Synthesis of 1-(2-bromo-1-arylethyl)-1H-benzotriazoles via NBS promoted addition of 1H-benzotriazole to alkene: Relevance in benzotriazole ring cleavage

Article abstract of DOI:10.1016/j.tet.2020.131078

A cost-effective and expeditious one-pot synthesis of 1-(2-bromo-1-arylethyl)-1H-benzotriazoles has been devised by reacting styrene and their derivatives with 1H-benzotriazole in presence of N-bromosuccinimide in anhydrous CH₂Cl₂ at room temperature. Further, the resulted compounds undergo E₂-ellimination to afford the respective 1-(1-aryl-vinyl)-1H-benzotriazoles. At the end, 1-(1-phenylvinyl)-1H-benzotriazoles underwent benzotriazole ring cleavage (BtRC) under free radical condition to produce phenanthridine as the final product.

Full text of DOI:10.1016/j.tet.2020.131078

Journal Pre-proof
Synthesis of 1-(2-bromo-1-arylethyl)-1H-benzotriazoles via NBS promoted addition of
1H-benzotriazole to alkene: Relevance in benzotriazole ring cleavage
Mala Singh, Priyanka Bose, Anoop S. Singh, Vinod K. Tiwari
PII:
S0040-4020(20)30198-8
https://doi.org/10.1016/j.tet.2020.131078
TET 131078
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 11 December 2019
Revised Date: 21 February 2020
Accepted Date: 22 February 2020
Please cite this article as: Singh M, Bose P, Singh AS, Tiwari VK, Synthesis of 1-(2-bromo-1-
arylethyl)-1H-benzotriazoles via NBS promoted addition of 1H-benzotriazole to alkene: Relevance in
benzotriazole ring cleavage, Tetrahedron (2020), doi: https://doi.org/10.1016/j.tet.2020.131078.
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Synthesis of 1-(2-bromo-1-arylethyl)-1H-benzotriazoles via
NBS promoted addition of 1H-benzotriazole to alkene:
Relevance in Benzotriazole Ring Cleavage
Mala Singh, Priyanka Bose, Anoop S. Singh, Vinod K. Tiwari*
N
NBS (2.0 eq.),
N
Elimination
BtH (2.0 eq.)
N
R
R
Dry DCM, RT
BtRC
N
R
Br
A cost-effective and expeditious one-pot synthesis of 1-(2-bromo-1-arylethyl)-1H-benzotriazoles has been devised by reacting styrene and their
derivatives with 1H-benzotriazole in presence of N-bromosuccinimide in anhydrous CH2Cl2 at room temperature. Further, the resulted
compounds undergo E2-ellimination to afford the respective 1-(1-aryl-vinyl)-1H-benzotriazoles. At the end, 1-(1-phenylvinyl)-1H-benzotriazoles
underwent benzotriazole ring cleavage (BtRC) under free radical condition to produce phenanthridine as the final product.

1
Author Name / Tetraheron
Tetrahedron
journal homepage: www.elsevier.com/locate/jestch
Synthesis of 1-(2-bromo-1-arylethyl)-1H-benzotriazoles via NBS promoted addition of 1H-
benzotriazole to alkene: Relevance in Benzotriazole Ring Cleavage
Mala Singh, Priyanka Bose, Anoop S. Singh, and Vinod K. Tiwari*
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi-221005, India
ABS TRACT
ARTICLE INFO
A cost-effective and expeditious one-pot synthesis of 1-(2-bromo-1-arylethyl)-1H-benzotriazoles
has been devised by reacting styrene and their derivatives with 1H-benzotriazole in presence of
N-bromosuccinimide in anhydrous CH2Cl2 at room temperature. Further, the resulted compounds
undergo E2-ellimination to afford the respective 1-(1-aryl-vinyl)-1H-benzotriazoles. At the end, 1-
(1-phenylvinyl)-1H-benzotriazoles underwent benzotriazole ring cleavage (BtRC) under free
radical condition to produce phenanthridine as the final product.
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
N-acyl benzotriazole
1H-benzotriazole
Benzotriazole Ring Cleavage (BtRC)
N-bromosuccinimide
E2-ellimination
2009 Elsevier Ltd. All rights reserved.
Ring Opening
important role in drug discovery and development.31-43
1. Introduction
Several research groups, most notably, the Katritzky group
explored the interesting features of benzotriazole
methodology since last four decades span. Previously, we also
have employed BtRC approach for the synthesis of diverse
benzothiazoles, benzoxazoles and amide derivatives.18-23
Therefore, to achieve a new method to prepare benzotriazole-
based reagents is of paramount importance, where
benzotriazole ring cleavage has been successfully explored as
a starting material in several reactions.16-23 Herein, we wish to
disclose a new synthetic protocol for the facile synthesis of 1-
Benzotriazole is pertaining significantly diverse role in organic
chemistry since last four decades in the form of reagents,
ligands, and intermediates to produce a large number of
molecules having widespread applications.1-3 The industrial
applications in terms of utilizing the scaffold can be
demonstrated as corrosion inhibitor and anti-tarnish agent for
copper and its alloys4 as fixing agents in photographic
emulsions, as reagent in analytical determination of silver and
use in anti-freeze and water coolant systems.3 Remarkable
bio-importance of benzotriazole moiety, such as, antibacterial,
antifungal, antiviral, anti-inflammatory, antihypertensive,
analgesic, etc., shows its broad relevance in the synthesis of
biologically active compounds.5-11 Molecules containing
benzotriazole ring have also exhibited potential anti-
tubercular,12 anti-tumor,13 anti-proliferative,14 and anesthetic
activities.15 Benzotriazole ring cleavage (BtRC) methodology is
one of the trending methods with the advantage of time as it is
an interesting approach for an easy access to diverse series of
compounds having pharmaceutical importance.1-3,16-23 Pre-
reported reaction conditions includes thermolysis, photolysis,
organo-metallic reagents oxidants and free radical reagents,
resulted in the desired cleavage of the relatively stable
benzotriazole ring, exhibiting fascinating chemistry.24-30
Economically effective, chemically stable and non-toxic nature
of benzotriazole moiety makes it appreciable approach in the
organic synthesis which has been explored to generate galaxy
of libraries comprising of diverse scaffolds. Thus it plays
(2-bromo-1-arylethyl)-1H-benzotriazoles
from
styrene.
