Electrochemical synthesis and antimicrobial evaluation of some N-phenyl α-amino acids

Abstract

In the present report, the authors describe a synthetic route for the generation of N-phenyl amino acid derivatives using CO₂ via a C–C coupling reaction in an undivided cell containing a combination of Mg–Pt electrodes. The reactions were completed in a short time without the formation of any other side product. The final products were purified via a simple recrystallization procedure. The structures of the newly prepared compounds were established using advanced spectroscopic techniques including ¹H, ¹³C NMR, IR, and ESI-MS. All the prepared derivatives show good-to-excellent activity when tested against bacterial and fungal strains. Interestingly, it was observed that the presence of polar groups (capable of forming H-bonds) such as –OH (4d) and –NO₂ (4e) at the para position of the phenyl ring show activity equivalent to the standard drugs.

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1. Introduction

Amino acids are a type of biomolecule that contains an amino group, a carboxylate group, and a side chain.¹ Although hundreds of different amino acid side chains have been described and synthesized in the literature, only 20 amino acids have been identified as common building blocks of proteins.² In addition to their role as protein building blocks, amino acids are used for a variety of other applications in organic chemistry.¹ Because of their inherent chirality, they can act as ligands,³ organocatalysts^4,5 and as structural linkers in agrochemicals⁶ and pharmaceuticals.⁷

Furthermore, these 20 important amino acids have been connected to a number of substrates to achieve potent biological activities, such as antibacterial,⁸ antifungal,⁹ anticancer¹⁰ inhibition of Type B monoamine oxidase,¹¹ and anti-fibrotic¹² activities. It has also been observed that N-substituted amino acids are more potent for a number of activities, such as PPAR γ-agonist,¹³ hyperalphalipoproteinaemic,¹⁴ anti-inflammatory,⁹ anti-phlogistic,¹⁰ anti-hypertensive,¹⁵ anti-oxidant,¹⁶ and anti-phlogistic activity.¹⁷

To date, a number of methods have been proposed for the formulation of amino acids, including simple refluxing,¹⁸ visible light irradiation,¹⁹ microwave irradiation,²⁰ and ultraviolet irradiation,²¹ but these methods all have limitations. In continuation of our work on electro-carboxylation,^22–24 in the present report, we introduced a synthetic route for the production of N-substituted amino acids via an environmentally friendly electrochemical C–C coupling method using carbon dioxide.

2. Results and discussion

2.1. Optimization of reaction conditions

2.1.1. Investigating the effect of the concentration of the substrate and electrode materials. Under optimized conditions, the effect of sacrificial anodes, such as Al, Ni, and Mg, was critically studied using an electrolyzed mixture of compound 3a (0.54 mmol), MeCN (100 mL), TPAC (5 mmol), and CO₂ at 20 °C with Pt as the cathode (Table 1). The use of Mg as a sacrificial anode gave the final product 4a in a high yield of 92% (Table 1, entry 9), while maximum yields of 64% and 72% were obtained using Ni and Al electrodes, respectively (Table 1, entries 1 and 5). In the present research, it was concluded that the use of Pt as an inert cathode with Mg as an anode was the only suitable combination to carry on with exploring other conditions.

Table 1 Effect of SRP of concentration on the electrocarboxylation and sacrificial electrodes 4a

Entry	Sacrificial anode	Conc. (mmol L^−1)	SRP (volts)	Yield (%)
1	Ni	0.64	−0.19	64
2	Ni	1.12	−0.19	53
3	Ni	1.53	−0.19	41
4	Ni	2.15	−0.19	30
5	Al	0.59	−1.62	72
6	Al	1.05	−1.62	63
7	Al	1.61	−1.62	59
8	Al	2.15	−1.62	47
9	Mg	0.54	−2.36	92
10	Mg	1.05	−2.36	89
11	Mg	1.59	−2.36	77
12	Mg	2.15	−2.36	65

