Ionic liquid-supported benzyl azide: an efficient soluble scavenger for alkynes

Abstract

An ionic liquid functionalized with benzyl azide was synthesized and its synthetic utility was evaluated by scavenging excess alkynes in the synthesis of 2,4-disubstituted quinoline via the Povarov reaction. The ionic liquid-supported benzyl azide gave excellent efficiency in alkyne scavenging (85–100%). Purification of the products without column chromatography, ease of monitoring, high loading of the scavenger and shorter scavenging time are some of the advantages of this approach over solid-supported scavengers.

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Introduction

Combinatorial and parallel synthetic techniques are frequently used in organic synthesis to generate a library of pharmaceutically active compounds for biological screening.¹ Need for chromatographic separation of excess reagents and byproducts from the reaction mixture at each step of the synthesis of the product is a serious issue with previous approaches. To overcome these problems, immobilized reagents and scavengers were developed as effective tools for impurity removal/product purification. Scavengers are the materials which detain excess reagent or byproduct after completion of the reaction.² Polymer supported scavengers were introduced as successful tools which selectively eradicated nucleophiles,^2b,3 electrophiles,⁴ dienophiles,⁵ metal ions,⁶ and alkynes⁷ from the reaction mixture leading to pure products. Products can be purified by simply filtering the scavenged compounds and thus avoids column chromatography. These scavengers are widely used but some limitations associated with these scavengers such as slow reaction rate due to biphasic nature of the reaction mixture, requirement of large volume of the solvent to swell the polymer resin and less number of equally accessible active sites in polymer resin to capture the excess reagent led to look for newer alternatives.⁸ Consequently, some new scavengers such as PEG-supported,⁹ fluorous-supported,¹⁰ silica-supported¹¹ and ionic liquid-supported¹² scavenger have been developed. Recently, Otvos et al. have reported iron powder as a readily available copper scavenger in continuous-flow azide–alkyne cycloaddition reaction.¹³ Among these, ionic liquid-supported scavengers have received greater attention in recent years due to their homogenous reaction conditions, high loading capacity, and ease of preparation and easy monitoring of the reaction by various analytical techniques such as NMR, IR and mass spectroscopy.¹⁴ BASIL™ is the first commercial process using ionic liquids for scavenging acids.^14a Some ionic liquid based scavengers are depicted in Fig. 1. Ionic liquid [Cmmim][x] (1) and [2-Aemim][PF₆] (2) have been utilized to scavenge electrophiles like benzyl chlorides, sulfonyl chlorides, isocyanates and isothiocyanates in solution phase synthesis.^3,15 Ionic liquid (3) has been used as a recyclable scavenger for CO₂ from the natural gas.¹⁶ A diol functionalized ionic liquid (4) has been used as an efficient and recyclable scavenger for aldehydes¹⁷ while ionic liquids functionalized with a Michael acceptor has been used to scavenge H₂S gas and thiols.¹⁸ We have also demonstrated that ionic liquid-supported aldehyde (5) can be used as an effective scavenger for primary amine in the synthesis of secondary amines.¹⁹ In continuation of our interest in application of ionic liquids in organic synthesis, herein, we report our finding on use of ionic liquid-supported benzyl azide as an efficient soluble scavenger for alkynes.

Fig. 1 Structure of some ionic liquid based scavengers.

In continuation to our interest towards ionic liquid supported reagents in organic synthesis,^12a,19,20 herein we report the synthesis of ionic liquid functionalized with benzyl azide and its utility as scavenger for excess alkyne in the synthesis of 2,4-disubstituted quinolones.

Result and discussion

The ionic liquid-supported benzyl azide (12) was synthesized from 4-hydroxybenzaldehyde (6) as shown in Scheme 1. Initially, reaction of 6 with 3-chloro-1-bromopropane (7) followed by reduction with sodium borohydride gave (8) in 86% yield. Quaternization of 1,2-dimethylimidazole (9) with 8 followed by anion exchange resulted in the formation of ionic liquid-supported benzyl alcohol (10).²¹ Reaction of 10 with sulfonyl chloride gave ionic liquid-supported benzyl chloride (11) in 93% yield. Finally, substitution of chloro group with azide in 11 resulted in the formation of ionic liquid-supported benzyl azide (12) in 92% yield. The structure of 12 was confirmed by IR, ¹H and ¹³C NMR spectroscopic analysis. The IR spectra of 12 showed the characteristic band for azide group at 2106 cm⁻¹ along with peaks at 1612 and 1427 cm⁻¹ for C=N and C=C stretching, respectively. In the ¹H NMR spectra of 12, a singlet at 4.36 ppm for benzylic protons and doublets at 7.67 and 7.62 ppm for imidazolium proton were observed along with other protons. Similarly, a characteristic peak for benzylic carbon appeared at 53.6 ppm in the ¹³C NMR spectrum of 12 along with peaks for other carbons.

