Organic Chemistry International

Volume 2014 (2014), Article ID 260726, 9 pages

http://dx.doi.org/10.1155/2014/260726

Research Article

Glycerol Containing Triacetylborate Mediated Syntheses of Novel 2-Heterostyryl Benzimidazole Derivatives: A Green Approach

Ashok Kumar Taduri, P. N. Kishore Babu, and B. Rama Devi

Department of Chemistry, College of Engineering, Jawaharlal Nehru Technological University Hyderabad, Kukatpally, Hyderabad Andhra Pradesh 500 085, India

Received 9 November 2013; Revised 20 February 2014; Accepted 24 March 2014; Published 27 April 2014

Academic Editor: Robert Salomon

Copyright © 2014 Ashok Kumar Taduri et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A very simple, mild, efficient, and novel green methodology has been developed for the syntheses of some 2-hetero/styryl-benzimidazoles. Title compounds were synthesized by the condensation of -phenylenediamine with cinnamic acids at 150–180°C for 5-6 h using glycerol containing triacetylborate (10–20 mol%) as the reaction medium. In an alternative approach, condensation of 2-methylbenzimidazole derivatives with aromatic aldehydes was done using glycerol containing triacetylborate (10–20 mol%) as the reaction medium.

1. Introduction

Using solvents in chemical synthesis represents a greater challenge in terms of green chemistry and among them solvents like water, PEG-600, and ionic liquids have proved to be potential green solvents for organic synthesis. In the past [1], glycerol as solvent has not been used extensively for organic synthesis and in recent years, various developments have been made to prove that glycerol can be used as an alternate solvent and can feasibly be considered as an efficient green solvent [2, 3]. Cinnamic acids and its derivatives (i-iv) (Figure 1) such as ethyl cinnamate, sodium cinnamate, and benzylcinnamate have century old history as potential antituberculosis agents [4–7]. Benzimidazole scaffold being an important pharmacophore and privileged structure in medicinal chemistry [8, 9], a new series of 5-(bromo/nitro)-2-styryl-benzimidazoles were synthesized earlier using cinnamic acids in ethylene glycol under reflux conditions and showed good antimicrobial and antitubercular activities (vii) [10, 11]. 2-Styryl-benzimidazoles and its analogues (v) were also reported as MAO-B inhibitors [12], in which the styryl compounds were synthesized using 2-methylbenzimidazole condensed with aromatic aldehydes refluxing at 180°C for 24 h (Figure 1).

Figure 1: Cinnamic acid and its derivatives showing various biological activities.

Recently, some new styryl benzimidazoles were reported as probes for imaging neurofibrillary tangles in Alzheimer’s disease [13], in which new iodo derivatives of styryl-benzimidazoles were prepared by the condensation of substituted -phenylenediamines with cinnamaldehydes refluxing in DMF [13]. Other methods for the preparation of 2-styrylbenzimidazoles involves the use of PPA at 200°C gave a low 30% yield [10] and the conventional Phillips method, in which -phenylenediamine condensed with cinnamic acids refluxing in 4 N HCl, for the synthesis of 2-styrylbenzimidazoles, resulted in the recovery of starting material [10].

The widespread interest in benzimidazole containing skeletons having a wide spectrum of activity has promoted extensive studies for their synthesis. While there are many strategies available for benzimidazole synthesis, there are a few methods available for the preparation of 2-styryl type of benzimidazoles. The earlier methods involve nongreen [14–17] routes like using PPA or high temperatures, long reaction times, various work-up methods, and low yields made to look for other methods for the synthesis of 2-styryl type of benzimidazoles. Though the above methods were effective in completion of the reaction, but they suffer from eco-friendly practise. Therefore, the discovery of practicable greener routes utilizing easily available starting materials like glycerol and triacetylborate which were considered as environmentally friendly for the synthesis of 2-hetero/styrylbenzimidazoles continues to attract the attention of researchers.

