Lewis-Acid-Mediated Benzotriazole Ring Cleavage (BtRC) Strategy

Aug 28, 2017 - A Lewis-acid-mediated ring cleavage of acylbenzotriazoles (RCOBt) followed by cyclization to corresponding benzoxazoles was achieved in...
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Lewis-Acid-Mediated Benzotriazole Ring Cleavage (BtRC) Strategy for the Synthesis of 2‑Aryl Benzoxazoles from N‑Acylbenzotriazoles† Anoop S. Singh, Nidhi Mishra, Dhananjay Kumar,‡ and Vinod K. Tiwari* Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi, U.P. 221005, India S Supporting Information *

ABSTRACT: A Lewis-acid-mediated ring cleavage of acylbenzotriazoles (RCOBt) followed by cyclization to corresponding benzoxazoles was achieved in good to excellent yields. The reaction was found consistent with the milligram to gram scale.

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INTRODUCTION Benzotriazole methodology has been emerged as an efficient, economical, and quick way for common organic transformations during the last four decades.1 The benzotriazole ring cleavage (BtRC) approach proficiently assists the development of a wide spectrum of benzoheterocycles via either Dimroth rearrangement, reagent-supported ring opening, free radical path, photolysis, or thermolysis.2−6 Because of benzoxazole being one of the most fascinating moieties having an extensive range of biological, pharmaceutical, industrial, and analytical importance,7 we intended to explore our standard protocols of benzotriazole methodology for the synthesis of benzoxazole derivatives. Influenced by our previously reported benzothiazole synthesis,2 which followed free radical mechanism of BtRC, we applied a similar protocol using tributyltin hydride for ring cleavage of N-phenylacylbenzotriazole. However, the reaction resulted in N-phenylbenzamide8 instead of 2-phenyl benzoxazole. Then, we switched our tryout from free-radical reagents to Lewis acid reagents and finally became successful when we refluxed N-phenyl benzotriazole with anhydrous AlCl3 in dry toluene (Scheme 1).

Scheme 1. Comparison of Benzotriazole Ring Cleavage (BtRC) Processes

Best results were achieved with high-boiling-point, aprotic nonpolar solvents, such as toluene and benzene. The optimum temperature for the reaction was found to be 140 °C, and below 130 °C, the reaction does not proceed at all. A total of 1.2 equiv of dry toluene under inert atmospheric conditions yielded the maximum product (65%) in 2 h. Among the various substrates, aryl-group-containing N-acylbenzotriazoles afforded benzoxazoles (Table 2), whereas alkyl-containing N-acylbenzotriazoles underwent traditional Friedel−Crafts acylation. No specific variation of yield was found with the electronwithdrawing- or electron-donor-group substitution on the aryl ring. The substrate containing iodo substituent at ortho postion 2q did not give any product, and the starting compound was recovered after column chromatography (SiO2). o-Methoxy-

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RESULTS AND DISCUSSION Our synthetic strategy was initiated with the formation of Nacylbenzotriazoles (2) from corresponding carboxylic acids (1) following two standard protocols, i.e., either by reaction with thionyl chloride and 1H-benzotriazole or by treatment with PPh3/NBS and 1H-benzotriazole.4e,9 The resulting N-acylbenzotriazoles (2) were subjected to ring cleavage by employing 1 equiv of anhydrous AlCl3 at 140 °C, which resulted in benzoxazole derivative 3 (Scheme 2). For the reaction optimization study, a sequence of reactions were carried out with compound 2a, in the presence of different Lewis acids in various solvents at diverse ranges of temperature to afford benzoxazole 3a (Table 1). © 2017 American Chemical Society

Received: July 10, 2017 Accepted: August 14, 2017 Published: August 28, 2017 5044

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the yields were found to be 64% and 81% for 3a and 3k, respectively (Scheme 4). The importance of developed benzoxazoles may be visualized in terms of its presence as a structural core in a diverse range of natural and synthetic molecules, for example, antimycobacterial agents pseudopteroxazole, UK-1, and AJI9561; HIV reverse transcriptase inhibitor L-697,661; estrogen receptor-β agonist ERB-041; selective peroxisome proliferator-activated receptor γ antagonist JTP-426467; anticancer agent NSC-693638; and also Tafamidis meglumine, a transthyretin amyloid inhibitor (developed by SCRIPPS/Pfizer in 2014) marketed for the treatment of neorodegenerative disorder.7,10 Mechanistic Consideration. A plausible mechanism of the reaction is shown in Scheme 5, where AlCl3 initiates the process by a rapid reaction with N-acylbenzotriazole to form intermediate A. This can give the traditional Friedel−Crafts product, which is generally obtained in the case of aliphatic Nacylbenzotriazoles, but in the case of aryl N-acylbenzotriazole, intermediate A favors cyclization via formation of carbocation complex intermediates B and C with the loss of molecular nitrogen (N2). The extended π-conjugative interactions involved in the carbocation system develop its stability and make the system stable enough to give the cyclized oxazole product. The stabilization of intermediate C could be governed via the delocalization of positive charge over the aryl ring and carbonyl functionality (Scheme 5).

