw connectors increase the chemical tolerance of the 3D-printed reactor as well as its chemical compatibility, compared to our previous devices [5]. The connectors at the device inlets were equipped with check valves (made of PEEK with a Chemraz® O-ring, which is compatible with organic solvents and compounds) to prevent potential backflow issues. The reactor inlets were connected to the syringe pumps containing the starting material solutions, whilst the outlets were connected to the in-line ATR-IR flow cell (see Figure 2). These improvements are a considerable step forward compared to our previous report on 3D printing fluidics [5], as they facilitate the integration of the devices, increase the chemical compatibility, improve the range of pressure that can be handled by the system, and enable the easy configuration for the use of ancillary equipment.

Figure 2: Flow system setup, where a R1 is connected to the syringe pumps and the ATR-IR flow cell with standard connectors. Figure 2: Flow system setup, where a R1 is connected to the syringe pumps and the ATR-IR flow cell with stand... Jump to Figure 2

Device 1: Imine formation

Here we show the 3D-printed device as a millifluidic reactor for the synthesis of imines under flow conditions. We monitored the reaction progress with the help of an in-line ATR-IR flow cell, which is a very useful technique for the monitoring of organic reactions under flow conditions [10,21-26]. The flow setup used for these syntheses consists of two syringe pumps, each of them connected to one of the inlets of the 3D-printed reactionware device R1. The syringe pumps were filled with the starting materials with a carbonyl compound (1a–c) being placed in syringe pump no. 1 and with a primary amine (2a–d) being placed in syringe pump no. 2 (Figure 3).

Figure 3: Carbonyl compounds and primary amines used in the syntheses reported in this work. Carbonyl compounds: benzaldehyde (1a); R-(−)-myrtenal (1b); 3-pentanone (1c). Aniline derivatives: aniline (2a); 3-(trifluoromethyl)aniline (2b); 3-chloroaniline (2c); 3,5-dimethylaniline (2d). Figure 3: Carbonyl compounds and primary amines used in the syntheses reported in this work. Carbonyl compoun... Jump to Figure 3

The experiments were conducted using 2 M methanolic solutions of the different substrates. This is convenient from a processing point of view, since high concentrations favor increased reaction kinetics [26] whilst minimizing the amount of waste generated during the downstream work-up [27]. The reactor output was connected with a length of tubing with a volume 0.1 mL to the IR flow cell. Hence, the total flow reactor volume (V R ) was 0.5 mL. The syntheses of the imines were monitored by an in-line ATR-IR flow cell and were conducted at a total flow rate of 0.25 mL min−1, where two equimolar methanolic solutions of 1 and 2 were flowed into R1 at the same flow rate. The residence time was calculated as the time taken for the solutions to go from the mixing point inside the 3D-printed reactor to the analytical device, thus taking into account the subsequent pieces of tubing employed, and resulted to be 2 minutes. The choice of a short residence time is to allow for a more reliable comparison of the imines synthesized and also to avoid the formation of the Michael addition adduct [28] (the thermodynamic compound) in the reaction between compounds 1b and 2a.

For the first experiment, we studied the reaction of benzaldehyde (1a) with the aniline derivatives 2a–d (Figure 3), to synthesize the N-benzylideneanilines 3a–d (see Table 2). The different substituents on the amine compounds have an electronic effect on the reactive center, thus influencing the observed conversion, i.e., an electron-donating group (EDG) in the meta-position of the aniline ring gives a higher percentage conversion than does an electron-withdrawing group (EWG) [28]. In fact, Table 2 shows that the conversion of benzaldehyde (1a) to imine 3a (Table 2, entry 1; obtained by reacting 1a with 2a), is higher than with the conversion of 1a to imine 3b (Table 2, entry 2; obtained by reacting 1a with 2b). The conversion of 1a to imine 3c (Table 2, entry 3; reaction of 1a with 2c), is the same as the formation of 3a, whilst the formation of 3d (reaction of 1a with 2d) has the highest conversion %.