Scheme 1 Synthesis of ionic liquid-supported benzyl azide 12.

The differential scanning calorimetric (DSC) analysis of 12 displayed that it was thermally stable up to 230 °C with melting point of 92 °C (Fig. 2).^20b,22 It is worth to mention that we did not observe any decomposition or loss in reactivity of 12 even after storage for one month at room temperature.

Fig. 2 DSC analysis of ionic liquid-supported benzyl azide 12.

Next, to study the efficacy of 12 as a scavenger, the Povarov reaction between aniline (13), benzaldehyde (14), and phenylacetylene (15) was selected as model reaction. Initially, mixture of 13 (1.0 mmol), 14 (1.0 mmol), 15 (2.0 mmol) and CuI (10 mol%) in ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]), was irradiated under microwave at 80 °C for 10 min (Scheme 2).²³ After completion of the reaction, 12 was added to scavenge excess of 15 and reaction mixture was again irradiated for 10 min at 40 °C, 40 W and 250 psi. On complete scavenging of excess 15, the product 2,4-diphenylquinoline (16) was extracted in ethyl acetate/hexane (1 : 1, v/v) mixture leaving behind scavenged 15 in [Bmim][BF₄] as ionic liquid-supported triazole (17). Capturing of 15 by 12 was monitored by IR and HPLC analysis. Fig. 3 shows an overlap of IR spectra of the reaction mixture before reaction (black), after reaction (red) and after treating with 12 (blue). Vanishing of the peak at 2106 cm⁻¹ for carbon–carbon triple bond stretching on treating the reaction mixture with 12 indicated complete scavenging of 15. The purity of the product was analyzed by HPLC analysis (ESI, Fig. S1†) and it was found that after scavenging the reaction mixture with 12 for 10 min the purity of the product was more than 95%. Similarly, excess alkyne was trapped using 12 for Povarov reaction of 3-methyl-, 4-methyl- and 4-tert-butylpheylacetylene with benzaldehyde and aniline. The HPLC analysis showed that the product was more than 95% pure after treating the reaction mixture with 12 (ESI, Fig. S2–S4†).

Scheme 2 Scavenging of excess alkyne using 12 in Povarov reaction.

Fig. 3 IR spectra of reaction mixture before & after MW irradiation and after treating with 12.

To further understand the scope of alkyne scavenging using 12, it was allowed to react with different type of pure alkynes under similar conditions. The results for capturing of different alkyne using 12 are shown in Table 1. Aliphatic alkynes, aromatic alkynes as well as heterocyclic alkynes were effectively scavenged under these conditions. In general, the scavenging ability of 12 was not much affected by electronic effects on aryl ring of aromatic alkynes. However, internal alkynes remained as such and they could not be scavenged under these conditions. The scavenged products (17) were characterized by ¹H and ¹³C NMR analysis (see ESI†). It is important to note that these ionic liquid-supported triazoles can be cleaved to give corresponding 1,2,3-triazoles.²⁴

Table 1 Scavenging of different alkynes using 12^a

Entry	R	Time (min)	Product	Conversion^b	% Yield^c
a Reaction conditions: 15 (1.0 mmol), CuI (10 mol%), 12 (1.0 mmol) and [Bmim][BF4] (3 mL), microwave irradiation at 40 °C, 40 W and 250 psi.b Based on HPLC-analysis.c Isolated yield of 17.
1	C6H5	10	17a	90	65
2	3-CH3C6H4	10	17b	100	88
3	4-CH3C6H4	10	17c	92	75
4	4-C2H5C6H4	15	17d	85	64
5	4-C(CH3)3C6H4	15	17e	98	79
6	4-OCH3C6H4	15	17f	92	65
7	3,4-(OCH3)2C6H3	10	17g	100	94
8	4-OC7H7C6H4	10	17h	95	76
9	2-CHOC6H4	10	17i	100	82
10	4-COCH3C6H4	10	17j	100	89
11	4-FC6H4	15	17k	94	67
12	–CH2(CH2)2C6H5	10	17l	99	89
13	–C3H7	15	17m	91	63
14	–C10H7	10	17n	100	85
15	–C14H11N2	10	17o	100	94