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2.1.2. Investigation of the relationship of current density with temperature variation and the pressure of carbon dioxide. Other characteristics, such as the CO₂ pressure, temperature, and current density, were studied in order to optimize the reaction conditions. In electrocarboxylation, the current density is a critical factor (Table 2). The experiment was carried out at three densities i.e., 10, 15 and 20 mA cm⁻²; among them, the current densities of 10 and 20 mA cm⁻² gave lower yields of the tested compound 4a; however, at 15 mA cm⁻², a better yield of the same compound was obtained. Determination of the most suitable temperature in combination with an appropriate CO₂ pressure was the next required step. After testing the reaction at temperatures ranging from 0 to 25 °C, it was observed that the yield of the formed product was lower in the low-temperature range and at a current density of 10 mA cm⁻² (Table 2, entry 1), while a better yield was obtained at a slightly high temperature (25 °C) and a current density of 20 mA cm⁻² (Table 2, entry 17). Thus, a current density of 15 mA cm⁻², temperature of 20–25 °C, and CO₂ pressure of 1 atm were the ideal conditions (Table 2, entries 11 and 12) to obtain a higher yield of the product, i.e., 92% (Table 2, entry 12).

Table 2 Standardization of current density and temperature (°C) for the synthesis of 4a

Entry	Current density (mA cm^−2)	Temperature (°C)	Yield^a (%)
a Yield refers to combined yield from all the crops.
1	10	0	61
2	10	5	65
3	10	10	69
4	10	15	75
5	10	20	79
6	10	25	75
7	15	0	63
8	15	5	74
9	15	10	79
10	15	15	80
11	15	20	90
12	15	25	92
13	20	0	65
14	20	5	69
15	20	10	73
16	20	15	75
17	20	20	81
18	20	25	79

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2.1.3. Effect of solvents and supporting electrolyte. The results for different supporting solutes (TPAC, TPAB, and TBABF₄) with different solvents (MeCN, n-butanol, and n-pentanol) on the production of the reference compound are summarized in Table 3. Out of the three solvents, MeCN with TPAC as the supporting electrolyte was found to be the best combination, producing 4a in 92% yield. However, the other two solvents were difficult to remove; it was assumed that the yield of the product was also reduced while dissipating those solvents.

Table 3 Standardization of solvent and supporting electrolyte for the synthesis of 4a

Entry	Solvent	Supporting electrolyte	Yield^a (%)
a Yield refers to combined yield from all the crops.
1	MeCN	TBABF4	89
2	n-Butanol	TBABF4	83
3	n-Pentanol	TBABF4	82
4	MeCN	TPAC	92
5	n-Butanol	TPAC	84
6	n-Pentanol	TPAC	79
7	MeCN	TPAB	81
8	n-Butanol	TPAB	75
9	n-Pentanol	TPAB	72

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Various substitutions on the aldehyde moiety in the imine derivatives were reacted with CO₂ under similar reaction conditions for the generalization of the reaction, and it was discovered that the reactions proceeded smoothly and the desired molecules were collected in high yield as well as high purity. Finally, all the collected synthesized compounds were purified by simple recrystallization in ethanol.

2.2. Chemistry

Primary analysis of the products was conducted by comparing their melting points (MP), and the spectra later assisted in the illustration of the synthesized compounds. The IR spectra of compound 3a in series 3 shows three considerable absorptions at 3149, 3029, and 1597 cm⁻¹ corresponding to the C–H, Ar–H, and C=N groups. In the ¹H NMR spectrum (500 MHz, DMSO-d₆), the deshielded signal at δ 8.44 (s) is assigned to the proton of C-1 and the multiplet peak at δ 7.44–7.13 (m) is assigned to the aromatic protons. Furthermore, the presence of [M+1] and [M+2] peaks at 216 and 217 m/z, respectively, in the mass spectrum validates the formation of the expected compound.

The synthesis of amino acid derivative 4a was confirmed by the downfield shift in the signal of the aromatic protons from δ 7.25 to δ 7.60, and also the singlet peak for the proton of N–H at δ 9.59 ppm. ¹³C-NMR exhibits signals at δ 180.5 for the carboxylic group, with other peaks δ 129.5, 129.2, 128.9, 120.8, 113.5, and 64.3 confirming the formation of the targeted compound. In the IR spectrum, an additional broad peak at 3356 cm⁻¹ demonstrates the –OH of the carboxylic group and a peak at 2873 cm⁻¹ validates the C–H group. Furthermore, ESI-MS fragmentation generated [M+1] and [M+2] peaks at 262 and 263 m/z respectively, confirming the formation of the desired amino acid derivative.

