Retracted Article: An efficient one pot three-component synthesis of 2,4,6-triarylpyridines using triflimide as a metal-free catalyst under solvent-free conditions.

A simple and efficient protocol developed for one pot three-component synthesis of 2,4,6-triarylpyridines from aromatic aldehydes, substituted acetophenones and ammonium acetate using the versatile super Brønsted acid triflimide (HNTf2) as an effective catalyst is described. The reactions proceed well in the presence of 1 mol% of HNTf2 at 80 °C under solvent-free conditions and provide the corresponding triarylpyridines in good to excellent yields. The method reported has several advantages such as a metal-free and commercially available catalyst, mild reaction conditions and lower loading of catalyst. This journal is © The Royal Society of Chemistry.

Introduction

Substituted 2,4,6-triarylpyridines have been extensively exploited as chemosensors,[1] photosensitizers,[2] and intermediates in the synthesis of therapeutic drugs, insecticides, herbicides, and surfactants.[3] Consequently, various synthetic methods have been developed for the synthesis of 2,4,6-triarylpyridines,[4] such as the coupling of aryl aldehydes with aromatic keto-oxime acetates,[5] oxidative cleavage of C–N bonds of benzyl amines with aromatic ketones,[6] and the reactions of aromatic ketones with benzyl amines,[7] acetophenoneoximes with aldehydes[8] or epoxy styrenes,[9] benzyl halides with acetophenones,[10] and chalcones with enaminones.[11] The cyclo-condensation reaction of acetophenones, benzaldehydes and ammonium acetate is known as one of the most conventional pathways for the synthesis of 2,4,6-triarylpyridines in the presence of various catalysts such as PEG1000-DAIL,[12] ionic liquids,[13] PFPAT,[14] TrCl,[15] MIL-101-SO3H,[16] DPTA,[17] ZrOCl2,[18] TiO2–SO3H,[19] TCT,[20] or Fe3O4@TiO2@O2PO2(CH2)2NHSO3H,[21] or without a catalyst.[22] Although each of the above methods has its own merits, most of these methods are associated with certain drawbacks such as commercially unavailable catalysts, use of metals as catalysts and high reaction temperatures. To avoid such drawbacks, development of more simple and efficient protocols is still in demand.

Nowadays, one pot multicomponent reactions (MCRs) have received much attention for the synthesis of diverse compounds and contribute to sustainability by simplifying the synthetic route.[23] MCRs combine three or more starting reagents at a time in the same pot to create the target molecule since there is no need of separating intermediate which help to reduce the energy consumption, solvent waste and reaction time and thus have the advantages of synthetic efficiency, simplicity, atom economy.

Triflimide (HNTf2), also known as bis(trifluoromethanesulfonyl)imide, is a commercially available white crystalline solid and highly versatile super Brønsted acid. Owing to its strong acidity as well as good compatibility with organic solvents, it has been widely employed as an exceptional catalyst in a wide range of organic reactions.[24] Because of the presence of two strongly electron-withdrawing trifluoromethanesulfonyl groups, triflimide belongs to the superacid family. In recent years, specifically, triflimide has been extensively demonstrated as a superb catalyst in miscellaneous cycloaddition reactions such as [2 + 2],[25] [3 + 2],[26] [4 + 2],[27] [3 + 3],[28] [4 + 3],[29] [6 + 2] (ref. 30) and [2 + 2 + 2] (ref. 31) cycloadditions. It is found that triflimide has also been used as a catalyst in numerous other organic reactions such as formation of oxime ethers,[32] Mukaiyama aldol reaction,[33] alkylation,[34] synthesis of indanes and tetralins,[35] allyl–allyl cross-coupling,[36] C–C bond formation[37] and Nazarov reaction.[38] In view of the efforts toward development of the potential of HNTf2 as a catalyst, herein, we report an efficient one pot three-component synthesis of 2,4,6-triarylpyridines (Scheme 1).

Scheme 1

HNTf2 catalyzed one pot three-component synthesis of 2,4,6-triarylpyridines.

Results and discussion

In order to optimize the reaction conditions, the reaction of acetophenone, benzaldehyde and ammonium acetate was selected as a model reaction. The effect of solvent (Table 1), catalyst loading (Table 2), and temperature (Table 3) were monitored. The model reaction was examined in various solvent such as DMF, DMSO, PEG-400, toluene and under solvent-free conditions. Studies of solvent effect on the activity of catalyst reveal that the best yield was achieved under solvent-free conditions (entry 8, Table 1). Only 21% yield of the product was obtained in the absence of the catalyst (entry 1, Table 2). When 0.1 mol% of catalyst was used in model reaction, the corresponding 2,4,6-triphenylpyridine product was isolated in 43% yield. When the amount of catalyst has been increased from 0.1 to 0.5 mol%, 2,4,6-triphenylpyridine was obtained in 79% yield and the best performance was obtained when 1 mol% of catalyst was used (entry 4, Table 2). Further increase in catalyst concentration did not show any significant effect on yield (entry 5, Table 2). At room temperature a useful conversion was not observed (entry 1, Table 3). However, the productivity increased on raising the temperature. The best results were obtained at 80 °C (entry 4, Table 3).

