ZrO₂-supported Cu(II)–β-cyclodextrin complex: construction of 2,4,5-trisubstituted-1,2,3-triazoles via azide–chalcone oxidative cycloaddition and post-triazole alkylation

Abstract

An efficient one-pot three-component stepwise approach for the synthesis of N-2-substituted-1,2,3-triazoles from chalcones, sodium azide and esters has been developed using a recoverable and reusable ZrO₂ nanoparticle-supported Cu(II)–β-cyclodextrin complex as a catalyst. N-2 alkylation of triazoles using different aryl–alkyl esters without any additives has been achieved for the first time. The one-pot operation, atom-economical nature, regioselectivity and good yields are the noteworthy features of this protocol. The reusability of the prepared nanocatalyst was successfully examined four times without any appreciable loss in catalytic activity.

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Introduction

During the last few decades, great attention has been given to click¹ and green chemistry² for the development of efficient, ecofriendly and environmentally benign protocols. Recently nano-particles in catalysis have emerged as a sustainable and competitive alternative to conventional catalysis. Specifically, nanoparticles supported on a metal oxide have been extensively studied in the field of medicine for drug delivery systems³ and for the detection of cancer cells in the early stages.⁴ Mesoporous nano-materials have also gained increasing importance in their use as catalysts in various organic reactions,^5,6 as sensors for the detection of hydrazine,⁷ in optoelectronics,⁸ for enhancing up-conversion luminescence,⁹ and as electron transfer mediators in bio-electrochemical systems.¹⁰ The properties of metal oxide nanoparticles are very attractive compared to bulk catalysts due to their high surface to volume ratio and their surface atoms are more active and have attracted much attention because of wide spread applications in the fields of science and technology.¹¹

On the other hand, 1,2,3-triazoles are an important class of five membered nitrogen containing heterocycles which have wide spread applications in various fields including organic synthesis, pharmaceutical agents, agrochemicals, dyes, corrosion-inhibitors, photo stabilizers and photographic materials.¹² 1,3-dipolar cycloaddition of organic azides with alkynes via the Huisgen method is the most common approach for the synthesis of triazoles.¹³ However, the above said, this method suffers from serious drawbacks such as high temperature, low yields and low regioselectivity. In recent years, several research groups have reported a one-pot protocol for the synthesis of 1,2,3-triazoles using various copper catalysts,¹⁴ but the existing synthetic methodologies have mainly focused on N-1 substituted 1,2,3-triazoles.¹⁵

Only a few methods have been reported for N-2 substituted 1,2,3-triazoles.¹⁶ Generally, synthesis of N-2-substituted 1,2,3-triazoles is achieved from hydroxy ketones and hydrazine,¹⁷ but the synthesis of α-hydroxy ketone is a challenging task. In addition, several research groups have demonstrated different approaches for the synthesis of N-2-substituted 1,2,3-triazoles.^18–23 Most of the reported methods suffer from serious drawbacks such as limited substrate scope, high temperature, long duration and using toxic reagents and non-economical chemicals like PdCl₂. It is noteworthy to mention that none of the above mentioned literature methods have used esters as an alkylating agent. Therefore, the development of an efficient and safe protocol for the synthesis of N-2-substituted triazoles is still in demand. In this direction, for the first time, we have developed a one-pot protocol for the regioselective synthesis of 2,4,5-trisubstituted-1,2,3-triazoles via 1,3-dipolar cycloaddition using a novel ZrO₂ nanoparticle supported Cu(II)–β-cyclodextrin complex (without any additives) followed by alkylation using esters as an alkylating agent in DMF at 100 °C (Scheme 1, eqn (4)). The main advantages of this protocol are the simple starting materials, one pot operation, easy workup procedure and high regioselectivity.

Scheme 1 N-2-substituted-1,2,3-triazoles in the presence of ZrO₂–Cu₂–β-CD.

Results and discussion

The ZrO₂ nanoparticle supported Cu(II)–β-cyclodextrin complex (ZrO₂–Cu₂–β-CD) was prepared by a simple one-pot co-precipitation method using ZrOCl₂·8H₂O, NH₄OH and the Cu(II)–β-cyclodextrin complex (Scheme 2).^24,25

Scheme 2 Preparation of ZrO₂-supported Cu(II)–β-cyclodextrin complex catalyst.