Further their representative application in benzotriazole ring
cleavage (BtRC) aimed to respective phenanthridine under
free radical condition (Scheme 1).
Scheme 1. Overall synthetic route for the phenanthridine

2
Scope of Alkenes. aReaction conditions: Styrene 1 (1.0 equiv.),
benzotriazole 2 (X equiv.), NBS (Y equiv.), and anhydrous DCM (5
2. Results and Discussion
A prototype reaction for the synthesis of 1-(2-bromo-1-
phenylethyl)-1H-benzotriazole
phenylethyl)-2H-benzotriazole 3a’ was set-up. For which
styrene (1.0 equiv.) was treated with 1.0 equiv. of N-
bromosuccinimide (NBS) in anhydrous dichloromethane and
after 30 min, 1.0 eq. of 1H-benzotriazole was added and the
reaction was further stirred for 5 hrs. White solid compounds
3a and 3a´ were isolated in 27% yield (Scheme 2).
c
mL) at r.t. for 5 h. b Yield of isolated product (3a + 3a’). reaction at
3a
and
1-(2-bromo-1-
40 °C.
The best optimized condition for the 1-(2-bromo-1-
phenylethyl)-1H-benzotriazoles established that involves the
reaction of styrene (1.0 equiv.) with NBS (2.0 equiv.) and
benzotriazole (2.0 equiv.) in anhydrous CH2Cl2 at room
temperature for 5 hrs. Thus, we have established an efficient
path for the amino-bromination of styrene after the
standardization of the reaction condition (Table 1). The scope
of styrene was explored at room temperature and the outcome
has been summarized in Figure 1. Different derivatives of
styrene such as electron-donating groups (methyl-, methoxy-
and t-butyl-) and electron-withdrawing groups (chloro, fluoro
and acetate) were used for the conversion and the product of
amino-bromination was in good yield.
Scheme 2. Prototype reaction for the synthesis of 1-(2-bromo-1-
phenylethyl)-1H (3a) or 2H-benzotriazoles (3b)
Then after, we look forward for the optimization, thus the
loading quantity of NBS and BtH were varied in the above
prototype reaction in various solvent, where we get optimum
reaction yield (90%) in entry 3. Further for the optimum
reaction condition, other halogenation reagents such as I2, NIS,
NCS, KI, and KBr were trialed. The reagents NCS, NIS and I2
provided average reaction yields, however, as expected KI and
KBr failed to afford the desired product. Furthermore, in order
to get the best optimum condition, we tried the reaction at
high temperature and also for longer reaction time to improve
the reaction yield of compound 3a, where the result is again
not the favorable one.
N
NBS (2.0 eq.),
BtH (2.0 eq.)
N
N
R
N
N
+
N
Dry DCM, R.T.
R
Br
R
Br
3a
3a'
Developed 1-(2-bromo-1-phenylethyl)-1H-benzotriazoles (3a-h) via amino halogenation
N
N
N
N
N
N
N
N
N
Br
Br
Br
H3CO
3c, 66%
3b, 50%
3a, 65%
N
N
N
N
N
N
N
N
N
Table 1: Optimization for the synthesis of 1-(2-bromo-
1-phenylethyl)-1H-benzotriazoles
Br
F
Br
Br
O
O
3d, 57%
3e, 47%
3f, 44%
N
N
N
N
N
N
Entrya Reagent
(equiv.)
BtH
(equiv.)
1.0
1.5
2.0
2.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
Solvent
Time Yieldb
3a + 3a’
I
Cl
3g, 36%
3h, 40%
1
2
3
4
5
6
7
8
NBS (1.0)
NBS (1.5)
NBS (2.0)
NBS (2.5)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NIS (2.0)
NCS (2.0)
I2 (2.0)
KBr (2.0)
KI (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
DCM
DCM
DCM
DCM
EtOAc
DMF
THF
CH3OH
DCM
DCM
DCM
DCM
DCM
CHCl3
CCl4
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
27%
59%
90%
90%
36%
ND
10%
ND
66%
56%
40%
ND
Developed 1-(2-bromo-1-phenylethyl)-2H-benzotriazoles (3a'-h') via amino halogination
Br
N
N
Br
N
N
N
Br
N
N
N
N
OCH3
3b', 30%
3c', 23%
3a' , 25%
9
10
11
12
13
14
15
16
17
Br
N
N
Br
Br
N
N
N
N
N
N
N
O
F
ND
O
80%
55%
36%
ND
3f', 34%
3d', 24%
3e', 30%
EtOAc
DMF
I
N
N
Cl
N
N
N
N
18
19
20c
21
NBS (2.0)
NBS (2.0)
NBS (2.0)
NBS (2.0)
2.0
2.0
2.0
2.0
THF
5
5
10%
ND
CH3OH
DCM
DCM
5
15
45%
88%
3g', 20%
3h', 26%

3
Author Name / Tetraheron
Figure 1: Scope of Alkenes. Reaction conditions: Styrene 1 (1.0
mmol), benzotriazole 2 (2.0 mmol), NBS (2.0 mmol), and anhydrous
DCM (5 mL) at r.t. for 5 h. % Yield of isolated product after column
chromatography (SiO2).
