Ionic liquid 1,3-disulfonic acid imidazolium hydrogen sulfate: a novel and highly efficient catalyst for the preparation of 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols

Abstract

In this work, a novel ionic liquid, 1,3-disulfonic acid imidazolium hydrogen sulfate {[Dsim]HSO₄}, with a Brønsted acidic property, is synthesized, and used as a highly efficient, green, recyclable and homogeneous catalyst for the preparation of 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols under solvent-free and relatively mild conditions. The one-pot multi-component condensation of β-naphthol with arylaldehydes and alkyl carbamates or amides affords the title compounds in high yields and in short reaction times.

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Ionic liquids (ILs) have attracted increasing interest from chemists in the last few decades, because of their unique properties including non-flammability, non-volatility, wide liquid-state temperature range, high thermal and chemical stability, large electrochemical window and favorable salvation behavior.^1–3 These compounds have been extensively used in electrochemistry, spectroscopy, extraction and separation processes,² and also as a solvent, catalyst and reagent in organic synthesis.^1–15 Brønsted acidic ILs are an important class of these salts, which have offered the possibility for development of environmentally friendly acidic catalysts for organic transformations, due to the combined advantages of liquid and solid acids, their operational simplicity, efficacy and selectivity coupled with their green natures.^9–15

1-Carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols are of importance as they can be easily hydrolyzed to 1-aminoalkyl-2-naphthol derivatives, which are biologically interesting compounds. 1-Aminoalkyl-2-naphthols have been frequently used as hypotensive and bradycardiac agents.^16,17 1-Amidoalkyl-2-naphthols can be also converted to 1,3-oxazine derivatives.¹⁸ Several pharmaceutical properties have been reported for 1,3-oxazines, e.g., antibiotic,¹⁹ antitumor,²⁰ and analgesic activities.²¹

The one-pot multi-component condensation reaction between β-naphthol, aromatic aldehydes and alkyl carbamates^22–25 or amides^15,26–35 has been used as a practical synthetic route towards 1-carbamatoalkyl-2-naphthol^22–25 and 1-amidoalkyl-2-naphthol^15,26–35 derivatives. Although some catalysts and methods for the synthesis of the title compounds are known, newer catalysts and methods continue to attract attention for their differences with the others in terms of generality and effectiveness. Furthermore, most of the reported methods for the synthesis of the 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols are associated with one or more of the following disadvantages: (i) the use of large amount of catalyst, (ii) the use of expensive and toxic catalysts, (iii) non-availability of the catalyst or its starting materials, (iv) poor agreement with the green chemistry protocols, (v) moderate yield, (vi) long reaction time, (vii) harsh reaction conditions, and especially (viii) poor generality (in most of the reported procedures, the synthesis of one type of the title compounds has been reported).

Having the above points in mind, we report here the synthesis of a new Brønsted acidic ionic liquid, 1,3-disulfonic acid imidazolium hydrogen sulfate {[Dsim]HSO₄}, from available and inexpensive starting materials (Scheme 1),³⁶ and its application as a highly efficient, homogeneous, recyclable and green catalyst for the preparation of 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols. The one-pot multi-component condensation of β-naphthol with arylaldehydes and alkyl carbamates or amides under solvent-free and relatively mild reaction conditions affords the compounds mentioned in Scheme 2.³⁷ It is noteworthy that our method has none of the above-mentioned drawbacks.

Scheme 1 The synthesis of 1,3-disulfonic acid imidazolium hydrogen sulfate {[Dsim]HSO₄}.

Scheme 2 The synthesis of 1-carbamatoalkyl-2-naphthols (1a–g) and 1-amidoalkyl-2-naphthols (2a–i) using [Dsim]HSO₄.