XRD analysis of ZrO₂ and the ZrO₂-nanoparticle supported Cu(II)–β-cyclodextrin complex indicates five characteristic peaks at 2θ = 30.2°, 35.15°, 50.44°, 60.14°, and 62.98° corresponding to (111), (200), (220), (311), and (331) planes, respectively, as shown in Fig. 1. All diffraction peaks and positions match those from the JCPDS card (Joint Committee on Powder Diffraction Standards no. 37-1484), calculated from the Scherrer’s equation,

where d is the average grain size of the crystallites, λ is the incident wavelength, θ is the Bragg angle and β is the diffracted full-width at half-maximum (FWHM) in radians, caused by the crystallites. The values provided by the above equation for the ZrO₂ and ZrO₂–Cu(II)–β-CD nanoparticles were 20 and 1.6 nm, respectively. This reveals that Cu(II)–β-cyclodextrin plays a surfactant role, which assists in reducing the size of the nanoparticles.

Fig. 1 (a) XRD pattern of ZrO₂. (b) XRD pattern of ZrO₂-supported Cu(II)–β cyclodextrin.

SEM and TEM images of the prepared ZrO₂–Cu₂–β-CD nanoparticles of the ZrO₂-supported Cu(II)–cyclodextrin complex are shown in Fig. 2. The TEM image of the catalyst shows that the nanoparticles are highly aggregated. The average size of these particles is about 1.2 nm, which shows a close agreement with the values calculated from XRD data. The SEM image of the supported catalyst confirms that these nanoparticles are unevenly sized due to deposition of Cu(II)–β-cyclodextrin complex nanoparticles on the surface of ZrO₂, and most of them are almost spherical in shape.

Fig. 2 (a) SEM and (b) TEM images of ZrO₂-supported Cu(II)–β-cyclodextrin.

The catalytic activity of the ZrO₂–Cu₂–β-CD nanoparticles was studied for the synthesis of 1,2,3-triazoles from chalcones. We initiated our studies with (E)-3-(4-ethylphenyl)-1-(thiophen-2-yl) prop-2-en-1-one (1h) as a representative substrate. Treatment of 1h and sodium azide with 40 mol% of ZrO₂–Cu₂–β-CD nanoparticles in DMF at 100 °C produced the triazole anion, and subsequent addition of 2-NO₂-1,4-difluoro benzene afforded the N-2 arylated product in trace amounts. Furthermore, we carried out the reaction for N-2 substitution using different aryl halides like 1,4-dibromo and 1,4-dichloro benzenes, in both cases ending up with trace amounts of the N-2 arylated product. Then, accidentally, when we used 2-bromo-methyl benzoate for N-2-aryl substitution, the product spot was prominent in TLC; surprisingly, spectral analysis of the product confirms the N-methylated compound instead of the corresponding N-arylated product. Inspired by this finding, we elaborated to different esters for alkylation. We found, exclusively, the N-alkylated product in all cases without any traces of the N-arylated product (Table 1, entries 1–7).

Table 1 N-alkylation using different esters^a

Entry	R	R^3	Yield^b (%)	Product
a 1h (1 mmol), NaN3 (1.2 mmol), 40 mol% of ZrO2–Cu2–β-CD (50 mg, 0.036 mmol) in DMF (3 mL) at 100 °C in air for 14 h first, then ester (1 mmol) was added to the mixture and the reaction continued for 16–48 h.b Isolated yields.
1	CH3	CH2CH3	—	—
2	C6H5	CH3	77	2k
3	C6H5	CH2CH3	60	2n
4	C6H5	Isopropyl	52	2o
5	C6H5	t-Butyl	49	2p
6	C6H5	n-Butyl	46	2q
7	C6H5	Isoamyl	45	2s

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We then applied our interest to screen the reaction using different catalysts such as CuO, CuSO₄, α-Fe₃O₄, ZrO₂–β-CD, Cu₂–β-CD, and ZrO₂–Cu₂–β-CD (Table 2, entries 1–15). Among all, ZrO₂–Cu₂–β-CD was found to be very effective for the formation of the N-2-alkylated product. CuO, CuSO₄ and α-Fe₃O₄ gave only the triazole anion without undergoing N-alkylation. CuSO₄ and Cu₂–β-CD gave a mixture of products with low yields (Table 2, entries 2 and 5). We then screened the reaction using ZrO₂–Cu₂–β-CD as a catalyst with different solvents such as DMF, DMSO, EtOH and THF. Among them, DMF proved to be a better solvent for the formation of N-2-alkylated triazoles (Table 2, entry 9) at 100 °C.