Scheme 2. Synthesis of Benzoxazole Derivative 3 from Corresponding N-Acylbenzotriazoles 2 through Cleavage of Benzotriazole Ring

Table 1. Optimization with (1H-Benzo[d][1,2,3]triazol-1yl)(phenyl)methanone

entrya

reagent

amount (equiv)

time (h)

solvent

temp (°C)

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 ZnCl2 ZnCl2 CuCl2 HgCl2 FeCl3

0.5 0 1 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2

1 4 2 2 4 4 2 2 2 2 2 2 2 2 2 2 2 2

toluene toluene toluene toluene toluene toluene toluene toluene MeCN DMF DCM THF benzene toluene toluene toluene toluene toluene

140 140 140 140 90 110 130 150 45 140 45 45 140 100 140 140 140 140

47 0 61 65 0 0 59 63 0 0 0 0 43 0 15 0 0 53

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CONCLUSIONS In summary, we introduced a convenient one-step synthetic route to convert N-acylbenzotriazole to benzoxazole via benzotriazole ring cleavage (BtRC). The reaction was best assisted by AlCl3 and possibly followed a ionic mechanism. The synthetic path showed equal compatibility with both electronwithdrawing and electron-donating substituents on the aromatic ring as well as with the quantity on a milligram or a gram scale.

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EXPERIMENTAL SECTION

General Remarks. All of the reactions were performed under an argon atmosphere in anhydrous solvents. Glasswares were dried in oven at 100 °C, 1 h prior to use. All reagents and solvents were of pure analytical grade. Thin layer chromatography (TLC) was carried out using 60 F254 silica gel-precoated aluminum plates, and visualization was under a UV lamp (λmax = 254 nm). 1H and 13C NMR spectra were recorded in CDCl3 solvent at 300 or 500 MHz and 75 or 125 MHz, respectively. Chemical shifts are reported in parts per million downfield from TMS as an internal standard; J values are reported in hertz. Mass spectra were recorded using electrospray ionization mass spectrometry (ESI-MS). IR spectra were recorded as Nujol mulls in KBr pellets. Elemental analysis was performed by a C, H, N analyzer with the results within ±0.4% of the calculated values. Procedure for the Synthesis of Arylcarbonylbenzotriazoles (RCOBt, 2a, 2c−2s) and Their Characterization Data. To a solution of carboxylic acid (1a, 7.34 mmol) in anhydrous dichloromethane (100 mL), thionyl chloride (8.27 mmol) was added at room temperature and stirred for 15 min. 1H-1,2,3-Benzotriazole (25.69 mmol) was then added to this mixture portionwise, and the resulting mixture was stirred for 2−3 h at room temperature. The completion of the reaction was monitored by TLC, and the reaction mass was washed with

a

Reaction carried out at a higher temperature in a sealed tube under argon. bYields reported after column chromatography (SiO2).

substituted substrate 2f underwent demethylation and formed compound 3h (91%); however, the m-methoxy group did not give such type of product, and target benzoxazole was isolated in good yield. The optimized protocol successfully transformed various acylbenzotriazoles 2a−s to their respective benzoxazoles 3a−s in good to excellent yields ranging from 43 to 91% (Table 2). However, when we carried out this reaction with benzylic N-acylbenzotriazoles, we obtained corresponding ketones in excellent yield, which suggests that the reaction followed the traditional Friedel−Crafts path with alkyl substrates (Scheme 3). BtRC-Mediated Cyclization Reaction on a Gram Scale. With the aim of performing quantity-based generalization of the reaction, we carried out the reaction with a larger quantity of starting materials. Scaling up the quantity from a milligram to a gram scale did not affect the course and yield of reaction, and 5045

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Table 2. Synthesis of Benzoxazole Derivative 3a−s from Corresponding N-Acylbenzotriazoles 2a−sa,b

a

Molar ratios: anhydrous AlCl3 (1.2 equiv); yield reported after purification by column chromatography (SiO2). bYield of demethylated product 3h.

