TBHP/I2-promoted oxidative coupling of azoles with benzyl compounds via cleavage of nonactivated C(sp3)-H bonds under solvent-free conditions

Article abstract of DOI:10.1055/s-0033-1338871

A novel and efficient TBHP/I₂-promoted oxidative coupling of azoles with benzyl compounds via cleavage of nonactivated C(sp³)-H bonds under metal-free, base-free, and solvent-free conditions for the synthesis of N-alkylated azoles has been developed. The procedure, using I₂ as the catalyst, is a simple, economical, and environmentally friendly protocol, which could be applied to various available substrates in moderate to good yields. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0033-1338871

1588▌
LETTER
letter
TBHP/I2-Promoted Oxidative Coupling of Azoles with Benzyl Compounds via
Cleavage of Nonactivated C(sp3)–H Bonds under Solvent-Free Conditions
Oxidative Coupling of Azoles with Benzyl Compounds
Xiang Liu,a,b Guiqin Yu,a,b Jihui Li,a,b Dong Wang,a,b Yongxin Chen,a,b Keqin Shi,a,b Baohua Chen*a,b
a
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Gansu, Lanzhou, 730000, P. R. of China
b
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, Lanzhou, 730000, P. R. of China
Fax +86(931)8912582; E-mail: chbh@lzu.edu.cn
Received: 09.04.2013; Accepted after revision: 02.05.2013
In recent years, molecular iodine has been identified as an
Abstract: A novel and efficient TBHP/I2-promoted oxidative cou-
important and effective catalyst to carry out various or-
pling of azoles with benzyl compounds via cleavage of nonactivated
C(sp3)–H bonds under metal-free, base-free, and solvent-free con-
ditions for the synthesis of N-alkylated azoles has been developed.
ganic reactions because of its inexpensive, eco-friendly
nature and ready availability.8 Our group has been work-
The procedure, using I2 as the catalyst, is a simple, economical, and ing on the investigation of N-alkylation of azoles9 for
environmentally friendly protocol, which could be applied to vari-
ous available substrates in moderate to good yields.
many years. Earlier in this year, our group developed an
efficient, metal-free, convenient, and relatively cheap
Key words: solvent-free, metal-free, oxidative coupling, N-alkyl-
ated azoles, free-radical reaction
method for iodine-induced direct alkylation of azoles via
in situ formed alkyliodide.10 However, the reactions were
carried out in the presence of two equivalents of iodine
and activated C(sp3)–H. It is obvious that they are nucleo-
During the past few years, N-alkylated azoles were of philic reactions. We are interesting to investigate the io-
great importance in the field of medicinal chemistry1 dine-induced direct alkylation of azoles using a catalytic
(used as antiviral, anticonvulsant, and antiulcer drugs) and amount of iodine and nonactivated C(sp3)–H bonds. We
sciences.2 In addition, they have been widely used as the first presented the TBHP/I2-promoted oxidative coupling
backbone in dyes3 and agricultural chemistry. As a conse- of azoles with benzyl compounds via cleavage of nonacti-
quence, developing efficient methodologies for the syn- vated C(sp3)–H bonds under metal-free, base-free, and
thesis of N-alkylated azoles and their derivatives have solvent-free conditions for the synthesis of N-alkylated
caused multitudinous attentions, and many methods have azoles. The modified method is not a nucleophilic reac-
been reported.4 Among them, the most common and ear- tion but a free-radical reaction, and it showed several ad-
liest route for N-alkylation of azoles is a nucleophilic sub- vantages comparing to our previous method, such as
stitution reaction of azoles with halides (Scheme 1, A). metal-free, solvent-free, base-free, easy operation, envi-
Although the most of the methods are apparently useful, ronmental friendliness, mild reaction conditions and high
their applications are limited by severe reaction condi- yields.
tions, low yields, long reaction time, and use of toxic sol-
In order to develop a convenient and selective reaction
vents, strong bases, or catalysts. Recently, direct arylation
procedure for N-alkylated azoles, we chose the reaction of
of the C–H/N–H bond in azoles has emerged as a hot
theme in organic synthetic chemistry5 due to their poten-
A) general strategy for the synthesis of N-alkylation of azoles
tial possibilities for diverse transformation into a variety
of useful derivatives. Great progress has been achieved
R2
H
N
N
N
strong bases
R2X
for direct arylation of N–H bonds in azoles via cleavage
of sp3 C–H bonds.6 These studies mainly focused on pal-
ladium-, iron-, ruthenium-, or copper-catalyzed arylation.
For example, Chen and co-workers first reported direct
C–N coupling of imidazoles with benzylic compounds via
iron(III)-catalyzed oxidative activation of C(sp3)–H
bonds.7 However, this methodology was not eco-friendly
because it involves direct or indirect use of toxic and cor-
rosive organic solvent and iron catalyst. Therefore, devel-
opment of an eco-friendly and metal-free N–H/C–H
bond-coupling protocol is still greatly desired for the N-
alkylation of azoles.
+
N
N
organic solvent
N
R1
R1
X = Cl, Br, I
B) previous work in our group
O
I
2 (2 equiv)
N
N
H
O
R2
NaHCO3 (2 equiv)
N
X
+
X
R2
DCE, 80 °C
R1
N
R1
X = C, N
C) this work
H
N
I2 (10 mol%)
TBHP (2 equiv)
N
R2
R2
X +
X
neat, 100 °C
N
N
R1
R1
SYNLETT 2013, 24, 1588–1594
Advanced online publication: 14.06.2013
X = C, N
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1338871; Art ID: ST-2013-W0317-L
© Georg Thieme Verlag Stuttgart · New York
Scheme 1 N-Alkylation of azoles synthesis