Initially, the ionic liquid 1,3-disulfonic acid imidazolium hydrogen sulfate {[Dsim]HSO₄} was synthesized (Scheme 1) and identified by IR, ¹H NMR, ¹³C NMR and mass spectra.³⁶ Furthermore, to confirm that 1,3-disulfonic acid imidazolium chloride {[Dsim]Cl} has been completely converted to our catalyst [Dsim]HSO₄, a solution of AgNO₃ in distilled water was added to a solution of the catalyst in distilled water. The absence of AgCl precipitate indicated complete conversion of the [Dsim]Cl to [Dsim]HSO₄.^9,11

In another study, thermal gravimetric (TG) and derivative thermal gravimetric (DTG) analysis of [Dsim]HSO₄ was investigated at range of 25 to 600 °C, with a temperature increase rate of 10 °C min⁻¹ in a nitrogen atmosphere (Fig. 1). The TG and DTG data of the catalyst showed two major weight losses; the first was observed after 350 °C, and the second appears after 540 °C. Therefore, the molecular decomposition of the ionic liquid occurred after 350 °C. The weight losses, which corresponded to about 24%, 8%, 13% and 24% of the ionic liquid weight, can be related to the loss of SO₃, CH₂=CH₂, CH₃CN and SO₃, respectively.

Fig. 1 The TG and DTG diagrams of the ionic liquid.

After characterization of the ionic liquid, its efficacy to catalyze the preparation of 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols was examined. For this purpose, as a model reaction, effect of different molar ratios of [Dsim]HSO₄ and temperature on the one-pot multi-component condensation of β-naphthol with benzaldehyde and methyl carbamate in the absence of solvent was studied. The results showed that 5 mol% of the catalyst was sufficient to promote the reaction efficiently at 80 °C; in these conditions, the product was obtained in 98% yield after 11 min.

In order to assess the efficiency and the generality of the catalyst, β-naphthol was reacted with different arylaldehydes (including benzaldehydes and arylaldehydes possessing electron-withdrawing and electron-releasing substituents as well as halogens) and alkyl carbamates (methyl and benzyl carbamates) under the optimal reaction conditions. The results are displayed in Table 1. Ascan be seen in Table 1, all reactions proceeded effectively and afforded the corresponding 1-carbamatoalkyl-2-naphthol derivatives in excellent yields in short reaction times (Table 1, compounds 1a–g). Interestingly, our catalyst, [Dsim]HSO₄, worked well when amides instead of alkyl carbamates were used; in these conditions, 1-amidoalkyl-2-naphthols were obtained in high yields and in short reaction times from β-naphthol, various arylaldehydes (including benzaldehydes and arylaldehydes bearing electron-withdrawing and electron-releasing substituents as well as halogens) and various amide derivatives (such as acetamide, acrylamide and benzamide) (Table 1, compounds 2a–i). Thus, the catalyst was highly efficient and general for the synthesis of the mentioned compounds.

Table 1 The solvent-free synthesis of 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols from β-naphthol, arylaldehydes and alkyl carbamates or amides using [Dsim]HSO₄ at 80 °C (Scheme 2)

Ar	RO or R′	Product	Time^a/Yield^b	M.p. (°C) (Lit.)
a Reaction time in min. b Isolated yield in %.
C6H5	CH3O	1a	11/98	220–222
				(222–224)^23
3-NO2C6H5	CH3O	1b	7/98	249–251
				(253–255)^23
4-NO2C6H5	CH3O	1c	8/98	203–205
				(200–202)^23
4-ClC6H5	CH3O	1d	15/96	201–203
				(198–200)^25
4-BrC6H5	CH3O	1e	17/98	172–174
C6H5	C6H5CH2O	1f	14/95	177–179
				(179–180)^25
3-CH3OC6H5	C6H5CH2O	1g	25/91	181–183
				(182–184)^25
C6H5	CH3	2a	9/98	239–240
				(239–240)^26
4-NO2C6H5	CH3	2b	10/97	248–250
				(247–249)^26
4-CH3OC6H5	CH3	2c	25/98	182–184
				(184–186)^27
4-BrC6H5	CH3	2d	14/96	226–228
				(228–230)^28
4-FC6H5	CH3	2e	12/97	206–208
				(206–208)^26
4-CH3C6H5	CH2=CH	2f	18/97	216–217
				(214–216)^29
4-NO2C6H5	CH2=CH	2g	10/98	218–220
				(223–225)^29
3-NO2C6H5	C6H5	2h	20/97	215–217
				(214–216)^26
4-CH3OC6H5	C6H5	2i	35/95	202–204
				(208–210)^30

