Microwave-assisted C–N and C–S bond-forming reactions: an efficient three-component domino sequence for the synthesis of sulfoether-decorated imidazo[1,2-a]pyridines

Abstract

An efficient and simple microwave-assisted three-component reaction for the formation of imidazo[1,2-a]pyridine derivatives in the presence of F₃CCO₂H has been described. The three-component reaction of 3-phenylpropiolaldehyde, pyridin-2-amines and thiols gives the desired products in good yields. It represents an efficient approach for the formation of C–N and C–S bonds under microwave irradiation.

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The formation of carbon-heteroatom bonds continues to be an active and challenging field of chemical research.¹ Transition metal-catalyzed reactions have emerged as a powerful methodology for the formation of carbon-heteroatom bonds over the past decades.² Many significant and favorable processes have been developed in this field; particularly Pd,³ Ag,⁴ Cu,⁵ Au,⁶ Rh (ref. 7) and Ru (ref. 8) catalysts are favorable for the formation of carbon-heteroatom bonds. However, those reactions suffer from having to use expensive catalysts and ligands. The cost, toxicity and environmental impact of these catalysts have hindered their further application on an industrial scale. These problems are of particular environmental and economic concern in large-scale syntheses. Therefore, the discovery of efficient processes that do not require a metal catalyst will be of great importance because such procedures provide an efficient route to avoid and solve these problems. It represents one of the most fundamental and economic strategies, and has attracted critical attention of organic chemists in recent years.⁹ In spite of the utility of transformation in the preparation of complex molecules, environmental sustainability and economy remains challenging.

Microwave-assisted chemistry¹⁰ has matured into a promising strategy for the formation of useful moleculars. It has shown tremendous advantages including simple easy work up procedure, decreased reaction time, saving energy and cost as well as providing clean products in good to excellent yields in comparison to conventional methods. Multicomponent reactions (MCRs) are very interested transformation because of their significant advantages.¹¹ The strategy of MCRs has been developed to enable the rapid preparation of diverse structures with an optimal number of new bonds and functionalities from readily accessible starting materials in a single operation under mild conditions.

On the other hand, imidazo[1,2-a]pyridines are recognized as an important class of heterocycles which exhibited remarkable biological activities as privileged scaffolds in drug discovery and development.¹² The core structure of imidazo[1,2-a]pyridines has been found in many drugs such as zolpidem, alpidem, zolimidine, olprinone, saripidem, necopidem.¹³ Therefore, organic chemists have been making extensive efforts to prepare imidazo[1,2-a]pyridine derivatives¹⁴ by developing novel and convenient organic transformations. Although many elegant processes have been reported to form those compounds,¹⁵ the development of new microwave-assisted domino reactions for synthesis of sulfoether-decorated imidazo[1,2-a]pyridines is still highly favorable the use of thiols and alkynes as starting materials. Our recent efforts were including the construction of imidazo[1,2-a]pyridines¹⁶ by direct C–H functionalization or multicomponent reaction. In this context, we described a novel microwave-assisted domino reactions for the formation of C–N and C–S bonds to prepare sulfoether-decorated imidazo[1,2-a]pyridine derivatives via three-component reactions of ynals, pyridin-2-amines and thiols.

Very recently, we have developed efficient three-component reaction of ynals, pyridin-2-amines and alcohol to construct functionalized imidazo[1,2-a]pyridine (Scheme 1).^16e In our further exploration of the scope of this novel domino reaction, we employed 3-phenylpropiolaldehyde, pyridin-2-amine and 2-methyl propane-2-thiol as the substrates and surprisingly found that the desired product was obtained with lower yield in the presence of AcOH in CH₃CN at 80 °C for 8 h.

Scheme 1 Synthesis of imidazo[1,2-a]pyridines.

Initially, 3-phenylpropiolaldehyde 1a, pyridin-2-amine 2a and 2-methylpropane-2-thiol 3a were chosen as model substrates to screen the catalysts and determine suitable reaction conditions for the MCRs. The results are summarized in Table 1. The desired product 4a was formed in 10% yield in the presence of AcOH in DMF at 80 °C for 24 h(Table 1, entry 1). Interestingly, the expected product 3-(tert-butylthio(phenyl)methyl) imidazo[1,2-a]pyridine 4a was obtained in 39% yield (Table 1, entry 2) under microwave irradiation condition at 120 °C in the presence of AcOH for 30 min. Encouraged by the result that microwave irradiation could promote the reaction, other catalyst, such as PhCO₂H, TsOH, F₃CCO₂H, HCl, and H₂SO₄, were next to examined (entries 3–7, Table 1). To our delight, the product 4a was formed in 83% yield in the presence of F₃CCO₂H under microwave heating condition. The results indicated that PhCO₂H, TsOH, HCl or H₂SO₄ could catalyze the transformation for yielding the product in moderate yields. It was interestingly found that the three-component reaction was highly sensitive to temperature variations (Table 1, entries 8–10). When the reaction carried out decreasing the reaction temperature from 120 °C to 100 °C, the corresponding product 4a was obtained in 69% yield. Furthermore, the effects of solvents were surveyed (Table 1, entries 11–14). The results clearly indicated that best result presented when DMF was used as solvent. An increase of reaction time was studied and the same efficiency was obtained after 50 min of microwaves activation, while the decrease of reaction time lead to lower yield (Table 1, entries 15–16).