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Experimental

General

The NMR spectra were recorded on 300 MHz, 400 MHz and 500 MHz spectrometers using CDCl₃ and DMSO-d₆ as solvents. The chemical shifts were expressed in ppm. The IR spectra were recorded on ABB Bomem MB3000 FTIR spectrophotometer. Reactions were monitored by thin-layer chromatography (TLC) carried out on silica-coated aluminum plates (60-F₂₅₄) using UV light as visualizing agent. All the chemicals and reagents were purchased at the highest commercial quality and used without further purification, unless otherwise stated.

General procedure for the synthesis of imidazolium-supported benzyl alcohol (10). A round bottom flask containing 1,2-dimethylimidazole (2.87 g, 30 mmol) and (4-(3-chloropropoxy)phenyl)methanol²¹ (6.00 g, 30 mmol) was heated at 110 °C for 3 h. A thick viscous liquid was obtained which was washed with ethyl acetate (3 × 20 mL) to remove unreacted starting materials. After washing 8.56 g (96%) of pure chloride salt was obtained. Ion metathesis of chloride with hexafluorophosphate was performed using aqueous potassium hexafluorophosphate solution (20 mL, 6.37 g, 35 mmol) at room temperature for 1 h. After filtrating solid precipitate, the filtrate was washed with water and dried in vacuum to get pure 10.

Yield: 97%; white solid; mp 115 °C; ¹H NMR (300 MHz, DMSO-d₆) δ 7.65 (d, J = 2.0 Hz, 1H), 7.60 (d, J = 2.0 Hz, 1H), 7.23 (d, J = 8.5 Hz, 2H), 6.86 (d, J = 8.6 Hz, 2H), 5.05 (t, J = 5.6 Hz, 1H), 4.41 (d, J = 5.6 Hz, 2H), 4.29 (t, J = 6.8 Hz, 2H), 3.97 (t, J = 5.9 Hz, 2H), 3.73 (s, 3H), 2.55 (s, 3H), 2.19 (p, J = 6.4 Hz, 2H); ¹³C NMR (75 MHz, DMSO-d₆) δ 157.4, 144.9, 135.3, 128.4, 122.8, 121.4, 114.4, 64.5, 62.9, 45.3, 35.1, 29.1, 9.5.

General procedure for the synthesis of ionic liquid-supported benzyl chloride (11). Thionyl chloride (1.61 mL, 22.15 mmol) was added dropwise to the suspension of ionic liquid supported benzyl alcohol (3 g, 7.38 mmol) in DCM at 0 °C. The reaction mixture was allowed to stir at room temperature till complete consumption of ionic liquid supported benzyl alcohol (2 h). After completion of reaction, excess of thionyl chloride was removed on rotatory-evaporator to obtain white solid as product (2.90 g, 93%).

General procedure for the synthesis of ionic liquid-supported benzyl azide (12). Ionic liquid supported benzyl chloride (2.90 g, 6.82 mmol) and sodium azide (0.533 g, 8.19 mmol) were dissolved in acetonitrile (15 mL). The reaction mixture was heated at 60 °C for 12 h and then cooled to room temperature. The resulting mixture was filtered by simple filtration method to remove NaCl formed. The filtrate was concentrated under high vacuum to get white solid as product (2.70 g, 92%).

Yield 92%; white solid; mp 95–97 °C; ¹H NMR (300 MHz, DMSO-d₆) δ 7.67 (d, J = 2.1 Hz, 1H), 7.61 (d, J = 2.1 Hz, 1H), 7.31 (d, J = 8.7 Hz, 2H), 6.94 (d, J = 8.7 Hz, 2H), 4.36 (s, 2H), 4.28 (d, J = 6.9 Hz, 2H), 4.00 (d, J = 6.0 Hz, 2H), 3.74 (s, 3H), 2.55 (s, 3H), 2.20 (p, J = 6.5 Hz, 2H); ¹³C NMR (75 MHz, DMSO-d₆) δ 158.4, 144.9, 130.5, 128.2, 122.8, 121.4, 115.0, 64.6, 53.6, 45.2, 35.1, 29.1, 9.5.