2.3. Plausible mechanism for the synthesis of amino acid derivatives

Several reports on electrocarboxylation of unsaturated organic imines are available in which an undivided cell with magnesium as a stable sacrificial anode and platinum or silver as a cathode gives a much higher yield and carboxylation selectivity.^25,26 When the carboxylate ions are reduced and dissociated, a reactive radical is formed. These dissociated carboxylate ions react with the anode immediately to form Mg(OH)₂. The reactive intermediate II is then reduced again to form an intermediate III anion. The nucleophile III later reacts with CO₂ to produce carboxylate anion IV (Fig. 1). Finally, during the work-up, intermediate IV absorbs protons from the solution to produce the final compound.

Fig. 1 Plausible reaction mechanism for the synthesis of amino acids.

2.4. Antimicrobial activity

The anti-microbial potencies of the synthesized compounds 4a–l were investigated using the Minimum Inhibitory Concentration (MIC) method. The findings were compared to the reference drugs fluconazole and amoxicillin in their respective areas at 4 g mL⁻¹ and 2 g mL⁻¹, respectively. Table 4 shows that series 4a–l has good-to-great activity against the preferred strain. Only five amino acids (4a, 4d, 4e, 4f, and 4g) were found to be effective against various bacterial (B. subtilis, E. coli, S. aureus S. pyogenes, and K. pneumonia) and fungal strains (A. janus and A. niger). Further, it was observed that the presence of polar groups that are capable of forming H-bonds, such as –OH (4d) and –NO₂ (4e) at the para position of the phenyl ring show maximum resistance against the microbes, which is equivalent to the standard drugs at MIC 4 μg mL⁻¹.

Table 4 Minimum inhibitory concentration (MIC in μg mL⁻¹) of synthesized amino acids derivatives 4a–l against various microbial agents

Compound	Gram (+ve) bacteria		Gram (−ve) bacteria			Fungi
	B. subtilis	S. pyogenes	E. coli	K. pneumonia	S. aureus	A. janus	A. niger	A. sclerotiorum
4a	16	8	8	8	16	8	16	8
4b	16	16	8	16	16	16	8	16
4c	16	32	32	16	32	16	8	32
4d	8	4	8	4	8	16	8	16
4e	4	4	4	8	4	4	4	8
4f	8	8	16	8	16	16	16	8
4g	16	16	8	8	16	8	16	—
4h	64	16	32	16	32	32	32	16
4i	16	32	32	32	16	32	32	32
4j	32	16	8	—	16	32	16	16
4k	8	16	16	32	—	16	16	32
4l	8	8	8	16	8	16	32	16
Amoxicillin	4	4	4	4	4	—	—	—
Fluconazole	—	—	—	—	—	2	2	2

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3. Materials and methods

The commercial supplier Sigma Aldrich provided all of the compounds utilized in the current methods, which were all used without any further purification. 4A molecular sieves were used to preserve the commercially available solvent CH₃CN (Merck) overnight. It was collected for further use in the reaction after distillation at a temperature of 80–82 °C. The distillate was placed in P₂O₅ for about 24 h before being diluted once to produce dry and pure CH₃CN. Acetonitrile was stored in a dark-coloured and airtight bottles. All of the aqueous solutions were created using double-distilled water. We bought all of the solvents of analytical grade from Loba-Chemie. The melting points of all the developed heterocyclic compounds were measured using the open capillary method and a digital melting point instrument. Recording of IR spectra was conducted using a PerkinElmer Spectrum II with a diamond ATR. Using a Bruker Advanced NMR spectrometer, CDCl₃ as the solvent and TMS as the reference, ¹H and ¹³C NMR data were both collected at 500 MHz. The mass spectra of the compounds was recorded on the LC-MS Spectrometer Model Q-ToF-Micromass, Waters. The purity of compounds was determined using the Thin Layer Chromatography (TLC) method and UV light.

3.1. Electrochemical instrumentation involved in setup

3.1.1. Power source. Direct current (DC) was used to power the electrocarboxylation process, and the electrophoresis power supply (Toshniwal) was equipped with a voltmeter that reads from 0–300 V and an ammeter that can read 0–100 mA.

3.1.2. Undivided cell. An undivided three-necked electrochemical chamber made of Pyrex glass was utilized for the electrocarboxylation procedure. The cathode and anode electrodes were both submerged through two different openings in the proposed cell, and CO₂ gas was continuously passed through this third hole throughout the reaction.

As an inert cathode and sacrificial anode electrode, platinum gauze and magnesium electrodes with dimensions of 1 cm × 1 cm × 0.1 cm and 1 cm diameter and 5 cm length, respectively, were employed. Using a DC power supply, the cathode and anode were eventually connected to the positive and negative ends of the electric circuit, respectively.

