Copper-catalyzed domino reaction of 2-haloanilines with hydrazides: A new route for the synthesis of benzo[e][1,2,4]triazine derivatives

Article abstract of DOI:10.1055/s-0031-1290615

A copper-catalyzed domino reaction for the synthesis of benzo[e][1,2,4]triazine derivatives has been developed using readily available substituted 2-haloanilines and hydrazides as the starting materials. The procedure of the present method is mild, facile

Full text of DOI:10.1055/s-0031-1290615

LETTER
903
Copper-Catalyzed Domino Reaction of 2-Haloanilines with Hydrazides:
A New Route for the Synthesis of Benzo[e][1,2,4]triazine Derivatives
Benzo[e][1,2,4]triazine
Deriv
n
ative
s
Guo,a,b,1 Jin Liu,a,b,1 Xianxue Wang,a,b Guosheng Huang*a,b
a
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. of China
Fax +86(931)8912582; E-mail: hgs@lzu.edu.cn
b
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou 730000, P. R. of China
Received 30 November 2011
readily available.10c–10h In recent years, domino reactions
have attracted a great deal of interest11 for the reason that
they have many advantages, such as effective, environ-
mental friendly, and resource preservation, etc.12 Herein,
we wish to report an efficient copper-catalyzed domino
strategy for the synthesis of benzo[e][1,2,4]triazine.
Abstract: A copper-catalyzed domino reaction for the synthesis of
benzo[e][1,2,4]triazine derivatives has been developed using readi-
ly available substituted 2-haloanilines and hydrazides as the starting
materials. The procedure of the present method is mild, facile, and
efficient.
Key words: benzo[e][1,2,4]triazine derivatives, 2-haloanilines,
hydrazides, copper catalyst, domino reaction
Initially, 2-iodoaniline (1a) and benzohydrazide (2a) were
used as the model substrates to optimize the reaction con-
ditions including catalysts, bases, solvents, and reaction
temperatures under nitrogen. As shown in Table 1, four
copper catalysts (0.1 equiv) were tested with two equiva-
lents of Cs2CO3 (relative to the amount of 2-iodoaniline)
as the base and DMF–1,4-dioxane as the solvent at 90 °C
(Table 1, entries 1–4), and Cu2O provided the highest
yield (Table 1, entry 4). Other bases, KOt-Bu, K2CO3, py-
ridine, MeONa, and Bu4NBr (Table 1, entries 5–9), were
screened, and Cs2CO3 showed the best result (Table 1,
compare entries 4, 5–9). The effect of solvents was also
investigated, and DMF–1,4-dioxane (1:1) was the optimal
solvent choice (Table 1, compare entries 4 and 10–13).
We attempted different reaction temperatures (Table 1,
entries 14 and 15), and 90 °C was chosen to be the best
choice. In addition, to optimize further the reaction condi-
tions, the same reaction was carried out under air, but it
failed to improve the yield (Table 1, entry 17). However,
the yield did not increase when the reaction time pro-
longed to eight hours (Table 1, entry 16).
1,2,4-Triazines are a well-known class of heterocyclic
compounds and they have many applications including
aza-nucleotides and -nucleosides,2 herero Diels–Alder re-
actants,3 ligands,4 herbicidal,5 metal ion adsorption.6
Moreover, the most noteworthy property of 1,2,4-triaz-
ines is their biological activity, a large number of the re-
lated literature have been reported.7 For example, 3-
amino-1,2,4-benzotriazine-1,4-dioxide,
(tirapazamine
TPZ), one of the most promising prodrugs, is a bioreduc-
tive antitumor agent that is selectively toxic to hypoxic
cells. Indeed, the study of TPZ has been a central occupa-
tion for many scientists.8 Therefore, as the main body of
TPZ, benzo[1,2,4]triazines have drawn extensive and en-
during attentions.
In our continuous study on the synthesis of bioactive N-
heterocycles, we have developed many simple routes to
synthesize several compounds of N-heterocycles.9 Ac-
cordingly, we directed our attention more commonly to-
wards synthesizing benzo[1,2,4]triazine derivatives. To
the best of our knowledge, several strategies for the syn-
thesis of benzo[e][1,2,4]triazines have been reported.
Typically, in 2006, Khodja et al. reported the two-step
syntheses of 3-methyl and 3-phenyl-1,2,4-benzotriaz-
ines.10a This method was clean and not strenuous, howev-
er, it needed a reductive cyclization by PtO2- or Pt/C-
catalyzed hydrogenation. In 2010, Wang’s group ob-
served a pathway for the synthesis of 3-aryl-1,2,4-benzo-
triazine via rearrangement of bis(benzotriazol-1-
yl)methylarenes.10b It is worth mentioning that this reac-
tion also included two steps and required allylsamarium
bromide as the catalyst. Other methods generally suffer
from some limitations such as complicated procedures,
harsh reaction conditions, and substrates that are not
Under the optimized reaction conditions, we then extend-
ed the scope of this reaction, and the results are illustrated
in Table 2. Generally, the reaction of 2-iodoanilines with
hydrazides proceeded smoothly and led to the desired
product 3 with moderate yields. As shown in Table 2, a
variety of 2-iodoanilines were efficient to give the desired
benzo[e][1,2,4]triazine products (Table 2, entries 2–7). It
is obvious that the electron density on 2-iodoanilines dras-
tically affects the yields of this reaction (Table 2, compare
entries 2–5 and 6, 7). Then the reaction scope of benzohy-
drazide was studied. From 2b to 2i, the electron-donating
or electron-withdrawing groups on the benzene ring did
not have distinct influence on the yields of 3 except 2e and
2f. In addition, 2-iodobenzohydrazide (2k), possibly be-
cause of its steric hindrance, gave only 22% yield of prod-
uct with 1a (Table 2, entry 17). In these experiments, we
also tried the reaction of 1a with furan-2-carbohydrazide
(2j), and we got the desired product. Notably, there was a
special case as shown in Scheme 1. The target product of
SYNLETT 2012, 23, 903–906
x
x.
x
x.
2
0
1
2
Advanced online publication: 15.03.2012
DOI: 10.1055/s-0031-1290615; Art ID: W73811ST
© Georg Thieme Verlag Stuttgart · New York

