A smooth access to benzotriazoles via azide-benzyne cycloaddition

Article abstract of DOI:10.1016/j.tet.2008.08.115

A facile synthesis of 1-alkyl benzotriazoles is achieved through fluoride triggered azide-benzyne cycloaddition. Various alkyl azides were treated with 2-(trimethylsilyl) phenyl triflate in the presence of CsF in acetonitrile to afford the corresponding substituted benzotriazoles in good yield.

Full text of DOI:10.1016/j.tet.2008.08.115

Tetrahedron 64 (2008) 11325–11327
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A smooth access to benzotriazoles via azide-benzyne cycloaddition
*
S. Chandrasekhar , M. Seenaiah, Ch. Lohitha Rao, Ch. Raji Reddy
Organic Division-I, Indian Institute of Chemical Technology, Hyderabad 500 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2008
Received in revised form 26 August 2008
Accepted 27 August 2008
Available online 4 October 2008
A facile synthesis of 1-alkyl benzotriazoles is achieved through fluoride triggered azide-benzyne cyclo-
addition. Various alkyl azides were treated with 2-(trimethylsilyl) phenyl triflate in the presence of CsF in
acetonitrile to afford the corresponding substituted benzotriazoles in good yield.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Benzotriazole
Cycloaddition
Benzyne
Azide
Cesium fluoride
1. Introduction
cycloaddition of azides and alkynes, which received a significant
applications after the development of copper(I)-catalyzed cyclo-
Benzotriazoles are important structural motifs of a variety of
biologically active compounds for antitumor, antifungal, anti-in-
flammatory and antidepressant activities, although could not be
found in nature.1 In particular, 1-alkyl benzotriazoles show excep-
tional binding affinities to a range of proteins.2 Further, benzo-
triazoles have great significance in synthetic organic chemistry as
valuable intermediates, protecting groups etc.3 In light of this im-
portance, a facile and efficient method for the synthesis of benzo-
triazoles is an attractive objective.
addition (click reaction).10,11
TMS
N
N
CsF, CH3CN, rt
2.5 - 7 h
N
R
N3
+
TfO
1
3
R
2
9-examples
64-84%
R= 1°-alkyl, 2°-alkyl
Scheme 1. Synthesis of 1-alkyl benzotriazoles.
The commonly used method for the preparation of benzo-
triazoles involves the diazotation reaction of ortho-phenylenedi-
amines.4 The alternative method, cycloaddition of azides with
arynes, to benzotriazoles is little explored5 with limited examples
having inconvenience in generating the arynes.6 In the recent
times, benzyne was generated simply from 2-(trimethylsilyl)phenyl
trifluoromethanesulfonate under mild conditions and used in var-
ious cycloaddition reactions.7 This prompted us to study the prac-
ticability of obtaining benzotriazoles from 2-(trimethylsilyl)phenyl
triflate, which was not reported previously. In continuation of our
work on cycloadditions, 8 herein we report the efficient synthesis of
1-alkyl benzotriazoles via cycloaddition of azide and benzyne un-
der mild reaction conditions (Scheme 1). Larock et al. have parallely
explored the similar strategy for the synthesis of benzotriazoles.9
This reaction is an analogous to the Huisgen 1,3-dipolar
2. Results and discussions
Accordingly, the treatment of 2-(trimethylsilyl)phenyl tri-
fluoromethanesulfonate (2) with benzyl azide (1a) and cesium
fluoride in acetonitrile at room temperature afforded the corre-
sponding benzotriazole in 82% yield (entry 1). With this success,
a variety of alkyl azides were studied to investigate the scope of the
cycloaddition reaction (Table 1). The results revealed that the re-
actions of structurally different primary alkyl azides 1b to 1e were
successfully participated in the cycloaddition reaction with 2, un-
der the described reaction conditions to provide the corresponding
1-alkyl benzotriazoles 3b to 3e in good yield (entries 2–5). Simi-
larly, the reactions of various interesting secondary azide substrates
1f to 1i (derived from glucose di-acetonide, AZT and others) with 2
were proceeded smoothly to afford the furanolinked triazoles
having structural resemblance to nucleosides 3h, 3i and others 3f,
3g (entries 6–9). Notably, several functionalities/protecting groups
* Corresponding author. Tel.: þ9140 27193210; fax: þ9140 27160512.