Procedure for the Synthesis of Arylcarbonylbenzotriazole (2b). To a stirring solution of PPh3 (1.1 g, 4.41 mmol) and NBS (0.78 g, 4.41 mmol) in anhydrous dichloromethane (25 mL), compound 1b (0.5 g, 3.67 mmol) was added at 0 °C and stirred for 15 min. To this mixture, 1H-benzotriazole (0.5 g, 4.2 mmol) was added portionwise, and the mixture was allowed to stir at room temperature for 1 h. The completion of the reaction was monitored by TLC, and the reaction mass was concentrated under reduced pressure till dry. The crude was purified by flash column chromatography using gradient mixtures of ethyl acetate and n-hexane to afford pure product 2b (0.84 g, 3.56 mmol).

Scheme 3. Formation of the Ketone Product from the Corresponding Benzylic N-Acylbenzotriazole via the Traditional Friedel−Crafts Reaction

10% Na2CO3 solution, followed by water and brine solution. The organic layer was separated, dried over anhydrous Na2SO4, and concentrated under reduced pressure. The crude was purified by flash column chromatography using ethyl acetate and n-hexane to afford pure products 2a. 5046

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(1H-Benzo[d][1,2,3]triazol-1-yl)(m-tolyl)methanone (2c).11 White crystalline solid, yield 83%; 1.44 g, Rf = 0.8 (10% ethyl acetate/n-hexane); mp = 75−76 °C; lit., mp 77−79 °C MS: m/ z 238 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.31 (d, J = 8.5 Hz, 1H), 8.10 (d, J = 7.5 Hz, 1H), 7.96 (bs, 2H), 7.62 (t, J = 7.5 Hz, 1H), 7.49−7.40 (m, 3H), 2.41 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 166.9, 145.7, 138.3, 134.5, 132.4, 132.1, 131.4, 130.3, 129.0, 128.3, 126.3, 120.1, 114.8, and 21.4 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(o-tolyl)methanone (2d).11 White crystalline solid, yield 74%; 1.28 g, Rf = 0.8 (10% ethyl acetate/n-hexane); mp = 110−111 °C; lit., mp 110−111 °C MS: m/z 238 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.27 (d, J = 8.5 Hz, 1H), 8.02 (d, J = 7.5 Hz, 1H), 7.58−7.55 (m, 1H), 7.51 (d, J = 6.5 Hz, 1H), 7.42−7.36 (m, 2H), 7.24−7.20 (m, 2H), 2.31 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 168.1, 145.9, 137.8, 132.1, 131.7, 131.5, 130.9, 130.2, 129.9, 126.2, 125.3, 120.1, 114.4, and 19.9 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(3-methoxyphenyl)methanone (2e).4f White crystalline solid, yield 72%; 1.34 g, Rf = 0.7 (10% ethyl acetate/n-hexane); mp = 80−84 °C; MS: m/z 254 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.35 (d, J = 7.5 Hz, 1H), 8.14 (d, J = 7.5 Hz, 1H), 7.79 (d, J = 7.5 Hz, 1H), 7.70−7.66 (m, 2H), 7.53−7.50 (m, 1H), 7.47−7.24 (m, 1H), 7.21−7.19 (m, 1H), 3.87 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 166.6, 159.5, 145.8, 132.6, 132.4, 130.4, 129.5, 126.4, 124.3, 120.3, 120.2, 116.2, 114.8, and 55.6 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(2-methoxyphenyl)methanone (2f).11 Colorless solid, yield 89%; 1.65 g, Rf = 0.6 (10% ethyl acetate/n-hexane); mp = 95−96 °C; lit., mp 96−97 °C MS: m/z 254 [M + H]+; 1H NMR (300 MHz, CDCl3): 1H NMR (300 MHz, CDCl3): δ 8.32 (d, J = 8.5 Hz, 1H), 8.05 (d, J = 7.5 Hz, 1H), 7.61−7.56 (m, 2H), 7.50−7.42 (m, 2H), 7.05− 6.98 (m, 2H), 3.68 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 166.9, 157.9, 146.1, 133.6, 131.5, 130.4, 130.3, 126.2, 122.8, 120.5, 120.1, 114.4, 111.8, and 55.8 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(3-(trifluoromethyl)phenyl)methanone (2i).4f White crystalline solid, yield 53%; 1.13 g, Rf = 0.7 (10% ethyl acetate/n-hexane); mp = 52−53 °C;

