A green strategy for the synthesis of sulfone derivatives of p-methylaminophenol: Kinetic evaluation and antibacterial susceptibility

This is one of the few examples in which the diverse products have been synthesized just by changing the applied potential. The synthesis of sulfonyl derivatives of p-methylaminophenol were carried out by reaction of the electrogenerated p-methylquinoneimine with sulfinic acids. Various types of mono (MSP), bis (BSP) and tris (TSP) sulfonyl p-methyl aminophenols were obtained by changing the electrode potential, in one pot under green conditions. The mono sulfonyl-p-(methylamino)phenol derivatives (MSP) were assessed for their in vitro antibacterial activity against the gram positive (Staphylococcus aureus) and gram negative (Escherichia coli) strains. It was found that the tested compounds were more active against Staphylococcus aureus than Escherichia coli. We also found that the antimicrobial activity of MSP derivatives to vary in the order MSP4 (R = CH3) > MSP1 (R = p-tolyl) ≈ MSP2 (R = phenyl) > MSP3 (R = p-ClC6H4). Moreover, the observed homogeneous rate constants (k obs) of the reaction of p-methyl quinoneimine with sulfinic acids were estimated in various pH values, based on the EC and ECEC mechanisms, by comparing the simulated cyclic voltammograms with the experimental ones.

We assume that the compounds containing both aminophenol and diphenylsulfone moieties, may show promising biological activities. So, the synthesis of these type of compounds by a simple method without the above discussed disadvantages using a green and controllable strategy is the main object of this paper. Figure 1a shows the cyclic voltammogram (CV) of p-methylaminophenol (MAP) in aqueous phosphate buffer (c = 0.2 M, pH = 2.0). It includes a well-defined anodic (A 1 )/cathodic (C 1 ) peak couple at E 1/2 = 0.44 V versus Ag/ AgCl. These peaks (A 1 and C 1 ) are attributed to two-electron oxidation of MAP to p-methylquinoneimine (MQI) and vice versa, respectively. Under these conditions, I pC1 /I pA1 is near to one, which can be regarded as a criterion for the consistency of electrogenerated MQI under the experimental conditions. Figure 1b shows the CV of MAP in the presence of benzensulfinic acid (BS) under the same conditions as Fig. 1a. Comparison of CVs 1a and 1b shows three important differences: (i) the I pC1 /I pA1 is near to one in Fig. 1a, but is about 0.06 in Fig. 1b, (ii) contrary to Fig. 1b shows two couples of peaks (A 1 /C 1 and A 2 /C 2 ), (iii) E pA1 in Fig. 1b is less positive than that of Fig. 1a. In addition, our data show that two factors: (i) potential scan rate and (ii) BS concentration are effective in the peak current ratio (I pC1 /I pA1 ) of MAP in the presence of BS, so that it decreases with increasing BS concentration and decreasing the potential scan rate.