Experimental procedure for scavenging alkyne using ionic liquid-supported benzyl azide 12

In microwave vial, a mixture of benzaldehyde (1.0 mmol), aniline (1.0 mmol), phenylacetylene (2.0 mmol) and CuI (10 mol%) in [Bmim]BF₄ (3 mL) was irradiated in close vessel at 80 °C, 50 W, 250 psi for 10 min. The reaction was monitored by TLC. IL-benzyl azide (12) (1.0 mmol) was added in same reaction mixture after complete consumption of benzaldehyde and aniline. The resulting mixture was again irradiated in microwave for 10 min at 40 °C, 40 W, 250 psi to remove excess of alkyne. Organic phase was separated by simple extraction by ethyl acetate/hexane (1 : 1, v/v) mixture leaving ionic liquid-supported triazole in [Bmim][BF₄] at bottom of vial. To isolate IL-triazole we added H₂O to reaction mixture, [Bmim][BF₄] dissolved in H₂O, leaving behind IL-triazole at the bottom of microwave vial. The resulting mixture was washed with toluene (3 × 5 mL) to remove H₂O to obtain the desired compound.

Physical and spectroscopic data for 17a–o.
17a. Yield 65%; green solid; mp 98–101 °C; ¹H NMR (300 MHz, DMSO-d₆) δ 8.60 (s, 1H), 7.82 (d, J = 7.4 Hz, 2H), 7.61 (d, J = 13.7 Hz, 2H), 7.43 (t, J = 7.1 Hz, 2H), 7.33 (d, J = 7.5 Hz, 3H), 6.92 (d, J = 7.8 Hz, 2H), 5.55 (s, 2H), 4.26 (t, J = 6.2 Hz, 2H), 3.97 (t, J = 4.8 Hz, 2H), 3.71 (s, 3H), 2.52 (s, 3H), 2.19–2.13 (m, 2H); ¹³C NMR (75 MHz, DMSO-d₆) δ 158.4, 147.0, 144.9, 131.1, 130.0, 129.3, 128.7, 128.3, 125.5, 122.8, 121.7, 121.4, 115.1, 64.6, 52.9, 45.2, 35.1, 29.0, 9.5.
17b. Yield 88%; brown liquid; ¹H NMR (300 MHz, DMSO-d₆) δ 8.58 (s, 1H), 7.71–7.56 (m, 5H), 7.34 (d, J = 8.1 Hz, 3H), 7.14 (d, J = 7.1 Hz, 1H), 6.93 (d, J = 7.9 Hz, 2H), 5.55 (s, 2H), 4.28 (t, J = 6.1 Hz, 2H), 3.99 (s, 2H), 3.72 (s, 3H), 2.54 (s, 3H), 2.35 (s, 3H), 2.24–2.11 (m, 2H); ¹³C NMR (75 MHz, DMSO-d₆) δ 158.4, 144.9, 138.4, 131.0, 130.0, 129.5, 129.4, 129.2, 128.9, 128.7, 126.1, 122.8, 122.7, 121.7, 121.3, 115.1, 64.6, 53.0, 46.2, 45.2, 35.1, 29.0, 21.4, 9.5.
17c. Yield 75%; white solid; mp 136–140 °C; ¹H NMR (300 MHz, DMSO-d₆) δ 8.54 (s, 1H), 7.72 (d, J = 7.2 Hz, 2H), 7.62 (d, J = 13.6 Hz, 2H), 7.34 (d, J = 7.7 Hz, 2H), 7.24 (d, J = 7.2 Hz, 2H), 6.93 (d, J = 7.8 Hz, 2H), 5.55 (s, 2H), 4.28 (s, 2H), 3.99 (s, 2H), 3.72 (s, 3H), 2.54 (s, 3H), 2.32 (s, 3H), 2.19 (s, 2H); ¹³C NMR (75 MHz, DMSO-d₆) δ 158.4, 147.1, 144.9, 137.6, 130.0, 129.8, 128.7, 128.3, 125.5, 122.8, 121.4, 121.3, 115.1, 64.6, 52.9, 45.2, 35.1, 29.0, 21.2, 9.5.