4. Experimental

4.1. Synthesis of (E)-N,1-diphenylmethanimine (3a)

Imine derivative 3a was prepared by the reaction of 4-chlorobenzaldehyde (1.40 g, 10 mmol) with aniline (0.91 mL, 10 mmol) at 85 °C in ethanol for 2 min using a microwave reactor. The progress of the reaction was observed via thin layer chromatography (TLC) and finally worked up in chilled water. The recrystallization was conducted with ethanol to yield colorless crystalline product 3a (Scheme 1). Similarly, other imine derivatives have been synthesized using aldehydes 1b–l with aniline 2 using the same procedure to obtain 3b–l in excellent yield (Table 5).

Scheme 1 Synthesis of imine derivatives 3a–l.

Table 5 Derivatives 3a–l with observed results

Entry	Product	R	Rf	Yield^a (%)	Melting point (°C)	Literature melting point (°C)
a Yield refers to total mass of collection from different crops.
1	3a	4-Cl C6H4	0.61	98	62	62–64 (ref. 25)
2	3b	C6H5	0.66	98	55	54 (ref. 25)
3	3c	4-Me C6H4	0.63	96	39–41	38–40 (ref. 25)
4	3d	4-OH C6H4	0.69	95	96–97	94–96 (ref. 25)
5	3e	4-NO2 C6H4	0.71	97	90–93	90–92 (ref. 25)
6	3f	3-NO2 C6H4	0.68	98	64–65	65–66 (ref. 25)
7	3g	4-OMe C6H4	0.62	93	64–66	63–65 (ref. 25)
8	3h	4-Br C6H4	0.63	95	75–77	76–77 (ref. 25)
9	3i	C6Hs CH=CH	0.65	89	103–105	106–108 (ref. 25)
10	3j	2-Furyl	0.67	91	56–58	55–57 (ref. 25)
11	3k	2-Thiophenyl	0.63	92	61–63	—
12	3l	2-Pyridyl	0.59	90	65–66	—

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3a: yield 98%, colourless solid, mp 62 °C. IR spectrum, ν, cm⁻¹: 3149 (sp² C–H), 3029 (Ar–H), 1597 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.44 (s, 1H, CH), 7.13–7.44 (m, 9H, Ar–H). Mass spectrum, m/z (I_rel, %): 216 (M+1), 217 (M+2).

3b: yield 98%, colourless solid, mp 55 °C. IR spectrum, ν, cm⁻¹: 3060 (sp², C–H), 3028 (Ar–H), 1590 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.13 (s, 1H, CH), 7.10–7.35 (m, 10H, Ar–H). Mass spectrum, m/z (I_rel, %): 182 (M+1).

3c: yield 96%, colourless solid, mp 39–41 °C. IR spectrum, ν, cm⁻¹: 3140 (sp² C–H), 3030 (Ar–H), 1586 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.28 (s, 1H, CH), 7.06–7.26 (m, 9H, Ar–H), 2.14 (s, 3H, CH₃). Mass spectrum, m/z (I_rel, %): 196 (M+1).

3d: yield 95%, colourless solid, mp 96–97 °C. IR spectrum, ν, cm⁻¹: 3315 (O–H), 3142 (sp² C–H), 3010 (Ar–H), 1584 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 9.1 (s, 1H, OH), 8.28 (s, 1H, CH), 7.09–7.27 (m, 9H, Ar–H). Mass spectrum, m/z (I_rel, %): 198 (M+1).

3e: yield 97%, pale yellow solid, mp 90–93 °C. IR spectrum, ν, cm⁻¹: 3170 (sp² C–H), 3039 (Ar–H), 1610 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.59 (s, 1H, CH), 7.23–7.84 (m, 9H, Ar–H). Mass spectrum, m/z (I_rel, %): 227 (M+1).

3f: yield 98%, pale yellow solid, mp 64–65 °C. IR spectrum, ν, cm⁻¹: 3152 (sp² C–H), 3033 (Ar–H), 1608 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.52 (s, 1H, CH), 7.22–7.79 (m, 9H, Ar–H). Mass spectrum, m/z (I_rel, %): 227 (M+1).