Table 2 Effect of catalyst and solvent on the reaction^a

Entry	Catalyst	Temp (°C)	Mol%	Solvent	Time (h)	Yield^b (%)
a 1h (1 mmol), NaN3 (1.2 mmol), 40 mol% of ZrO2–Cu2–β-CD (50 mg, 0.036 mmol) in DMF (3 mL) at 100 °C in air for 14 h first, then 3-Cl–C6H4COOCH3 (1 mmol) was added to the mixture and the reaction continued for 16–48 h.b Isolated yields.
1	—	125	—	DMF	24	—
2	CuO	125	10	DMF	18	50
3	CuSO4	100	40	DMF	20	25
4	α-Fe3O4	120	10	DMSO	20	50
5	ZrO2–β-CD	100	40	DMF	16	35
6	Cu2–β-CD	100	40	DMF	20	30
7	ZrO2–Cu2–β-CD	75	10	DMF	16	58
8	ZrO2–Cu2–β-CD	100	20	DMF	13	72
9	ZrO2–Cu2–β-CD	100	40	DMF	14	77
10	ZrO2–Cu2–β-CD	120	40	DMF	14	55
11	CuO	100	40	DMF	12	67
12	ZrO2–Cu2–β-CD	100	20	EtOH	13	None
13	ZrO2–Cu2–β-CD	100	20	THF	13	None
14	ZrO2–Cu2–β-CD	75	40	DMF	20	55
15	CuO	100	20	DMF	16	50

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With the optimized reaction conditions in hand, we next explored the generality and scope of the protocol using several substituted aryl–alkyl esters (Table 3). The reaction of the 2,4,5 trisubstituted 1,2,3 triazole anion with an ester containing a chlorine atom at the m-position of the aryl group proceeded smoothly and regioselectively produced the corresponding 2,4,5-trisubstituted 1,2,3-triazole with an excellent yield (Table 3, entry 8). With methyl benzoate as an alkylating agent, a moderate yield of the desired product was observed (Table 3, entry 2) and the reaction with ethylbromoacetate yielded the expected product in good yield (Table 3, entry 3). When we carried out the reaction with different alkyl benzoates containing halogens at different positions the corresponding 2,4,5-trisubstituted 1,2,3-triazoles were produced with low yields (Table 3, entries 4–12). Disappointingly, 2-chloroethyl 3-chlorobenzoate did not promote the reaction (Table 3, entry 12).

Table 3 Effect of different halogen substituents for the synthesis of triazoles^a

Entry	R	R^3	Yield^b (%)	Product
a 1d (1 mmol), NaN3 (1.2 mmol), 40 mol% of ZrO2–Cu2–β-CD (50 mg, 0.036 mmol) in DMF (3 mL) at 100 °C in air for 14 h first, then ester (1 mmol) was added to the mixture and the reaction continued for 16–48 h.b Isolated yields.
1	CH3	CH2CH3	—	—
2	C6H5	CH3	63	2m
3	BrCH2	CH2CH3	78	2m
4	2-BrC6H4	CH3	53	2d
5	2-IC6H4	CH3	Trace	2d
6	2-FC6H4	CH3	44	2d
7	2-ClC6H4	CH3	60	2d
8	3-ClC6H4	CH3	81	2d
9	4-ClC6H4	CH3	57	2d
10	2,4-ClC6H4	CH3	58	2d
11	3-ClC6H4	CH2CH3	68	2m
12	3-ClC6H4	CH2CH2Cl	—	—

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Furthermore, varieties of chalcones were used to investigate the substrate scope of the reaction as shown in Table 4. Generally, chalcones with electron-withdrawing substituents at R¹ and/or R² led to higher yields (Table 4, entries 1–3, 6–12, 14–18 and 20). In contrast, chalcones bearing a thiophene ring gave moderate yields (Table 4, entries 1–3, 7–11, 14–18 and 20). When we used methyl 3-chlorobenzoate the reaction underwent substitution at a faster rate and we observed moderate to good yields (Table 4, entries 1–7, 9–12 and 14). With increasing alkyl chain length, as with isopropyl, t-butyl and isoamyl aromatic esters, the reactions proceeded more slowly with lower yields of the desired products (Table 4, entries 16–18, 20). When isopropyl 3-chlorobenzoate and phenyl 2,4-dichlorobenzoate were explored, only traces of the desired alkylated products were isolated.

