Catalysis by FeF₃ in water: a green synthesis of 2-substituted 1,3-benzazoles and 1,2-disubstituted benzimidazoles

Abstract

FeF₃ in water facilitated the reaction of 1,2-phenylenediamine and 2-aminothiophenol with 1 equivalent of an alkyl- or aryl-aldehyde leading to 1,3-benzazoles in open air. The process afforded 1,2-disubstituted benzimidazoles when 1,2-phenylenediamine was reacted with 2 equivalents of aryl aldehyde. The methodology is operationally simple, free from the use of hazardous organic solvents and chemoselective. The products were isolated by simple filtration and the catalyst can be recovered and recycled.

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Introduction

Development of atom-efficient chemical processes using environmentally benign chemistry that minimizes or eliminates the formation of by-products has become a frontier area of research in modern organic synthesis.¹ Thus a number of transition metal catalysts have been developed that enable chemical synthesis to proceed via simple addition reactions.^1e

As an important class of heterocyclic compounds, 1,3-benzazoles (A, Fig. 1) i.e. benzimidazole and benzothiazole, are considered to be privileged structures in the area of medicinal chemistry.² This is exemplified by a range of commonly used drugs such as the proton-pump inhibitor Omeprazole (B), TTX-sensitive sodium channels blocker Riluzole (C), H1 receptor antagonist Mizolastin (D), AT1 receptor antagonist Telmisartan (E) and direct thrombin inhibitor Dabigatran (Fig. 1).

Fig. 1 Examples of 1,3-benzazole based drugs.

The most commonly used synthetic methods for accessing benzimidazole derivatives include condensation of 1,2-phenylenediamines with (i) carboxylic acids^3,4 or their derivatives⁵ or (ii) aldehydes followed by oxidative cyclodehydrogenation.^6,7 Similarly, 2-substituted benzothiazoles can be synthesized⁸via condensation of 2-aminothiophenol with (i) carboxylic acids⁹ followed by dehydration or (ii) aldehydes under oxidative conditions.¹⁰ While many of these methods are quite effective and useful most of them however suffer from the use of acidic or similar reagents or hazardous organic solvents that are not environmentally compatible and produce a large amount of waste. Moreover, the requirement for longer reaction times, higher temperatures and expensive reagents and catalysts are the other drawbacks of these methodologies. In some of these cases the formation of side products, e.g. 1,2-disubstituted benzimidazoles, along with the desired 2-substituted derivatives was also observed.

For several years iron salts have been reported as promising and alternative transition-metal catalysts that have received much attention due to their low price, sustainability, ready availability, nontoxicity, and environmentally friendly properties.¹¹ More recently, in addition to its commercial use in the production of ceramics, FeF₃ has also received attention in organic synthesis.¹² This includes its use in the chemoselective addition of cyanide to aldehydes, synthesis of polyhydroquinolines, direct thiolation of an arene C–H bond, etc. Additionally, the use of water as an inexpensive and environmentally friendly solvent in commercially available and water stable FeF₃ catalyzed reactions has also been explored.^12a This is remarkable as the use of large volumes of volatile hazardous organic solvents in industrial processes poses a serious threat to the environment. Thus, procedures involving alternative benign solvents for reaction, isolation and purification are of high priority in industry. This prompted us to develop a green and atom-efficient one-pot synthesis of 1,3-benzazoles (benzimidazole and benzothiazole) via an FeF₃ catalyzed reaction in water. Herein we report our preliminary findings on the FeF₃ mediated condensation of 1,2-phenylenediamine and 2-aminothiophenol (1) with 1.0 equivalent of an alkyl- or aryl-aldehyde (2) leading to 1,3-benzazoles (3) in open air (Scheme 1). The synthesis of 1,2-disubstituted benzimidazoles is also presented.

Scheme 1 FeF₃ catalyzed synthesis of 1,3-benzazoles in water.

Results and discussion

Initially, we examined the reaction of 1,2-phenylenediamine (1a) with p-chloro benzaldehyde (2a) in the presence of FeCl₃ in water at 60 °C for 7 h when the desired benzimidazole (3a) was isolated in 75% yield (entry 1, Table 1). While the yield of 3a was increased when FeCl₃·6H₂O was used in place of FeCl₃ (entry 2, Table 1) the maximum yield however was achieved using FeF₃ (entry 3, Table 1) indicating it is the most suitable for the present reaction. The use of other catalysts e.g. NH₄F and TBAF was examined but found to be less effective (entries 4–5, Table 1). The yield of 3a was decreased when the reaction was carried out at a lower temperature (entry 6, Table 1). The use of other solvents was also examined (entries 7–8, Table 1) among which 1,4-dioxane, EtOH and MeOH were found to be effective. Nevertheless, being an inexpensive and readily available green solvent, water was chosen for further study of the present FeF₃ mediated condensation reaction. To test the recyclability of the catalyst used FeF₃ was recovered by simple filtration followed by evaporating the filtrate to dryness (see the experimental section) and reused in the same reaction when 3a was isolated without significant loss of yield. The yield of 3a was found to be 83, 81 and 78 after the 1st, 2nd and 3rd recovery and reuse of the catalyst. A comparison of the XRD spectrum obtained for fresh FeF₃ and the reused catalyst indicated no change in its crystalline nature (Fig. 2). Notably, all these reactions were carried out in open air and therefore free from the use of an inert and anhydrous atmosphere thereby avoiding possible pressure development in a closed reaction vessel especially in scale-up synthesis. Overall, the combination of FeF₃ in water was found to be optimal for the preparation of 3a.