Table 1 Optimization of reaction conditions^a

Entry	Catalyst	Thermal/MW	Solvent	t	Yield^b (%)
a Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), 3a (1.2 mmol), catalyst (2.0 mol%); solvent (3.0 mL), under microwave 500 W.b GC-yield.
1	AcOH	80 °C	CH3CN	24h	10
2	AcOH	120 °C(MW)	DMF	30 min	39
3	PhCO2H	120 °C(MW)	DMF	30 min	22
4	TsOH	120 °C(MW)	DMF	30 min	46
5	F3CCO2H	120 °C(MW)	DMF	30 min	83
6	HCl	120 °C(MW)	DMF	30 min	45
7	H2SO4	120 °C(MW)	DMF	30 min	17
8	F3CCO2H	100 °C(MW)	DMF	30 min	69
9	F3CCO2H	130 °C(MW)	DMF	30 min	84
10	F3CCO2H	150 °C(MW)	DMF	30 min	76
11	F3CCO2H	130 °C(MW)	DMSO	30 min	80
12	F3CCO2H	130 °C(MW)	Toluene	30 min	34
13	F3CCO2H	130 °C(MW)	DMA	30 min	79
14	F3CCO2H	130 °C(MW)	Dioxane	30 min	36
15	F3CCO2H	130 °C(MW)	DMF	20 min	72
16	F3CCO2H	130 °C(MW)	DMF	50 min	82

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Under the optimized reaction conditions, we evaluated the scope of this novel transformation synthesis of sulfoether-decorated imidazo[1,2-a]pyridines. The results are summarized in Table 2. First, 1a and 3a fixed as substrates to test various substituted pyridin-2-amines. As shown in Table 2, a variety of substituted pyridin-2-amine derivatives were effective substrates for this transformation and the corresponding products (4a–f) were obtained in good yields in all the cases. To our delight, the sensitive functionalized groups on the pyridine ring, such as Cl, Br and I, were also tolerated to afford the corresponding in good yields. Subsequently, cyclohexanethiol (3b) and propane-2-thiol (3c) were also employed for this transformation. The results also indicated that all of the reactions proceeded smoothly under the optimized conditions and provided the thioether-decorated imidazo[1,2-a]pyridine derivatives (4g–n) in good yields.

Table 2 Microwave-assisted synthesis of imidazo[1,2-a]pyridines^a

a Isolated yields.

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Encouraged by the successful syntheses of substituted imidazo[1,2-a]pyridines, our attention turned to the possibility of synthesizing functionalized imidazo[1,2-a]pyridine derivatives by using primary thiols in this reaction under the optimized conditions. And the results are shown in Table 3. Dodecane-1-thiol (3d) was firstly examined. Various substituted pyridin-2-amines reacted well with 1a, 3d and led to the desired product 5aa–al in good to high yields. Furthermore, multisubstituted pyridin-2-amine, such as 5-chloro-3-iodopyridin-2-amine, 3,5-dibromo-4-methylpyridin-2-amine or 3,5-diiodo-6-methylpyridin-2-amine reacted with 1a and 3d smoothly under the optimized conditions. Other commercially available aliphatic primary thiols, such as, propane-1-thiol(3e), ethanethiol (3f), butane-1-thiol (3g), and phenylmethanethiol (3f), were also tested. We were pleased to find that substituted thiols as substrates were also tolerated in the reaction and provided the desired imidazo[1,2-a]pyridines 5ba–ec in good to excellent yields. To our delight, substituted thiols reacted with 1a and multisubstituted or electron-poor groups substituted (CF₃) pyridin-2-amines smoothly and afforded the desired product in good yields. It is worth pointing out that multi-halogen-substituted pyridin-2-amines could also be achieved upon carrying out the experiment under the optimized conditions. The results clearly indicated that this strategy can be extended to a variety of substituted thiols to form functionalized imidazo[1,2-a]pyridine derivatives. Notably, the sole products were detected in the reaction, which was indicated that this one pot transformation was regioselective and chemoselective.

Table 3 F₃CCO₂H-catalyzed synthesis of imidazo[1,2-a]pyridines^a

a Isolated yields.

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To our delight, other nucleophiles, such as CH₃OH and C₂H₅OH, were also performed very well and offered the corresponding products in 95% and 86% yields respectively (Scheme 2).

Scheme 2 MCRs of 1a, 2a and ethanol for the synthesis of imidazo[1,2-a]pyridines.

In summary, we have reported an efficient microwave-assisted three-component reactions synthesis of sulfoether-decorated imidazo[1,2-a]pyridines. This procedure offers easy access to highly functionalized imidazo[1,2-a]pyridines which are useful structural motif in pharmaceuticals and natural compounds. This transformation provides a convenient strategy for the formation of C–N and C–S bonds to prepare sulfoether-decorated imidazo[1,2-a]pyridines. This methodology has several advantages: (1) it does not require expensive substrates or catalysts; (2) it decrease reaction time, saves energy and cost as well as provides clean products in good to excellent yields; (3) it has provided a wide range of substrates. Further studies and applications of metal-free multicomponent reactions are ongoing in our laboratory.

Acknowledgements

This research was financially Supported by National Natural Science Foundation of China (21302023) and the Project of Department of Education of Guangdong Province (2013KJCX0111).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental protocols and NMR spectra for products 4aa–4eg. See DOI: 10.1039/c5ra05377c

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