Styrene with different substituent, e.g. 4-methoxy, 4-methyl, 4-
t-butyl has given the products in good yield (70-80%) where
N1/N2 ratio was found to be around 1.5:1, here, N1 has been
assigned for the yield of 1-(2-halo-1-phenylethyl)-1H-
benzotriazoles and N2 has been assigned for the yield of 1-(2-
halo-1-phenylethyl)-2H-benzotriazoles, respectively. The
Chloro and acetate group at para position of styrene gave
products in 68% and 78% yield respectively in which N1/N2
ratio was around to be 1.4:1. In case of 3-fluoro-styrene, the
yield of the reaction was found to be 77% having N1/N2
(1.56:1).
Scheme 5: Possible mechanism for synthesis of 1-(2-bromo-1-
arylethyl)-1or 2H-benzotriazoles
Based on our previous experience of the benzotriazole ring
cleavage, benzothiazole and benzamide were obtained from N-
thioacyl benzotriazole and N-acyl benzotriazole, respectively
through the cleavage of benzotriazole ring under the influence
of free radical development (Figure 2), so we have interest to
explore the resulted R(Bt)C=CH2 in order to have the desired
benz-fused heterocycle i.e. indole derivative, however to our
surprise the isolated compound was phenanthridine, instead
of the indole one.
For more exploration of the work, a set of benzotriazole
derivatives were explored in the pre-established protocol
(Scheme 3). When 5-chlorobenzotriazole was reacted with
styrene having chloro and t-butyl at p- position, the reaction
yield was 68% (N1/N2 ratio 1.42) and 79% (N1/N2 ratio
1.19), respectively.
Previous experience on BtRC:
S
O
N
N
N
N
N
N
(Ref: 16-21)
(Ref: 22,23)
Scheme 3: Scope of reaction by varying substitution on
benzotriazole ring.
Present work with an aid of BtRC:
Furthermore, by altering the aryl alkene with aliphatic alkene
in the same reaction conditions same results have been
achieved (Scheme 4).
CH2
N
N
N
N
Br
NBS (2.0 eq.),
BtH (2.0 eq.).
Figure 2: Exploration of benzotriazole ring cleavage (BtRC)
N
N
N
N
N
Br
Anhydrous DCM,
R.T.
Thus, the end product, 1-(2-bromo-1-phenylethyl)-1H-
benzotriazole 3a also has been utilized for the synthesis of
phenanthridine. For this, first of all E2-ellimination on the 1-
(2-bromo-1-phenylethyl)-1H-benzotriazole is carried out with
the help of base for the formation of 1-(1-phenylvinyl)-1H-
benzotriazole which when heated in seal tube with 2.0 equiv.
of nBuSnH in presence of 0.2 equiv. of azobisisobutyronitrile
(AIBN) in anhydrous toluene undergoes free radical
benzotriazole ring opening to afford phenanthridine 5 as final
product (Scheme 6).
3k', 33%
3k
, 47%
Scheme 4: Scope of reaction by altering the aryl alkene with
aliphatic alkene
Initially, the alkene substrate reacts with Br+ to afford the
bromonium ion intermediate I,44-45 which undergoes
regioselective ring opening. Further, it reacts with
benzotriazole affording the desired bromo products 3. The
path of reaction proceed to the final product also has been
evaluated through DFT analysis utilizing the zero point
vibrational energy value supports the results obtained in the
our protocol (see, the supporting information),

4
3. Conclusions
In conclusion, we have developed a practical synthesis of 1-(2-
bromo-1-phenylethyl)-1H-benzotriazole using styrenes and
1H-benzotriazole, which further undergoes E2-ellimination
and free radical benzotriazole ring opening to produce
phenanthridine. This approach is straightforward, easy to
execute, and metal-free condition and thus can be explored in
further investigation for the development of biologically
relevant heterocycles.
4. Experimental
All the solvents and reagents were taken of pure grade. Thin-
layer chromatography (TLC) was performed on, pre-coated
aluminium plates and spots were evaluated using UltraViolet
lamp (λ max = 254 nm). The solvents were evaporated at
temperature < 50oC. Column chromatography was carried out
using silica gel (230-400 mesh, Merck). Distilled n-hexane and
ethyl acetate were used for the column chromatography. 1H
and 13C NMR were recorded at 500 and 125 MHz, respectively.
The chemical shifts are given in ppm downfield from internal
TMS; J values in Hz. The Infrared spectra have been recorded
as Nujol mulls in KBr palettes.
Scheme
6
.