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To compare the efficiency of our catalyst with the reported catalysts for the synthesis of 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols, we have tabulated the results of these catalysts to promote the synthesis of compounds 1a and 2a, in Table 2. As shown in Table 2, our catalyst is superior to the previously reported catalysts in terms of reaction temperature, reaction time and yield. Furthermore, in our presented work, the preparation of both 1-carbamatoalkyl-2-naphthol and 1-amidoalkyl-2-naphthol derivatives have been achieved.

Table 2 Comparison of the results of the synthesis of 1-carbamatoalkyl-2-naphthol 1a and 1-amidoalkyl-2-naphthol 2a using our catalyst with those obtained by the reported catalysts

Catalyst, reaction temperature (°C)	Compound 1a		Compound 2a
	Time (min)	Yield (%)	Time (min)	Yield (%)
a Our catalyst. b In this paper, the synthesis of 1-carbamatoalkyl-2-naphthols have not been reported. c In this paper, the synthesis of 1-amidoalkyl-2-naphthols have not been reported.
[Dsim]HSO4, 80^a	11	98	9	98
1,3-Disulfonic acid imidazolium chloride, 120^15	—^b	—^b	1	95
4-(1-Imidazolium) butanesulfonate, 80^22	120	78	120	80
Preyssler nanoparticles/SiO2, 90^23	3	84	—^c	—^c
NaHSO4/SiO2, 100^25	3.5	81	—^c	—^c
H3PMo12O40.xH2O/SiO2, 120^26	—^b	—^b	15	91
Ce(SO4)2, 85^27	—^b	—^b	2160	72
Montmorillonite K10 clay, 125^28	—^b	—^b	90	89
PEG based dicationic acidic ionic liquid, 80^29	—^b	—^b	5	91
Copper p-toluene sulfonate, 80^30	—^b	—^b	90	94
HClO4/SiO2, 110^31	—^b	—^b	40	89
Sulfamic acid (ultrasound), 30^32	—^b	—^b	15	89
Silphox, 120^33	—^b	—^b	30	92
Silphos, 120^33	—^b	—^b	20	93
[Bmim]Br (microwave), 130^34	—^b	—^b	25	94
Silica chloride (ultrasound), 30^35	—^b	—^b	9	95

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To increase the catalyst worth, its recyclability was tested upon the synthesis of compound 1a. After completion of the reaction, the product was extracted by ethyl acetate (the product is soluble in ethyl acetate, but [Dsim]HSO₄ is not soluble in this solvent), and the remaining catalyst was used for the next run of the reaction. The catalytic activity of [Dsim]HSO₄ was restored within the limits of the experimental errors for four successive runs (see Table 3).

Table 3 Recyclability of [Dsim]HSO₄ in the preparation of 1-carbamatoalkyl-2-naphthol 1a

Run	Amount of recycled catalyst (%)	Time (min)	Yield^a (%)
a Isolated yield. b First use of the catalyst.
1	—^b	11	98
2	96	12	97
3	96	14	94
4	95	18	91