Table 1 The effect of reaction conditions on the condensation of 1a with 2a^a

Entry	Catalyst (mmol)	Solvent	T (°C); t (h)	Yield (%)^b
a All the reactions were carried out using 1,2-phenylenediamine 1a (1.0 mmol), aldehyde 2a (1.0 mmol), a catalyst (0.02 mmol) in a solvent (5 mL) in open air. b Isolated yields. c Catalyst was reused for additional three runs and figures within parentheses indicate the corresponding yield for each run.
1	FeCl3 (0.05)	H2O	60; 7	75
2	FeCl3·6H2O (0.05)	H2O	60; 7	81
3	FeF3 (0.02)	H2O	60; 7	85 (83, 81, 78)^c
4	NH4F (0.02)	H2O	60; 7	50
5	TBAF (0.02)	H2O	60; 7	65
6	FeF3 (0.02)	H2O	25; 7	78
7	FeF3 (0.02)	Toluene	110; 12	73
8	FeF3 (0.02)	1,4-Dioxane	100; 8	80
9	FeF3 (0.02)	MeOH	65; 2	83
10	FeF3 (0.02)	EtOH	80; 2	85

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Fig. 2 XRD spectrum of fresh FeF₃ and the reused catalyst after the first cycle.

Having the optimized reaction conditions in hand we then examined the generality and scope of the present FeF₃ mediated reaction in water. Thus, a range of aldehydes (2) was initially reacted with 1a and the results are summarized in Table 2. Various electron donating groups, e.g. Cl, Br, ^tBu, Me and NMe₂ (entries 1–5, Table 2), or electron withdrawing groups, e.g. CF₃ and COOH (entries 6 and 7, Table 2), present on the aryl ring of the aldehyde were well tolerated. The use of heteroaromatic (entries 8 and 9, Table 2) and aliphatic aldehydes (entry 10, Table 2) was also successful and afforded the desired 2-substituted benzimidazoles in good yields. The reaction proceeded smoothly with other 1,2-phenylenediamines as well, e.g.1b–d (entries 11–14, Table 2).

Table 2 FeF₃ catalyzed synthesis of 2-substituted benzimidazoles and benzothiazoles in water (Scheme 1)^a

Entry	o-Phenylenediamine/o-aminobenzenethiol (1)	Aldehyde (2)	Product (3)^b	Yield (%)^c
a All the reactions were carried out using 1,2-phenylenediamine or 2-aminobenzenethiol 1 (1.0 mmol), aldehyde 2 (1.0 mmol), FeF3 (0.02 mmol) in water (5 mL) at 60 °C for 7–8 h in open air. b Identified by ^1H NMR, IR and MS. c Isolated yields. d The reaction was carried out using 1.2 mmol of aldehyde.
1	1a	2a	3a	85
2	1a	2b	3b	86
3	1a	2c	3c	88
4	1a	2d	3d	80
5	1a	2e	3e	81
6	1a	2f	3f	83
7	1a	2g	3g	88
8	1a	2h	3h	89
9	1a	2i	3i	90
10	1a	2j	3j	90
11	1b	2b	3k	84
12	1b	2g	3l	78
13	1c	2c	3m	80
14	1d	2b	3n	75
15	1e	2g	3o	92^d
16	1e	2k	3p	92^d
17	1e	2l	3q	90^d
18	1a	2m	3r	85
19	1a	2n	3s	81
20	1a	2o	3t	79

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To extend the scope of this methodology further we examined the use of 2-aminothiophenol (1e) in place of 1a and the corresponding 2-aryl substituted benzothiazoles were obtained in good yields (entries 15–17, Table 2). Furthermore, the present method afforded 2-alkyl benzimidazoles in good yields (entries 18–20, Table 2), the preparation of which was not very convenient previously as most of the reported methods were either less effective or ineffective in terms of product yields.

Prompted by the fact that 1,2-disubstituted benzimidazoles can be accessed by a direct one-step condensation of 1,2-phenylenediamines with aryl aldehydes¹³ we decided to explore the potential of the present FeF₃ catalyzed reaction for the synthesis of these compounds in a single pot. Accordingly, 2.0 equiv of aldehyde were reacted with 1.0 equiv of 1,2-phenylenediamine under the conditions mentioned earlier (entry 3, Table 1). To our satisfaction the reaction proceeded smoothly to give the desired 1,2-disubstituted benzimidazoles (4) in good yields (Table 3). Thus, the present methodology can be used to prepare mono- or di-substituted derivatives depending on the conditions employed. The experimental procedure is simple and the products can be isolated by filtration followed by purification through crystallization.