Application of 1-(2-bromo-1-arylethyl)-1H-
benzotriazoles for the systematic formation of phenanthridine 5
via free radical reaction path.
Mechanistic consideration for BTRC:
The 1-(1-phenylethenyl)-1H-1,2,3-benzotriazole derivatives
can easily proceed to ring cleavage by reaction with nBu3SnH
in presence of azobisisobutyronitrile (AIBN), a well-known
radical initiator. In such reactions, the 2-cyanoprop-2-yl
radical generated from AIBN initiates the process by rapidly
reacting with tin hydride to produce a tin radical, which
attacks the vinylic system of 1-(1-phenylethenyl)-1H-1,2,3-
benzotriazole 4 and propagates the reaction. Decomposition of
1-(1-phenylethenyl)-1H-1,2,3-benzotriazole via N-N bond
cleavage through removal of the molecular nitrogen (N2)
followed by the rearrangement of the resulted biradical
afforded species B. In the species B, free radical hydrogen
abstracted from the another attached phenyl ring forms two
phenyl radical centers as shown in species C.46,47 At the end,
both of the phenyl radicals in species C undergo cyclization to
give the respective phenanthridine 5 (Figure 3). Moreover,
the reaction carried out in presence of TEMPO, the final
cyclized product was not obtained which further supports the
free radical generation mechanism as presented in proposed
reaction.
General procedure for synthesis of 1-(2-bromo-1-
arylethyl)-1H-benzotriazoles
Styrene (1.0 equiv.) in anhydrous dichloromethane (5 mL) and
NBS (2.0 equiv.) was taken into RB and reaction mixture was
stirred at room temperature. After 30 mins benzotriazole (2.0
equiv) dissolved in anhydrous DCM was added drop wise to
the reaction mixture. After the completion of reaction as
monitored by TLC, solvent was evaporated under vacuum.
The reaction mixture was extracted with ethyl acetate and
washed with hypo and brine solution. The organic layer
obtained was dried over sodium sulphate, filtered and then
evaporated under reduced pressure. Further purification was
done by using flash column chromatography (SiO2).
1-(2-Bromo-1-phenylethyl)-1H-benzo[d][1,2,3]triazole
(3a). White Solid, yield: 188mg (65%); Rf = 0.3 (5% ethyl
acetate/n- hexane); m.p. = 130-134oC; 1H NMR (500 MHz,
CDCl3): δ 8.07 (d, J = 8.5Hz, 1H), 7.43-7.40 (m, 4H), 7.36-7.34
(m, 4H), 6.02 (dd, J = 5.5Hz, 3.5Hz, 1H), 4.66 (t, J = 10.5Hz, 1H),
4.16 (dd, J = 5.5Hz, 5.0Hz, 1H); 13C NMR (125 MHz, CDCl3): δ
146.1, 136.7, 133.2, 129.3, 129.2, 127.7, 127.0, 124.2, 120.2,
109.4, 64.9 and 32.8 ppm; HRMS m/z (M + H) calcd for
C14H13BrN3 302.0214; Found, 302.0275.
2-(2-Bromo-1-phenylethyl)-2H-benzo[d][1,2,3]triazole
(3a’). White Solid, yield: 72mg (25%); Rf = 0.7 (5% ethyl
acetate/n-hexane); m.p. = 100-104oC; 1H NMR (500 MHz,
CDCl3): δ 7.90-7.88 (m, 2H), 7.49-7.47 (m, 2H), 7.39-7.34 (m,
5H), 6.22 (dd, J = 4.5Hz, 5.5Hz, 1H), 4.60 (t, J = 10.5Hz, 1H), 4.06
(dd, J = 4.5Hz, 6.0Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 144.4,
136.6, 129.4, 129.1, 127.1, 126.7, 118.4, 72.2 and 32.6 ppm.
1-(2-Bromo-1-(4-methoxyphenyl)ethyl)-1H-benzo[d]
[1,2,3]triazole (3b). oily, yield: 123mg (50%); Rf = 0.4 (10%
ethyl acetate/n-hexane); 1H NMR (300 MHz, CDCl3): δ 8.05 (d, J
Figure 3. Plausible mechanism for the synthesis of
phenanthridine 5.

5
Author Name / Tetraheron
= 8.1Hz, 1H), 7.42-7.40 (m, 2H), 7.35-7.32 (m, 3H), 6.84 (d, J =
8.7Hz, 2H), 6.01-5.96 (m, 1H), 4.62 (t, J = 10.2Hz, 1H), 4.15-4.09
(m, 1H), 3.72 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 159.9, 145.8,
132.9, 128.5, 128.1, 127.4, 123.9, 119.8, 114.3, 109.3, 64.1,
55.1 and 32.8 ppm; HRMS m/z (M + H) calcd for C15H15BrN3O
332.0320; Found, 332.0404.
2-(2-Bromo-1-(3-fluorophenyl)ethyl)-2H-benzo[d]
[1,2,3]triazole (3e’). White Solid, yield: 78mg (30%); Rf = 0.7
(5% ethyl acetate/n-hexane); m.p. = 76-78oC; 1H NMR (500
MHz, CDCl3): δ 7.90-7.88 (m, 2H), 7.40-7.38 (m, 2H), 7.34-7.20
(m, 3H), 7.04-7.01 (m, 1H), 6.21 (dd, J = 5.0Hz, 4.5Hz, 1H), 4.55
(t, J = 10.5Hz, 1H), 4.06 (dd, J = 4.5Hz, 6.0Hz, 1H); 13C NMR (125
MHz, CDCl3): δ 163.7, 161.7, 144.3, 138.66, 138.6, 130.67,
130.6, 126.7, 122.9, 122.8, 118.2, 116.4, 116.2, 114.3, 114.1,
71.2 and 32.1 ppm.