Scheme 4. Synthesis of Benzoxazole Derivatives 3a and 3n on a Gram Scale via the BtRC Approach

(1H-Benzo[d][1,2,3]triazol-1-yl)(phenyl)methanone (2a).11 White crystalline solid, yield 95%; 1.55 g, Rf = 0.6 (10% ethyl acetate/n-hexane); mp = 112−113 °C; lit., mp 110−112 °C, MS: m/z 224 [M + H]; 1H NMR (300 MHz, CDCl3): δ 8.35 (d, J = 8.5 Hz, 1H), 8.20 (d, J = 7.5 Hz, 2H), 8.14 (d, J = 8.0 Hz, 1H), 7.68−7.65 (m, 2H), 7.56−7.49 (m, 3H); 13C NMR (75 MHz, CDCl3): δ 166.7, 145.7, 133.7, 132.3, 131.8, 131.5, 130.4, 128.5, 126.4, 120.2, and 114.8 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(p-tolyl)methanone (2b).11 White crystalline solid, yield 97%; 0.84 g, Rf = 0.7 (10% ethyl acetate/n-hexane); mp = 123−124 °C; lit., mp 122−124 °C MS: m/z 238 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.31 (d, J = 8.5 Hz, 1H), 8.10−8.08 (m, 3H), 7.63−7.60 (m, 1H), 7.46 (t, J = 7.5 Hz, 1H), 7.32 (d, J = 7.5 Hz, 2H), 2.42 (s, 3H); 13 C NMR (75 MHz, CDCl3): δ 166.5, 145.7, 144.8, 132.4, 132.0 (2C), 130.2, 129.2 (2C), 128.6, 126.2, 120.1, 114.8, and 21.8 ppm.

Scheme 5. Proposed Mechanism for Benzotriazole Ring Cleavage under Ionic Conditions Using Anhydrous AlCl3