E-mail address: srivaric@iict.res.in (S. Chandrasekhar).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.08.115

11326
S. Chandrasekhar et al. / Tetrahedron 64 (2008) 11325–11327
Table 1
3. Conclusion
Cycloaddition of alkyl azides with 2 to 1-alkyl benzotriazolesa
In conclusion, we have successfully developed an extremely
facile, new and efficient method for the synthesis of 1-alkyl ben-
zotriazoles by the cycloaddition of 2-(trimethylsilyl) phenyl tri-
fluoromethanesulfonate with alkyl azide in the presence of CsF at
room temperature.
Entry
Azide
Time
(h)
Product
Yield
(%)b
N
N
N
1
3
82 (62)c
84 (65)c
Ph
N3
1a
3a
Bn
N
4. Experimental
4.1. General
N
N3
2
3
2.5
1b
3b
BnO
N
OBn
N
All the solvents and reagents were purified by standard tech-
niques. Crude products were purified by column chromatography
on silica gel of 60–120 mesh. IR spectra were recorded on Perkin–
Elmer 683, Nicolet Nexus 670 spectrometer. 1H NMR and 13C NMR
spectra were recorded in CDCl3, DMSO-d6 solvent on a Varian
Gemini 200 and Bruker AV-300 NMR spectrometer. Chemical shifts
were reported in parts per million (ppm) with respect to internal
TMS. Coupling constants (J) are quoted in hertz (Hz). Optical rota-
tions were measured on Perkin Elmer-343 digital polarimeter using
a 1 mL cell with a 1 dm path length, the concentration c is given in
g/100 mL. Mass spectra were obtained on Finnegan MAT1020B,
micromass VG 70–70H or agilent technologies LC/MSD treapSL
spectrometer operating at 70 eV using direct inlet system.
N
N3
BocN
N
2.5
66
1c
3c
O
O
BocN
N
N
N3
N
4
5
2.5
69
74
1d
3d
N
Boc
BocN
N
N
N3
N
3
1e
3e
4.2. General experimental procedure (for 3a to 3i)
N
To
a
stirred solution of 2-(trimethylsilyl) phenyl tri-
N
N3
fluoromethanesulfonate 2 (1.0 mmol) in acetonitrile (15 mL), alkyl
azide (1.0 mmol), and CsF (2.0 mmol) were added. The reaction
mixture was stirred for the given time (see Table 1) at room tem-
perature. After completion of the reaction (monitored by TLC), the
reaction mixture was diluted with ethyl acetate (10 mL) and washed
with brine solution (1ꢀ15 mL). The organic layer was dried over
Na2SO4 and evaporated in vacuo. The crude compound was purified
bycolumn chromatography (hexanes and ethyl acetate) to afford the
corresponding 1-alkyl benzotriazole.
N
6
4
73
3f
1f
MeO
MeO
OMe
OMe
N
N
O
O
N3
O
N
7
8
3
5
74
82
1g
3g
OPMB
5
O
OPMB
5
4.2.1. 1-Benzyl-1H-benzo[d][1,2,3]triazole (3a)
O
O
O
O
White solid; yield: 29 mg (82%); Rf (hexanes/EtOAc, 9:1)¼0.2;
O
O
O
O
O
mp¼113–115 ꢁC; IR (KBr):
n ;
3426, 2924, 1449, 1224, 746, 718 cmꢂ1
O
N
1h
3h
N
N
1H NMR (200 MHz, CDCl3):
d
8.07 (d, J¼7.8 Hz, 1H, ArH), 7.44–7.23
N3
(m, 8H, ArH), 5.86 (s, 2H, Ar-CH2); 13C NMR (75 MHz, CDCl3):
d
146.5, 134.9, 129.4 (2C), 129.2, 128.6, 127.8 (2C), 127.6, 124.1, 120.3,
109.9, 52.5; ESI (MS): 209 (MþH)þ; HRMS (ESI) calcd for
O
C13H11N3Na: 232.0845 [MþNa]þ, found: 232.0849 [MþNa]þ.
O
N
NH
NH
O
4.2.2. 1-(4-(Benzyloxy) butyl)-1H-benzo[d][1,2,3]triazole (3b)
Viscous liquid; yield: 40 mg (84%); Rf (hexanes/EtOAc,
O
N
O
9
7
64
1i
3i
O
HO
HO
9:1)¼0.15; IR (KBr):
n ;
2926, 2856,1452, 1100, 743 cmꢂ1 1H NMR
N
N3
(300 MHz, CDCl3):
d
8.06 (d, J¼8.3 Hz, 1H, ArH), 7.51–7.23 (m, 8H,
N
N
ArH), 4.69 (t, J¼6.7 Hz, 2H, BtCH2), 4.47 (s, 2H, ArCH2), 3.52 (t,
J¼6.0 Hz, 2H, ArCH2OCH2), 2.18 (qt, J¼7.5 Hz, 2H, BtCH2CH2), 1.68
a
Reaction conditions: CsF, CH3CN, rt.