Table 3 FeF₃ catalyzed synthesis of 1,2-disubstituted benzimidazoles in water^a

Entry	o-Phenylenediamine (1)	Aldehyde (2)	Product (4)^b	Yield (%)^c
a All the reactions were carried out using 1,2-phenylenediamines 1 (1.0 mmol), aldehyde 2 (2.0 mmol), FeF3 (0.02 mmol) in water (5 mL) at 60 °C for 10 h in open air. b Identified by ^1H NMR, IR and MS. c Isolated yields.
1	1a	2m	4a	85
2	1a	2n	4b	80
3	1b	2c	4c	78
4	1f	2c	4d	77
5	1a	2o	4e	80
6	1a	2p	4f	73

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Mechanistically, the reaction seems to proceed via a sequence (Scheme 2) involving the FeF₃ promoted formation of the Schiff base E-2 (via the intermediate E-1) followed by intramolecular ring closure leading to E-3. The oxidative dehydrogenation of E-3 by air affords the desired product 3. In the presence of 2.0 equivalents of the diamine (1a–b and 1f), the reaction undergoes FeF₃ mediated formation of the Schiff base E-4 (i.e. N¹,N²-bis(arylidene)benzene-1,2-diamine) which, after intramolecular cyclization followed by 1,3-hydride migration, affords the 1,2-disubstituted benzimidazole 4 (Scheme 3).¹³ While air seems to have no role in this case its presence however did not affect the reaction.

Scheme 2 Proposed mechanism for the formation of 1,3-benzazoles (3).

Scheme 3 Proposed mechanism for the formation of 1,2-disubstituted benzimidazoles (4).

Conclusions

In conclusion, a green, efficient and simple method has been developed for the facile and one-pot synthesis of 2-substituted benzimidazoles, 2-substituted benzothiazoles and 1,2-disubstituted benzimidazoles. The methodology involves catalysis by FeF₃ in water, which facilitates the reaction of 1,2-phenylenediamine and 2-aminothiophenol with 1 equivalent of an alkyl- or aryl-aldehyde leading to 1,3-benzazoles (A, Fig. 1) in open air. The process afforded 1,2-disubstituted benzimidazoles when 1,2-phenylenediamine was reacted with 2 equivalents of aryl aldehyde. The products were isolated by simple filtration and the catalyst can be recovered and recycled. The operational simplicity, excellent yields of the products, and high chemoselectivity are the main advantages of this method, and furthermore, this procedure is inexpensive, safe and environmentally benign. The present example of the first FeF₃ mediated synthesis of 1,3-benzazoles and 1,2-disubstituted benzimidazoles therefore would find wide applications.

Experimental section

General methods

Unless stated otherwise, reactions were performed in open air. Reactions were monitored by thin layer chromatography (TLC) on silica gel plates (60 F254), visualizing with ultraviolet light or iodine spray. Flash chromatography was performed on silica gel (230–400 mesh) using distilled hexane, ethyl acetate or dichloromethane. ¹H NMR and ¹³C NMR spectra were determined in DMSO-d₆ solution using 400 or 100 MHz spectrometers, respectively. Proton chemical shifts (δ) are relative to tetramethylsilane (TMS, δ = 0.00) as internal standard and expressed in ppm. Spin multiplicities are given as s (singlet), d (doublet), t (triplet) and m (multiplet) as well as b (broad). Coupling constants (J) are given in hertz. Infrared spectra were recorded on a FT-IR spectrometer. Melting points were determined using melting point B-540 apparatus and are uncorrected. MS spectra were obtained on a mass spectrometer. HRMS was determined using waters LCT premier XETOF ARE-047 apparatus. The catalyst FeF₃ (light green powder; purity 98%; CAS No: 7783-50-8) used is commercially available.

General procedure for the preparation of compounds 3a–t. A mixture of 1,2-phenylenediamine or 2-aminobenzenethiol 1 (1 mmol), aldehyde 2 (1 mmol for the preparation of benzoimidazole, 1.2 mmol for the preparation of benzothiazoles), FeF₃ (0.02 mmol), and water (5 mL) was heated at 50–60 °C for 7–8 h. After completion of the reaction (indicated by TLC), the mixture was cooled to room temperature and filtered. The solid obtained was treated with water (10 mL) and filtered under vacuum. The crude product was recrystallized from ethanol to afford the desired product.

2-(4-Chlorophenyl)-1H-benzo[d]imidazole (3a). White solid; mp 291–292 °C (lit¹⁴ 290–292 °C); ¹H NMR (DMSO-d₆, 400 MHz): δ 12.99 (bs, 1H), 8.19 (d, J = 8.0 Hz, 2H), 7.65 (q, J = 8.0 Hz, 3H), 7.54 (d, J = 8.0 Hz, 1H), 7.21 (t, J = 8.0 Hz, 2H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 150.1, 143.7, 135.0, 134.4, 129.0 (3C), 128.1 (2C), 122.7, 121.9, 118.9, 111.4; IR (KBr): 3051, 1586, 1490, 1448, 1429, 1272, 831, 745, 728 cm⁻¹; HRMS (ESI): calcd for C₁₃H₁₀N₂Cl (M+H)⁺ 229.0533, found 229.0525; MS (ESI): m/z ([M+H]+): 229.1.

