Acidic tributyl phosphonium-based ionic liquid: an efficient catalyst for preparation of diverse pyridine systems via a cooperative vinylogous anomeric-based oxidation

In this study, an acidic phosphonium-based ionic liquid, namely tributyl(3-sulfopropyl)phosphonium trifluoroacetate, was designed and synthesized via a facile and green route. From an accurate perspective, the structure of the prepared ionic liquid was investigated using FT-IR, 1H NMR and 13C NMR spectroscopies, and EDX, elemental mapping and TGA/DTG analysis. In this intensive research, catalytic application of tributyl(3-sulfopropyl)phosphonium trifluoroacetate was explored for the preparation of diverse pyridine systems such as triaryl pyridines, 2-amino-3-cyanopyridines and indolyl pyridines via a cooperative vinylogous anomeric-based oxidation (CVABO). The observed results proved that prepared acidic phosphonium-based ionic liquid is a effective catalyst for the multicomponent synthesis of pyridines.


Introduction
Ionic liquids (ILs), as remarkable class of chemical compounds, have attracted a tremendous deal of interest. [1][2][3] Many of researchers in modern sciences are engaged in exploration of the exceptional properties of these versatile systems. 4,5 These materials, due to irreplaceable privileges such as suitable chemical and thermal stability, controlled miscibility, tunability, designability, low vapor pressure, high ionic conductivity, etc., are an integral part of catalysis, biotechnology, medical sciences, energetic materials, separation and extraction processes, electrochemistry, surfactants and functional materials. 2,[6][7][8][9][10][11][12] In subsequent and intensive study, acidic ILs have been investigated in catalytic processes such as multicomponent reactions and CO 2 xation, biomass processing, and electrolytes. 13,14 The cationic section of ILs plays a decisive role in viscosity, solubility, melting point, density and other characteristic properties of ionic liquids. 15 Among varied type ILs, phosphonium-based ionic liquid (PILs) are more thermal and chemical stable and less viscous than other ILs. 16,17 PILs due to environment compatibility, low cost and simplicity of synthesis, have an excellent potential for academic and industrial chemical processes. [18][19][20][21][22][23][24][25][26] It goes without saying that there are several reports for variety applications of PILs. Wittig reagents, 26 catalytic applications, 27 desulfurization and denitrogenation of liquid fuels, 18,28 extraction of phenolic compounds from water 29 and recovery of metals 30 are a selection of applications of PILs. In recent years, catalytic applications of PILs have received much attentions. 16 These systems, have a good catalytic response to multicomponent reactions, 16 CO and CO 2 xation, 31-33 Mukaiyama-aldol reaction, 33 hydroformylation acetalization 34 and Heck reactions. 35 A number of catalytic applications of PILs are shown in Scheme 1.
Triaryl pyridines and nicotinonitriles as the most important subunits of pyridine families are ubiquitous subsets of pyridine families. [36][37][38][39][40] Triaryl pyridines are popular building blocks in supramolecular chemistry, 41 advanced devices as components of OLEDs, 42 sensors 43 and photo sensitizers 44 and have good medicinal potentials such as antidepressant, anticancer and antimicrobial activities. 45,46 Noticeably, nicotinonitrile derivatives by pharmaceutical and medicinal functions, have a crucial role and created new insights in the design of anticancer compounds, antihistaminic, antimicrobial and etc. [47][48][49] Multicomponent strategy for the synthesis of pyridine families has a fundamental attitude of synthetic efficiency, utility high atom economy and selectivity. 50-52 A number of pharmaceutical applications of triaryl pyridines and nicotinonitriles are sketched in Scheme 2.
In this study, we devoted our focus on the design and synthesis of tributyl(3-sulfopropyl)phosphonium tri-uoroacetate and its catalytic application in the preparation of triaryl pyridines, indolyl pyridines and nicotinonitriles Also, 1 H NMR and 13 C NMR spectra are well consistent with the structure of the catalyst (see ESI †).
Elemental composition of target catalyst was evaluated by using EDX analysis. According to the observed results, the expected elements in the structure of tributyl(3-sulfopropyl) phosphonium triuoroacetate including C, S, P, O and F are approved (Fig. 1). In addition, elemental mapping analysis was used for further conrmation of the obtained data from EDX  analysis. Also, due to this technique, scattering of elements are well observed (Fig. 2).
Thermogravimetric/differential thermogravimetric analysis displayed that PIL 1 was stable up to 223°C. Fiddling weight loss up to 163°C is related to the separation of trapped solvents. Also, it seems that the two extreme weight loss respectively in 223 and 416°C which indicate the decomposition of -(CH 2 ) 3 -SO 3 H and butyl chains of the catalyst 57 (Fig. 3).  In a deliberate study, aer characterization of PIL 1, its catalytic application was investigated for the synthesis of diverse pyridine systems. In this respect, 4-chlorobenzaldehyde, 1-(naphthalen-2-yl)ethan-1-one, malononitrile and ammonium acetate as typical compounds were selected for the optimization of the reaction conditions. Initially, the role of temperature was examined and several temperatures consist of 40, 60, 70, 80 and 90°C were exerted for the model reaction. The obtained experimental data proved that 80°C is the best operational temperature. Aer that, the reaction was performed in the presence of different solvents including polar (protic and aprotic) and nonpolar solvents. The reaction has a substantial yield by using EtOH and CHCl 3 as solvent. Then, the different amounts of catalyst were used in model reaction that due to the observed results, 10 mol% of catalyst gives the desired product by good yield. As conrmed by the obtained data, the best results for the model reaction achieved by the performing of the reaction in the presence of 10 mol% of catalyst and solvent free conditions at 80°C (Table 1).
The plausible mechanistic route for the synthesis of nicotinonitrile derivatives is revealed in Scheme 5. Firstly, activated 1-(naphthalen-2-yl)ethan-1-one by catalyst was reacted by ammonia (from NH 4 OAc) and leads to imine intermediate. Likewise, Knoevenagel intermediate was achieved by reaction of 4-chlorobenzaldehyde and malononitrile. In the second step, these two intermediates were reacted with each other to produce intermediate a. Then, through an intermolecular cyclization and subsequent tautomerization processes, the intermediate d was produced. Aer that, in the key step, with the participation of a cooperative vinylogous anomeric-based

