Efficient N-Aroylation of Substituted Indoles with N-Aroylbenzotriazoles

Synthesis, 2007, 23, 3673-3677 

Efficient N-Aroylation of Substituted Indoles with N-Aroylbenzotriazoles 

Alan R. Katritzky, *,a Levan Khelashvili, a Prabhu P. Mohapatra a and Peter J. Steel b 

a Center for Heterocyclic Compounds, Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA, bChemistry 

Department, University of Canterbury, Christchurch, New Zealand 

Fax: +1(352)3929199 

E-mail: katritzky@chem.ufl.edu 

Received: The date will be inserted once the manuscript is accepted. 

Abstract: Stable and easily accessible N-aroylbenzotriazoles 

react with indoles in the presence of a base to afford the corresponding 

N-aroylindoles in yields averaging 70%. This method is 

effective even when both coupling reagents possess electrondonating 

substituents. 

Key words: N-aroylbenzotriazoles, indoles, N-aroylindoles, electron-donating 

substituents 

N-Aroylindoles are well-known non-steroidal antiinflammatory 

drugs (NSAIDS) (e.g. indomethacin, a 5methoxyindole 

derivative). 1 N-Aroyl-5-nitro-1H-indoles 

are utilized as intermediates for the synthesis of 2-aryl-5nitro-1H-indoles 

effective as bacterial NorA efflux pump 

inhibitors. 2 5-Substituted N-aroylindoles also exhibit 

selective cyclooxygenase-2 inhibitor activity. 3 and 5amino-1-(3,5-dimethylbenzoyl)-1H-indole 

possesses 

anti-angiogenic activity. 4 Furthermore, suitably substituted 

N-aroylindoles are used as precursors for the synthesis 

of benzannulated indolizidines. 5 

A variety of methods have been reported for the direct 

N-aroylation of indoles. When neither the aroylating 

agent nor the indole contains an electron donating substituent, 

several procedures are satisfactory. 6 However, 

useful N-aroylations of indoles to a product in which 

either the aryl group or indole nucleus contains a strong 

electron donor substituent are rare. N-Aroylindoles have 

been prepared by (i) indolide anion formation with sodium 

hydride followed by reaction with an acid chloride 

(36−82%), 6a (ii) direct aroylation of indoles with carboxylic 

acids using boric acid (50−82%) 6c and (iii) 

phase-transfer catalyzed aroylation of indoles 

(77−94%). 6d Most recently, Bremner et al 6b N-aroylated 

5-substituted indoles with carboxylic acids via 

DCC/DMAP coupling with an electron-withdrawing 

substituent (NO2, CN or F) in the 5-position to give 

73−95% yields of the N-aroylindoles. However, the 

yields for the 5-unsubstituted indoles, were 32−46% and 

for 5-methoxyindole only 0−15%. 

N-Acylbenzotriazoles are easily prepared activated derivatives 

of carboxylic acids. 7 Applications of Nacylbenzotriazoles 

include (i) N-acylation of amines, 

amides 8 and sulfonamides, 9 (ii) O-acylation of aldehydes, 

10 steroids, 11 hydroxyterpenes and alcohols, 12 (iii) 

many C-acylations viz: pyrroles and indoles, 13 ketones 

and heteroaromatics, 14 alkyl sulfones, 15 alkyl cyanides, 16 

alkyl azines, 17 α-nitroalkanes, 18 furan and thiophene; 19 

syntheses of (iv) peptides, 20 (v) oxazolines, 21 (vi) esters, 

22 (vii) benzodioxin-4-ones, 23 (viii) ketones, 24 (ix) 

benzodiazepin-2-ones, 25 (x) thiol esters, 26 (xi) alcohols, 27 

(xii) hydrazides 28 and (xiii) heteroaromatics. 29 

Earlier we 13a,14b and others 12 reported regiospecific Cacylation 

of pyrroles and indoles using Nacylbenzotriazoles. 

Herein, we report the efficient synthesis 

of electron-rich N-aroylindoles from Naroylbenzotriazoles 

in the presence of a base via Naroylation 

of indoles. 