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lit., mp 52−54 °C MS: m/z 292 [M + H]+; 1H NMR (500 MHz, CDCl3): δ 8.47 (s, 1H), 8.42 (d, J = 7.5 Hz, 1H), 8.37 (d, J = 8.5 Hz, 1H), 8.16 (d, J = 8.5, 1H), 7.93 (d, J = 7.5 Hz, 1H), 7.72−7.69 (m, 2H), 7.55 (t, J = 7.5 Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 165.4, 145.8, 134.9, 132.3, 132.2, 130.8, 130.2, 130.1, 129.1, 128.7, 128.6, 126.7, 120.4, and 114.8 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(3-fluorophenyl)methanone (2j).4c White crystalline solid, yield 71%; 1.26 g, Rf = 0.6 (10% ethyl acetate/n-hexane); mp = 102−103 °C; lit., mp 102 °C MS: m/z 242 [M + H]+; 1H NMR (500 MHz, CDCl3): δ 8.34 (d, J = 8.5 Hz, 1H), 8.14 (d, J = 8.5 Hz, 1H), 8.03 (d, J = 7.5 Hz, 1H), 7.93 (d, J = 9.5 Hz, 1H), 7.68 (t, J = 7.5 Hz, 1H), 7.54 (dd, J = 7.5, 7.0 Hz, 2H), 7.38 (t, J = 7.5 Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 165.3, 162.2 (d, J = 237.5 Hz), 145.7, 133.2 (d, J = 8.4 Hz), 132.2, 130.6, 130.1 (d, J = 8.4 Hz), 127.6 (d, J = 4.0 Hz), 126.5, 120.8 (d, J = 20 Hz), 120.3, 118.6 (d, J = 24 Hz), and 114.7 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(4-chlorophenyl)methanone (2l).11 White crystalline solid, yield 71%; 1.34 g, Rf = 0.5 (10% ethyl acetate/n-hexane); mp = 137−138 °C; lit., mp 138 °C MS: m/z 258 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.30 (d, J = 8.5 Hz, 1H), 8.13−8.08 (m, 3H), 7.63 (t, J = 7.5 Hz, 1H), 7.49−7.46 (m, 3H); 13C NMR (75 MHz, CDCl3): δ 165.6, 145.8, 140.5, 133.2 (2C), 132.3, 130.6, 129.8, 128.9 (2C), 126.6, 120.3, and 114.8 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(3-chlorophenyl)methanone (2m).11 White crystalline solid, yield 64%; 1.20 g, Rf = 0.5 (10% ethyl acetate/n-hexane); mp = 122−124 °C; lit., mp 120−121 °C MS: m/z 258 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.31 (d, J = 8.5 Hz, 1H), 8.15−8.06 (m, 3H), 7.67− 7.60 (m, 2H), 7.52−7.45 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 165.3, 145.7, 134.6, 133.7, 133.1, 132.2, 131.6, 130.6, 129.9, 129.7, 126.6, 120.3, and 114.8. (1H-Benzo[d][1,2,3]triazol-1-yl)(2-chlorophenyl)methanone (2n).11 White crystalline solid, yield 81%; 1.53g, Rf = 0.5 (15% ethyl acetate/n-hexane); mp = 81−83 °C; lit., mp 82−84 °C MS: m/z 258 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.35 (d, J = 8.5 Hz, 1H), 8.09 (d, J = 7.5 Hz, 1H), 7.68−7.62 (m, 2H), 7.51−7.48 (m, 3H), 7.41−7.38 (m, 1H); 13 C NMR (75 MHz, CDCl3): δ 165.8, 146.2, 132.9, 132.7, 132.3, 131.3, 130.8, 130.2 (2C), 126.8, 126.7, 120.4, and 114.4 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(4-bromophenyl)methanone (2p).4g Light yellow solid, yield 83%; 1.83 g, Rf = 0.5 (10% ethyl acetate/n-hexane); mp = 130−132 °C; lit., mp 142−145 °C MS: m/z 302 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.23 (d, J = 8.5 Hz, 1H), 8.04 (d, J = 8.0 Hz, 1H), 8.00−7.97 (m, 2H), 7.60−7.56 (m, 3H), 7.44−7.41 (m, 1H); 13 C NMR (75 MHz, CDCl3): δ 165.5, 145.5, 133.0 (2C), 132.0, 131.6 (2C), 130.4, 130.0, 129.0, 126.4, 120.1, and 114.6 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(2-bromophenyl)methanone (2r).12,13 Solid, Rf = 0.7 (5% ethyl acetate/nhexane); MS: m/z 302 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.35 (d, J = 8.4 Hz, 1H), 8.10 (d, J = 8.1 Hz, 1H), 7.70−7.60 (m, 3H), 7.53−7.38 (m, 3H); 13C NMR (75 MHz, CDCl3): δ 166.0, 145.8, 134.7, 132.9, 132.3, 130.9, 130.4, 129.8, 127.0, 126.4, 120.2, 120.0, and 114.0 ppm. (1H-Benzo[d][1,2,3]triazol-1-yl)(3-bromophenyl)methanone (2s).14 Solid, yield 87%; 1.92 g, Rf = 0.7 (5% ethyl acetate/n-hexane); MS: m/z 302 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.37(d, J = 9.0 Hz, 2H), 8.17 (d, J = 8.4 Hz, 2H), 7.81 (d, J = 8.1 Hz, 1H), 7.71 (t, J = 7.5 Hz, 1H), 7.58−