904
H. Guo et al.
LETTER
1g with 2a was 7-bromo-3-phenylbenzo[e][1,2,4]triazine. Table 2 Synthesis of Benzo[e][1,2,4]triazines from 2-Iodoanilines
and Hydrazidesa,13
However, by NMR, MS, and IR, the compound examined
was 3-phenylbenzo[e][1,2,4]triazine (3aa), the substitu-
I
N
N
O
Cu2O, Cs2CO3
N
R1
R1
ent of Br queerly disappeared. The reason is not clear at
the current stage, and investigations are ongoing in our
laboratory.
+
R2
N NH2
DMF–1,4-dioxane (1:1)
N2, 90 °C, 6 h
R2
NH2
H
1
2
3
Entry R1
R2
Product Yield (%)b
Table 1 Optimization of Reaction Conditions with 1a and 2aa
1
2
1a H
2a Ph
3aa
3ba
3ca
3da
3ea
3fa
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
75
57
56
56
64
43
30
52
54
56
40
36
65
62
61
46
22
52
65
60
0
O
N
N
I
NH2
N
1b 4-Me
1c 4-Et
1d 4-i-Pr
1e 4,5-Me2
1f 4-Cl
1g 4-Br
1a H
2a Ph
Cu cat., base
solvent, temp
H
+
N
NH2
3
2a Ph
1a
2a
3aa
4
2a Ph
Entry Catalyst
Base
Solvent
Temp Yield
5
2a Ph
(°C)
90
90
90
90
90
90
90
90
90
90
90
90
90
80
(%)b
6
2a Ph
1
2
3
4
5
6
7
8
9
Cu(OAc)2 Cs2CO3 DMF–1,4-dioxane (1:1)
39
7
2a Ph
CuBr
CuI
Cs2CO3 DMF–1,4-dioxane (1:1)
Cs2CO3 DMF–1,4-dioxane (1:1)
Cs2CO3 DMF–1,4-dioxane (1:1)
KOt-Bu DMF–1,4-dioxane (1:1)
K2CO3 DMF–1,4-dioxane (1:1)
pyridine DMF–1,4-dioxane (1:1)
MeONa DMF–1,4-dioxane (1:1)
Bu4NBr DMF–1,4-dioxane (1:1)
Cs2CO3 DMSO
41
8
2b 4-MeC6H4
2c 3-MeC6H4
2d 3,5-Me2C6H4
2e 4-MeOC6H4
2f 3,4-(MeO)2C6H4
2g 3-ClC6H4
2h 4-ClC6H4
2i 3-O2NC6H4
2j furan-2-yl
2k 2-IC6H4
2b 4-MeC6H4
2g 3-ClC6H4
2h 4-ClC6H4
2l i-Bu
52
9
1a H
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
75
10
11
12
13
14
15
16
17
18
19
20
21
1a H
56
1a H
34
1a H
n.r.c
40
1a H
1a H
n.r.c
50
1a H
10 Cu2O
11 Cu2O
12 Cu2O
13 Cu2O
14 Cu2O
15 Cu2O
16 Cu2O
17 Cu2O
1a H
3aj
3ak
3bb
3bg
3eh
–
Cs2CO3 THF
21
1a H
Cs2CO3 PhMe
n.r.c
55
1b 4-Me
1b 4-Me
1e 4,5-Me2
1a H
Cs2CO3 DMF
Cs2CO3 DMF–1,4-dioxane (1:1)
37
Cs2CO3 DMF–1,4-dioxane (1:1) 110
52
Cs2CO3 DMF–1,4-dioxane (1:1)
Cs2CO3 DMF–1,4-dioxane (1:1)
90
90
75d
36e
a Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), Cs2CO3 (0.4
mmol), Cu2O (0.02 mmol), N2, DMF (0.5 mL) with 1,4-dioxane (0.5
mL), 90 °C, 6 h.
a Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), base (0.4
mmol), catalyst (0.02 mmol) in 1 mL of solvent for 6 h under nitrogen
b Isolated yield after column chromatography.
atmosphere
b Isolated yield.
O
N
N
c n.r. = no reaction.
d Yield for a reaction time of 8 h.
e Under air.
N
Br
I
N
NH2
+
H
Cu2O, Cs2CO3,
DMF–1,4-dioxane (1:1)
N2, 90 °C, 6 h
NH2
1g
2a
3aa
Encouraged by the results obtained, the reaction scope of
2-bromoanilines was studied. On the basis of the opti-
mized reaction conditions above, we attempted to look for
the optimal conditions of 2-bromoanilines with hy-
drazides. 2-Bromoaniline (4a) and benzohydrazide (2a)
were used as the model substrates, and the procedure is il-
lustrated in Table 3. The result of some different sub-
strates is illustrated in Table 4.
Scheme 1 A special case
It is obvious from Table 2 and Table 4 that the result of 2-
iodoanilines was superior to 2-bromoanilines in this reac-
tion. On the other hand, the reaction of 2-bromoanilines
with hydrazines explored the scope of this protocol. The
structure of compound 3bg (Figure 1) was confirmed by
single-crystal X-ray analysis.
Synlett 2012, 23, 903–906
© Thieme Stuttgart · New York