17d. Yield 64%; colorless solid; mp 100–104 °C; ¹H NMR (400 MHz, CDCl₃) δ 8.13 (s, 1H), 7.71 (d, J = 8.0 Hz, 2H), 7.48 (d, J = 1.9 Hz, 1H), 7.46 (d, J = 1.8 Hz, 1H), 7.33 (d, J = 8.5 Hz, 2H), 7.23 (d, J = 7.9 Hz, 2H), 6.89 (d, J = 8.6 Hz, 2H), 5.53 (s, 2H), 4.33 (t, J = 6.8 Hz, 2H), 4.00 (t, J = 5.6 Hz, 2H), 3.79 (s, 3H), 2.65 (q, J = 7.6 Hz, 2H), 2.59 (s, 3H), 2.32–2.23 (m, 2H), 1.24 (t, J = 7.6 Hz, 3H); ¹³C NMR (101 MHz, CDCl₃) δ 158.0, 144.0, 143.9, 129.6, 128.1, 127.9, 125.3, 122.6, 121.1, 120.1, 114.6, 63.7, 53.1, 45.2, 34.9, 28.8, 28.3, 15.4, 9.2.
17e. Yield 79%; light yellow solid; mp 82–86 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.77 (s, 1H), 7.71 (d, J = 8.1 Hz, 2H), 7.40 (d, J = 8.3 Hz, 2H), 7.22 (d, J = 8.5 Hz, 2H), 7.05 (d, J = 2.0 Hz, 1H), 7.02 (d, J = 1.9 Hz, 1H), 6.80 (d, J = 8.5 Hz, 2H), 5.43 (s, 2H), 4.17 (t, J = 7.2 Hz, 2H), 3.88 (t, J = 5.3 Hz, 2H), 3.62 (s, 3H), 2.43 (s, 3H), 2.22–2.12 (m, 2H), 1.31 (s, 9H); ¹³C NMR (101 MHz, CDCl₃) δ 158.2, 151.3, 144.0, 129.7, 127.7, 127.6, 127.5, 125.7, 125.3, 122.4, 120.8, 114.8, 63.5, 53.5, 45.2, 34.9, 34.6, 31.2, 28.7, 8.9.
17f. Yield 65%; green solid; mp 67–70 °C; ¹H NMR (400 MHz, DMSO-d₆) δ 8.49 (s, 1H), 7.76 (d, J = 7.5 Hz, 2H), 7.62 (d, J = 18.2 Hz, 2H), 7.33 (d, J = 7.3 Hz, 2H), 7.00 (d, J = 7.5 Hz, 2H), 6.93 (d, J = 7.3 Hz, 2H), 5.54 (s, 2H), 4.28 (s, 2H), 3.98 (s, 2H), 3.78 (s, 3H), 3.72 (s, 3H), 2.54 (s, 3H), 2.19 (s, 2H); ¹³C NMR (101 MHz, DMSO-d₆) δ 159.4, 158.4, 147.0, 144.9, 130.0, 128.8, 126.9, 123.7, 122.8, 121.4, 120.8, 115.1, 114.7, 64.6, 55.6, 52.9, 45.2, 35.1, 29.0, 9.5.
17g. Yield 94%; yellow solid; mp 80–84 °C; ¹H NMR (300 MHz, DMSO-d₆) δ 8.51 (s, 1H), 7.60 (d, J = 13.6 Hz, 2H), 7.39 (s, 1H), 7.31 (d, J = 7.2 Hz, 3H), 6.99 (d, J = 8.0 Hz, 1H), 6.91 (d, J = 6.8 Hz, 2H), 5.53 (s, 2H), 4.26 (s, 2H), 3.97 (s, 2H), 3.79 (s, 3H), 3.76 (s, 3H), 3.71 (s, 3H), 2.52 (s, 3H), 2.17 (s, 2H); ¹³C NMR (75 MHz, DMSO-d₆) δ 158.4, 149.4, 149.0, 144.9, 130.0, 128.8, 123.8, 123.4, 122.8, 121.4, 121.0, 117.9, 115.1, 112.4, 109.2, 64.6, 55.9, 52.9, 45.2, 35.1, 29.0, 9.5.