7.53 (m, 1H), 7.45 (d, J = 7.8 Hz, 1H); anal. calcd for C13H8BrN3O: C, 51.68; H, 2.67; N, 13.91. Found: C, 51.47; H, 2.71; N, 13.84. Procedure for the Synthesis of Benzoxazoles and Their Characterization Data. A stirring solution of compound 2b (0.8 mmol) in toluene was added with AlCl3 (1 mmol) under an inert atmosphere. The reaction was stirred under heating at 140 °C. After the completion of the reaction (monitored by TLC), the reaction mixture was concentrated in vacuo and extracted with CH2Cl2, water, and brine solutions. After drying over anhydrous Na2SO4, the organic layer was concentrated in vacuo. Purification using flash column chromatography afforded the benzoxazole derivative. 2-Phenyl benzo[d]oxazole (3a).15 Crystalline solid, yield 65%; 0.101 g, Rf = 0.6 (2% ethyl acetate/n-hexane); mp = 103− 104 °C; lit., mp 102−104 °C MS: m/z 196 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.26 (dd, J = 3.9, 2.1 Hz, 2H), 7.79−7.76 (m, 1H), 7.60−7.53 (m, 4H), 7.35 (dd, J = 3.3,2.4 Hz, 2H); 13C NMR (75 MHz, CDCl3): δ 163.0, 150.7, 142.0, 131.5, 128.8(2C), 127.5(2C), 127.1, 125.0, 124.5, 119.9, and 110.5 ppm. 2-(p-Tolyl)benzo[d]oxazole (3b).15 White solid, yield 59%; 0.98 g, Rf = 0.6 (2% ethyl acetate/n-hexane); mp = 116−118 °C; lit., mp 116−117 °C MS: m/z 210 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.12 (d, J = 8.1 Hz, 2H), 7.76−7.73 (m,1H), 7.55−7.52 (m, 1H), 7.32−7.27 (m, 4H), 2.40 (s, 3H); 13 C NMR (75 MHz, CDCl3): δ 163.2, 150.7, 142.2, 142.0, 129.6 (2C), 127.5 (2C), 124.8, 124.4, 124.4, 119.8, 110.4, and 21.6 ppm. 2-(m-Tolyl)benzo[d]oxazole (3c).15 Crystalline solid, yield 61%; 0.102 g, Rf = 0.7 (2% ethyl acetate/n-hexane); mp = 84− 86 °C; lit., mp 82−83 °C MS: m/z 210 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.00−7.94 (m, 2H), 7.68 (bs, 1H), 7.47 (bs, 1H), 7.33−7.16 (m, 4H), 2.35 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 163.2, 150.7, 142.1, 138.6, 132.3, 128.7, 128.1, 127.0, 124.9, 124.7, 124.4, 119.8, 110.5, and 21.2 ppm. 2-(o-Tolyl)benzo[d]oxazole (3d).15 Crystalline solid, yield 60%; 0.100 g, Rf = 0.7 (2% ethyl acetate/n-hexane); mp = 67− 68 °C; lit., mp 67 °C MS: m/z 210 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.16 (d, J = 7.5 Hz, 1H), 7.80−7.77 (m, 1H), 7.58−7.55 (dd, J = 3.3, 2.7 Hz, 1H), 7.41−7.26 (m, 5H), 2.80 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 163.3, 150.2, 142.1, 138.8, 131.7, 130.8, 129.8, 126.2, 125.9, 124.9, 124.3, 120.1, 110.4, and 22.1 ppm. 2-(3-Methoxyphenyl)benzo[d]oxazole (3e).16 Solid, yield 57%; 0.103 g, Rf = 0.6 (2% ethyl acetate/n-hexane); mp = 72− 74 °C; lit., mp 71−75 °C MS: m/z 226 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 7.85 (d, J = 7.8 Hz, 1H), 7.79−7.76 (m, 2H), 7.59 (dd, J = 3.3, 2.7 Hz, 1H), 7.42 (t, J = 7.8, 1H), 7.37− 7.32 (m, 2H), 7.10−7.06 (m,1H), 3.91 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 161.9, 158.9, 149.7, 141.0, 128.9, 127.3, 124.1, 123.5, 119.1, 119.0, 117.3, 110.9, 109.5, and 54.5 ppm. 2-(3-Phenoxyphenyl)benzo[d]oxazole (3g).17 Colorless crystalline solid, yield 43%; 0.098 g, Rf = 0.6 (3% ethyl acetate/n-hexane); mp = 86−88 °C; lit., mp 84−86 °C MS: m/ z 288 [M + H]+ 1H NMR (300 MHz, CDCl3): δ 7.97 (d, J = 7.8 Hz, 1H), 7.88 (s, 1H), 7.75−7.72 (m, 1H), 7.53−7.41 (m, 2H), 7.37−7.29 (m, 4H), 7.14−7.10 (m, 2H), 7.07−7.04 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 162.3, 157.8, 156.7, 150.7, 141.9, 130.3, 129.8(2C), 128.7, 125.2, 124.5, 123.7, 122.3, 121.8, 120.0, 119.1(2C), 117.5, and 110.6 ppm. 2-(Benzo[d]oxazol-2-yl)phenol (3h).18 Crystalline solid, yield 73%; 0.123 g, Rf = 0.5 (2% ethyl acetate/n-hexane); mp 5048