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In a plausible mechanism (Scheme 3), which is supported by the literature;^{15,22,25,29–31} at first, β-naphthol attacks the activated aldehyde by [Dsim]HSO₄ to give intermediate I. Afterward, ortho-quinone methide (o-QM) is produced by removing one molecule of H₂O from I. Subsequently, alkyl carbamate or amide reacts with o-QM as a Michael acceptor to afford 1-carbamatoalkyl-2-naphthols or 1-amidoalkyl-2-naphthols. Because of the specific structure of the catalyst, it can form several hydrogenic bonds with the starting materials by its Brønsted acidic and Lewis basic sites. Due to the formation of these hydrogenic bonds, the starting materials could be positioned in a suitable state to react with each other. On the other hand, 1,3-disulfonic acid imidazolium hydrogen sulfate is more acidic than 1,3-disulfonic acid imidazolium chloride (pH of a 0.02 M solution of [Dsim]HSO₄ and [Dsim]Cl were 1.8 and 2.3, respectively); thus, the former ionic liquid can activate aldehydes and o-QM better than the latter. For these reasons, [Dsim]HSO₄ is a more successful catalyst than [Dsim]Cl for this reaction.

Scheme 3 The proposed mechanism for the preparation of 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols using [Dsim]HSO₄.

Conclusions

In summery, we have prepared and applied a novel ionic liquid, namely 1,3-disulfonic acid imidazolium hydrogen sulfate {[Dsim]HSO₄}, as a highly efficient, homogeneous, recyclable and green catalyst for the synthesis of 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols from β-naphthol, arylaldehydes and alkyl carbamates or amides. This method has solved most of the drawbacks accompanied with previously reported methods.

Acknowledgements

The authors gratefully acknowledge financial support of this work by Research Council of Payame Noor University.

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36. Procedure for the preparation of ionic liquid 1,3-disulfonic acid imidazolium hydrogen sulfate {[Dsim]HSO₄} (Scheme 1): to a round-bottomed flask (100 mL) containing imidazole (0.340 g, 5 mmol) in dry CH₂Cl₂ (50 mL), was added chlorosulfonic acid (1.1885 g, 10.2 mmol) dropwise over a period of 20 min at room temperature. After the addition was completed, the reaction mixture was stirred for 12 h under pressure of nitrogen gas, stood for 5 min, and the CH₂Cl₂ was decanted. The residue was washed with dry CH₂Cl₂ (3×50 mL) and dried under vacuum to give 1,3-disulfonic acid imidazolium chloride {[Dsim]Cl} as a viscous pale yellow oil in 95% yield.¹⁵ Then, sulfuric acid (99.99%) (0.49 g, 5 mmol) was added dropwise to [Dsim]Cl (1.63 g, 5 mmol) over a period of 5 min at room temperature under pressure of nitrogen gas (to remove HCl produced during the reaction). The resulting mixture was stirred for 24 h at 60 °C under a continuous flow of nitrogen gas to give [Dsim]HSO₄ as a viscous yellow oil in 99% yield. IR (Nujol): 624, 1031, 1053, 1085, 1285, 1324, 3100–3400 cm⁻¹; ¹H NMR (300 MHz, DMSO-d₆): δ 7.22 (s, 2H), 8.44 (s, 1H), 11.95 (s, 1H), 13.55 (s, 2H); ¹³C NMR (75 MHz, DMSO-d₆): δ 119.5, 134.0; MS (m/z): 326 (M⁺).
37. General procedure for the synthesis of 1-carbamatoalkyl-2-naphthols and 1-amidoalkyl-2-naphthols (Scheme 2): to a mixture of β-naphthol (0.288 g, 2 mmol), arylaldehyde (2 mmol) and alkyl carbamate or amide (2.4 mmol) in a test tube, was added [Dsim]HSO₄ (0.033 g, 0.1 mmol), and the resulting mixture was firstly stirred magnetically, and after solidification of the reaction mixture with a small rod, at 80 °C. After completion of the reaction, as monitored by TLC, the reaction mixture was cooled to room temperature, H₂O (5 mL) was added, stirred for 3 min and filtered. The solid residue was recrystallized from hot EtOH (95%) to give the pure product.

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Footnote

† Electronic Supplementary Information (ESI) available: General experimental details for the starting materials and instruments, and selected spectral data of the products. See DOI: 10.1039/c2ra20679j/

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