17h. Yield 76%; brown liquid; ¹H NMR (400 MHz, DMSO-d₆) δ 8.48 (s, 1H), 7.75 (d, J = 7.9 Hz, 2H), 7.61 (d, J = 18.2 Hz, 2H), 7.45 (d, J = 6.7 Hz, 2H), 7.39 (t, J = 6.8 Hz, 2H), 7.32 (d, J = 7.5 Hz, 3H), 7.07 (d, J = 7.8 Hz, 2H), 6.91 (d, J = 7.9 Hz, 2H), 5.53 (s, 2H), 5.12 (s, 2H), 4.26 (t, J = 6.1 Hz, 2H), 3.97 (s, 2H), 3.71 (s, 3H), 2.53 (s, 3H), 2.23–2.13 (m, 2H); ¹³C NMR (101 MHz, DMSO-d₆) δ 158.5, 158.4, 144.9, 137.4, 130.0, 128.9, 128.8, 128.3, 128.1, 126.9, 123.9, 122.8, 121.4, 120.9, 118.5, 115.6, 115.1, 69.7, 64.6, 52.9, 45.2, 35.1, 29.0, 9.5.
17i. Yield 82%; brown liquid; ¹H NMR (400 MHz, DMSO-d₆) δ 10.33 (s, 1H), 8.73 (s, 1H), 7.89 (d, J = 4.2 Hz, 1H), 7.76 (s, 2H), 7.70–7.52 (m, 3H), 7.38 (d, J = 5.9 Hz, 2H), 6.94 (d, J = 5.5 Hz, 2H), 5.62 (s, 2H), 4.28 (s, 2H), 3.99 (s, 2H), 3.72 (s, 3H), 2.54 (s, 3H), 2.19 (s, 2H); ¹³C NMR (101 MHz, DMSO-d₆) δ 192.8, 158.5, 144.9, 134.4, 133.8, 133.4, 130.2, 130.0, 129.5, 128.9, 128.5, 128.1, 125.0, 122.8, 121.4, 115.1, 64.6, 53.1, 45.2, 35.1, 29.0, 9.5.
17j. Yield 89%; light yellow solid; mp 180–184 °C; ¹H NMR (400 MHz, DMSO) δ 8.78 (s, 1H), 8.01 (d, J = 7.8 Hz, 4H), 7.62 (d, J = 18.5 Hz, 2H), 7.36 (d, J = 4.8 Hz, 2H), 6.94 (d, J = 4.6 Hz, 2H), 5.59 (s, 2H), 4.28 (s, 2H), 3.99 (s, 2H), 3.73 (s, 3H), 2.60 (s, 3H), 2.54 (s, 3H), 2.19 (s, 2H); ¹³C NMR (101 MHz, DMSO) δ 197.8, 158.5, 146.0, 144.9, 136.3, 135.4, 130.1, 129.4, 128.5, 125.5, 123.0, 122.8, 121.4, 115.1, 64.6, 53.1, 45.2, 35.1, 29.0, 27.1, 9.5.
17k. Yield 67%; brown liquid; ¹H NMR (300 MHz, DMSO-d₆) δ 8.60 (s, 1H), 7.88 (s, 2H), 7.62 (d, J = 13.2 Hz, 2H), 7.41–7.20 (m, 4H), 6.93 (d, J = 6.7 Hz, 2H), 5.56 (s, 2H), 4.28 (s, 2H), 3.99 (s, 2H), 3.72 (s, 3H), 2.54 (s, 3H), 2.18 (s, 2H); ¹³C NMR (75 MHz, DMSO-d₆) δ 158.4, 156.9, 146.2, 144.9, 130.0, 128.6, 127.7, 127.5, 122.8, 121.7, 121.4, 116.4, 116.1, 115.8, 115.1, 64.6, 53.0, 45.2, 35.1, 29.0, 9.5.
17l. Yield 89%; light yellow solid; mp 124–126 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.66 (s, 1H), 7.51–7.41 (m, 2H), 7.26 (d, J = 6.6 Hz, 4H), 7.17 (s, 3H), 6.87 (d, J = 6.6 Hz, 2H), 5.46 (s, 2H), 4.36 (t, J = 6.5 Hz, 2H), 4.00 (d, J = 4.7 Hz, 2H), 3.82 (s, 3H), 2.66 (s, 4H), 2.62 (s, 3H), 2.29 (d, J = 4.2 Hz, 2H), 1.98 (s, 2H); ¹³C NMR (101 MHz, CDCl₃) δ 158.0, 143.9, 141.6, 129.5, 128.2, 128.1, 127.8, 125.6, 122.6, 121.1, 114.5, 63.6, 53.1, 45.2, 35.0, 35.0, 30.8, 28.8, 25.1, 9.2.