LETTER
Oxidative Coupling of Azoles with Benzyl Compounds
1589
benzotriazole (1a) with toluene (2a) as the model case to crease of the yield (Table 1, entries 7–10). Then, we in-
optimize the reaction conditions (Table 1). The reaction vestigated the reaction at different loading of I2. We found
was carried out with 1a (0.5 mmol), 2a (1.5 mmol), in the that 10 mmol% was the best catalytic amount. Finally, the
presence of I2 (10%), tert-butyl hydroperoxide (TBHP, 2 N-alkylation reactions were carried out with different ox-
equiv) in DCE at 80 °C under air for eight hours. The de- idants. 6-Dicyanobenzoquinone (DDQ), Ag2CO3, O2, and
sired product 3aa was obtained in 15% yield (Table 1, en- t-BuOOt-Bu showed no or slight effect to promote the re-
try 1). In this preliminary experiment, the N-alkylation action. Moreover, the yield of the desired product 3aa
reaction was carried out in different solvents, such as DM- could be slightly affected by gradually increasing the
SO, DMF, MeCN, PhCl, and PhNO2, and the reactions did amount of TBHP. Thus, two equivalents of TBHP, 10
not provide higher yields (Table 1, entries 2–6), but the mmol% I2, 100 °C without solvent-free are the optimal
solvent-free reaction gave good performance (Table 1, en- conditions for this reaction.
try 7). Based on the above-studied results, we decided to
Once the viability of the method was established, this ap-
develop an environmentally friendly reaction, and sol-
proach was then applied to the coupling of benzotriazole
vent-free conditions are clearly the most favorable. Next,
to a variety of benzyl compounds, and the results are
we tested the reaction at other temperature. We found the
shown in Table 2. Various benzyl compounds were suit-
able for the solvent-free and metal-free reaction condi-
yield of the desired product 3aa could be improved by
gradually increasing the reaction temperature. We were
tions. The electron-donating-substituted toluenes
pleased to find that the yield of 3aa was enhanced to 90%
(including p-xylene, m-xylene, o-xylene, and mesitylene)
at 100 °C, and the higher temperature (>100 °C) led to de-
could be easily converted into N-alkylated azoles in good
yields (Table 2, entries 1–5). On the contrary, the elec-
tron-withdrawing-substituted toluenes (including 4-bro-
Table 1 Optimization of the Reaction Conditionsa
mo-1-methylbenzene, 4-chloro-1-methylbenzene, and 2-
chloro-1-methylbenzene) gave lower but still acceptable
H
N
I2, TBHP
N
N
yields (Table 2, entries 6–8). However, the strongly elec-
tron-withdrawing-substituted toluenes such as 4-nitro-1-
methylbenzene and 4-methylbenzonitrile had not given
the expected product (Table 2, entries 15 and 16). The re-
actions of benzotriazole with benzylic compounds also
produced the corresponding cross-coupling products in
moderate to good yields (Table 2, entries 9–11, 13).
+
N
N
neat, 100 °C
N
1a
2a
3aa
Entry
Catalyst
(mmol%)
Temp
Solvent
Oxidant
Yield
(%)b
(°C)
1
2
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (10)
I2 (20)
I2 (0)
80
80
DCE
DMSO
DMF
MeCN
PhCl
PhNO2
neat
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
O2
15
0
However, the cumene had not given the expected product
(Table 2, entry 14). The 2-substituted benzyl compounds
showed slightly lower yields than the 4-substituted ones
(Table 2, entries 2–4, 7 and 8) that was probably caused
by steric effect. In addition, when the reaction was carried
out with benzimidazole and toluene, only 53% yield was
provided (Table 2, entry 12).
3
80
0
4
80
trace
0
5
80
6
80
0
After testing the coupling reaction of benzotriazole with
benzyl compounds, other azoles were investigated. As
shown in Table 3, a variety of azoles were found to be
coupling partners, and the desired products were formed
in satisfactory to excellent yields. The 5-substituted azoles
1 containing electron-withdrawing or electron-donating
groups could provide the desired products in perfect
yields (Table 3, entries 1–5). Unfortunately, poor regiose-
lectivities were observed in this transformation: substrates
substituted by Cl or NO2 groups gave a mixture of regioi-
somers. The combined yields ranged from 94–99%, and
the ratio of isomers varies from 1:1.10 to 1:1.22. Steric
hindrance at the 2-position of benzimidazoles had an ob-
vious impact on the yields. Compounds 3fa, 3ga, 3ha, and
3ia were obtained in moderate yields unless using more
TBHP, prolonging the reaction times or higher tempera-
ture (Table 3, entries 6–9).
7
80
68
90
88
60
60
0
8
100
120
60
neat
9
neat
10
11
12
13
14
15
16
neat
100
100
100
100
100
100
neat
neat
I2 (10)
I2 (10)
I2 (10)
I2 (10)
neat
0
neat
Ag2CO3
trace
neat
BuOOt-Bu 34
DDQ
neat
0
a Reaction conditions: 1a (0.5 mmol), 2a (2.0 mmol), catalyst (10
mol%), oxidant (2.0 equiv), solvent (1 mL), 100 °C, 8 h.
b Isolated yields.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1588–1594