Results and Discussion 

Indole 1a and 5-methoxyindole 1b were purchased and 

used as substrates for N-aroylation reactions. N- 

Aroylbenzotriazoles 2a–c were readily prepared from 

1H-benzotriazole (BtH) and the corresponding carboxylic 

acid in yields of 85 to 95% according to a literature 

procedure 20 (Figure 1). Reaction of 2,6dimethoxybenzoic 

acid with BtH and thionyl chloride in 

dichloromethane at RT for 1 h gave benzotriazol-1-yl- 

(2,6-dimethoxyphenyl)methanone 2d (95%) characterized 

by 1 H, 13 C NMR and CHN analysis (Figure 1). 

Figure 1. N-Acylbenzotriazoles (R 1 COBt) 2a–d. 

Bt 

O 

Bt 

OMe 

MeO 

Bt 

MeO 

Bt 

O 

O 

O 

MeO 

2a 2b 2c 2d 

Bt = 

N 

N 

N 

Reaction of the sodium salt of indole 1a (1 mmol) with 

benzotriazol-1-yl-phenylmethanone 2a (1 mmol) in THF 

at 25 o C for 12 h gave the corresponding 1-benzoyl-1Hindole 

3a in 81% yield (Table 1). Previously 3a was 

prepared in 32% yield from 1a and benzoyl chloride. 6a 

Reaction of indoles 1a and 1b with electron-rich Nacylbenzotriazoles 

2a−d (Figure 1) tested the generality 

of this method. Reaction of 1a with 2b and 2c gave 1-(4methoxybenzoyl)-1H-indole 

3b and 1-(2methoxybenzoyl)-1H-indole 

3c, in 91% (lit. 6b 46%) and 

90% yields (lit. 6b 34%) respectively (Table 1). 

Reaction of 1b with 2a, 2b and 2c gave 1-benzoyl-5methoxy-1H-indole 

3d (87%, lit. 6b 15%), 5-methoxy-1- 

(4-methoxybenzoyl)-1H-indole 3e (77%, lit. 6b 9%) and 

novel 5-methoxy-1-(2-methoxybenzoyl)-1H-indole 3f 

(42%, lit. 6b 0%) (Table 1). A previous attempt to prepare 

3f from the corresponding acid chloride was unsuccessful. 

6c In order to confirm unambiguously the structures 

of these products the X-ray crystal structure of a representative 

example, 3f, was determined. 30 Figure 2 shows 

1

Synthesis, 2007, 23, 3673-3677 

a perspective view of the structure which is in accord 

with that proposed and confirms the N-aroylation. 

Figure 2. X-ray crystal structure of 3f. 

Table 1 Preparation of N-aroylindoles 3a–h 

Reactions of 1a and 1b with more electron-rich (thus 

less reactive) N-aroylbenzotriazole 2d gave novel 1-(2,6dimethoxybenzoyl)-1H-indole 

3g and 1-(2,6dimethoxybenzoyl)-5-methoxy-1H-indole 

3h, in 60% 

and 36% yields respectively (Table 1). These results 

demonstrate that the present method of synthesis is compatible 

with up to a total of three electron-donating substituents 

in both the indole and phenyl rings. 

Conclusion 

Readily available electron-rich N-aroylbenzotriazoles 

under basic conditions efficiently N-aroylate indoles, 

including those with a 5-electron donor substituent. 

entry indole RCOBt product structure yield 

(%) a 

yield (%) 

a 1a 2a 

3a d 81 32 

b 1a 2b 

c 1a 2c 

d 1b 2a 

e 1b 2b 

f 1b 2c 

g 1a 2d 

h 1b 2d 

MeO 

MeO 

MeO 

MeO 

a Isolated yields after column purification and determined from a 

O 

N 

N 

O OMe 

O 

O 

N 

MeO 

N 

MeO 

single experiment, b TLC profile (ethyl acetate/hexanes = 1:4), c SiO2 

column chromatography (5-20% EtOAc in hexanes), d H-3' proton 

O 

N 

N 

O OMe 

O 

N 

MeO 

OMe 

O 

N 

OMe 

lit. 6b 

3b d 91 46 

3c 90 34 

3d 87 15 

3e 77 9 

3f 42 0 

3g 60 new 

3h 36 new 

MeO 

appears as singlet because the coupling constants between H-3' and H- 

2' are 2-4 Hz (low resolution) 