2-(2-Bromo-1-(4-methoxyphenyl)ethyl)-2H-benzo[d]
[1,2,3]triazole (3b’). White Solid, yield: 74mg (30%); Rf = 0.7
(10% ethyl acetate/n-hexane); m.p. = 64-68oC; 1H NMR (300
MHz, CDCl3): δ 7.88-7.86 (m, 2H), 7.44-7.33 (m, 4H), 6.86-6.83
(m, 2H), 6.18-6.17 (m, 1H), 4.57 (t, J = 10.5Hz, 1H), 4.05-4.01
(m, 1H), 3.72 (s, 3H) ; 13C NMR (75 MHz, CDCl3): δ 160.1,
144.2, 128.5, 128.3, 126.4, 118.4, 114.2, 71.5, 55.1 and 32.7
ppm.
4-(1-(1H-benzo[d][1,2,3]triazol-1-yl)-2-bromoethyl)
phenyl acetate (3f). White Solid, yield: 97mg (44%); Rf = 0.3
(10% ethyl acetate/n-hexane); m.p. = 148-152oC; 1H NMR
(500 MHz, CDCl3): δ 8.10 (d, J = 8.5Hz, 1H), 7.49-7.43 (m, 4H),
7.39-7.35 (m, 1H), 7.12-7.10 (m, 2H), 6.08 (dd, J = 5.0Hz, 4.5Hz,
1H), 4.64 (t, J = 10.5Hz, 1H), 4.15 (dd, J = 5.0Hz, 5.5Hz, 1H), 2.28
(s, 3H); 13C NMR (125 MHz, CDCl3): δ 169.0, 150.9, 145.6,
133.9, 132.9, 128.0, 127.6, 124.1, 122.2, 119.8, 109.1, 63.8,
32.5 and 20.8 ppm; HRMS m/z (M + H) calcd for C16H15BrN3O2
360.0269; Found, 360.0326.
1-(2-Bromo-1-(p-tolyl)ethyl)-1H-benzo[d][1,2,3]triazole
(3c). oily, yield: 176mg (66%); Rf = 0.5 (10% ethyl acetate/n-
hexane); 1H NMR (300 MHz, CDCl3): δ 8.09-8.05 (m, 1H), 7.42-
7.36 (m, 2H), 7.34-7.28 (m, 3H), 7.15-7.12 (m, 2H), 6.03-5.97
(m, 1H), 4.69-4.60 (m, 1H), 4.18-4.11 (m, 1H), 2.29 (s, 3H); 13
C
NMR (75 MHz, CDCl3): δ 145.9, 139.1, 133.5, 133.0, 129.7,
127.4, 126.7, 124.0, 119.9, 109.3, 64.5, 32.7 and 21.0 ppm;
HRMS m/z (M + H) calcd for C15H15BrN3 316.0371; Found,
316.0448.
4-(1-(2H-benzo[d][1,2,3]triazol-2-yl)-2-bromoethyl)
phenyl acetate (3f’). White Solid, yield: 75mg (34%); Rf = 0.6
(10% ethyl acetate/n-hexane); m.p.
Formula = C16H14BrN3O2
1H NMR (500 MHz, CDCl3): δ 7.88-
=
126-130oC; mol.
;
7.86 (m, 2H), 7.53-7.50 (m, 2H), 7.39-7.37 (m, 2H), 7.09-7.06
(m, 2H), 6.21 (dd, J = 4.5Hz, 6.0Hz, 1H), 4.57 (t, J = 10.5Hz, 1H),
4.03 (dd, J = 4.5Hz, 6.5Hz, 1H), 2.26 (s, 3H); 13C NMR (125 MHz,
CDCl3): δ 169.2, 151.3, 144.4, 134.0, 128.4, 126.8, 122.3, 118.4,
71.5, 32.5 and 21.1 ppm.
2-(2-Bromo-1-(p-tolyl)ethyl)-2H-benzo[d][1,2,3]triazole
(3c’). White Solid, yield: 61mg (23%); Rf = 0.8 (10% ethyl
acetate/n-hexane); m.p. = 74-76oC; H NMR (300 MHz, CDCl3):
δ 7.87-7.84 (m, 2H), 7.36-7.28 (m, 4H), 7.09 (d, J = 7.8Hz, 2H),
6.22-6.17 (m, 1H), 4.58 (t, J = 10.5Hz, 1H), 4.05-4.01 (m, 1H),
2.23 (s, 3H) ; 13C NMR (75 MHz, CDCl3): δ 144.1, 139.0, 133.4,
129.4, 126.8, 126.3, 118.1, 71.6, 32.7 and 20.9 ppm.