Isolated yields.
Yield from TBAF mediated reaction.
(qt, J¼6.7 Hz, 2H, OCH2CH2); 13C NMR (75 MHz, CDCl3):
d 145.9,
b
c
138.2, 132.9, 128.3 (2C), 127.6 (3C), 127.1, 123.7, 119.9, 109.3, 73.0,
69.3, 47.9, 26.8, 26.7; ESI (MS): 282 (MþH)þ; HRMS (ESI) calcd for
C17H20N3O: 282.1600 [MþH]þ, found: 282.1607 [MþH]þ.
such as ester, ether, amide, tert-butylcarbamate, benzyl, p-
methoxybenzyl, and acetonide were found to be stable under the
present reaction conditions. Finally, the reaction of 2 with 1a and
1b was tested in the presence of tetrabutylammonium fluoride
(TBAF), a metal-free fluorinating agent, and found to be efficient to
afford the benzotriazoles 3a (62%) and 3b (65%), respectively,
however with lower yields.
4.2.3. (S)-tert-Butyl 4-((1H-benzo[d][1,2,3]triazol-1-yl)methyl)-
2,2-dimethyloxazolidine-3-carboxylate (3c)
Viscous liquid; yield: 36 mg (66%); Rf (hexanes/EtOAc, 7:3)¼0.6;
[a
]
þ10.8 (c 0.75, CHCl3); IR (KBr):
n ;
2925, 2855, 761 cmꢂ1 1H
D
NMR (300 MHz, CDCl3):
d
8.15–8.03 (m, 1H, ArH), 7.81 (d, J¼8.3 Hz,
1H, ArH), 7.66–7.34 (m, 2H, ArH), 4.97–4.74 (m, 2H, BtCH2), 4.47–
4.32 (m, 1H, NCH), 4.24–3.87 (m, 2H, OCH2), 1.55 (s, 9H, (CH3)3C),

S. Chandrasekhar et al. / Tetrahedron 64 (2008) 11325–11327
11327
1.48 (s, 3H, CH3),1.46 (s, 3H, CH3); 13C NMR (75 MHz, CDCl3):
d
152.1,
1062, 747 cmꢂ1; 1H NMR (200 MHz, CDCl3):
d
8.05 (d, J¼8.0 Hz, 1H,
145.8, 127.5, 124.0, 119.8 (2C), 109.9, 93.9, 80.9, 65.0, 56.7, 48.4, 27.2
(3C), 26.9, 24.0; ESI (MS): 333 (MþH)þ; HRMS (ESI) calcd for
C17H25N4O3: 333.1921 [MþH]þ, found: 333.1906 [MþH]þ.
ArH), 7.64–7.22 (m, 3H, ArH), 6.37 (d, J¼3.6 Hz, 1H, BtCHCHCHOO),
5.20 (dd, J¼3.6 Hz, 2H, BtCH, BtCHCHCHO), 4.41 (dd, J¼3.6 Hz,
1H, BtCHCHCHOO), 3.86 (d, J¼5.1 Hz, 2H, BtCHCHCHO,
BtCHCHCHCHxHY), 3.02–2.85 (m, 1H, BtCHCHCHCHxHY), 1.63 (s, 3H,
CH3), 1.42 (s, 3H, CH3), 1.40 (s, 3H, CH3), 0.90 (s, 3H, CH3); 13C NMR
4.2.4. (S)-tert-Butyl 2-((1H-benzo[d][1,2,3]triazol-1-
yl)methyl)pyrrolidine-1-carboxylate (3d)
(75 MHz, CDCl3): d 145.5,133.9,127.3,124.3,119.9,112.7,110.8,109.6,
Yellow viscous liquid; yield: 35 mg (69%); Rf (hexanes/EtOAc,
106.8, 84.6, 81.1, 72.3, 67.8, 63.8, 27.1, 27.0, 26.4, 24.9; ESI (MS): 362
(MþH)þ; HRMS (ESI) calcd for C18H24N3O5: 362.1710 [MþH]þ,
found: 362.1722 [MþH]þ.