Effect of solvent on the synthesis of 2,4,6-triphenylpyridinea

Entry	Solvent	Yieldb (%)
1	Acetonitrile	23
2	DMF	64
3	DMSO	56
4c	CH2Cl2	57
5	PEG-400	66
6	Toluene	52
7	H2O	62
8	None	93

Reaction condition: benzaldehyde (1 mmol), acetophenone (2 mmol), ammonium acetate (1.5 mmol), solvent (3 mL), HNTf2 (1 mol%) at 80 °C for 50 min.

Isolated yield.

Reaction was performed under reflux condition.

Effect of catalyst loading on synthesis of 2,4,6-triphenylpyridinea

Entry	Catalyst loading (mol%)	Yieldb (%)
1	None	21
2	0.1	43
3	0.5	79
4	1	93
5	1.5	93

Reaction condition: benzaldehyde (1 mmol), acetophenone (2 mmol), ammonium acetate (1.5 mmol), HNTf2 catalyst at 80 °C for 50 min under solvent-free conditions.

Isolated yield.

Effect of temperature on the synthesis of 2,4,6-triphenylpyridinea

Entry	Temp. (°C)	Yieldb (%)
1	Room temp.	—
2	45	36
3	60	67
4	80	93
5	85	93

Reaction condition: benzaldehyde (1 mmol), acetophenone (2 mmol), ammonium acetate (1.5 mmol), HNTf2 (1 mol%) at different temperature for 50 min under solvent-free conditions.

Isolated yield.

On the basis of the above results, the catalytic activity of HNTf2 was further explored for various aromatic aldehydes and substituted acetophenones in the presence of 1 mol% of catalyst at 80 °C under solvent-free conditions. The results are summarized in Table 4. These results show that the reactions are equally facile with both electron-donating and electron-withdrawing substituents on the aromatic ring, and all products are obtained in good yields.

Synthesis of 2,4,6-triarylpyridines catalysed by HNTf2a,b

Reaction conditions: HNTf2 (1 mol%), aldehydes (1 mmol), acetophenones (2 mmol), ammonium acetate (1.5 mmol), 80 °C.

Isolated yields.

A probable mechanism for the synthesis of 2,4,6-triarylpyridines has been proposed in Scheme 2. HNTf2 acts as an efficient Brønsted acid catalyst by activating the carbonyl group of an aromatic aldehyde and a substituted acetophenone to undergo aldol condensation to form a 1,3-diaryl-2-propen-1-one A, which further underwent Michael addition with substituted acetophenone to yield an intermediate B. Intermediate B is then cyclized and dehydrogenated to afford the title product 3.

Scheme 2

A plausible reaction mechanism.

The efficiency of the catalyst was determined by comparison with other catalytic systems. It gives a better yield at lower temperature (Table 5).

Comparison of efficiency of HNTf2 with reported catalytic methods under solvent-free conditions

Entry	Conditions	Yield (%)	Ref.
1	120 °C, 3.5–7 h, H14[NaP5W30O110]	50–98	39
2	120 °C, 4–6 h, HClO4/SiO2	68–88	40
3	120 °C, 15–120 min, BaCl2/nano-SiO2	70–94	41
4	120 °C, 6 h, I2 (20 mol%)	48–61	42
5	120 °C, 1.5–3.5 h, [HO3S(CH2)4mim]HSO4 (20 mol%)	82–93	13a
6	130 °C, 4–7.5 h, wet TCT (5 mol%)	58–86	20
7	120 °C, 4–7 h, DPAT (2 mol%)	51–96	17
8	130 °C, 1–6 h, without catalyst	60–86	22
9	110 °C, 5–9 min, MW, without catalyst	80–95	43
10	110 °C, 80–150 min, n-TSA	82–96	19
11	80 °C, 30–60 min, HNTf2 (1 mol%)	81–96	This work

Conclusions

In summary, we have disclosed that HNTf2 works as a powerful Brønsted acid catalyst for the one pot three-component synthesis of 2,4,6-triarylpyridines from aromatic aldehydes, substituted acetophenones and ammonium acetate under solvent-free conditions. Advantages of this methodology are use of metal-free and commercially available catalyst, simple experimental and work up procedure, high yields, mild reaction conditions and lower loading of catalyst compared with other methods. The methodology requires 1 mol% of HNTf2 catalyst, but it is not reused. This is a huge limitation.

Experimental

Materials and instrumentation

Materials obtained from commercial suppliers were used as received unless mentioned otherwise. All solvents were dried by standard procedures. Melting points were determined on an XT4A electrothermal apparatus equipped with a microscope and are uncorrected. NMR spectra were recorded on a Bruker Avance 400 spectrometer in CDCl3.

General procedure for the synthesis of 2,4,6-triarylpyridine

A mixture of aromatic aldehyde (1 mmol), acetophenone (2 mmol), ammonium acetate (1.5 mmol) and HNTf2 (1 mol%) was stirred at 80 °C for 30–60 min. The reaction was monitored by TLC (petroleum ether : ethyl acetate = 7 : 3, v/v). After completion, the reaction mixture was poured in ice water (5 mL) and the precipitated solid was collected by filtration, washed with distilled water (20 mL) and dried. The crude product was recrystallized from 95% ethanol (5 mL) to give the corresponding pure product. All the products were characterized by 1H NMR and 13C NMR spectroscopy.

Conflicts of interest

There are no conflicts to declare.