2-(4-Bromophenyl)-1H-benzo[d]imidazole (3b). White solid; mp 295–296 °C (lit^6c 291–294 °C); ¹H NMR (DMSO-d₆, 400 MHz) δ 13.02 (br s, 1H), 8.13–8.11 (m, 2H), 7.77 (d, J = 6.80 Hz, 2H), 7.66 (d, J = 8.0 Hz, 1H), 7.53 (s, 1H), 7.23–7.18 (m, 2H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 150.0, 143.5, 134.7, 131.8 (2C), 129.0, 128.4 (2C), 123.1 (2C), 121.7, 118.7, 111.4; IR (KBr): 2846, 2745, 1447, 1427, 1274, 1011, 963, 828, 745 cm⁻¹; HRMS (ESI): calcd for C₁₃H₁₀N₂Br (M+H)⁺ 273.0027, found 273.0033; MS (ESI): m/z ([M+H]+): 273.1.

2-(4-(Tert-butyl)phenyl)-1H-benzo[d]imidazole (3c). White solid; mp 255–257 °C (lit¹⁵ 250–251 °C); ¹H NMR (DMSO-d₆, 400 MHz) δ 12.82 (s, 1H), 8.10 (d, J = 8.0 Hz, 2H), 7.64 (d, J = 8.0 Hz, 1H), 7.57 (d, J = 8.0 Hz, 2H), 7.51 (d, J = 8.0 Hz, 1H), 7.22–7.15 (m, 2H), 1.33 (s, 9H); ¹³C NMR (DMSO-d₆, 100 MHz) δ 152.5, 151.3, 143.8, 134.9, 127.4, 126.2 (2C), 125.7 (2C), 122.3, 121.5, 118.7, 111.2, 34.5, 30.9 (3C); IR (KBr): 3696, 2959, 1431, 1269, 968, 840, 739 cm⁻¹; HRMS (ESI): calcd for C₁₇H₁₉N₂ (M+H)⁺ 251.1548, found 251.1546; MS (ESI): m/z ([M+H]+): 251.3.

2-Mesityl-1H-benzo[d]imidazole (3d). White solid; mp 252–254 °C (lit¹⁶ 253–254 °C); ¹H NMR (DMSO-d₆, 400 MHz): δ 12.4 (s, 1H), 7.61–7.58 (m, 2H), 7.20–7.17 (m, 2H), 7.00 (s, 2H), 2.31 (s, 3H), 2.06 (s, 6H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 151.2, 138.3, 137.0 (2C), 128.7, 127.9 (3C), 121.4 (3C), 114.8 (2C), 20.8, 19.6 (2C); IR (KBr): 3055, 2920, 2676, 1450, 1417, 1223, 851, 753 cm⁻¹; HRMS (ESI): calcd for C₁₆H₁₇N₂ (M+H)⁺ 237.1392, found 237.1393; MS (ESI): m/z ([M+H]+): 237.2.

4-(1H-Benzo[d]imidazol-2-yl)-N,N-dimethylaniline (3e). Yellow solid; mp 285–286 °C (lit¹⁷ 288–290 °C); ¹H NMR (DMSO-d₆, 400 MHz): δ 12.5 (brs, 1H), 7.98 (d, J = 8.0 Hz, 2H), 7.67–7.51 (m, 2H), 7.14–7.08 (m, 2H), 6.82 (d, J = 8.0 Hz, 2H), 2.99 (s, 6H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 151.8, 150.7, 139.1 (2C), 127.3 (2C), 120.8 (2C), 116.7 (3C), 111.3 (2C), 39.1 (2C); IR (KBr): 3418, 3053, 1610, 1508, 1439, 1372, 1203, 819, 747 cm⁻¹; HRMS (ESI): calcd for C₁₅H₁₆N₃ (M+H)⁺ 238.1344, found 238.1340; MS (ESI): m/z ([M+H]+): 238.2.

2-(4-(Trifluoromethyl)phenyl)-1H-benzo[d]imidazole (3f). White solid; mp 262–264 °C (lit¹⁸ 262–264 °C); ¹H NMR (DMSO-d₆, 400 MHz) δ 13.17 (bs, 1H), 8.39 (d, J = 8.0 Hz, 2H), 7.94 (d, J = 8.0 Hz, 2H), 7.71 (d, J = 8.0 Hz, 1H), 7.57 (d, J = 8.0 Hz, 1H), 7.29–7.21 (m, 2H); ¹³C NMR (DMSO-d₆, 100 MHz) δ 149.6, 143.7, 133.9, 129.6 (q, 1C, J = 32.1), 127.0 (4C), 125.9 (2C), 123.2, 122.0, 119.2, 111.6; IR (KBr): 3426, 1433, 1321, 1170, 1140, 1064, 850, 746 cm⁻¹; HRMS (ESI): calcd for C₁₄H₁₀N₂F₃ (M+H)⁺ 263.0796, found 263.0799; MS (ESI): m/z ([M+H]+): 263.1.