2

Synthesis, 2007, 23, 3673-3677 

Melting points are uncorrected. 1 H NMR and 13 C NMR spectra 

were recorded on a FT-Bruker AT-300 instrument using TMS as an 

internal standard and CDCl3 as solvent. Compounds were characterized 

by elemental analysis using a Carlo-Erba EA1112 instrument. 

Benzotriazol-1-yl-(2,6-dimethoxyphenyl)methanone 

(2d): 2,6-Dimethoxybenzoic acid (1.82 g, 10 mmol) was 

placed in a 100 mL Schlenk flask and dry dichloromethane 

(20 mL) was added. Then benzotriazole (3.63 g, 

30.5 mmol) and dichloromethane (20 mL) were added 

followed by thionyl chloride (0.77 mL, 10.5 mmol). The 

solid precipitate was difficult to stir and so it was shaken 

in a New Brunswick Scientific apparatus at 40 speed at 

RT for 1 h. The white precipitate was filtered and the 

filtrate was evaporated under reduced pressure to give 

the crude product which was then washed with dry diethyl 

ether (2 x 10 mL) to obtain pure benzotriazol-1-yl- 

(2,6-dimethoxyphenyl)methanone (2.62 g, 95%) as white 

crystals. 

Mp 154−155 o C. 

1 H NMR (300 MHz, CDCl3): δ = 8.45 (d, J = 8.2 Hz, 1 

H), 8.13 (d, J = 8.2 Hz, 1 H), 7.70 (t, J = 7.3 Hz, 1 H), 

7.53 (t, J = 7.3 Hz, 1 H), 7.45 (t, J = 8.4 Hz, 1 H), 6.67 

(d, J = 8.4 Hz, 2 H), 3.76 (s, 6H). 

13 C NMR (75 MHz, CDCl3): δ = 165.6, 158.2, 146.4, 

132.9, 131.4, 130.3, 126.3, 120.2, 114.8, 112.7, 104.1, 

56.2. 

Anal. Calcd for C15H13N3O3: C, 63.60; H, 4.63; N, 

14.83. Found: C, 63.37; H, 4.44; N, 14.73. 

General procedure for preparation of N-aroylindoles 

3: The sodium salt of the appropriate indole was prepared 

by adding 1.1 equiv of NaH portionwise to a 

stirred solution of the indole in dry THF (10 mL) at RT. 

After the salt had formed, 1.1 equiv of the appropriate Nacylbenzotriazole 

in dry THF (10 mL) was added dropwise 

and the mixture was allowed to stir at RT until the 

reaction was complete (followed by TLC, hexanes/ethyl 

acetate, 5:1). The resulting mixture was concentrated in 

vacuo and chromatographed on silica gel [10−20% 

EtOAc in hexanes] to give the N-(hetero)aroylindole 3. 

Sometimes additional purification was achieved by 

washing the product with hexanes or recrystallization 

from hexanes. 

1-Benzoyl-1H-indole (3a) 

Yield 0.22 g (81%); white crystals; mp = 63−64 o C 

(Lit. 6b 63−64 o C). 


H), 7.72 (d, J = 6.9 Hz, 2 H) , 7.61−7.49 (m, 4 H), 

7.40−7.29 (m, 3 H), 6.61 (s, 1 H). 

13 C NMR (75 MHz, CDCl3): δ = 168.7, 135.9, 134.5, 

131.8, 130.7, 129.1, 128.5, 127.6, 124.9, 123.9, 120.8, 

116.3, 108.5. 

Anal. Calcd for C15H11N1O1: C, 81.43; H, 5.01; N, 6.33. 

Found: C, 81.50; H, 4.95; N, 6.37. 

1-(4-Methoxybenzoyl)-1H-indole (3b) 


(Lit. 6b 138−139 o C). 