17m. Yield 63%; yellow liquid; ¹H NMR (400 MHz, DMSO-d₆) δ 7.88 (s, 1H), 7.64 (d, J = 2.0 Hz, 1H), 7.60 (d, J = 2.0 Hz, 1H), 7.26 (d, J = 8.4 Hz, 2H), 6.90 (d, J = 8.5 Hz, 2H), 5.45 (s, 2H), 4.28 (t, J = 6.8 Hz, 2H), 3.97 (t, J = 5.9 Hz, 2H), 3.72 (s, 3H), 2.56 (d, J = 3.8 Hz, 2H), 2.54 (s, 3H), 2.23–2.13 (m, 2H), 1.58 (dd, J = 14.4, 7.2 Hz, 2H), 0.89 (t, J = 7.3 Hz, 3H); ¹³C NMR (101 MHz, DMSO-d₆) δ 158.3, 144.9, 129.9, 129.0, 122.8, 121.4, 115.0, 64.6, 52.6, 45.2, 35.1, 29.0, 27.5, 22.6, 14.0, 9.5.
17n. Yield 85%; light yellow solid; mp 84–86 °C; ¹H NMR (400 MHz, CDCl₃) δ 8.40 (s, 1H), 8.07 (s, 1H), 7.89 (d, J = 3.1 Hz, 2H), 7.72 (s, 1H), 7.52 (d, J = 2.6 Hz, 3H), 7.37 (d, J = 2.7 Hz, 4H), 6.90 (d, J = 3.5 Hz, 2H), 5.62 (s, 2H), 4.33 (d, J = 3.3 Hz, 2H), 3.99 (d, J = 2.9 Hz, 2H), 3.79 (s, 3H), 2.59 (s, 3H), 2.28 (d, J = 1.8 Hz, 2H); ¹³C NMR (101 MHz, CDCl₃) δ 148.8, 140.2, 139.7, 138.5, 135.6, 134.6, 133.6, 133.1, 132.4, 131.8, 131.4, 130.8, 130.1, 127.6, 126.0, 119.5, 68.3, 58.3, 50.2, 39.9, 33.7, 14.1.
17o. Yield 94%; brown solid; mp 150–153 °C; ¹H NMR (400 MHz, DMSO-d₆) δ 9.09 (s, 1H), 8.67 (s, 1H), 8.45 (s, 1H), 7.90 (d, J = 4.3 Hz, 2H), 7.75 (d, J = 3.9 Hz, 2H), 7.62 (d, J = 16.9 Hz, 2H), 7.36 (s, 2H), 7.25 (s, 2H), 6.95 (s, 2H), 5.60 (s, 2H), 4.27 (s, 2H), 3.99 (s, 2H), 3.72 (s, 3H), 3.36 (s, 3H), 2.32 (s, 3H), 2.18 (s, 2H); ¹³C NMR (101 MHz, DMSO-d₆) δ 158.4, 144.8, 144.2, 144.1, 137.7, 130.1, 129.7, 128.5, 125.8, 123.7, 123.5, 122.8, 121.8, 121.3, 116.8, 115.1, 64.6, 53.1, 45.2, 35.1, 29.0, 21.3, 9.5.

Conclusion

In summary, we have described the synthesis of ionic liquid-supported benzyl azide and explored its application as a soluble scavenger to scavenge alkynes. Purification of the reaction product without column chromatography, requirement of lesser amount of scavenger and rapid scavenging process, ease in monitoring progress of the reaction are some of the advantages of this approach over analogous polymer-supported scavenger.

Acknowledgements

We acknowledge the Council for Scientific and Industrial Research (CSIR), New Delhi for the financial support (01(115)/13/EMR-II). SC thanks CSIR, New Delhi for a senior research fellowship, Saroj and KP thank UGC-BSR & UGC, New Delhi for junior research fellowship, respectively.

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Footnote

† Electronic supplementary information (ESI) available: HPLC analysis and copies of ¹H & ¹³C NMR of 10, 12, 17a–o. See DOI: 10.1039/c5ra09407k

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