1590
X. Liu et al.
LETTER
TBHP
R
t-BuO• or OH•
2
+
I2
t-BuOH or H2O
•
R
+
I2
TBHP
t-BuOH or H2O
+
A
HI
I
t-BuOH or H2O
+
R
B
R
H
N
N
N
N
N
N
1
3
HI
Scheme 2
Table 2 Reactions of Azoles with Various Benzyl Compoundsa
I2 (10 mol%)
TBHP (2 equiv)
H
N
R1
N
N
X +
X
neat, 100 °C
X = C, N
N
R1
1
2
3
Entry
1
2
Product
Yield (%)b
H
N
N
N
1
2
3
3aa11
90
2a
1a
1a
1a
1a
H
N
N
N
3ab
3ac
80
2b
2c
H
N
N
N
75.8
H
N
N
N
4
3ad
82
2d
Synlett 2013, 24, 1588–1594
© Georg Thieme Verlag Stuttgart · New York

LETTER
Oxidative Coupling of Azoles with Benzyl Compounds
1591
Table 2 Reactions of Azoles with Various Benzyl Compoundsa (continued)
I2 (10 mol%)
TBHP (2 equiv)
H
N
R1
N
N
X +
X
neat, 100 °C
X = C, N
N
R1
1
2
3
Entry
1
2
Product
Yield (%)b
H
N
N
N
5
3ae
93
1a
2e
2f
H
N
N
N
6
3af
3ag
3ah
3ai
3aj
81
74
77
97
92
Br
Cl
1a
1a
1a
1a
1a
H
N
N
N
7
2g
H
N
N
N
8
Cl
2h
H
N
N
N
9
2i
H
N
N
N
10
Bu
Bu
2j
H
N
N
N
11
3ak
85
1a
2k
2a
H
N
12
13
14
3al
53
80
0
N
1e
1a
1a
H
N
N
N
3am
3an
2m
2n
H
N
N
N
i-Pr
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1588–1594