4-(1H-Benzo[d]imidazol-2-yl)benzoic acid (3g). White solid, mp 327–329 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 13.16 (bs, 1H), 13.02 (s, 1H), 8.20 (d, J = 8.0 Hz, 2H), 8.05 (d, J = 8.0 Hz, 2H), 7.73 (d, J = 8.0 Hz, 2H), 7.23–7.19 (m, 2H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 166.9, 150.1, 134.4, 133.9, 131.5, 129.9 (2C), 129.8, 129.6 (2C), 129.4, 129.2, 126.4 (2C); IR (KBr): 3061, 2925, 1708, 1387, 1289, 1116, 737 cm⁻¹; HRMS (ESI): calcd for C₁₄H₁₁N₂O₂ (M+H)⁺ 239.0821, found 239.0810; MS (ESI): m/z ([M+H]+): 239.1.

2-(Furan-2-yl)-1H-benzo[d]imidazole (3h). Light yellow solid; mp 285–287 °C (lit¹⁴ 287–288 °C); ¹H NMR (DMSO-d₆, 400 MHz): δ 12.9 (s, 1H), 7.94 (s, 1H), 7.55 (s, 2H), 7.21–7.19 (m, 3H), 6.73 (s, 1H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 145.5, 144.5 (3C), 143.6, 122.1 (2C), 112.2 (2C), 110.4 (2C); IR (KBr): 3059, 2661, 1630, 1525, 1417, 1279, 1119, 1011, 980, 738 cm⁻¹; HRMS (ESI): calcd for C₁₁H₉N₂O (M+H)⁺ 185.0715, found 185.0708; MS (ESI): m/z ([M+H]+): 185.1.

2-(Thiophen-2-yl)-1H-benzo[d]imidazole (3i). Light yellow solid; mp 341–342 °C (lit¹⁴ 341–343 °C); ¹H NMR (DMSO-d₆, 400 MHz): δ 12.90 (brs, 1H), 7.83 (d, J = 8.0 Hz, 1H), 7.72 (d, J = 8.0 Hz, 1H), 7.56–7.54 (m, 2H), 7.24–7.18 (m, 3H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 147.0 (2C), 133.6, 128.7 (2C), 128.2 (2C), 126.6 (2C), 122.1 (2C); IR (KBr): 3446, 1621, 1569, 1450, 1423, 1275, 1234, 1073, 944, 850, 742 cm⁻¹; HRMS (ESI): calcd for C₁₁H₉N₂S (M+H)⁺ 201.0486, found 201.0481; MS (ESI): m/z ([M+H]+): 201.1.

2-Cyclopropyl-1H-benzo[d]imidazole (3j). White solid; mp 232–234 °C (lit¹⁹ 231–232 °C); ¹H NMR (DMSO-d₆, 400 MHz): δ 12.10 (s, 1H), 7.40–7.38 (m, 2H), 7.08–7.06 (m, 2H), 2.11–2.08 (m, 1H), 1.05–1.01 (m, 4H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 156.8, 121.0 (2C), 120.9 (2C), 108.1 (2C), 9.3, 8.6 (2C); IR (KBr): 3474, 1632, 1420, 1263, 997, 749 cm⁻¹; HRMS (ESI): calcd for C₁₀H₁₁N₂ (M+H)⁺ 159.0922, found 159.0916; MS (ESI): m/z ([M+H]+): 159.1.

2-(4-Bromophenyl)-6-methyl-1H-benzo[d]imidazole (3k). Light yellow solid; mp 242–243 °C; ¹H NMR (DMSO-d₆, 400 MHz) δ 12.84 (d, J = 16.0 Hz, 1H), 8.09 (d, J = 8.0 Hz, 2H), 7.75 (d, J = 8.0 Hz, 2H), 7.53 (d, J = 8.0 Hz, 1H), 7.48–7.31 (m, 1H), 7.06–7.01 (m, 1H), 2.43 (s, 3H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 149.6, 132.1, 131.9 (3C), 129.5, 128.2 (2C), 124.2, 123.4, 122.9, 118.5, 111.1, 21.3; IR (KBr): 3440, 3200, 2919, 1631, 1416, 1316, 1011, 830, 714 cm⁻¹; HRMS (ESI): calcd for C₁₄H₁₂N₂Br (M+H)⁺ 287.0184, found 287.0186; MS (ESI): m/z ([M+H]+): 287.1.

4-(5-Methyl-1H-benzo[d]imidazol-2-yl)benzoic acid (3l). Light brown solid; mp 281–282 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 13.15 (s, 1H), 13.02 (s, 1H), 8.21 (d, J = 8.0 Hz, 2H), 8.05 (d, J = 8.0 Hz, 2H), 7.71–7.69 (m, 1H), 7.56–7.54 (m, 1H), 7.23–7.21 (m, 1H), 2.33 (s, 3H); ¹³C NMR: Not available due to poor solubility. IR (KBr): 3451, 1615, 1559, 1409, 868, 789, 715 cm⁻¹; HRMS (ESI): calcd for C₁₅H₁₁N₂O₂ (M-H)⁺ 251.0821, found 251.0828; MS (ESI): m/z ([M+H]+): 251.2.