Results and Discussion
All of these results are in agreement with the participation of electrogenerated MQI in the Michael addition reaction with BS [28][29][30][31][32][33][34] . Fig. 1c is the cyclic voltammogram of BS which indicates that in the studied potential range, BS is not electroactive.
Controlled-potential coulometry at E app < E pA1 (E app = 0.40 V vs. Ag/AgCl) was used to determine the number of the electrons transferred (n) for the anodic oxidation of MAP in the presence of BS (Fig. 2I). The cyclic voltammetry during the progress of CPC indicates two significant changes: (i) decreasing I pA1 and (ii) increasing I pA2 . The I pA1 reaches zero when the charge consumption was about 2e − (n = 1.95) per molecule of MAP.
The CPC was also performed at E app > E pA2 (E app = 0.55 V vs. Ag/AgCl) (Fig. 2II). In comparison with CPC at E app = 0.40, it shows two significant differences: first, the number of electrons is increased from two to four electrons (n = 4.5). Second, both anodic peaks have been removed. Another CPC was performed in the same solvent/electrolyte system containing MAP (0.25 mmol) in the presence of methanesulfinic acid (MS) (0.75 mmol) at E app = 0.55 V vs. Ag/AgCl) (Fig. 3). Contrary to Fig. 2, during the progress of CPC, a new anodic peak (A 3 ) appeared at more positive potentials than the main wave. Under these conditions, all anodic and cathodic peaks disappear at the end of CPC after consumption of 151 C (n = 6.3) of electricity. The following evidences along with the spectroscopic data of the final products were used to propose the following mechanism for the electrochemical oxidation of MAP at different applied potentials in the presence of sulfone nucleophiles.
As indicated in Fig. 4, when the applied potential is 0.40 V vs. Ag/AgCl, the nucleophilic attack of the sulfone compounds on the electrogenerated MQI would result in INT1-4 which undergoes aromatization to afford the mono sulfonyl-p-(methylamino)phenol derivatives (MSP1-4) as the final products. MSP1-4 is structurally a p-methylaminophenol derivative, however, because of the presence of an electron-withdrawing sulfonyl group in its structure, its oxidation is more difficult than the oxidation of MAP (Fig. 5) and thus, its oxidation at the anode is avoided.
With increasing the applied potential to 0.55 V vs. Ag/AgCl, the oxidation of MSP1-4 becomes possible and MSP ox 1-4 is formed. The addition of sulfone nucleophile to MSP ox 1-4 followed by aromatization, converts MSP ox 1-4 into bis sulfonyl-p-(methylamino)phenol derivatives (BSP1-4). Since the oxidation of BSP1-4 is more difficult than the oxidation of MSP1-4 and MAP (Fig. 5), its oxidation was stopped due to the presence of the two electron-withdrawing sulfonyl groups as well as the insolubility of BSP1-4 in the electrolysis medium. A remarkable finding in this study is related to the role of methanesulfinic acid (MS) as a nucleophile. Despite several attempts to synthesize BSP4, no bis sulfonyl derivative has been isolated and unexpectedly a tris sulfonyl compound, TSP, which is a new sulphonamide molecule was obtained via the oxidation of BSP4 and attack of MS to electrogenerated BSP ox 4. The oxidation of BSP4 at 0.55 V vs. Ag/AgCl, can be related to the lower electron-withdrawing ability of the methyl-sulfony group compared with aryl-sulfony groups that causes the oxidation potentials of bis and tris sulfonyl compounds do not have any significant differences. In addition, higher nucleophilicity and lower steric effect of MS compared with aryl-sulfone nucleophiles and more solubility of the    MQI is a bis-Michael acceptor (ortho and meta of the phenolic group) and can be attacked by the sulfone nucleophiles from two sites to yield two isomers (for example, 4-(methylamino)-m-(methylsulfonyl)phenol and 4-(methylamino)-o-(methylsulfonyl) phenoltypes). However, the comparison of simulated 1 H NMR 35 for the possible compounds and experimental 1 H NMR of the final product (See SI), confirms synthesis of ortho derivative. The protonation of the amino group in MAP at pH = 2.0, makes the ortho position in MQI more reactive than meta position for the addition reaction.
The compounds MSP1-4 were tested for the antibacterial activity against the Gram positive (Staphylococcus aureus) (Fig. 6) and Gram negative (Escherichia coli) strains. The results indicated that Staphylococcus aureus was more sensitive to MSP1-4 than Escherichia coli (See SI). The existence of an exterior membrane along with a collection of resisting pumps against drugs in the negative gram bacteria, makes a very effective barrier from this group of bacteria against antibacterial growth and operation. We also found that the antimicrobial activity of MSP derivatives to vary in the order MSP 4 (R=CH 3 The observed homogeneous rate constants (k obs ) of the reaction of MQI with the sulfinic nucleophiles was studied based on ECEC (Fig. 7, parts I-IV) and EC (Fig. 7, parts V-VIII) mechanisms, by computer simulation of the experimental cyclic voltammograms. Our data shows that, k obs is strongly dependent to pH so that, it increases with decreasing pH value. The protonation of the nitrogen atom in MQI, makes it more reactive toward the addition reaction and is responsible for increasing k obs . In addition, the results displays that, k obs is also dependent directly to the electron donating ability of the nucleophile, so that, it varies in the order methanesulfinic acid > p-toluenesulfinic acid > banzenesulfinic acid > p-chlorosulfinic acid (Fig. 8).