3g: yield 93%, colourless solid, mp 64–66 °C. IR spectrum, ν, cm⁻¹: 3122 (sp² C–H), 3019 (Ar–H), 1582 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.24 (s, 1H, CH), 7.09–7.23 (m, 9H, Ar–H), 3.92 (s, 3H, OCH₃). Mass spectrum, m/z (I_rel, %): 212 (M+1).

3h: yield 95%, colourless solid, mp 75–77 °C. IR spectrum, ν, cm⁻¹: 3148 (sp² C–H), 3024 (Ar–H), 1599 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.27 (s, 1H, CH), 7.15–7.42 (m, 9H, Ar–H). Mass spectrum, m/z (I_rel, %): 261 (M+1), 262 (M+2).

3i: yield 89%, yellow solid, mp 103–105 °C. IR spectrum, ν, cm⁻¹: 3149 (sp² C–H), 3015 (Ar–H), 1581 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.27 (s, 1H, CH), 6.89 (s, 1H, =CH), 7.27 (s, 1H, =CH), 7.11–7.34 (m, 10H, Ar–H). Mass spectrum, m/z (I_rel, %): 208 (M+1).

3j: yield 91%, yellow solid, mp 56–58 °C. IR spectrum, ν, cm⁻¹: 3144 (sp² C–H), 3033 (Ar–H), 1601 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.43 (s, 1H, CH), 7.17–7.45 (m, 8H, Ar–H). Mass spectrum, m/z (I_rel, %): 172 (M+1).

3k: yield 92%, yellow-brown solid, mp 61–63 °C. IR spectrum, ν, cm⁻¹: 3140 (sp² C–H), 3025 (Ar–H), 1599 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.47 (s, 1H, CH), 7.16–7.48 (m, 8H, Ar–H). Mass spectrum, m/z (I_rel, %): 188 (M+1).

3l: yield 90%, brown solid, mp 65–66 °C. IR spectrum, ν, cm⁻¹: 3132 (sp² C–H), 3029 (Ar–H), 1597 (C=N). ¹H NMR spectrum, δ, ppm (J, Hz): 8.46 (s, 1H, CH), 7.14–7.51 (m, 9H, Ar–H). Mass spectrum, m/z (I_rel, %): 183 (M+1).

4.2. Synthesis of anilino(phenyl)acetic acid (4a)

To obtain amino acid derivative 4a, an undivided electrochemical cell consisting of Mg as a sacrificial anode and Pt as an inert cathode was used, which was cleaned with diluted HNO₃, rinsed with distilled water and dried in an oven. Then, 0.54 mmol of imine derivative 3a was added to a 100 mL quantity of CH₃CN containing 5 mmol of TPAC as a supporting electrolyte. In the next step, the designed electrolytic solution mixture was electrolyzed at 25 °C by maintaining a constant current density of 15 mA cm⁻². A continuous flow of CO₂ gas was also passed into the solution to maintain the required pressure (1 atm) over a 10 hours period to obtain compound 4a.

Following this, the excess solvent was reduced under low pressure, while the solid residue was retained. Furthermore, to eliminate ionic residues from the solid, the extraction was carried out in a separating funnel with diethyl ether, and the product was left to dry using anhydrous MgSO₄. Finally, compound 4a was obtained by recrystallizing the isolated crude product with ethanol (Scheme 2). Similarly, the other imine derivatives 3b–l were converted into amino acid derivatives 4b–l using the same procedure (Table 6).

Scheme 2 Synthesis of N-substituted amino acids 4a–l.

Table 6 Derivatives 4a–l with observed results

Entry	Product	R1	Rf	Yield^a (%)	Melting point (°C)
a Yield refers to total mass of collection from different crops.
1	4a	4-Cl C6H4	0.66	92	196–197
2	4b	C6H5	0.69	91	183–185
3	4c	4-Me C6H4	0.61	86	174–175
4	4d	4-OH C6H4	0.72	84	216–217
5	4e	4-NO2 C6H4	0.70	91	210–212
6	4f	3-NO2 C6H4	0.64	89	191–192
7	4g	4-OMe C6H4	0.66	86	206–208
8	4h	4-Br C6H4	0.69	85	178–180
9	4i	C6Hs CH = CH	0.73	82	225–226
10	4j	2-Furyl	0.66	87	199–201
11	4k	2-Thiopehyl	0.67	86	188–190
12	4l	2-Pyridyl	0.63	83	178–181

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4a: yield 92%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3356 (OH), 2970 (Ar–H), 2873 (C–H), 1619 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.85 (s, 1H, OH), 9.59 (s, 1H, NH), 8.54 (s, 1H, CH), 7.14–7.80 (m, 9H, ArH). ¹³C NMR spectrum, δ, ppm: 180.5, 145.9, 135.0, 133.1, 129.5, 129.2, 128.9, 120.8, 113.5, 64.3. Mass spectrum, m/z (I_rel, %): 262 (M+1), 263 (M+2).