7:3)¼0.3; [
a
]
D ꢂ8.8 (c 1, CHCl3); IR (KBr):
n
2924, 2854, 1690, 1393,
8.12–8.02 (m, 1H,
1168, 748 cmꢂ1
;
1H NMR (300 MHz, CDCl3):
d
ArH), 7.76 (d, J¼8.3 Hz, 1H, ArH), 7.57–7.33 (m, 2H, ArH), 4.96–4.63
(m, 2H, BtCH2), 4.38–4.25 (m, 1H, NCH), 3.47–2.98 (m, 2H, NCH2),
2.14–1.56 (m, 4H, CHCH2CH2), 1.53 (s, 9H, (CH3)3C); 13C NMR
4.2.9. 1-((2R,4S,5S)-4-(1H-Benzo[d][1,2,3]triazol-1-yl)-5-
(hydroxymethyl)-tetrahydrofuran-2-yl)-5-methyl pyrimidine-
2,4(1H,3H)-dione (3i)
(75 MHz, CDCl3): d 153.8, 144.8, 132.6, 126.3 (2C), 122.9, 118.6, 78.8,
55.6, 48.7, 45.8, 28.6, 27.7 (3C), 13.1; ESI (MS): 303 (MþH)þ; HRMS
(ESI) calcd for C16H23N4O2: 303.1815 [MþH]þ, found: 303.1829
[MþH]þ.
[
a
]
þ2.8 (c 0.75, MeOH); white solid; yield: 37 mg (64%); Rf
D
(hexanes/EtOAc, 4:6)¼0.1; mp¼205–207 ꢁC; IR (KBr):
n
3436, 2924,
1643, 760 cmꢂ1 1H NMR (200 MHz, DMSO-d6):
; d 9.37 (s, 1H, NH),
8.07 (d, J¼7.9 Hz, 1H, ArH), 7.78–7.23 (m, 4H, ArH, CH]C), 6.36 (t,
J¼6.3 Hz, 1H, BtCHCH2CH), 5.83–5.67 (m, 1H, BtCH), 4.59–4.48 (m,
1H, BtCHCH), 4.10–3.95 (m, 1H, CHxHYOH), 3.77–3.64 (m, 1H,
CHxHYOH), 2.35–2.22 (m, 1H, BtCHCHxHY), 2.06–1.99 (m, 1H,
BtCHCHxHY), 1.95 (s, 3H, CH]CCH3); 13C NMR (75 MHz, DMSO-d6):
4.2.5. 1-(2-(4-Isobutylphenyl)propyl)-1H-benzo[d][1,2,3]-
triazole (3e)
White solid; yield: 36 mg (74%); Rf (hexanes/EtOAc, 9:1)¼0.1;
mp¼63–65 ꢁC; IR (KBr):
n ;
3448, 2924, 1634, 1457, 761 cmꢂ1 1H
8.03 (d, J¼7.5 Hz, 1H, ArH), 7.36–6.98 (m,
NMR (300 MHz, CDCl3):
d
d 160.5, 137.8, 128.0, 124.6, 120.2, 114.1, 111.2, 109.8, 109.2 (2C), 88.8,
7H, ArH), 4.86–4.63 (m, 2H, BtCH2), 3.53–3.49 (m, 1H, ArCH), 2.42
(d, J¼6.7 Hz, 2H, ArCH2), 1.89–1.74 (m, 1H, (CH3)2CH), 1.36 (d,
J¼6.7 Hz, 3H, ArCHCH3), 0.88 (s, 3H, CH3), 0.86 (s, 3H, CH3); 13C NMR
85.1, 61.7, 56.9, 32.0, 14.2; ESI (MS): 344 (MþH)þ; HRMS (ESI) calcd
for C16H18N5O4: 344.1353 [MþH]þ, found: 344.1358 [MþH]þ.
(75 MHz, CDCl3): d 145.5, 140.5, 139.8, 133.2, 129.4 (2C), 126.9, 126.7
(2C), 123.5, 119.7, 109.3, 55.5, 44.9, 40.3, 30.1, 22.3 (2C), 18.4;
ESI(MS): 294 (MþH)þ; HRMS (ESI) calcd for C19H24N3: 294.1964
[MþH]þ, found: 294.1978 [MþH]þ.
Acknowledgements
M.S. and C.L.R. thank the CSIR, New Delhi for financial
assistance.