2-(4-(Tert-butyl)phenyl)-6-chloro-1H-benzo[d]imidazole (3m). White solid, mp 252–254 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 13.03 (d, J = 8.0 Hz, 1H), 8.09 (d, J = 8.0 Hz, 2H), 7.70 (s, 1H), 7.58 (d, J = 8.0 Hz, 2H), 7.52 (d, J = 8.0 Hz, 1H), 7.21 (t, J = 8.0 Hz, 1H), 1.33 (s, 9H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 152.9 (2C), 126.9 (3C), 126.3 (4C), 125.7 (2C), 122.0 (2C), 34.6, 30.9 (3C); IR (KBr): 3696, 2961, 1616, 1422, 1308, 1061, 964, 846, 809, 710 cm⁻¹; HRMS (ESI): calcd for C₁₇H₁₈N₂Cl (M+H)⁺ 285.1159, found 285.1155; MS (ESI): m/z ([M+H]+): 285.2.

2-(4-Bromophenyl)-5-nitro-1H-benzo[d]imidazole (3n). Yellow solid; mp 176–177 °C (lit²⁰ 161–164 °C); ¹H NMR (DMSO-d₆, 400 MHz) δ 13.69 (brs, 1H), 8.84 (s, 1H), 8.04 (d, J = 8.0 Hz, 3H), 7.93 (d, J = 8.0 Hz, 1H), 7.73 (d, J = 8.0 Hz, 2H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 158.5, 151.1, 135.8, 135.2, 133.4, 132.1, 132.0, 131.6, 131.4, 130.9, 128.8, 128.4, 125.1, 124.4, 118.9, 113.0, 112.8, 101.7, 98.4; (Note: A mixture of tautomers observed); IR (KBr): 3483, 3373, 1601, 1488, 1340, 1069, 816, 746 cm⁻¹; HRMS (ESI): calcd for C₁₃H₇N₃O₂Br (M-H)⁺ 315.9722, found 315.9729; MS (ESI): m/z ([M+H]+): 317.1.

4-(Benzo[d]thiazol-2-yl)benzoic acid (3o). Light yellow solid; mp 294–295 °C (lit²¹ 294.1–295.5 °C); ¹H NMR (DMSO-d₆, 400 MHz): δ 13.26 (br s, 1H), 8.23–8.20 (m, 3H), 8.18–8.10 (m, 3H), 7.60–7.56 (m, 1H), 7.51 (t, J = 8.0 Hz, 1H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 165.8, 153.2, 136.2, 134.5, 132.8, 130.0, 127.1, 126.6 (2C), 125.7, 123.0 (2C), 122.2 (2C); IR (KBr): 3058, 2827, 1681, 1609, 1427, 1293, 971, 860, 755 cm⁻¹; HRMS (ESI): calcd for C₁₄H₁₀NO₂S (M+H)⁺ 256.0432, found 256.0428; MS (ESI): m/z ([M+H]+): 256.2

2-(2-Nitrophenyl)benzo[d]thiazole (3p). Light green solid; mp 128–129 °C (lit²² 127–129 °C); ¹H NMR (DMSO, 400 MHz) δ 8.23 (d, J = 7.2 Hz, 1H), 8.09–7.99 (m, 3H), 7.90–7.85 (m, 2H), 7.65–7.54 (m, 2H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 162.2, 153.0, 148.4, 135.1, 133.0, 131.9, 131.7, 126.8, 126.2, 126.0, 124.5, 123.2, 122.4; IR (KBr): 3083, 1608, 1536, 1367, 1305, 1231, 973, 761 cm⁻¹; HRMS (ESI): calcd for C₁₃H₉N₂O₂S (M+H)⁺ 257.0385, found 257.0387; MS (ESI): m/z ([M+H]+): 257.2.

4-(Benzo[d]thiazol-2-yl)benzonitrile (3q). Greenish yellow solid; mp 171–173 °C (lit²³ 170.5–172.5 °C); ¹H NMR (DMSO-d₆, 400 MHz) δ 8.28 (d, J = 8.0 Hz, 2H), 8.22 (d, J = 8.0 Hz, 1H), 8.13 (d, J = 8.0 Hz, 1H), 8.05 (d, J = 8.0 Hz, 2H), 7.61 (t, J = 8.0 Hz, 1H), 7.53 (t, J = 8.0 Hz 1H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 165.0, 153.2, 136.4, 134.7, 133.1 (2C), 127.6 (2C), 126.8, 126.0, 123.2, 122.4, 118.1, 113.1; IR (KBr): 3422, 2227, 1480, 1315, 967, 839, 764 cm⁻¹; HRMS (ESI): calcd for C₁₄H₉N₂S (M+H)⁺ 237.0486, found 237.0482; MS (ESI): m/z ([M+H]+): 237.1

2-Cyclohexyl-1H-benzo[d]imidazole (3r). Colorless solid; mp 272–273 °C (lit²⁴ sublimes at 250 °C); ¹H NMR (DMSO-d₆, 400 MHz): δ 12.13 (s, 1H), 7.50 (d, J = 8.0 Hz, 1H), 7.39 (d, J = 8.0 Hz, 1H), 7.12–7.06 (m, 2H), 2.82 (d, J = 8.0 Hz, 1H), 2.03–1.98 (m, 2H), 1.82–1.78 (m, 2H), 1.72–1.67 (m, 1H), 1.64–1.53 (m, 2H), 1.44–1.35 (m, 2H), 1.30–1.25 (m, 1H); MS (ESI): m/z ([M+H]+): 200.3.