4b: yield 91%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3455 (OH), 3052 (Ar–H), 2825 (C–H), 1628 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.79 (s, 1H, OH), 9.62 (s, 1H, NH), 8.23 (s, 1H, CH), 7.22–7.53 (m, 10H, ArH). ¹³C NMR spectrum, δ, ppm: 188.3, 145.9, 136.9, 129.7, 129.5, 129.1, 127.5, 120.8, 113.5, 64.3. Mass spectrum, m/z (I_rel, %): 228 (M+1).

4c: yield 86%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3440 (OH), 3031 (Ar–H), 2978 (C–H), 1610 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.83 (s, 1H, OH), 9.51 (s, 1H, NH), 8.37 (s, 1H, CH), 7.14–7.739 (m, 9H, ArH), 2.23 (s, 3H, CH₃). ¹³C NMR spectrum, δ, ppm: 180.5, 155.9, 147.2, 143.9, 139.6, 139.5, 139.4, 130.8, 123.5, 74.3, 31.3. Mass spectrum, m/z (I_rel, %): 242 (M+1).

4d: yield 84%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3428 (OH), 3031 (Ar–H), 2978 (C–H), 1625 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.87 (s, 1H, OH), 9.55 (s, 1H, NH), 8.42 (s, 1H, CH), 7.21–7.43 (m, 9H, ArH). ¹³C NMR spectrum, δ, ppm: 180.5, 157.3, 145.9, 129.5, 128.9, 120.8, 116.3, 113.5, 64.3. Mass spectrum, m/z (I_rel, %): 244 (M+1).

4e: yield 91%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3478 (OH), 3057 (Ar–H), 3020 (C–H), 1612 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 13.15 (s, 1H, OH), 9.87 (s, 1H, NH), 8.69 (s, 1H, CH), 7.38–8.03 (m, 9H, ArH). ¹³C NMR spectrum, δ, ppm: 180.5, 146.7, 145.9, 143.0, 129.5, 128.9, 127.9, 120.8, 113.5, 64.3. Mass spectrum, m/z (I_rel, %): 273 (M+1).

4f: yield 89%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3470 (OH), 3053 (Ar–H), 3017 (C–H), 1628 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 13.03 (s, 1H, OH), 9.83 (s, 1H, NH), 8.63 (s, 1H, CH), 7.37–7.99 (m, 9H, ArH). ¹³C NMR spectrum, δ, ppm: 180.5, 148.3, 145.9, 136.8, 135.8, 130.0, 129.5, 123.5, 122.7, 120.8, 113.5, 63.3. Mass spectrum, m/z (I_rel, %): 273 (M+1).

4g: yield 86%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3421 (OH), 3039 (Ar–H), 2983 (C–H), 1605 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.80 (s, 1H, OH), 9.51 (s, 1H, NH), 8.37 (s, 1H, CH), 7.22–7.53 (m, 9H, ArH), 3.98 (s, 3H, OMe). ¹³C NMR spectrum, δ, ppm: 180.5, 159.4, 145.9, 129.5, 129.2, 128.5, 120.8, 114.7, 113.5, 64.3, 55.8. Mass spectrum, m/z (I_rel, %): 258 (M+1).

4h: yield 85%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3446 (OH), 3037 (Ar–H), 2989 (C–H), 1622 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.89 (s, 1H, OH), 9.62 (s, 1H, NH), 8.42 (s, 1H, CH), 7.22–7.57 (m, 9H, ArH), 5.34 (d, 2H). ¹³C NMR spectrum, δ, ppm: 180.5, 145.9, 135.9, 132.0, 131.9, 129.5, 121.9, 120.8, 113.5, 64.3. Mass spectrum, m/z (I_rel, %): 307 (M+1), 308 (M+2).