H), 7.74 (d, J = 8.5 Hz, 2 H), 7.60 (d, J = 7.2 Hz, 1 H), 

7.36−7.25 (m, 3 H), 7.00 (d, J = 8.4 Hz, 2 H), 6.61 (s, 1 

H), 3.89 (s, 3 H). 

13 C NMR (75 MHz, CDCl3): δ = 168.2, 162.6, 136.0, 

131.7, 130.6, 127.7, 126.5, 124.6, 123.6, 120.8, 116.1, 

113.8, 108.0, 55.5. 


Found: C, 76.34; H, 5.08; N, 5.83. 

1-(2-Methoxybenzoyl)-1H-indole (3c) 

Yield 0.225 g (90%); colorless glassy solid (Lit. 6b pinkish-red 

glassy solid). 

1 H NMR (300 MHz, CDCl3): δ = 8.37 (br d, J = 7.7 Hz, 

1 H), 7.51−7.18 (m, 5 H), 7.03−6.94 (m, 3 H), 6.47 (d, J 

= 3.7 Hz, 1 H), 3.71 (s, 3 H). 

13 C NMR (75 MHz, CDCl3): δ = 167.2, 156.3, 135.5, 

132.1, 130.9, 129.0, 127.4, 124.8, 124.8, 123.9, 120.8, 

120.7, 116.5, 111.4, 108.6, 55.7. 


Found: C, 76.21; H, 4.83; N, 5.67. 

1-Benzoyl-5-methoxy-1H-indole (3d) 


(Lit. 6b 106−107 o C). 


H), 7.65 (dd, J = 6.9, 1.5 Hz, 2 H), 7.55−7.41 (m, 3H), 

7.18 (dd, J = 3.0, 0.5 Hz, 1 H), 6.99 (d, J = 2.5 Hz, 1 H), 

6.92 (dd, J = 8.9, 2.5 Hz, 1 H), 6.47 (d, J = 3.7 Hz, 1 H), 

3.80 (s, 3H). 

13 C NMR (75 MHz, CDCl3): δ = 168.4, 156.6, 134.5, 

131.7, 130.6, 129.1, 128.5, 128.2, 117.2, 113.3, 108.5, 

103.5, 55.6. 


Found: C, 76.36; H, 5.15; N, 5.50. 

5-Methoxy-1-(4-methoxybenzoyl)-1H-indole (3e) 


(Lit. 6b 105−106 o C). 


H), 7.78 (tt, J = 9.7, 2.7, 2.0 Hz, 2 H), 7.38 (d, J = 3.7 

Hz, 1 H), 7.12 (d, J = 2.5 Hz, 1 H), 7.07−7.02 (m, 3 H), 

6.56 (d, J = 3.7 Hz, 1 H), 3.94 (s, 3 H), 3.92 (s, 3 H). 

3

Synthesis, 2007, 23, 3673-3677 

13 C NMR (75 MHz, CDCl3): δ = 167.8, 162.5, 156.4, 

131.6, 131.5, 130.7, 128.3, 126.4, 116.9, 113.7, 113.2, 

107.9, 103.3, 55.6, 55.4. 


Found: C, 72.84; H, 5.32; N, 4.85. 

5-Methoxy-1-(2-methoxybenzoyl)-1H-indole (3f) 

Yield 0.119 g (42%); white crystals; mp = 93−95 o C. 


1 H, H-2), 7.50 (td, J = 9.0, 1.6 Hz, 1 H, H-6’), 7.43 (dd, 

J = 7.5, 1.6 Hz, 1 H, H-4’), 7.08 (d, J = 7.4 Hz, 1 H, H- 

7), 7.05−7.00 (m, 3H, H-4, H-3’, H-5’), 6.97 (dd, J = 

8.9, 2.4 Hz, 1 H, H-3), 6.47 (d, J = 3.7 Hz, 1 H, H-6), 

3.87 (s, 3H, OCH3), 3.79 (s, 3H, OCH3). 