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= 122−124 °C; lit., mp 123.5 °C MS: m/z 212 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 11.4 (s, 1H), 7.95 (d, J = 6.9 Hz,1H), 7.67−7.64 (m, 1H), 7.54−7.51 (m, 1H), 7.40−7.29 (m, 3H), 7.08 (d, J = 7.1 Hz, 1H), 6.95 (t, J = 7.5 Hz, 1H); 13C NMR (75 MHz, CDCl3): δ 162.7, 158.6, 148.9, 139.8, 133.4, 127.0, 125.2, 124.8, 119.4, 119.1, 117.2, 110.5, and 110.4 ppm. 2-(3-(Trifluoromethyl)phenyl)benzo[d]oxazole (3i).19 Crystalline solid, yield 67%; 0.14 g, Rf = 0.5 (3% ethyl acetate/nhexane); mp = 119−122 °C; lit., mp 119−120 °C MS: m/z 264 [M + H]+ 1H NMR (300 MHz, CDCl3): δ 8.52 (s, 1H), 8.42 (d, J = 7.8 Hz,1H), 7.78−7.76 (m, 2H), 7.67−7.58 (m, 2H), 7.39−7.36 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 161.4, 150.8, 141.8, 131.4, 130.5, 129.5, 128.0, 127.9, 125.7, 124.9, 124.5, 120.3, and 110.7; 19F NMR (470 MHz, CDCl3): −62.77 ppm. 2-(3-Fluorophenyl)benzo[d]oxazole (3j).16 White solid, yield 69%; 0.117 g, Rf = 0.6 (3% ethyl acetate/n-hexane); mp = 92−94 °C; lit., mp 92−94 °C MS: m/z 214 [M + H]+; 1H NMR (500 MHz, CDCl3): δ 8.05 (d, J = 8.5 Hz, 1H), 7.96− 7.93 (m, 1H), 7.79−7.75 (m, 1H), 7.60−7.58 (m, 1H), 7.52− 7.47 (m, 1H), 7.39−7.36 (m, 2H), 7.25−7.21 (m, 1H); 13C NMR (75 MHz, CDCl3): δ 163.9, 162.0, 161.8, 150.8, 142.0, 130.7, 130.6, 129.3, 129.2, 125.6, 124.8, 123.4, 123.3, 120.3, 118.6, 118.5, 114.7, 114.4, 110.7; 19F NMR (470 MHz, CDCl3): −111.6 ppm. 2-(3,5-Difluorophenyl)benzo[d]oxazole (3k).20 Yellow solid, yield 83%; 0.153 g, Rf = 0.6 (4% ethyl acetate/n-hexane); MS: m/z 232 [M + H]+ 1H NMR (300 MHz, CDCl3): δ 7.64− 7.62 (m, 3H), 7.50−7.46 (m,1H), 7.32−7.28 (m, 2H), 6.90− 6.84 (m, 1H); 13C NMR (75 MHz, CDCl3): δ 164.9, 164.7, 161.6, 161.4, 160.7, 150.7, 141.7, 130.1, 129.9, 129.2, 125.8, 124.9, 120.4, 110.4 (m), 106.7 (m); 19F NMR (470 MHz, CDCl3): −107.9 ppm. 2-(4-Chlorophenyl)benzo[d]oxazole (3l).15 Colorless crystal solid, yield 67%; 0.122 g Rf = 0.6 (2% ethyl acetate/n-hexane); mp = 150−151 °C; lit., mp 150 °C MS: m/z 230 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.16 (d, J = 8.4 Hz, 2H), 7.76− 7.73 (m, 1H), 7.57−7.54 (m, 1H), 7.47 (d, J = 8.4 Hz, 2H), 7.37−7.33 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 162.0, 150.6, 141.9, 137.7, 129.2(2C), 128.7(2C), 125.6, 125.3, 124.7, 120.0, and 110.5 ppm. 2-(3-Chlorophenyl)benzo[d]oxazole (3m).16 Crystal solid, yield 57%; 0.104 g, Rf = 0.7 (3% ethyl acetate/n-hexane); mp = 122−124 °C; lit., mp 124−125 °C MS: m/z 230 [M + H]+; 1H NMR (500 MHz, CDCl3): δ 8.21 (s, 1H), 8.09 (d, J = 7.0 Hz, 1H), 7.76−7.74 (m, 1H), 7.56−7.54 (m, 1H), 7.47−7.35 (m, 2H), 7.34−7.33 (m, 2H); 13C NMR (125 MHz, CDCl3): δ 161.6, 150.8, 141.9, 135.1, 131.5, 130.2, 128.9, 127.6, 125.7, 125.6, 124.8, 120.3, and 110.7 ppm. 2-(2-Chlorophenyl)benzo[d]oxazole (3n).21 Crystal solid, yield 77%; 0.141 g, Rf = 0.7 (3% ethyl acetate/n-hexane); mp = 72−74 °C; lit., mp 72 °C MS: m/z 230 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.15−8.12 (m, 1H), 7.86−7.83 (m, 1H), 7.61−7.53 (m, 2H), 7.44−7.36 (m, 4H); 13C NMR (75 MHz, CDCl3): δ 160.8, 150.4, 141.6, 133.3, 131.8, 131.7, 131.3, 126.8, 126.1, 125.5, 124.5, 120.4, and 110.6 ppm. 2-(2,3-Dichlorophenyl)benzo[d]oxazole (3o).22 Crystal solid, yield 45%; 0.0942 g, Rf = 0.6 (3% ethyl acetate/nhexane); mp = 88−90 °C; lit., mp 90−92 °C MS: m/z 263 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.01 (d, J = 7.8, 1H), 7.85−7.82 (m, 1H), 7.62−7.60 (m, 2H), 7.42−7.30 (m, 3H); 13 C NMR (75 MHz, CDCl3): δ 160.2, 150.5, 141.4, 134.9, 132.6, 131.9, 130.0, 128.4, 127.2, 125.8, 124.7, 120.5, and 110.7