1592
X. Liu et al.
LETTER
Table 2 Reactions of Azoles with Various Benzyl Compoundsa (continued)
I2 (10 mol%)
TBHP (2 equiv)
H
N
R1
N
N
X +
X
neat, 100 °C
X = C, N
N
R1
1
2
3
Entry
1
2
Product
Yield (%)b
H
N
NO2
N
N
15
16
3ao
0
2o
2p
1a
1a
H
N
N
N
CN
3ap
0
a Reaction conditions: 1a (0.5 mmol), 2a (2.0 mmol), I2 (10 mmol%), TBHP (2.0 equiv), 100 °C, 8 h.
b Isolated yields.
To explore the mechanism of this reaction, we first ob-
tained 1-(iodomethyl)benzene (A) and 1-iodo-1H-benzo-
[d][1,2,3]triazole (B), then the reaction of A with benzo-
triazole and the reaction of B with toluene were investi-
gated, respectively. We found that the former reaction did
not afford the expected product. Then the reaction was
carried out with 1a (0.2 mmol), 2a (1.0 mmol), TBHP (2.0
equiv), I2 (10% mmol) in the presence of TEMPO (6.0
equiv) at 100 °C under solvent-free conditions for eight
hours. We were surprised to find that only a trace of prod-
uct was obtained. Based on the above observations, a rea-
sonable mechanism for this reaction was proposed in
Scheme 2. It may involve a free-radical process. The reac-
tion started with abstraction of H· from 2 by TBHP giving
RCH2· (A). Subsequently, I2 oxidized A to B. HI, pro-
duced during transformation of A to B, reacted with
TBHP giving I2 and t-BuOH or H2O to complete the io-
dine-catalyst cycle. Finally, nucleophilic reaction of 1
with B produced the target product 3.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
Table 3 Reactions of Diphenylmethane with Various Azolesa
R
H
N
X
N
R
N
I2 (10 mol%)
1
X
TBHP (2 equiv)
N
3
+
neat, 100 °C
X = C, N
N
2i
X
N
4
R
Entry
1
1
Product
Yield (%)b
In conclusion, we have developed a TBHP/I2-promoted
oxidative coupling of azoles with benzyl compounds via
cleavage of nonactivated C(sp3)–H bonds under metal-
free, base-free, and solvent-free conditions for the synthe-
sis of N-alkylated azoles. The use of a catalytic amount of
I2 as catalyst is a simple and economical protocol, which
have been successfully applied to the synthesis of various
available substrates. It is a useful strategy to develop new
arylating reagents for the preparation of N-alkylated
azoles.
H
N
N
3ai
97
N
1a
H
N
Cl
N
N
94
1:1.20
2
3
4
3ba and 4ba
3ca and 4ca
3da and 4da
1b
H
N
Cl
97
1:1.22
N
Acknowledgment
1c
We are grateful for financial support from the National Science
Foundation of P. R. of China (No.J11003307) and the Project of Na-
tional Science Foundation of Gansu Province P. R. China (No.
1208RJZA266).
H
N
O2N
96
1:1.10
N
1d
Synlett 2013, 24, 1588–1594
© Georg Thieme Verlag Stuttgart · New York

LETTER
Oxidative Coupling of Azoles with Benzyl Compounds
1593
Table 3 Reactions of Diphenylmethane with Various Azolesa
(continued)
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R
H
N
X
N
R
N
I2 (10 mol%)
TBHP (2 equiv)
1
X
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N
3
+
neat, 100 °C
X = C, N
N
2i
X
N
4
R
Entry
5
1
Product
Yield (%)b
90
H
N
3ea
N
1e
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H
N
CF3
6
3fa
73
N
1f
1g
1h
1i
H
N
7
3ga
3ha
3ia
62c
57c
62c
63c
N
H
N
8
N
H
N
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9
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10
3ja
N
1j
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(11) General Procedure of the Reaction between Azoles and
Toluenes – Synthesis of 1-Benzyl-1H-
a Reaction conditions: 1 (0.5 mmol), 2i (2.0 mmol), I2 (10 mol%),
TBHP (2.0 equiv), 100 °C, 8 h.
b Isolated yields.
C TBHP (5 equiv).
benzo[d][1,2,3]triazole (3aa)
All reactions were performed on a 0.50 mmol scale relative
to azoles. The benzotriazole (1a, 0.50 mmol), toluene (2a,
2.0 mmol), I2 (0.050 mmol), and TBHP (2 equiv) were taken
in a round-bottom flask equipped with a stirrer. The resulting
mixture was stirred for 8 h at 100 °C. After cooling to r.t., to
the reaction mixture was added H2O (2 mL) and extracted
with ester (3 × 10 mL). The combined organic phases were
washed with brine (2 × 5 mL), dried over anhyd MgSO4, and
concentrated in vacuo. The residue was subjected to flash
column chromatography with hexanes–EtOAc (10:1) as
eluent to obtain the desired 3aa a light yellow solid (90%
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1588–1594

1594
X. Liu et al.
LETTER
yield). The remaining substituted triazoles were prepared in
the similar manner and their characterization data are as
follows. Compound 3aa was purified by flash
CDCl3): δ = 8.04–8.06 (d, J = 7.8 Hz, 1 H), 7.25–7.38 (m, 8
H), 5.82 (s, 2 H). 13C NMR (100 MHz, CDCl3): δ = 146.2,
134.7, 132.7, 128.9, 128.3, 127.5, 127.3, 123.8, 119.9,
109.6, 52.1.
chromatography (hexane–EtOAc = 10:1, v/v) as light yellow
solid (yield 90%), mp 113–115 °C. 1H NMR (300 MHz,
Synlett 2013, 24, 1588–1594
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