Table 2: Conversion of benzaldehyde (1a) into imines 3a–d. Entry Product Conversion (%) 1

3a 96 2

3b 85 3

3c 96 4

3d 99

Figure 4 shows the effect of the EWG and EDG substituents of a phenyl ring through the IR spectra of compounds 3b (on the left) and 3d (on the right). In both graphs the imine spectrum (in red) is compared with the spectrum of the starting materials (dash line): the aldehyde peak of benzaldehyde (1a) at 1704 cm−1 (in black) disappears when it reacts with compound 2d (Figure 4, on the left), while it is still present when combined with compound 2b (Figure 4, on the right). 1H NMR spectroscopy was used to confirm the conversion rate of 1a to the N-benzylideneaniline derivatives 3a–d.

Figure 4: ATR-IR spectra of the synthesis of compounds 3b (on the left) and 3d (on the right). The spectrum on the left shows the reaction that does not go to completion due to the EWG substituent on the meta-position of the primary amine 2b (see Supporting Information File 1). Figure 4: ATR-IR spectra of the synthesis of compounds 3b (on the left) and 3d (on the right). The spectrum o... Jump to Figure 4

To calculate the conversion of the benzaldehyde (1a) into the imines 3a–d when combined with the amines 2a–d, a calibration of the IR spectra of benzaldehyde at known concentrations was obtained. The different concentrations of the substrates used for the IR analysis do not significantly affect the intensity of the area of the solvent band at 1022 cm−1 (A 1022 ). Hence, it is possible to use the solvent peaks to normalize the different spectra, allowing for comparison of the results. From this data a calibration curve can be obtained dividing the area of the benzaldehyde band at 1704 cm−1 (A 1704 ) by A 1022 , calculated for five different molar concentrations of the methanolic solutions of benzaldehyde. We used 2 M, 1 M, 0.5 M, 0.25 M and 0.125 M methanolic solutions of benzaldehyde, and the relative areas were calculated using the corrected solvent-band area (A s *) and adding A 1704 to it, in order to minimize the slight change of A 1022 with the concentration of the benzaldehyde (Figure 5).

Figure 5: (a) IR spectra of benzaldehyde at different concentrations. The solvent peak at 1022 cm−1 remains constant while the aldehyde peak at 1704 cm−1 increases with the concentration of benzaldehyde. (b) Calibration curve of the different molar concentrations of benzaldehyde is shown. Equation 1: [benzaldehyde] = −0.432 + 21.56 × A 1704 / (A 1022 + A 1704 ) and the R2 = 0.993. Figure 5: (a) IR spectra of benzaldehyde at different concentrations. The solvent peak at 1022 cm−1 remains c... Jump to Figure 5

Different flow rates were assayed to elucidate the effect of the reaction time. To synthesize imine 3a, equimolar amounts of benzaldehyde (1a) and aniline (2a) were mixed in ratio 1:1 (v/v) at different flow rates in the range 0.25–1.5 mL min−1. The reported spectra are focused in the region of the IR spectra where the conversion of aldehyde 1a to imine 3a can be followed (see Figure 6). Following the red spectra (synthesis of 3a with the shortest residence time) it can be seen that the imine band at 1627 cm−1 is more intense compared to the one in black (synthesis of 3a with the highest residence time). The observed conversion range found was between 94% and 97%. Under the studied conditions, very high conversions have been obtained with residence times as low as 20 seconds.

Figure 6: Comparison of the IR spectra of imine 3a, derived from benzaldehyde (1a) and aniline (2a), synthesized at different flow rates. The conversion of 3a at different flow rates was calculated using the equation of the calibration curve (see Figure 4), and for a flow rate of 0.25 mL min−1 was 97% and at a flow rate of 1.5 mL min−1, 94%. Figure 6: Comparison of the IR spectra of imine 3a, derived from benzaldehyde (1a) and aniline (2a), synthesi... Jump to Figure 6

Further imine syntheses in-flow were conducted with the 3D-printed millifluidic reactor R1 and monitored with the in-line ATR-IR (Table 3).

Table 3: Conversion of carbonyl compounds 1b and 1c with aniline (2a) into imines 3e and 3f. Entry Product Conversion (%) 1

3e 94 2

3f –

The results of these reactions are summarized in Table 3 where it can be seen that the reaction between aniline (2a) and R-(−)-myrtenal (1b) readily takes place to give imine 3e (Table 3, entry 1), whilst no product can be observed under these conditions for the reaction of 2a with 3-pentanone (1c), due to the lower reactivity of the latter. For details, see the IR spectrum of compound 3f in section 5 of Supporting Information File 1. 1H NMR spectra were used to calculate the conversion rate of aldehyde 1b into imine 3e.