LETTER
Benzo[e][1,2,4]triazine Derivatives
905
Table 3 Screening the Impact of Quantity of Base, Temperature,
Nevertheless, the yields of all products are not very high.
It is enlightening to recall that the coupling of hydrazides
with aryl iodides more easily forms N-arylated products,
and the yields of N¢-arylated products are moderate or
slightly low.14
and Timea
O
N
N
Br
Cs2CO3
N
+
N
NH2
temp, time
H
NH2
A plausible mechanism is proposed in Scheme 2 that is
based on previous reports15 combined with our experi-
mental results. Firstly, the Ullmann-type coupling of 2-
iodoaniline 1 with hydrazide 2 provides A. Then NH2 at-
tacks the carbonyl group affording the intermediate B,
which would lose H2O to form C. As a result of tending to
conjugation, the C=N bond of C may shift to the N=N
bond forming C¢. Finally, oxidative aromatization16 with
removal of hydrogen from C¢ in the presence of Cu2O
would yield the target product 3.
4a
2a
3aa
Entry
Cs2CO3 (equiv) Temp (°C) Time (h) Yield (%)b
1
2
3
4
5
6
7
8
2.0
3.0
3.0
3.0
4.0
6.0
6.0
8.0
90
90
6
6
0
0
120
120
120
120
120
120
6
10
8
17
8
19
O
H
H
I
8
34 (18)c
33
N
N
O
Cu2O, Cs2CO3
R2
R1
+
R1
R2
10
8
H2N
N
NH2
NH2
H
34
1
2
A
H
N
H
N
a Reaction conditions: 4a (0.2 mmol), 2a (0.3 mmol), Cs2CO3, Cu2O
(0.02 mmol), N2, DMF (0.5 mL) with 1,4-dioxane (0.5 mL), temp,
time.
NH
– H2O
N
R1
OH
R1
R2
N
H
R2
N
H
b Isolated yield after column chromatography.
c Yield for a reaction time of 6 h.
B
C
Table 4 Synthesis of Benzo[e][1,2,4]triazines from 2-Bromo-
N
N
N
anilines and Hydrazidesa
N
Cu2O
– H2
N
R1
R1
R2
O
R2
Br
N
N
N
N
Cu2O, Cs2CO3
H
H
R1
R1
+
3
C'
R2
N NH
2DMF–1,4-dioxane (1:1)
N2, 120 °C, 8 h
R2
H
NH2
Scheme 2 Proposed mechanism
4
2
3
Entry
R1
R2
2a
2a
2a
2b
2g
Product
3aa
Yield (%)b
In summary, we have developed a simple and efficient
copper-catalyzed method for the synthesis of benzo-
[e][1,2,4]triazine derivatives. The protocol uses cheap and
less toxic Cu2O as the catalyst, readily available 2-haloa-
nilines and hydrazides as the starting materials, and the re-
action conditions are mild. Many substrates are suitable
for this procedure, affording various benzo[e][1,2,4]triaz-
ine derivatives in moderate yields. This strategy may be
very useful for the synthesis of benzo[e][1,2,4]triazines
and corresponding natural products. A more detailed in-
vestigation on the mechanism and scope of this reaction as
well as the utilization of this reaction in the synthesis of
complex molecules is ongoing in our laboratory.
1
2
3
4
5
4a H
34
25
4
4b 4-Me
4c 4-Cl
3ba
3fa
4b 4-Me
4b 4-Me
3bb
20
10
3bg
a Reaction conditions: 4 (0.2 mmol), 2 (0.3 mmol), Cs2CO3 (1.2
mmol), Cu2O (0.02 mmol), N2, DMF (0.5 mL) with 1,4-dioxane (0.5
mL), 120 °C, 8 h.
b Isolated yield after column chromatography.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
The authors thank the State Key Laboratory of Applied Organic
Chemistry for financial support.
Figure 1 X-ray crystal structure of 3bg
© Thieme Stuttgart · New York
Synlett 2012, 23, 903–906

