Preparation of New α-Aminophosphonate Derivatives by Kabachnik-Fields Reaction Using a Recyclable Catalyst

A convenient and efficient synthetic method for the preparation of some new α-aminophosphonate derivatives via a one-pot threecomponent system has been achieved using Amberlite IRC-748 as a recyclable catalyst. This method not only provides an excellent complement for the synthesis of α-aminophosphonates but also avoids the use of hazardous acids or expensive/toxic Lewis acids and harsh reaction conditions. Most of the synthesized compounds (4a–o) exhibited activity against bacteria/fungi strains and moderate DPPH radical scavenging activity.


Results and Discussion
In the initial experiments, the one-pot, three-component reaction of aniline, benzaldehyde, and diethyl phosphite was chosen as the model reaction to optimize the reaction conditions. In the present work, the procedures followed for the synthesis of -aminophosphonates are conventional reflux in toluene, in the presence of catalyst (Amberlite IRC-748) and microwave irradiation (solvent-free). The data obtained are shown in Table 1, entries 21-23. A comparison of the catalysts used in the Kabachnik-Field reaction for the synthesis of 4 is listed in Table 1, serial numbers 1-20. The products -aminophosphonates were obtained by solvent-free microwave irradiation of aldehyde, amine, and diethyl phosphite for 1 min. In toluene, without any catalyst, the product formed was in a good yield, but the time taken was 4 to 5 h, which is considerably longer. Therefore, the reaction time has been reduced to 30 min by using Amberlite IRC-748 a recyclable catalyst. This catalyst is mildly acidic with iminodiacetic acid functional group. Amberlite IRC-748 acts as an efficient and recyclable acidic promoter which yields good results when compared to the catalysts reported earlier ( Table 1). The reaction mechanism proceeds as in case of acid catalysts. In optimization of reaction time, the yield of the product did not increase, when more than 5 mg of catalyst was used. This suggested the use of 5 mg of Amberlite catalyst for 0.005 mol of reactants. Thin layer chromatography (TLC) was employed to monitor reaction progress and to determine the purity of the products.
New -aminophosphonic acid esters (4a-o) were synthesized by a one-pot reaction using equimolar quantities of different substituted aromatic amines and aldehydes with diethylphosphite (Scheme 2). The reaction was carried out using catalytic amount of Amberlite IRC-748, in toluene for 30 min. All the title compounds are readily soluble in polar organic solvents.
The IR spectra of compounds (4a-o) showed the NH band in the range of 3338-3438 cm −1 . The sharp band observed in the range 1240-1291 cm −1 is due to the ] P=O , and a band for P-C stretching occurred in the range 740-770 cm −1 . All the stretching frequencies are compiled in Table 2. The 1 H NMR spectra of the compounds (4a-o) were recorded in the DMSO-d 6 solvent. The aromatic protons of -aminophosphonic acid esters appeared as a multiplet in the region 6.15-8.69. The P-C-H group proton resonated as a multiplet in the range 3.77-4.86 due to coupling with phosphorus and N-H. The N-H proton signal appeared at 4.58-5.90 as a multiplet. The protons of P-O-CH 2 -C that appeared as a quartet at 3.56-3.62 and P-O-C-CH 3 gave a triplet at 1.12-1. 19. The compounds were analyzed by mass spectroscopy, the M + 1 peak confirmed product formation, and compounds containing one chlorine atom showed molecular ion peaks in a 3 : 1 ratio.
Antibacterial activity was carried out by the well diffusion method using nutrient agar medium, DMSO as control, and chloramphenicol as a standard bactericide. The antifungal activity was carried out by well diffusion method using potato dextrose agar (PDA) medium, DMSO as control, and fluconazole as a standard fungicide [51][52][53][54]. The antioxidant activity of the synthesized derivatives was evaluated using the DPPH (diphenyl picrylhydrazyl) radical scavenging assay by standard methods [55].

Antioxidant
Activity. The novel compounds were checked for the free radical scavenging activity by the DPPH method, and the data are listed in Table 4. The graphical representation of the DPPH activity, indicated in Figure 1, showed that most of the compounds are good antioxidants with more than 50% scavenging activity. Among them, the compounds 4c, 4e, 4h, and 4j showed higher activity than the standard used. This may be attributed to the presence of substitutions like -NO 2 and -OH groups in the compounds synthesized.

Experimental Procedure.
All the reagents and solvents were used as received from commercial suppliers, unless otherwise stated. All chemicals used for the synthesis were of analytical grade or laboratory grade and purchased from HiMedia Laboratories Pvt. Ltd., Sigma Chemical Co., USA,  E. Merck, Germany, and Sarabhai Merck Company, India, and specialty chemicals are procured as samples from the commercial suppliers in India. Mass spectra of the synthesized compounds were recorded on Agilent 6320 Ion Trap mass spectrometer. IR spectra were recorded on a Shimadzu IR-470 spectrometer. 1 H NMR spectra were recorded on a Bruker DRX-300 Avance spectrometer (300 MHz).

Synthetic Procedure-a.
A mixture of benzaldehyde (0.005 mol), aniline (0.005 mol) and diethylphosphite (0.005 mol) in dry toluene was stirred for 10 min at room temperature. Then the temperature was raised to reflux for 5 h. The reaction was monitored by TLC. After completion of the reaction, toluene was removed by distillation and the residue was purified using column chromatography (6 : 4, ethyl acetate: hexane).

Synthetic Procedure b.
A mixture of benzaldehyde (0.005 mol), aniline (0.005 mol), diethylphosphite (0.005 mol), and 5 mg of Amberlite IRC-748 in dry toluene was stirred for 10 min at room temperature. Then it was refluxed for 30 min. The reaction was monitored by TLC. After completion of the reaction, the mixture was filtered to separate the solid catalyst. The filtrate was distilled to remove toluene, and the residue obtained was purified using column chromatography (6 : 4, ethyl acetate: hexane).

Experimental
Procedure for Antioxidant Activity. The antioxidant activity of the synthesized derivatives was evaluated using the DPPH free radical scavenging assay. 200 L of test sample solution (100 g/mL) was added to 4 mL of 100 M methanolic DPPH. The mixture was incubated for 20 minutes at room temperature, and the absorbance at 517 nm was measured. BHT was used as standard. A blank was prepared without adding standard or test compound. Lowering the absorbance of the reaction mixture indicates higher free radical scavenging activity. The capability to scavenge the DPPH radical was calculated using the following equation:

DPPH scakenged (%) =
Abs control − Abs test Abs control × 100, (1) where Abs control is the absorbance of the control reaction and Abs test is the absorbance in the presence of the test compounds. The antioxidant activities of the synthesized compounds are expressed comparing with standard BHT.

Conclusion
The synthesis of new -aminophosphonic acid esters was achieved in high yields through a one-pot three-component reaction process, a Kabachnik-Fields reaction. It involves the reactions among substituted anilines, substituted aromatic 7 aldehydes, and dialkyl phosphites in dry toluene at reflux temperature, in the presence of Amberlite IRC-748 as catalyst. Their structures were established by elemental analysis IR, 1 H and 31 P-NMR, and mass spectral data. All the title compounds were screened for their antibacterial and antioxidant activity. Most of the compounds exhibited moderate antimicrobial activity, and for some the activity was fairly good.