4.2.6. 1-(1-(3,4-Dimethoxyphenyl)but-3-enyl)-1H-
benzo[d][1,2,3]triazole (3f)
Viscous liquid; yield: 38 mg (73%); Rf (hexanes/EtOAc, 7:3)¼0.15;
n ;
2924, 1514, 1261, 748 cmꢂ1 1H NMR (300 MHz, CDCl3):
References and notes
IR (KBr):
d
8.18 (d, J¼7.5 Hz,1H, ArH), 7.60–7.40 (m, 3H, ArH), 7.14–6.89 (m, 3H,
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ArH), 6.00–5.76 (m, 2H, CH]CH), 5.36–5.08 (m, 2H, BtCH, CH]CH),
3.98 (s, 3H, OCH3), 3.94 (s, 3H, OCH3), 3.78–3.58 (m,1H, BtCHCHxHY),
3.41–3.28 (m, 1H, BtCHCHxHY); 13C NMR (75 MHz, CDCl3):
d 149.3,
149.0, 146.1, 133.3, 132.7, 131.1, 127.0, 123.8, 119.9, 119.3, 118.6, 110.9,
109.8 (2C), 63.3, 55.9, 55.8, 39.1; ESI (MS): 310 (MþH)þ; HRMS (ESI)
calcd for C18H20N3O2: 310.1550 [MþH]þ, found: 310.1555 [MþH]þ.
4.2.7. (2S,3R)-Methyl 3-(1H-benzo[d][1,2,3]triazol-1-yl)-9-(4-
methoxybenzyloxy)-2-methylnonanoate (3g)
Viscous liquid; yield: 35 mg (69%); Rf (hexanes/EtOAc, 7:3)¼0.3;
[
a
]
ꢂ4.4 (c 1, CHCl3); IR (KBr):
n ;
2925, 2854, 1735, 758 cmꢂ1 1H
D
NMR (300 MHz, CDCl3):
d
8.10 (d, J¼8.3 Hz, 1H, ArH), 7.60–7.36 (m,
4. (a) Chan, M. S.; Hunter, W. E. U.S. Patent 4,299,965, 1981; (b) Muir, J. C.; Pat-
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3H, ArH), 7.31–7.20 (m, 2H, ArH), 6.88 (d, J¼8.3 Hz, 2H, ArH), 4.93
(dt, J¼3.7 Hz, 1H, BtCH), 4.38 (s, 2H, OCH2), 3.81 (s, 3H, OCH3), 3.74
(s, 3H, COOCH3), 3.39–3.30 (m, 3H, Ar-CH2-OCH2, CHCOOCH3),
2.43–2.29 (m, 1H, BtCHCHxHY), 1.97–1.84 (m, 1H, BtCHCHxHY), 1.53–
1.43 (m, 2H, OCH2CH2), 1.31–1.04 (m, 6H, OCH2CH2CH2CH2CH2),
0.89 (d, J¼6.7 Hz, 3H, CH3CHCOOCH3); 13C NMR (75 MHz, CDCl3):
6. The starting materials used for the generation of benzyne were either explosive
or commercially not available.
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T.; Yamamoto, Y. Angew. Chem., Int. Ed. 2007, 46, 3323–3325; (c) Liu, Z.; Shi, F.;
Martinez, P. D. G.; Raminelli, C.; Larock, R. C. J. Org. Chem. 2008, 73, 219–226; (d)
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d
174.8, 159.3, 145.8, 134.1, 130.8, 129.3 (2C), 127.6, 124.1, 120.3, 113.9
(2C), 109.5, 72.6, 70.1, 61.9, 55.4, 52.3, 45.3, 33.5, 29.6, 28.3, 26.2,
26.0, 14.8; ESI (MS): 440 (MþH)þ; HRMS (ESI) calcd for
C25H33N3O4Na: 462.2363 [MþH]þ, found: 462.2367[MþH]þ.
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4.2.8. 1-((3Ar,6S,6aR)-5-(2,2-Dimethyl-1,3-dioxolan-4-yl)-2,2-
dimethyl-tetrahydrofuro[2,3-d][1,3]dioxol-6-yl)-1H-
benzo[d][1,2,3]triazole (3h)
[
a]
D þ53.1 (c 1, CHCl3); white solid; yield: 49 mg (82%); Rf (hex-
anes/EtOAc, 9:1)¼0.15; mp¼125–127 ꢁC; IR (KBr):
n 3424, 2926,