4i: yield 82%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3442 (OH), 3050 (Ar–H), 2987 (C–H), 1616 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.92 (s, 1H, OH), 9.63 (s, 1H, NH), 8.61 (s, 1H, =CH), 7.48 (s, 1H, CH), 7.23–7.58 (m, 10H, ArH), 6.91 (s, 1H, =CH). ¹³C NMR spectrum, δ, ppm: 184.1, 147.6, 136.4, 129.5, 128.6, 128.5, 127.9, 123.3, 120.8, 113.5, 72.3. Mass spectrum, m/z (I_rel, %): 254 (M+1).

4j: yield 87%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3434 (OH), 3049 (Ar–H), 2997 (C–H), 1633 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.95 (s, 1H, OH), 9.57 (s, 1H, NH), 8.62 (s, 1H, CH), 7.28–7.68 (m, 8H, ArH). ¹³C NMR spectrum, δ, ppm: 178.5, 145.9, 142.8, 139.3, 129.5, 120.8, 118.6, 113.5, 110.7, 60.0. Mass spectrum, m/z (I_rel, %): 218 (M+1).

4k: yield 86%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3439 (OH), 3047 (Ar–H), 2998 (C–H), 1628 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.87 (s, 1H, OH), 9.58 (s, 1H, NH), 8.61 (s, 1H, CH), 7.23–7.62 (m, 8H, ArH). ¹³C NMR spectrum, δ, ppm: 178.5, 145.9, 137.5, 129.5, 128.1, 126.1, 121.3, 120.8, 113.5, 65.5. Mass spectrum, m/z (I_rel, %): 234 (M+1).

4l: yield 83%, colourless solid, mp 183–185 °C. IR spectrum, ν, cm⁻¹: 3445 (OH), 3055 (Ar–H), 3002 (C–H), 1615 (C=O). ¹H NMR spectrum, δ, ppm (J, Hz): 12.93 (s, 1H, OH), 9.65 (s, 1H, NH), 8.63 (s, 1H, CH), 7.34–7.79 (m, 9H, ArH). ¹³C NMR spectrum, δ, ppm: 178.5, 155.4, 148.6, 145.9, 136.2, 129.5, 121.9, 120.9, 120.8, 113.5, 72.9. Mass spectrum, m/z (I_rel, %): 229 (M+1).

4.3. Anti-microbial evaluation

The newly synthesized compounds 4a–l were tested against three Gram −ve (Escherichia coli MTCC 443, Klebsiella pneumonia MTCC 3384, and Staphylococcus aureus MTCC 96), two Gram +ve (Bacillus subtilis MTCC 441, and Streptococcus pyogenes MTCC 442) and Fungus (Aspergillus janus MTCC 2751, Aspergillus niger MTCC 281, and Aspergillus sclerotiorum MTCC 1008) samples. Nutrient broth was used to store the bacteria samples after they had been cultured at 37 °C for 24 hours. The fungal strains, on the other hand, were grown in malt extract for 72 hours at 28 °C before inoculation. A serial dilution procedure was used to test each produced chemical in triplicate after it was dissolved in DMSO at doses of 128, 64, 32, 16, 8, 4, and 2 g mL⁻¹.

5. Conclusion

In this work, the authors synthesized 12 potent N-phenyl amino acid derivatives from imines and CO₂ via direct C–C coupling reaction in an undivided cell containing a combination of Mg–Pt electrodes. The products are obtained in a single step with adaptability and diversity in excellent yield and superior purity. This procedure is efficient in terms of labor, cost, and waste production, as well as the absence of harsh reaction conditions. All the synthesized amino acid derivatives show good-to-excellent resistance against the microbes. However, those with polar groups like –OH and –NO₂ at the para position on the phenyl ring show equivalent resistance compared to the standard drugs amoxicillin and fluconazole.

Abbreviations

TPAC	Tetrapropylammonium chloride
TPAB	Tetrapropylammonium bromide
TBABF4	Tetrapropylammonium tetrafluoroborate
MIC	Minimum inhibitory concentration
SRP	Standard reduction potential
ATR	Attenuated total reflectance
TLC	Thin layer chromatography

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

The authors would like to thank the Researchers Supporting Project Number (RSPD2023R729), King Saud University, Riyadh, Saudi Arabia. The authors are also grateful to UGC-BSR (New Delhi) Research Fellowship Scheme 2014–2015 (No. 15510), Punjabi University, Patiala, and Chandigarh University, Gharuan for providing financial assistance for research.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra03592a

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