906
H. Guo et al.
LETTER
Chim. Fr. 1992, 129, 343. (f) Gelvin, C. R.; Turnbull, K.
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38, 1793.
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(13) Typical Procedure for the Synthesis of
Benzo[e][1,2,4]triazine
An oven-dried Schlenk tube was charged with Cu2O (2.9
mg, 0.02 mmol), Cs2CO3 (130.4 mg, 0.4 mmol), 1a (44 mg,
0.20 mmol) and 2a (41 mg, 0.3 mmol). The Schlenk tube
was sealed, evacuated, and backfilled with nitrogen (3
cycles). Then DMF (0.5 mL) and 1,4-dioxane (0.5 mL) were
added to the reaction tube. The reaction was stirred at 90 °C
under N2 for 6 h. After cooling to r.t. the solvent was diluted
with EtOAc (3 × 10 mL), washed with brine (3 × 10 mL),
and dried over anhyd Na2SO4. After the solvent was
evaporated in vacuo, the residues were purified by column
chromatography, eluting with PE–EtOAc (20:1) to afford 31
mg (75%) of 3-phenylbenzo[e][1,2,4]triazine (3aa) as a
yellow solid, mp 119–121 °C. 1H NMR (400 MHz, CDCl3):
d = 8.77–8.74 (m, 2 H), 8.51 (dd, J = 8.4, 0.8 Hz, 1 H), 8.07
(d, J = 8.8 Hz, 1 H), 7.96–7.92 (m, 1 H), 7.83–7.78 (m, 1 H),
7.59–7.57 (m, 3 H). 13C NMR (100 MHz, CDCl3): d = 160.0,
146.6, 141.2, 135.8, 135.6, 131.6, 130.3, 129.7, 129.3,
129.1, 128.9. IR (neat): 3368, 3060, 1506, 1329, 1016, 771,
699 cm–1. HRMS: m/z calcd for C13H10N3 [M + H]+:
208.0870; found: 208.0871.
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