In continuation of our earlier work on the synthesis of 2-styrylbenzimidazoles, now we wish to extend our approach by using other heterocyclic aldehydes like furfuraldehyde, piperonaldehyde and thiophene-2-aldehyde, and so forth, in addition to benzaldehyde derivatives, in a very green way by using glycerol and triacetylborate as green and as a recyclable reaction medium.

2. Materials and Methods

All the reagents used in this work were obtained from commercial suppliers. Solvents were freshly distilled before being used. Melting points were determined using a Buchi Melting Point B-545 apparatus and are uncorrected. TLC analyses were done on glass plates coated with silica gel GF-254 and spotting was done using Iodine/UV lamp. IR spectra were recorded on a Perkin-Elmer model 446 instrument in KBr phase. ¹H NMR were recorded in CDCl₃/DMSO using 400 MHz Varian Gemini spectrometer and mass spectra were recorded on LC-MS spectrometer, model HP5989A. ¹³C NMR was recorded in DMSO using 100 MHz spectrometer.

2.1. General Procedure for the Synthesis of 2-Styryl-benzimidazole Derivatives from O-PDA and Cinnamic Acids

An intimate mixture of o-penylenediamine 1 (1.08 g, 10 mM) was dissolved in a 100 mL round bottom flask. To this, triacetylborate (0.2 g, 10 mol%) was added, followed by the addition of cinnamic acids 2 (10 mM) and allowed it to boil at 160–180°C in oil-bath for 3 h using Dean-Stark apparatus. The completion of the reaction was monitored by checking TLC. At the end of this period, the reaction mixture was poured into ice cold water. The pH of the solution was adjusted to 8.0–10.0. The formed product was filtered, dried and recrystallized by using a suitable solvent.

2.2. General Procedure for the Synthesis of 2-Styryl-benzimidazole Derivatives from 2-Methylbenzimidazoles and Aromatic Aldehydes

An intimate mixture of 2-methylbenzimidazole 4(a-b) (1.32 g, 10 mM) was dissolved in 10 mL of glycerol in a 100 mL round bottom flask. To this, triacetylborate (0.2 g, 10 mol%) was added, followed by the addition of corresponding aromatic aldehydes 5(a-i) (10 mM) and allowed it to boil at 160–180°C in oil-bath for 3 h using Dean-Stark apparatus. The completion of the reaction was monitored by checking TLC. At the end of this period, the reaction mixture was poured into ice cold water and the pH of the solution was adjusted to 8.0–10.0. Filter the compound and recrystallize it by using a suitable solvent.

2.2.1. (E)-2-(2-(Benzo[d][1,3]dioxol-5-yl)vinyl)-1H-benzimidazole (3f)

Wheatish light brown crystals, Yield (2.2 g, 85%), m.p 200–204°C, IR (KBr, in cm⁻¹): 3435 (–NH), 2922 (=C–H), 1683 (C=N), 1620 (C=C), ¹H NMR (DMSO-, 400 MHz) ppm: 2.5 (s, 2H, –CH₂), 7.2–7.4 (d, 1H, –C=CH, J H-H = 16.4 Hz), 7.5–7.3 (m, 4 aryl, 3 phenyl protons), 7.8–8.0 (d, 1H, –CH=C, J H-H = 16.4 Hz), 10.0 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 102.29, 108.54, 109, 122, 124, 128, 130, 132, 134, 138, 148, 152 (1 dioxymethylene carbon, 6 aryl carbons, 6 phenylic carbons, 2 vinylic carbons and 1 imidazole quaternary carbon), MS (/): 265.10 (M+∙), Anal. Calcd. for C₁₆H₁₂N₂O₂: C, 72.72; H, 4.58; N, 10.60; O, 12.11% Found: C, 72.84; H, 4.70; N, 10.68; O, 12.25%.