1
1-(2-Chloro-1-phenylethyl)-1H-benzo[d][1,2,3]triazole
(3g). White Solid, yield: 88mg (36%); Rf = 0.5 (10% ethyl
acetate/n-hexane); m.p. = 118-122oC; 1H NMR (500 MHz,
CDCl3): δ 8.05 (d, J = 7.5Hz, 1H), 7.42-7.37 (m, 4H), 7.32-7.28
(m, 4H), 6.00 (dd, J = 6.5Hz, 4.0Hz, 1H), 4.80-4.76 (m, 1H), 4.28
(dd, J = 4.5Hz, 7.0Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 146.0,
136.1, 133.3, 129.3, 129.2, 127.8, 127.1, 124.3, 120.1, 109.5,
64.9 and 45.5 ppm; HRMS m/z (M + H) calcd for C14H13ClN3
258.0719; Found, 258.0782.
1-(2-Bromo-1-(4-(tert-butyl)phenyl)ethyl)-1H-benzo[d]
[1,2,3]triazole (3d).White Solid, yield: 127mg (57%); Rf = 0.4
1
(5% ethyl acetate/n-hexane); m.p. = 134-136oC; H NMR (500
MHz, CDCl3): δ 8.00 (d, J = 8.5Hz, 1H), 7.39-7.34 (m, 2H), 7.29-
7.25 (m, 5H), 5.94 (dd, J = 5.0Hz, 5.5Hz, 1H), 4.59 (t, J = 10.5Hz,
1H), 4.06 (dd, J = 4.5Hz, 6.0Hz, 1H), 1.19 (s, 9H); 13C NMR (125
MHz, CDCl3): δ 152.4, 146.0, 133.7, 133.3, 127.6, 126.7, 126.2,
124.2, 120.2, 109.4, 64.7, 34.7, 32.9 and 31.2 ppm; HRMS m/z
(M + H) calcd for C18H21BrN3 358.0840; Found, 357.0896.
2-(2-Chloro-1-phenylethyl)-2H-benzo[d][1,2,3]triazole
(3g’). White Solid, yield: 49mg (20%); Rf = 0.8 (10% ethyl
1
acetate/n-hexane); m.p. = 78-82oC; H NMR (500 MHz, CDCl3):
2-(2-Bromo-1-(4-(tert-butyl)phenyl)ethyl)-2H-benzo[d]
[1,2,3]triazole (3d’). Oily, yield: 53mg (24%); Rf = 0.8 (5%
ethyl acetate/n-hexane); 1H NMR (500 MHz, CDCl3): δ 7.81-
7.77 (m, 2H), 7.33-7.31 (m, 2H), 7.29-7.26 (m, 4H), 6.12 (dd, J =
5.0Hz, 5.5Hz, 1H), 4.52 (t, J = 10.5Hz, 1H), 3.96 (dd, J = 5.0Hz,
5.5Hz, 1H), 1.17 (s, 9H); 13C NMR (125 MHz, CDCl3): δ 152.4,
144.3, 133.6, 126.8, 126.6, 126.1, 118.4, 72.0, 34.7, 32.8 and
31.2 ppm.
δ 7.89-7.87 (m, 2H), 7.48-7.46 (m, 2H), 7.38-7.31 (m, 5H), 6.19
(dd, J = 6.0Hz, 4.5Hz, 1H), 4.78-4.64 (m, 1H), 4.20 (dd, J = 5.0Hz,
6.5Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 144.2, 135.8, 129.2,
129.0, 127.1, 126.5, 118.2, 72.1 and 45.4 ppm.
1-(2-Iodo-1-phenylethyl)-1H-benzo[d][1,2,3]triazole (3h).
White Solid, yield: 134mg (40%); Rf = 0.4 (5% ethyl acetate/n-
1
hexane); m.p. = 132-134oC; H NMR (500 MHz, CDCl3): δ 7.99
(d, J = 7.5Hz, 1H), 7.36-7.24 (m, 8H), 5.89 (dd, J = 5.5Hz, 5.0Hz,
1H), 4.37 (t, J = 10.5Hz, 1H), 3.94 (dd, J = 5.0Hz, 5.5Hz, 1H); 13C
NMR (125 MHz, CDCl3): δ 146.1, 137.5, 133.1, 129.3, 129.2,
127.7, 126.8, 124.2, 120.2, 109.4, 65.5 and 5.2 ppm; HRMS m/z
(M + H) calcd for C14H13IN3 350.0075; Found, 350.0139.
1-(2-Bromo-1-(3-fluorophenyl)ethyl)-1H-benzo[d][1,2,3]
triazole (3e).White Solid, yield: 123mg (47%); Rf = 0.3 (5%
ethyl acetate/n-hexane); m.p. = 68-72oC; 1H NMR (500 MHz,
CDCl3): δ 8.09 (d, J = 8.0Hz, 1H), 7.48-7.31 (m, 4H), 7.21 (d, J =
8.5Hz, 1H), 7.15-7.13 (m, 1H), 7.05-7.01 (m, 1H), 6.01 (dd, J =
5.5Hz, 5.0Hz, 1H), 4.64-4.59 (m, 1H), 4.17 (dd, J = 5.5Hz, 5.5Hz,
1H); 13C NMR (125 MHz, CDCl3): δ 163.8, 161.8, 145.8, 138.9,
138.8, 133.0, 130.8, 130.7, 127.8, 124.3, 122.7, 122.6, 120.1,
116.3, 116.2, 114.2, 114.0, 109.0, 64.0 and 32.2 ppm; HRMS
m/z (M + H) calcd for C14H12BrN3 320.0120; Found, 320.0187.