Experimental section
General experimental procedure for the synthesis of PIL 1 At the rst, zwitterion 1 was synthesized according to the reported procedure. 15 Hereupon, tributyl phosphine (5 mmol, 1.01 g), 1,2-oxathiolane 2,2-dioxide (5 mmol, 0.61 g) and acetonitrile as solvent were successively added into a 50 mL round-bottomed ask and reuxed under N 2 atmosphere for 72 h. Aer completing of the reaction, the solvent was removed and the remained oil was washed with diethyl ether to gives the oily product. In the second step, zwitterion 1 (5 mmol, 1.62 g) and triuoro acetic acid (5 mmol, 0.57 g) and 30 mL of toluene as catalyst were added into a 50 mL round-bottomed ask and the mixture of reaction was reuxed at 110°C for 24 h. Aer that, the oily product was separated from solvent by using extraction with n-hexane to gives PIL 1 as desired product.

Conclusion
In conclusion, we reported the tributyl(3-sulfopropyl) phosphonium triuoroacetate as a new phosphonium-based ionic liquid with acidic moiety. The prepared ionic liquid was characterized by using FT-IR, 1 H NMR and 13 C NMR spectroscopies, EDX, elemental mapping and TGA/DTG analysis. Aer that, PIL 1 has been shown good potential as an efficient catalyst for the multicomponent synthesis of triaryl pyridines, 2amino-3-cyanopyridines and indolyl pyridine. In this work, we used diverse starting materials such as different ketones and aldehydes that leads to formation of varied pyridine systems. Moreover, most of synthesized pyridine derivatives have a relatively good yields and short reaction times. In a separate study, a plausible mechanism based on a cooperative vinylogous anomeric-based oxidation was suggested for the synthesis of pyridine derivatives.