ppm; anal. calcd for C13H7Cl2NO: C, 59.12; H, 2.67; N, 5.30. Found: C, 59.37; H, 2.75; N, 5.21. 2-(4-Bromophenyl)benzo[d]oxazole (3p).22 White solid, yield 68%; 0.148 g, Rf = 0.6 (2% ethyl acetate/n-hexane); mp = 158−160 °C; lit., mp 158−159 °C MS: m/z 274 [M + H]+; 1 H NMR (300 MHz, CDCl3): δ 8.11 (d, J = 8.4 Hz, 2H), 7.77−7.73 (m,1H), 7.65 (d, J = 8.4 Hz, 2H), 7.56−7.55 (m, 1H), 7.38−7.34 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 162.1, 150.7, 142.0, 132.2, 129.0, 126.2, 126.1, 125.3, 124.7, 120.1, and 110.6 ppm. 2-(2-Bromophenyl)benzo[d]oxazole (3r).22 White solid, yield 71%; 0.155 g, Rf = 0.6 (2% ethyl acetate/n-hexane); mp = 52−54 °C; lit., mp 54−55 °C MS: m/z 274 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 8.06 (d, J = 6.9 Hz, 1H), 7.85− 7.82 (m, 1H), 7.75 (d, J = 7.8 Hz, 1H), 7.61−7.58 (m, 1H), 7.45−7.30 (m, 4H); 13C NMR (75 MHz, CDCl3): δ 161.4, 150.5, 141.5, 134.6, 132.0, 131.9, 128.3, 127.3, 125.4, 124.5, 121.8, 120.4, and 110.7 ppm. 2-(3-Bromophenyl)benzo[d]oxazole (3s).22 White solid, yield 63%; 0.137 g, Rf = 0.6 (2% ethyl acetate/n-hexane); mp = 128−130 °C; lit., mp 136−138 °C MS: m/z 274 [M + H]+; 1 H NMR (500 MHz, CDCl3): δ 8.40−8.39 (m, 1H), 8.16−8.15 (m, 1H), 7.77−7.75 (m,1H), 7.64−7.62 (m, 1H), 7.57−7.55 (m, 1H), 7.38−7.34 (m, 3H); 13C NMR (125 MHz, CDCl3): δ 161.5, 150.8, 141.9, 134.4, 130.5, 129.1, 126.1, 125.6, 124.8, 123.0, 120.3, and 110.7 ppm. 2-(4-Chlorophenyl)-1-(p-tolyl)ethanone (4). White solid, yield 96%; Rf = 0.6 (1% ethyl acetate/n-hexane); mp = 108− 110 °C; lit., mp 113 °C MS: m/z 245 [M + H]+; 1H NMR (300 MHz, CDCl3): δ 7.88 (d, J = 8.1 Hz, 2H), 7.27−7.15 (m, 6H), 4.20 (s, 2H), 2.38 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 196.6, 144.1, 133.8, 133.1, 130.8, 129.3, 128.6, 128.5, 44.4, and 21.5 ppm.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00965. Typical experimental procedure and characterization data for acylbenzotriazoles and benzoxazoles, copies of 1H and 13C NMR spectra for all of the developed compounds, and 19F NMR spectra of fluorinated compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Fax: (+91) 542-236817. ORCID

Vinod K. Tiwari: 0000-0001-9244-8889 Present Address ‡

Department of Chemistry, University of Delhi, New Delhi 110007, India (D.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Science Engineering and Research Board (SERB), Department of Science & Technology, New Delhi (Grant No. EMR/2016/001123) for the funding and CISC, Banaras Hindu University, for providing spectroscopy studies. 5049

DOI: 10.1021/acsomega.7b00965 ACS Omega 2017, 2, 5044−5051

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ADDITIONAL NOTE Manuscript Dedicated to (Late) Prof. Alan R Katritzky for his excellent contributions to benzotriazole chemistry. †

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