2.2.2. (E)-2-(2-(Furan-2-yl)vinyl)-1H-benzimidazole (3g)

Brown crystals, Yield (1.97 g, 94%), m.p 218–220°C, IR (KBr, in cm⁻¹): 3405 (–NH), 3101 (=C–H), 1894 (C=N), 1633 (C=C), ¹H NMR (DMSO-, 400 MHz) ppm: 6.91–6.95 (d, 1H, –C=CH (vinylic proton), J H-H = 16.4 Hz), 7.16–7.18 (t, 1H, furan proton), 7.23–7.28 (q, 2H, phenylic protons), 7.44–7.45 (d, 1H, furan proton), 7.57–7.61 (q, 2H, phenylic protons), 7.67-7.68 (d, 1H, furan proton), 7.92-7.96 (d, 1H, –CH=C–(vinylic proton), J H-H = 16.4 Hz), 10.0 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 114.47, 114.50, 122.87, 127.96, 128.51, 129.15, 129.48, 137.39, 140.33, 149.87 (6 aryl carbons, 4 furyl carbons, 2 vinylic carbons and 1 imidazole quaternary carbon), MS (/): 211.10 (M+∙), Anal. Calcd. for C₁₃H₁₀N₂O: C, 74.27; H, 4.79; N, 13.33; O, 7.61% Found: C, 74.34; H, 4.84; N, 13.60; O, 7.73%.

2.2.3. (E)-2-(2-(Thiophen-2-yl)vinyl)-1H-benzimidazole (3h)

Light green crystals, Yield (1.97 g, 92%), m.p 198–200°C, IR (KBr, in cm⁻¹): 3405 (–NH), 3101 (=C–H), 1894 (C=N), 1633 (C=C), ¹H NMR (DMSO-, 400 MHz) ppm: 6.91–6.95 (d, 1H, –C=CH (vinylic proton), J H-H = 16.4 Hz), 7.16–7.18 (t, 1H, thiophenyl proton), 7.23–7.28 (q, 2H, phenylic protons), 7.44-7.45 (d, 1H, thiophenyl proton), 7.57–7.61 (q, 2H, phenylic protons), 7.67–7.68 (d, 1H, thiophenyl proton), 7.92–7.96 (d, 1H, –CH=C–(vinylic proton), J H-H = 16.4 Hz), 10.0 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 114.47, 114.50, 122.87, 127.96, 128.51, 129.15, 129.48, 137.39, 140.33, 149.87 (6 aryl carbons, 4 thiophenyl carbons, 2 vinylic carbons and 1 imidazole quaternary carbon), MS (/): 227.07 (M+∙), Anal. Calcd. for C₁₃H₁₀N₂S: C, 69.00; H, 4.45; N, 12.38; S, 14.17% Found: C, 69.24; H, 4.52; N, 12.40; S, 14.27%.

2.2.4. (E)-Phenyl(2-styryl-1H-benzimidazol-6-yl)methanone (3i)

Light orange crystals, Yield (2.9 g, 90%), m.p 202–204°C, IR (KBr, in cm⁻¹): 3401 (–NH), 2956 (=C–H), 1891 (C=N), 1610 (C=C), 1644 (C=O), ¹H NMR (DMSO-, 400 MHz) ppm: 7.33–7.37 (d, 1H, –C=CH, vinylic proton, J H-H = 16.4 Hz), 7.48–7.67 (m, 8 aryl, 5 phenyl), 10.2 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 102, 106, 108, 110, 112, 120, 121, 122, 128, 129, 130, 132, 142, 148, 152, 190 (12 aryl carbons, 6 phenyl carbons, 2 vinylic carbons, 1 imidazole quaternary carbon and 1 carbonyl carbon), MS (/): 325.2 (M+∙), Anal. Calcd. for C₂₂H₁₆N₂O: C, 81.46; H, 4.97; N, 8.64; O, 4.93% Found: C, 81.54; H, 4.99; N, 8.76; O, 4.97%.