13 C NMR (75 MHz, CDCl3): δ = 166.8 (C=O), 156.6 (C- 

2’), 156.3 (C-5), 132.0 (C-4’), 131.9 (C-6’), 130.2 (C- 

3a), 129.0 (C-7a), 128.1 (C-2), 124.7 (C-1’), 120.7 (C- 

5’), 117.2 (C-3’), 113.0 (C-7), 111.4 (C-6), 108.5 (C-3), 

103.6 (C-4), 55.7 (OCH3). 


Found: C, 72.93; H, 5.35; N, 4.88. 

1-(2,6-Dimethoxybenzoyl)-1H-indole (3g) 

Yield 0.17 g (60%); Pinkish-white crystals; mp = 

141−142 o C. 


1 H, H-7), 7.64 (d, J = 8.1 Hz, 1 H, H-2), 7.46 (t, J = 8.4 

Hz, 2 H, H-4 and H-6), 7.37 (t, J = 7.1 Hz, 1 H, H-4’), 

7.03 (br s, 1 H, H-5), 6.69 (d, J = 8.4 Hz, 2 H, H-3,5 Ar), 

6.59 (br s, 1H, H-3’), 3.80 (s, 6H, OCH3). 


2’ and C-6’), 135.5 (C-4’), 131.9 (C-7a), 131.2 (C-3a), 

127.3 (C-2), 124.9 (C-5), 124.0 (C-6), 120.8 (C-4), 117.0 

(C-7), 114.0 (C-1’), 108.6 (C-3’ and C-5’), 104.1 (C-3), 

56.0 (OCH3). 


Found: C, 72.50; H, 5.32; N, 4.90. 

1-(2,6-Dimethoxybenzoyl)-5-methoxy-1H-indole (3h) 

Yield 0.113 g (36%); white crystals (from hexanes); mp 

= 134−135 o C. 


H, H-2), 7.39 (t, J = 8.4 Hz, 1 H, H-4’)), 7.04 (s, 1 H, H- 

7), 7.00 (s, 1 H, H-4), 6.94 (d, J = 2.1 Hz, 1 H, H-3), 

6.63 (d, J = 8.5 Hz, 2 H, H-6, H-3’)), 6.46 (d, J = 2.1 Hz, 

1 H, H-5’), 3.86 (s, 3H, OCH3), 3.74 (s, 6H, OCH3). 


2’ and C-6’), 156.8 (C-5), 132.2 (C-4’), 131.9 (C-3a), 

130.3 (C-7a), 128.0 (C-2), 117.7 (C-7), 113.9 (C-6), 

113.1 (C-1’), 108.6 (C-3’ and C-5’), 104.0 (C-3), 103.7 

(C-4), 56.0 (OCH3), 55.8 (OCH3). 


Found: C, 69.49. H, 5.47. N, 4.38. 

Acknowledgments 

We thank Dr. C. D. Hall for checking the manuscript. 

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(30) Crystal data for 3f: C 17H 15NO 3, MW 281.30, monoclinic, 

space group Cc, a = 15.1850(14), b = 4.3143(4), c = 

21.398(2) Å, β = 103.703(3) o , V = 1362.0(2) Å 3 , F(000) = 

592, Z = 4, T = -170 o C, µ (MoKα) = 0.095 mm -1 , D calcd = 

1.372 g.cm -3 , 2θ max 50 o APEX II CCD area detector, 

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wR(F 2 ) = 0.074 (all 2244 data), R = 0.029 (2039 data with 

I > 2σI). CCDC # 639667. 

5

Synthesis, 2007, 23, 3673-3677 

Graphical Abstract 

Short title of the article: Efficient N-Acylation of Substituted Indoles with N-Acylbenzotriazoles 

R 

N 

H 

1 2 

N 

NaH 

O 

1 

R 2 

R 3 

O 

Bt 

R 1 

3 

36-92% 

R1 = H, 5-OMe; R2 = R3 = H, p-OMe, o-OMe, 2,6-OMe; Bt = benzotriazol-1-yl 

R 2 

R 3 

6

Efficient N-Aroylation of Substituted Indoles with N-Aroylbenzotriazoles

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