Device 2: Imine reduction

To further prove the reliability of the 3D-printed devices as flow reactors, we decided to connect one reactor to the other and perform a two steps flow reaction in an automated way. To this end, we employed two R2 reactionware devices connected in series (Figure 7), to monitor the formation of the final product using the in-line ATR-IR flow cell. We ran the imine synthesis in the first of the two reactors (R2’), and once formed we subsequently reduced it in the second reactor (R2”). R2’ was connected to the syringe pumps containing the starting materials (compounds 1a and 2a–d) for the imine synthesis as previously described (but with a longer residence time than described above, to ensure a complete conversion of the substrates), before imines 3a–d were directly introduced to R2” for the subsequent reduction.

Figure 7: Representation of the setup for the two-step flow reaction employed in this work. The first reactor (R2’) is used to synthesize the imines under previously optimized conditions. The product is then directly introduced into the next reactor (R2”) and mixed with the reducing agent to produce the secondary amine. Figure 7: Representation of the setup for the two-step flow reaction employed in this work. The first reactor... Jump to Figure 7

The reduction of imines is a strategy to synthesize functionalized secondary amines [23,24], although only a few examples of reductions in microfluidic devices have been reported in the literature [5,23-25]. The condensation reactions were conducted using a 2 M solution of benzaldehyde (1a) in MeOH as before, which was pumped through inlet B’ into reactor R2’ at 0.0125 mL min−1 and mixed with a 2 M solution of the aniline derivatives 2a–d in MeOH introduced through inlet A’ at the same flow rate, keeping the aldehyde/amine ratio (1:1) (v/v) as described for the imine synthesis in R1. We selected this low flow rate to obtain a sufficient residence time (t R = 14 min) for a full conversion of 1a into imines 3a–d. Reactor R2’ was connected to the inlet A” of a second device (R2”) where the freshly formed imine was mixed with the reducing agent, cyanoborohydride (NaBH 3 CN) in MeOH (1 M), introduced through inlet B”, and the two equimolar solutions were pumped through R2” at the same flow rate. The molar and volumetric ratios hydride/imine were kept constant (1:1) to produce the corresponding amines with a residence time of 7 min. The reducing agent was selected because it is mild but effective, and it prevents the undesired formation of bubbles or problems related to over-reduction, which could be expected in this range of concentrations when using conventional reducing agents, such as NaBH 4 . Using this methodology, imines 3a–d were reduced affording the corresponding secondary amines 4a–d (Table 4).

Table 4: Table of the compounds used to study the imine reduction. Entry Product 4 Yield (%) 1

4a 78 2

4b 99 3

4c 96 4

4d 97

1H NMR spectroscopy and MS spectrometry confirmed the presence of the amines.

1H NMR spectra were used to calculate the conversion rate of the amines 4a–d.

The reactions were followed by monitoring the absence of the imine and aldehyde bands in the in-line ATR-IR flow cell, focusing the attention on the region of the IR spectrum between 1720 cm−1 and 1550 cm−1, where the disappearance of the imine band (around 1630 cm−1) can be observed. Figure 8 shows the spectra of imine 3b (red) and its corresponding reduced product, compound 4b (green) as an example; in the red spectrum a complete conversion of the aldehyde into imine 3b can be observed (due to the absence of the aldehyde peak at 1704 cm−1), and in the green spectrum the imine peak at 1632 cm−1 has completely disappeared.

Figure 8: Example of an ATR-IR graph in which an imine spectrum is compared with the reduced imine spectrum. Figure 8: Example of an ATR-IR graph in which an imine spectrum is compared with the reduced imine spectrum. Jump to Figure 8

In addition to the IR analysis, compounds 4a–d were collected and analyzed by mass spectrometry (MS), HPLC and 1H NMR spectroscopy. In all the studied cases, the analytical data confirmed full conversion of the substrates into the corresponding amines.