2.2.5. (E)-(2-(4-Fluorostyryl)-1H-benzimidazol-6-yl)(phenyl)methanone (3j)

Black crystals, Yield (1.97 g, 90%), m.p 178–180°C, IR (KBr, in cm⁻¹): 3432 (–NH), 3178 (=C–H), 1909 (C=N), 1628 (C=C), 1700 (C=O), ¹H NMR (DMSO-, 400 MHz) ppm: 7.09–7.13 (d, 1H, –C=CH– vinulic proton, J H-H = 16.4 Hz), 7.3–7.9 (m, 8 aryl, 4 phenyl), 8.09–8.13 (d, 1H, –CH=C–, vinylic proton, J H-H = 16.4 Hz), 10.2 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 102.29, 106.24, 108.53, 110, 112, 120, 121, 122, 128.51, 129, 130, 132, 142, 148, 152, 190.91 (12 aryl carbons, 6 phenyl carbons, 2 vinylic carbons, 1 imidazole quaternary carbon and 1 carbonyl carbon), MS (/): 343.2 (M+∙), Anal. Calcd. for C₂₂H₁₅FN₂O: C, 77.18; H, 4.42; F, 5.55; N, 8.18; O, 4.67% Found: C, 77.32; H, 4.69; F, 5.69; N, 8.27; O, 4.73%.

2.2.6. (E)-(2-(4-Chlorostyryl)-1H-benzimidazol-6-yl)(phenyl)methanone (3k)

Brown crystals, Yield (3.2 g, 90%), m.p 130–132°C, IR (KBr, in cm⁻¹): 3426 (–NH), 2898 (=C–H), 1909 (C=N), 1601 (C=C), 1680 (C=O), ¹H NMR (DMSO-, 400 MHz) ppm: 7.09–7.13 (d, 1H, –C=CH– vinulic proton, J H-H = 16.4 Hz), 7.3–7.9 (m, 8 aryl, 4 phenyl), 8.09–8.13 (d, 1H, –CH=C–, vinylic proton, J H-H = 16.4 Hz), 10.2 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 102.29, 106.24, 108.53, 110, 112, 120, 121, 122, 128.51, 129, 130, 132, 142, 148, 152, 190.91 (12 aryl carbons, 6 phenyl carbons, 2 vinylic carbons, 1 imidazole quaternary carbon and 1 carbonyl carbon), MS (/): 359.1 (M+∙), Anal. Calcd. for C₂₂H₁₅ClN₂O: C, 73.64; H, 4.21; Cl, 9.88; N, 7.81; O, 4.46% Found: C, 73.52; H, 4.39; Cl, 9.96; N, 7.87; O, 4.53%.

2.2.7. (E)-(2-(4-Nitrostyryl)-1H-benzimidazol-6-yl)(phenyl)methanone (3l)

Light yellow crystals, Yield (3.2 g, 90%), m.p 210–212°C, IR (KBr, in cm⁻¹): 3410 (–NH), 2960 (=C–H), 1680 (C=N), 1620 (C=C), 1670 (C=O), ¹H NMR (DMSO-, 400 MHz) ppm: 7.0–8.3 (m, 8 aryl, 4 phenyl and 2 vinylic protons, J H-H = 16.4 Hz), 10.2 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 102, 106, 108, 110, 112, 120, 121, 122, 128, 129, 130, 132, 142, 148, 152, 190 (12 aryl carbons, 6 phenyl carbons, 2 vinylic carbons, 1 imidazole quaternary carbon and 1 carbonyl carbon), MS (/): 370.2 (M+∙), Anal. Calcd. for C₂₂H₁₅N₃O₃: C, 71.54; H, 4.09; N, 11.38; O, 12.99% Found: C, 71.62; H, 4.29; N, 11.47; O, 13.05%.

2.2.8. (E)-(2-(4-Methylstyryl)-1H-benzimidazol-6-yl)(phenyl)methanone (3m)

Black crystals, Yield (3.2 g, 90%), m.p 198–200°C, IR (KBr, in cm⁻¹): 3420 (–NH), 3214 (=C–H), 1642 (C=N), 1603 (C=C), 1616 (C=O), ¹H NMR (DMSO-, 400 MHz) ppm: 2.4 (s, 3H, –CH₃), 7.0–8.2 (m, 8 aryl, 4 phenyl and 2 vinylic protons, J H-H = 16.4 Hz), 10.0 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 21, 102, 106, 108, 110, 112, 120, 121, 122, 128, 129, 130, 132, 142, 148, 152, 190 (12 aryl carbons, 6 phenyl carbons, 2 vinylic carbons, 1 imidazole quaternary carbon and 1 carbonyl carbon), MS (/): 370.2 (M+∙), Anal. Calcd. for C₂₂H₁₅N₃O₃: C, 71.54; H, 4.09; N, 11.38; O, 12.99% Found: C, 71.62; H, 4.29; N, 11.47; O, 13.05%.