2-(2-Iodo-1-phenylethyl)-2H-benzo[d][1,2,3]triazole
(3h’). White Solid, yield: 87mg (26%); Rf = 0.8 (5% ethyl
acetate/n-hexane); m.p. = 126-130oC; 1H NMR (500 MHz,
CDCl3): δ 7.89-7.87 (m, 2H), 7.48 (d, J = 7.5Hz, 2H), 7.39-7.31
(m, 5H), 6.19 (dd, J = 5.5Hz, 5.0Hz, 1H), 4.38 (t, J = 11.5Hz, 1H),

6
3.94 (dd, J = 5.5Hz, 6.0Hz, 1H); 13C NMR (125 MHz, CDCl3): δ
144.3, 137.4, 129.3, 129.1, 126.9, 126.7, 118.4, 72.8 and 4.9
ppm.
1H NMR (500 MHz, CDCl3): δ = 7.99-7.97 (m, 1H), 7.29-7.14 (m,
7H), 6.95-6.93 (m, 1H), 5.66 (m, 1H), 5.63 (s, 1H); 13C NMR
(125 MHz, CDCl3): 146.1, 142.6, 134.6, 133.0, 129.8, 128.9,
127.9, 126.9, 124.3, 120.1, 111.3 and 111.2 ppm; HRMS m/z
(M + H) calcd for C14H12N3 221.0953; Found, 222.0978.
1-(2-Bromo-1-(4-chlorophenyl)ethyl)-5-chloro-1H-benzo
[d][1,2,3]triazole (3i). Semi solid, yield: 107mg (40%); Rf =
1
0.3 (3% ethyl acetate/n-hexane); H NMR (500 MHz, CDCl3): δ
6-Methylphenanthridine (5).48 The 1-(1-phenylethenyl)-1H-
1,2,3-benzotriazole 4 (1.0 equiv.) was heated with nBu3SnH
(2.0 equiv.) in presence of azobisisobutyronitrile (5 mol%) in
anhydrous toluene (3 mL) under sealed tube condition for 3
hours at 140 0C. Reaction mixture was evaporated under
reduced pressure and subjected to column chromatography
(SiO2) using gradient of ethyl acetate and hexane to afford
compound 5. Oily, yield: 34mg (40%); (Rf = 0.5, 3 % ethyl
acetate/n-hexane; MS m/z 194 [M + H]+; 1H NMR (500 MHz,
CDCl3): δ = 8.51 (d, J = 8.5Hz, 1H), 8.43 (d, J = 7.5Hz, 1H), 8.10
(d, J = 8.5Hz, 1H), 8.03 (d, J = 7.5Hz, 1H), 7.74-7.71 (m, 1H),
7.64-7.50 (m, 3H), 2.95 (s, 3H); 13C NMR (125 MHz, CDCl3):
158.7, 143.6, 132.4, 130.3, 129.2, 128.5, 127.2, 126.4, 126.2,
125.8, 123.6, 122.2, 121.8 and 23.3 ppm; HRMS m/z (M + H)
calcd for C14H12N 193.0891; Found, 194.0926.
8.06-8.05 (m, 1H), 7.42-7.40 (m, 1H), 7.33-7.30 (m, 5H), 5.92
(dd, J = 5.5Hz, 4.0Hz, 1H), 4.56 (t, J = 10.5Hz, 1H), 4.11 (dd, J =
5.0Hz, 5.5Hz, 1H); 13C NMR (125 MHz, CDCl3): δ 146.6, 135.6,
134.7, 130.3, 129.6, 128.9, 128.4, 127.4, 119.7, 110.1, 64.4 and
32.0 ppm; HRMS m/z (M + H) calcd for C14H11BrCl2N3
368.9435; Found, 369.9501.
2-(2-Bromo-1-(4-chlorophenyl)ethyl)-5-chloro-2H-benzo
[d][1,2,3]triazole (3i’). Oily, yield: 75mg (28%); Rf = 0.7 (3%
ethyl acetate/n-hexane); 1H NMR (500 MHz, CDCl3): δ 7.76-
7.75 (m, 1H), 7.69 (d, J = 9.5Hz, 1H), 7.30 (d, J = 9.5Hz, 2H),
7.22-7.19 (m, 3H), 6.06 (dd, J = 5.5Hz, 4.5Hz, 1H), 4.39 (t, J =
10.5Hz, 1H), 3.91 (dd, J = 5.0Hz, 5.5Hz, 1H); 13C NMR (125 MHz,
CDCl3): δ 144.7, 142.9, 135.6, 134.7, 132.8, 129.4, 128.6, 128.5,
119.6, 117.5, 71.5 and 32.2 ppm.
1-(2-Bromo-1-(4-(tert-butyl)phenyl)ethyl)-5-chloro-1H-
benzo[d][1,2,3]triazole (3j) White Solid, yield: 105mg
(43%); Rf = 0.5 (10% ethyl acetate/n-hexane); m.p. = 118-
122oC; 1H NMR (500 MHz, CDCl3): δ 7.98 (d, J = 8.5Hz, 1H),
7.41-7.30 (m, 6H), 5.89 (dd, J = 6.0Hz, 4.5Hz, 1H), 4.65-4.61 (m,
1H), 4.09-4.07 (m, 1H), 1.26 (s, 9H); 13C NMR (125 MHz,
CDCl3): δ 152.7, 147.2, 144.6, 134.1, 133.3, 132.0, 128.6, 126.6,
126.3, 125.4, 64.9, 34.7, 32.7 and 31.2 ppm; HRMS m/z (M +
H) calcd for C18H20BrClN3 391.0450; Found, 392.0485.