Conclusions
In summary, the electrochemical synthesis of the title compounds has two advantages over conventional methods. Firstly, in this method, a variety of products could be formed just by changing the applied potential. In this work, the synthesis of some mono and bis (or tris) sulfonyl compounds were carried out by electrochemical oxidation of MAP in the presence of arylsulfinic acids only with changing the applied potential from 0.40 to 0.55 V vs. Ag/AgCl. Secondly, this method proceeds in a single step, in ambient conditions, without dealing with strong acids/base, organic solvent and catalysis, under green and mild conditions with high atom economy.

Materials and Methods
Apparatus and Reagents. Cyclic voltammetry, controlled-potential coulometry and preparative electrolysis were performed using a Zahner pp201 potentiostat/galvanostat. Macro-scale electrolysis and controlled-potential coulometry were carried out with a three electrode system, using a Behpajooh C 2056 potentiostat equipped with a digital coulometer. The working electrode used in the voltammetry experiments was a glassy carbon disc (1.8 mm 2 area) and platinum wire was used as counter electrode. The working electrode used in controlled-potential coulometry and synthesis was an assembly of three ordinary soft carbon plates (20 mm length, 10 mm width and 40 mm height), and large stainless steel cylinder (25 cm 2 area) constituted the counter electrode. The working electrode potentials were measured versus Ag/AgCl (all electrodes from AZAR electrode). The electrochemical synthesis was performed under controlled-potential condition in a simple cell equipped with a magnetic stirrer. More details are described in our previous paper 36 .
p-Methylaminophenol (MAP), p-toluenesulfinic acid, benzensulfinic acid, p-chlorosulfinic acid, methanesulfinic acid, phosphoric acid and ethanol were obtained from commercial sources. These chemicals were used without further purification. The glassy carbon electrode was polished using alumina slurry (from Iran Alumina Co.).    Antibacterial Susceptibility Assay: The samples were tested to determine the antibacterial susceptibility by Kirby-bauer disk diffusion method. Escherichia coli (E. coli) ATCC 35218 (gram negative) and Staphylococcus aureus (S.aureus) ATCC 6538 (gram positive) were used as test organisms. First, the colony of the bacterias of this study were cultured in the sterile Nutrient Broth and after 18-24 hours incubation at 37 °C microbial suspension was prepared from each bacteria balanced with turbusion 0.5 McFarland standard to 1.5 × 10 8 cfu.ml −1 with distillated water and was cultured on Muller Hinton agar (MHA) (Merck, KGaA) culture condition in the form of lawn culture. To solve the present samples, dimethyl sulfoxide 10% in distillated water was used. Then 5 mg from each sample was poured on sterile blank discs, and after setting discs in aseptic conditions on cultured Muller Hinton, bacterias were incubated 24 hours at 37 °C in the presence of studied derivatives. Afterwards, the diameter of the inhabitation zone surrounding of the discs were measured by special ruler. Also, to study the lack of influence of the solution containing 10% dimethyl sulfoxide on the bacterias of the study, 20 microlitre of the solution was poured on the discs and set on cultured Muller Hinton agar and incubated.

N-(4-Hydroxy-3,5-bis(methylsulfonyl)phenyl)-N-methylmethanesulfonamide
Scientific RepoRts | 7: 4436 | DOI:10.1038/s41598-017-04581-0 Digital Simulation. Digital simulation was performed using the DigiElch SB simulation software version 2.0 37 . The simulation was carried out assuming semi-infinite one-dimensional diffusion and planar electrode geometry. The experimental parameters entered for digital simulation consisted of the following: (i) the transfer coefficient (α) was assumed to be 0.5. (ii) The formal potentials were obtained experimentally as the midpoint potential between the anodic and cathodic peaks (E mid ). (iii) Analytical concentration of MAP, E start , E switch , E end and temperature = 25 °C. All these parameters were kept constant throughout the fitting of the digitally simulated voltammogram to the experimental data. Electrochemical oxidation of MAP was examined by digital simulation in the pH range 1-6. The simulation was performed based on the proposed EC and ECEC mechanisms (for simplifying, the proton transfers processes are not included). In these mechanisms, E is electrochemical oxidation of MAP to MAP OX , and C is the addition reaction that happens after the electrooxidation process. In these mechanisms, the peak current ratio (I pC /I pA1 ) is a criterion for the chemical reaction rate between MAP OX and sulfinic ion nucleophiles.