2.2.9. (E)-(2-(2-(Benzo[d][1,3]dioxol-5-yl)vinyl)-1H-benzimidazol-6-yl)(phenyl)methanone (3n)

Dark brown crystals, Yield (3.2 g, 90%), m.p > 240°C, IR (KBr, in cm⁻¹): 3422 (–NH), 2917 (=C–H), 1644 (C=N), 1575 (C=C), 1609 (C=O), ¹H NMR (DMSO-, 400 MHz) ppm: 2.5 (s, 2H, –CH₂), 6.2–6.26 (d, 1H, –C=CH–, vinylic proton, J H-H = 16.4 Hz), 7.0–8.0 (m, 8 aryl, 4 phenyl and 1 vinylic protons, J H-H = 16.4 Hz), 9.8 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 114.30, 115.04, 116.96, 125.42, 128.45, 128.68, 129.12, 129.30, 129.48, 129.59, 131.08, 132.22, 132.33, 133.76, 134.37, 137.58, 137.64, 137.74, 195.17 (1 dioxymethylene carbon, 12 aryl carbons, 6 phenylic carbons, 2 vinylic carbons, 1 imidazole quaternary carbon, 1 carbonyl carbon), MS (/): 370.2 (M+∙), Anal. Calcd. for C₂₂H₁₅N₃O₃: C, 71.54; H, 4.09; N, 11.38; O, 12.99% Found: C, 71.62; H, 4.29; N, 11.47; O, 13.05%.

2.2.10. (E)-(2-(2-(Furan-2-yl)vinyl)-1H-benzimidazol-6-yl)(phenyl)methanone (3o)

Black crystals, Yield (3.2 g, 90%), m.p 100–102°C, IR (KBr, in cm⁻¹): 3423 (–NH), 2921 (=C–H), 1642 (C=N), 1597 (C=C), 1617 (C=O), ¹H NMR (DMSO-, 400 MHz) ppm: 6.6–8.1 (m, 8 aryl, 3 furanyl and 2 vinylic protons, J H-H = 16.4 Hz), 10.2 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 111, 112, 118, 125.12, 128.45, 128.68, 128.91, 129.47, 130.43, 131.85, 132.27, 137.75, 139.96, 152.32, 195.27 (12 aryl carbons, 4 furanyl carbons, 2 vinylic carbons, 1 imidazole quaternary carbon and 1 carbonyl carbon), MS (/): 370.2 (M+∙), Anal. Calcd. for C₂₂H₁₅N₃O₃: C, 71.54; H, 4.09; N, 11.38; O, 12.99% Found: C, 71.62; H, 4.29; N, 11.47; O, 13.05%.

2.2.11. (E)-Phenyl(2-(2-(thiophen-2-yl)vinyl)-1H-benzimidazol-6-yl)methanone (3p)

Light green crystals, Yield (3.2 g, 90%), m.p 108–110°C, IR (KBr, in cm⁻¹): 3427 (–NH), 3060 (=C–H), 1633 (C=N), 1597 (C=C), 1614 (C=O), ¹H NMR (DMSO-, 400 MHz) ppm: 6.6–8.1 (m, 8 aryl, 3 thiophenyl and 2 vinylic protons, J H-H = 16.4 Hz), 10.2 (s, 1H, –NH of benzimidazole), ¹³C NMR (DMSO-, 100 MHz) ppm: 111, 112, 118, 125.12, 128.45, 128.68, 128.91, 129.47, 130.43, 131.85, 132.27, 137.75, 139.96, 152.32, 195.27 (12 aryl carbons, 4 thiophenyl carbons, 2 vinylic carbons, 1 imidazole quaternary carbon and 1 carbonyl carbon), MS (/): 370.2 (M+∙), Anal. Calcd. for C₂₂H₁₅N₃O₃: C, 71.54; H, 4.09; N, 11.38; O, 12.99% Found: C, 71.62; H, 4.29; N, 11.47; O, 13.05%.