Table 4 Substrate scope of different chalcones and esters^a

Entry	R^1	R^2	R^3	R	Time (h)	Yield^b (%)	Product
a Chalcone (1 mmol), NaN3 (1.2 mmol), 40 mol% of ZrO2–Cu2–β-CD (50 mg, 0.036 mmol) in DMF (3 mL) at 100 °C in air for 14 h first, then ester (1 mmol) was added to the mixture and the reaction continued for 16–48 h.b Isolated yields.
1	2-Thiophenyl	4-OCH3C6H4	CH3	3-ClC6H4	16	80	2a
2	2-Thiophenyl	3,4-(OCH3)2C6H3	CH3	3-ClC6H4	15	82	2b
3	2-Thiophenyl	C6H5	CH3	3-ClC6H4	16	76	2c
4	4-OCH3C6H4	C6H5	CH3	3-ClC6H4	14	81	2d
5	4-OCH3C6H4	4-CH3C6H4	CH3	3-ClC6H4	15	82	2e
6	4-OCH3C6H4	4-FC6H4	CH3	3-ClC6H4	17	78	2f
7	2-Thiophenyl	4-FC6H4	CH3	3-ClC6H4	18	72	2g
8	2-Thiophenyl	4-EthylC6H4	CH2CH3	BrCH2	12	79	2h
9	2-Thiophenyl	3,4,5-(OCH3)3C6H2	CH3	3-ClC6H4	14	81	2i
10	2-Thiophenyl	Benzo[1,3]dioxole	CH3	3-ClC6H4	16	75	2j
11	2-Thiophenyl	4-EthylC6H4	CH3	3-ClC6H4	17	77	2k
12	4-OCH3C6H4	4-BrC6H4	CH3	3-ClC6H4	16	79	2l
13	4-OCH3C6H4	C6H5	CH2CH3	3-ClC6H4	22	68	2m
14	2-Thiophenyl	4-FC6H4	CH3	C6H5	21	62	2g
15	2-Thiophenyl	4-EthylC6H4	CH2CH3	3-ClC6H4	26	60	2n
16	2-Thiophenyl	4-EthylC6H4	Isopropyl	3-ClC6H4	30	Trace	2o
17	2-Thiophenyl	4-EthylC6H4	t-Butyl	3-ClC6H4	36	49	2p
18	2-Thiophenyl	4-EthylC6H4	n-Butyl	3-ClC6H4	32	46	2q
19	4-OCH3C6H4	C6H5	C6H5	2,4-diClC6H4	48	Trace	2r
20	2-Thiophenyl	4-EthylC6H4	Isoamyl	3-ClC6H4	34	45	2s

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Finally, confirmation of the structure and the site of N-alkylation of the product were obtained by single crystal X-ray analysis of 2b, as shown in Fig. 3. The reusability of the ZrO₂-supported Cu(II)–β-cyclodextrin nanoparticles has been studied. The ZrO₂-supported Cu(II)–β-cyclodextrin nanoparticles were collected by filtration and washed four times with deionized water followed by methanol, dried in air and reused for the one-pot preparation of N-2-substituted 1,2,3-triazoles from chalcones. We haven’t found a noticeable decrease in the catalytic activity even after four catalytic cycles (Table 5).

Fig. 3 Single crystal XRD pattern of 2b with 50% probability.

Table 5 Recyclability test of ZrO₂-supported Cu(II)–β-cyclodextrin

Cycle	Catalyst recovery (wt%)	Yield of 2a (%)
1	90	81
2	88	79
3	87	77
4	85	74

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Conclusion

We have developed an efficient ZrO₂ nanoparticle-supported Cu(II)–β-cyclodextrin complex catalyzed protocol for regioselective synthesis of N-2-alkylated 1,2,3-triazoles with excellent yields from chalcones via cycloaddition and post-triazole alkylation using different aryl–alkyl esters without any additives for the first time. The regeneration of the ZrO₂ nanoparticle-supported Cu(II)–β-cyclodextrin complex in situ by air oxidation is the advantage of our protocol and it reduces the catalyst load and increases the efficiency. The catalyst was collected easily by filtration and the reusability of the prepared nanocatalyst was successfully examined over four runs and was found to be effective up to four cycles with only a very slight loss of catalytic activity. This new, efficient, protocol offers several advantages over many of the previously published procedures. Interestingly, this method could be highly useful to synthetic as well as medicinal chemists for the regioselective derivation of a variety of N-2-substituted-1,2,3-triazole derivatives. The scope of this novel catalyst for the synthesis of other nitrogen containing heterocycles is being studied in our research group.

Acknowledgements

YRG and KSS are thankful to UGC-BSR for meritorious fellowship Order no. DV-5/662/RFSMS/2012–13, New Delhi, India for financial support and we are also thankful to IOE, University of Mysore and NCBS, Bengaluru, Karnataka, India for their support in carrying out research work regarding the single crystal XRD and TEM analysis.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1009982. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra09970b

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