Acknowledgments
The author thanks Science and Engineering Research Board
(SERB), Department of Science & Technology, New Delhi
(Grant No. EMR/2016/001123) for the funding. The authors
sincerely thank CISC-Banaras Hindu University for providing
spectroscopic studies of the developed molecules. M.S.
acknowledge to BHU for the fellowship.
2-(2-Bromo-1-(4-(tert-butyl)phenyl)ethyl)-5-chloro-2H-
benzo[d][1,2,3]triazole (3j’). Oily, yield: 88mg (36%); Rf =
0.7 (10% ethyl acetate/n-hexane); H NMR (500 MHz, CDCl3):
δ 7.90 (s, 1H), 7.83 (d, J = 7.5Hz, 1H), 7.44-7.37 (m, 4H), 7.32
(d, J = 9.5Hz, 1H), 6.23-6.20 (m, 1H), 4.61 (t, J = 10.5Hz, 1H),
4.05 (dd, J = 5.0Hz, 5.5Hz, 1H), 1.27 (s, 9H); 13C NMR (125 MHz,
CDCl3): δ 152.6, 144.7, 142.8, 133.4, 132.5, 128.2, 126.8, 126.1,
119.6, 117.5, 72.2, 34.7, 32.7 and 31.3 ppm.
Supporting Information
1
Electronic Supplementary Information (ESI) available: Copies
1
of H and 13C NMR for all the developed compounds has been
provided. Supporting information for this article is available free
of charge via internet under www.
References and notes
1-(2-Bromocyclohexyl)-1H-benzo[d][1,2,3]triazole (3k).
Semi solid, yield: 160mg (47%); Rf = 0.4 (15% ethyl acetate/n-
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7.48 (d, J = 6.5Hz, 1H), 7.41-7.37 (m, 1H), 7.28-7.25 (m, 1H),
4.63-4.61 (m, 2H), 2.53-2.50 (m, 1H), 2.24-2.22 (m, 1H), 2.14-
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HRMS m/z (M + H) calcd for C12H15BrN3 279.0371; Found,
280.0411.
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Highlights
ꢀ A novel and expeditious one-pot synthesis of 1-(2-bromo-1-arylethyl)-1H-
benzotriazoles has been devised by reacting styrene with 1H-benzotriazole in presence
of NBS in anhydrous CH2Cl2 at room temperature.
ꢀ The resulted compounds undergo E2-ellimination to afford respective 1-(1-phenylvinyl)-
1H-benzotriazoles.
ꢀ 1-(1-Phenylvinyl)-1H-benzotriazole underwent benzotriazole ring cleavage (BtRC)
under free radical condition to produce phenanthridine.

Declaration of Interest statement
In addition to our research on Click chemistry in glycoscience for various applications (Ref:
Tiwari et al, Chem. Rev., 2016, 116, 3086) we also focus Benzotriazole Ring Cleavage (BtRC)
methodology. Recently, we have investigated n-Bu3SnH/silane mediated cyclization protocol of
thioacyl sugar alcohol to afford diverse 2-substituted benzothiazole glycoconjugates via
cyclative-cleavage of benzotriazole ring (J. Org. Chem. 2013, 899-909). The chemistry reported
was successfully extended with different amines, thiols and aryls leading to a general route for
an easy access of diverse benzothiazoles (J. Org. Chem. 2014, 251-265). However, the benzylic
alcohols undergo deoxygenation leading to deoxy sugar rather to cyclization (RSC Adv. 2015,
31584). This BtRC chemistry has been demonstrated to UG/PG students for chemistry
education purpose (J. Het. Chem., 2019). Recently, we presented an improved N-acylation of
1H-benzotriazole using 2,2'-dipyridylsulfide and Ph3P (SYNTHESIS, 2019, 470,
https://www.organic-chemistry.org/abstracts/ lit6/674.shtm) and a practical route via
Trichloroisocyanuric Acid Mediated High-Yielding Synthesis of N-Acylbenzotriazoles
(SYNTHESIS, 2019, 2183). Likewise, similar transformation would occur to provide N-
phenylamides using acylbenzotriazoles under heating in presence of Bu3SnH (ChemistrySelect,
2017), where AlCl3 mediated BtRC furnished respective benzoxazoles (ACS Omega, 2017).
Finally, A Practical and Green Synthesis of diverse Benzothiazoles utilizing a Silicon Industry
Waste Polymethylhydrosiloxane-mediated BtRC has been established (ACS Omega, 2019,
6681)
Herein, a cost-effective and high yield contributing method for the expeditious synthesis of 1-(2-
bromo-1-arylethyl)-1H-benzotriazoles has been devised using alkenes and 1H-benzotriazole.
Synthesis condition involves stirring of alkenes and 1H-benzotriazole in presence of NBS in one-
pot in anhydrous DCM at room temperature. Further the resulted compound undergoes E2-
ellimination to afford respective 1-(1-phenylvinyl)-1H-benzotriazoles, which finally go through
benzotriazole ring cleavage (BtRC) under free radical condition to produce phenanthridine as
the final product.
The work described in the manuscript has not been previously published and no portion of it is
being considered for publication elsewhere in any medium, including electronic journals,
computer databases, and publicly accessible preprint Web sites.