3. Results and Discussion

In continuation of our earlier strategies for the establishment of 2-styryl-benzimidazoles using green solvents like ethylene glycol [10], now we have developed a highly efficient and simple green methodology for the synthesis of 2-styryl-benzimidazole derivatives 3(a–p), by direct condensation of equivalent amounts of substituted -phenylenediamines 1(a-b) with various cinnamic acids 2(a–h) using 10–20 mol % of triacetylborate and glycerol (10 mL) as reaction medium at 160–180°C for 3–5 h (Scheme 1).

Scheme 1: Synthesis of 2-styrylbenzimidazoles 3(a–p) from 1(a-b) and 2(a–h).

In an alternative approach, 2-methylbenzimidazoles 4(a-b) were condensed with a variety of aromatic aldehydes 5(a–h) using glycerol (10 mL) as solvent and triacetylborate (10–20 mol %) at 150–180°C for 5-6 h resulted 2-styryl-benzimidazole derivatives 3(a–p) (Scheme 2). The most important advantages of our method are as follows: (a) it is totally green procedure that involves homogeneous catalysis; (b) only 10–20 mol % of triacetylborate is sufficient to complete the reaction; (c) the very simple workup does not involve the use of any acids; (d) the products, which in general possess stable high melting points, solidify readily and hence can be very easily collected and recrystallized from suitable solvent without need for further purification; (e) used triacetylborate is cheap and readily available; (f) the yields of all the products are good and the reaction procedure is highly a general one with 100% conversion in all the cases (no starting materials were apparent by TLC).

Scheme 2: Synthesis of 2-styrylbenzimidazoles 3(a–p) from 4(a-b) and 5(a–h).

In order to ascertain the necessity of triacetylborate, glycerol, or both, the correct solvent and the required temperature, the reaction of 4-chlorobenzaldehyde (5c) with 2-methylbenzimidazole (4a) was carried out separately in glycerol alone (entry 2), without glycerol and triacetylborate, that is, solvent free manner (entry 1), only triacetylborate (2–20 mol %) (entries 3, 4, and 5), or both triacetylborate and glycerol (10 mL) (entries 6 to 9) or in other solvents (entries 10, 11) at various temperatures (Table 1). It was found that only 10 mol% of triacetylborate and glycerol as reaction medium at 170°C (Table 1, entry 8#) provided the best conditions for the synthesis of 2-styryl type benzimidazoles in good yields. The method is further suitable for heteroaromatic aldehydes like furfural, thiophene-2-carboxaldehyde, and piperonaldehyde.

Table 1: Synthesis of 2-(2-(4-chloro-phenyl)-vinyl)-1-H-benzimidazoles.

Entry
Triacetylborate (mol-%)
glycerol* used (+)

not used (−)
Solvents used
Time (h)
Yield (%)

1.
—
−
—
05
50

2.
—
+
Glycerol
05
50

3.
2
−
—
05
60

4.
5
−
—
05
68

5.
10
−
—
05
70

6.
2
+
Glycerol
05
60

7.
5
+
Glycerol
05
78

8. #
10 #
+ #
Glycerol #
05 #
90 #

9.
20
+
Glycerol
05
84

10.
20
−
Ethylene glycol
06
60

11.
20
−
PEG-600
06
0

Reactions were carried out in oil bath using Dean-Stark apparatus at 160°C (entries 2, 6, 7, 8, and 9), 120°C (entry 11), 190°C (entry 10), or under solvent free conditions at 180°C (entries 1, 3, 4, and 5).

10 mL glycerol was used in all the above reactions.