2-(Pentan-3-yl)-1H-benzo[d]imidazole²⁵ (3s). Off white solid; mp 262–264 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 12.17 (s, 1H), 7.52 (d, J = 8.0 Hz, 1H), 7.40 (d, J = 8.0 Hz, 1H), 7.13–7.08 (m, 2H), 2.70 (d, J = 8.0 Hz, 1H), 1.79–1.72 (m, 4H), 0.77 (t, J = 8.0 Hz, 6H); MS (ESI): m/z ([M+H]+): 188.3.

2-Ethyl-1H-benzo[d]imidazole²⁶ (3t). Light yellow solid; mp 159–160 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 12.18 (bs, 1H), 7.42 (d, J = 8.0 Hz, 2H), 7.18 (d, J = 8.0 Hz, 2H), 2.81 (q, J = 8.0 Hz, 2H), 1.26 (t, J = 8.0 Hz, 3H); MS (ESI): m/z ([M+H]+): 146.2.

General procedure for the preparation of compounds 4a–4f. A mixture of 1,2-phenylenediamine 1 (1 mmol), aldehyde 2 (2 mmol), FeF₃ (0.02 mmol), and water (5 mL) was heated at 50–60 °C for 10 h. After completion of the reaction (indicated by TLC), the mixture was cooled to room temperature and filtered. The solid obtained was treated with water (10 mL) and filtered under vacuum. The crude product was recrystallized from ethanol to afford the pure product.

4-(1-(4-Hydroxybenzyl)-1H-benzo[d]imidazol-2-yl)phenol (4a). White solid; mp 254–256 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 9.94 (bs, 1H), 9.38 (bs, 1H), 7.67 (d, J = 8.0 Hz, 1H), 7.58 (d, J = 7.6 Hz, 2H), 7.41 (d, J = 8.0 Hz, 1H), 7.19 (d, J = 7.6 Hz, 2H), 6.88–6.83 (m, 4H), 6.62 (d, J = 7.6 Hz, 2H), 5.4 (s, 2H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 158.5, 156.3, 153.3, 142.4, 135.5, 130.3 (2C), 127.2 (2C), 126.8, 121.8 (2C), 121.6 (2C), 120.5, 118.5, 115.4, 115.3, 110.7, 46.8; IR (KBr): 3253, 2803, 1611, 1515, 1443, 1266, 1244, 1170, 839 cm⁻¹; HRMS (ESI): calcd for C₂₀H₁₇N₂O₂ (M+H)⁺ 317.1290, found 317.1288; MS (ESI): m/z ([M+H]+): 317.2.

4-(1-(3,4-Dihydroxybenzyl)-1H-benzo[d]imidazol-2-yl)benzene-1,2-diol (4b). Off white solid, mp 236–238 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 9.46 (bs, 1H), 9.28 (bs, 1H), 8.90–8.83 (m, 2H), 7.67 (d, J = 8.0 Hz, 1H), 7.35 (d, J = 8.0 Hz, 1H), 7.21–7.17 (m, 3H), 6.98 (d, J = 7.6 Hz, 1H), 6.82 (d, J = 7.6 Hz, 1H), 6.63 (d, J = 8.0 Hz, 1H), 6.39–6.31 (m, 2H), 5.36 (s, 2H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 153.6, 147.1, 145.4, 145.3, 144.5, 142.6, 135.8, 127.7, 122.0, 121.8, 121.0, 120.3, 118.7, 117.1, 116.6, 115.7, 115.5, 113.4, 110.9, 47.1; IR (KBr): 3378, 1605, 1483, 1453, 1282, 1248, 1125, 759 cm⁻¹; HRMS (ESI): calcd for C₂₀H₁₇N₂O₄ (M+H)⁺ 349.1188, found 349.1176; MS (ESI): m/z ([M+H]+): 349.2.

1-(4-(Tert-butyl)benzyl)-2-(4-(tert-butyl)phenyl)-5-methyl-1H-benzo[d]imidazole (4c). White solid; mp 231–233 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 7.66 (d, J = 8.0 Hz 2H), 7.59 (d, J = 8.0 Hz, 1H), 7.53 (d, J = 7.6 Hz, 1H), 7.32 (d, J = 8.0 Hz, 2H), 7.22 (s, 1H), 7.06 (d, J = 8.0 Hz, 2H), 6.92 (d, J = 8.0 Hz, 2H), 5.52 (s, 2H), 2.38 (s, 3H), 1.31 (s, 9H), 1.21 (s, 9H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 152.9, 150.5, 141.3, 136.4, 133.5, 132.7, 128.9 (2C), 127.2, 125.9 (2C), 125.6 (2C), 124.1 (2C), 119.3 (2C), 110.3 (2C), 48.0, 34.8, 34.5, 31.3 (3C), 31.2 (3C), 21.8; IR (KBr): 3673, 2962, 1462, 1332, 1268, 992, 843, 809 cm⁻¹; HRMS (ESI): calcd for C₂₉H₃₅N₂ (M+H)⁺ 411.2800, found 411.2798; MS (ESI): m/z ([M+H]+): 411.4.