Triacetylborate is prepared according to the reported procedure [18], which is soluble in glycerol, maintaining the optimum acidity of the reaction medium and at the same time it is not necessary to use acids like H₂SO₄ or HCl and oxalic acid to prepare salts or to recover the product during the work-up process. Both triacetylborate and glycerol, being highly soluble in water, can be recycled after the reaction. The water solution obtained as the filtrate can be lyophilized to reclaim the boric acid (tested by the usual way of obtaining the green flame of ethyl borate with ethanol without adding Conc. H₂SO₄) which, when dried, could be further reused for the preparation of triacetylborate.

The reaction proceeds by the formation of a solketal/glycerol aldehyde acetal [19, 20] type of intermediate (ix) formed by the condensation of glycerol (viii) and aromatic aldehyde (5) in presence of triacetylborate (vii), which act as a Lewis acid, is further reacted with 2-methylbenzimidazole 4(a-b), and forms 2-styryl-benzimidazole 3(a-p) by losing water molecules. At the end glycerol as a by-product can be collected as filtrate and purified for further usage (Scheme 3).

Scheme 3: Plausible mechanism for the formation of 2-styrylbenzimidazoles (3) from (4) and arylaldehyde (5) using glycerol and triacetylborate.

This reaction mechanism can be explained in another approach, in which triacetylborate (vii) and glycerol (viii) react together to form an intermediate by losing two acetate molecules and combine with cinnamic acid (4), where the boron loses its third acetate molecule and accepts electrons from the oxygen atom of cinnamic acid and forms intermediate (x), which is further reacts with o-pheneylenediamine (1) and undergoes intramolecular cyclisation to form the final product (3) (Scheme 4).

Scheme 4: The alternative proposed mechanism for the formation of (3) from (1) and (4) using glycerol and triacetylborate.

The above reactions using triacetylborate as the reagent was found to be general and has been extended to all other derivatives, yielding 3(a–p). All the reactions done can be summarized in Table 2.

Table 2: Synthesis of 2-hetero/styryl-benzimidazoles 3(a–p) from cinnamic acids 2(a–h) and benzaldehydes 5(a–h).

S.

number
Product 3(a–p)
Method-1 (diamine + cinnamic acid)
Method-2 (2-MeBz + aldehydes)
M.P (°C)

Time

(h)
Temp

(°C)
Yield

(%)
Time

(h)
Temp

(°C)
Yield

(%)

1.

4
160
90
6
160
88
205 (202) [16]

2.

3.5
150
86
6
160
92
114-115 (113) [15]

3.

3.5
160
90
6
160
90
221–223 (223-224) [16]

4.

4
150
90
6
160
92
>240 (262-263) [16]

5.

5
160
85
6
160
88
210–212 (214–216) [16]

6.

5
160
78
6
150
85
200–204

7.

5
150
92
5
150
94
218–220

8.

5
160
92
5
150
90
198–200

9.

5
160
90
6
160
88
202–204

10.

5
160
84
6
160
90
178–180

11.

5
160
90
6
160
90
130–132

12.

5
150
92
6
160
86
210–212

13.

5
150
88
6
160
86
198–200

14.

5
160
84
5
160
88
>240

15.

5
160
80
5
150
82
100–102

16.

5
150
78
5
150
80
108–110

4. Conclusions

We have highlighted the potential of triacetylborate and glycerol for the first time as a cheap, mild, highly efficient, nontoxic, and recyclable reaction medium for the high yielding synthesis of 2-hetero/styryl-benzimidazoles at 160–180°C with wide variations both in the aldehydes and cinnamic acids and also in the 2-methylbenzimidazoles. Thus, this green approach opens an important alternative to the use of volatile organic solvents and, therefore, should be highly beneficial to both academics and industry.

Conflict of Interests

The authors declare that there is no conflict of interests regarding publication of this paper.

Acknowledgments

The authors are thankful to the authorities of Jawaharlal Nehru Technological University, Hyderabad, for providing laboratory facilities. The authors are also grateful to the CSIR-CDRI, Lucknow, for providing financial support in the form of OSDD project. The authors are also thankful to CFRD, Osmania University, Hyderabad, for providing spectral analysis.

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