1-(4-(Tert-butyl)benzyl)-2-(4-(tert-butyl)phenyl)-4,5-dichloro-1H-benzo[d]imidazole (4d). White solid; mp 231–233 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 7.82 (d, J = 8.0 Hz 1H), 7.70 (d, J = 8.0 Hz, 2H), 7.57–7.52 (m, 3H), 7.32 (d, J = 8.0 Hz, 2H), 6.90 (d, J = 7.6 Hz, 2H), 5.60 (s, 2H), 1.35 (s, 9H), 1.22 (s, 9H); ¹³C NMR (CDCl₃, 100 MHz): δ 155.7, 152.9, 150.2, 142.0, 135.8, 132.6, 128.4 (2C), 126.0, 125.3 (2C), 125.3 (3C), 125.2 (3C), 119.9, 111.9, 47.4, 33.9 (2C), 30.8 (3C), 30.7 (3C); IR (KBr): 3433, 3059, 2963, 1609, 1460, 1445, 1304, 1118, 838 cm⁻¹; HRMS (ESI): calcd for C₂₈H₃₁N₂Cl₂ (M+H)⁺ 465.1864, found 465.1857; MS (ESI): m/z ([M+H]+): 465.3.

2-(Pyridin-4-yl)-1-(pyridin-4-ylmethyl)-1H-benzo[d]imidazole (4e). Light yellow solid; mp 188–189 °C; ¹H NMR (DMSO-d₆, 400 MHz) δ 8.48 (d, J = 8.0 Hz, 2H), 8.35 (d, J = 8.0 Hz, 2H), 7.48 (d, J = 8.0 Hz, 2H), 7.16 (d, J = 8.0 Hz, 2H), 6.53–6.42 (m, 3H), 6.18 (d, J = 8.0 Hz, 1H), 5.82 (s, 2H); ¹³C NMR (DMSO-d₆, 100 MHz) δ 150.5, 150.1, 150.0, 149.8, 149.5, 145.7, 142.5, 137.1, 136.1, 123.7, 123.0, 122.9, 122.6, 122.5, 121.1, 119.8, 111.1, 46.6; IR (KBr): 3415, 3036, 1603, 1414, 827, 749 cm⁻¹; HRMS (ESI): calcd for C₁₈H₁₅N₄ (M+H)⁺ 287.1297, found 287.1294; MS (ESI): m/z ([M+H]+): 287.2.

1-((1H-Indol-2-yl)methyl)-2-(1H-indol-2-yl)-1H-benzo[d]imidazole (4f). Light yellow solid; mp 232–234 °C; ¹H NMR (DMSO-d₆, 400 MHz): δ 11.7 (bs, 1H), 11.0 (bs, 1H), 8.31 (d, J = 8.0 Hz, 1H), 7.88 (s, 1H), 7.68 (d, J = 8.0 Hz, 1H), 7.57 (d, J = 7.6 Hz, 1H), 7.49 (d, J = 8.0 Hz, 1H), 7.32 (d, J = 8.0 Hz, 1H), 7.25–7.16 (m, 5H), 7.04 (t, J = 7.6 Hz, 2H), 6.85 (t, J = 7.6 Hz, 1H), 5.83 (s, 2H); ¹³C NMR (DMSO-d₆, 100 MHz): δ 149.5, 142.8, 136.3, 136.0, 135.4, 126.5, 126.4, 125.5, 123.5, 122.3, 121.5, 121.3 (2C), 121.2, 120.2, 118.8, 118.1 (2C), 111.8, 111.7, 110.5, 110.2, 104.8, 40.7; IR (KBr): 3418, 3047, 1617, 1569, 1452, 1391, 1242, 1013, 937, 746 cm⁻¹; HRMS (ESI): calcd for C₂₄H₁₉N₄ (M+H)⁺ 363.1610, found 363.1604; MS (ESI): m/z ([M+H]+): 363.2.

Recovery and reuse of FeF₃ catalyst. After completion of the reaction the mixture was cooled to room temperature and the crude product was separated by filtration. The crude product isolated was treated with water (10 mL) and filtered. The aqueous filtrates containing the catalyst FeF₃ were collected, combined, evaporated under reduced pressure and dried to give a pale pink colored solid. The XRD/IR spectrum of this recovered catalyst was identical to that of the commercially available catalyst (Aldrich). The recovered FeF₃ was reused for three times (see Table 1).

Acknowledgements

The author (TBK) thanks Dr V. Dahanukar for his encouragement. The authors thank the analytical group of DRL for spectral data.

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Footnote

† Electronic Supplementary Information (ESI) available: Copies of NMR spectra for all new compounds. For ESI see DOI: 10.1039/c2ra22302c

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