Reusable ionic liquid-catalyzed oxidative coupling of azoles and benzylic compounds via sp3 C-N bond formation under metal-free conditions

Article abstract of DOI:10.1039/c5ob00781j

The heterocyclic ionic liquid-catalyzed direct oxidative amination of benzylic sp³ C-H bonds via intermolecular sp³ C-N bond formation for the synthesis of N-alkylated azoles under metal-free conditions is reported for the first time. The catalyst 1-butylpyridinium iodide can be recycled and reused with similar efficacies for at least eight cycles.

Full text of DOI:10.1039/c5ob00781j

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DOI: 10.1039/C5OB00781J
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Reusable Ionic Liquid-Catalyzed Oxidative Coupling of Azoles and
Benzylic Compounds via sp3 C−N Bond Formation under Metal-
Free Conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Wenbo Liu, Chenjiang Liu,* Yonghong Zhang, Yadong Sun, Ablimit Abdukadera, Bin Wang, He Li,
Xuecheng Ma and Zengpeng Zhang
DOI: 10.1039/x0xx00000x
www.rsc.org/
The classical heterocyclic ionic liquid-catalyzed direct oxidative
amination of benzylic sp3 C−H bonds via intermolecular sp3 C−N
bond formation for the synthesis of N-alkylated azoles under
metal-free conditions has been reported firstly. The catalyst 1-
butylpyridinium iodide can be recycled and reused with the similar
efficacies for at least eight cycles.
Nitrogen-containing heterocycles and their derivatives are in the
focus of the chemical synthesis because they commonly exist in
pharmaceuticals, agrochemicals and natural products.1 As an usual
motif in bioactive compounds, developing effective strategies for
accessing nitrogen heterocycles remain an important objective.1d-f, 2
Many azole derivatives, especially the N-alkylation and N-arylation
azoles have also displayed interesting antimycobacterial and
antifungal activity.3 Nucleophilic substitution reaction of azoles with
alkyl halides is one of the most classical ways for the synthesis of N-
alkylation azoles. However, this method suffers from the use of
alkyl halides and strong bases which limits its applications.
Therefore, the development of more effective routes for
constructing the azole derivatives are highly desirable. In recent
years, transition-metal-catalyzed C−N bond formations have made
great achievements in preparing biologically and medicinally
important N-aryl/heteroaryl heterocycles.4 Despite their
remarkable potential utility, the toxicity and expense of transition
metals limit the practical applications.5 The discovery of green and
efficient C−H bonds transformation under metal-free conditions will
be of great value.
Scheme 1 Reported and designed routes for the formation of N-alkylation azoles
via oxidative coupling of azoles and benzylic compounds.
catalyzed amination of benzylic C−H bonds leading to imidazole
derivatives in an efficient and straightforward way. But the reaction
was conducted at higher reaction temperature in chlorobenzene
(Scheme 1a).7 More recently, Zhu’s group reported TBAI-catalyzed
highly selective benzylic C−H bond amination protocol using TBHP
as an oxidant under metal-free conditions, which had the
advantages of milder reaction conditions, shorter reaction times,
and higher yields (Scheme 1b).8
Ionic liquids (ILs), also called room-temperature ionic liquids
(RTILs), are usually made up of larger organic cations and smaller
organic or inorganic anions. Some unique properties such as good
thermal stabilities, tunable viscosities, negligible vapor pressures,
and generally an anion-dependent miscibility with water and
various organic solvents9 permit ILs to be extensively used in
catalysis.10 In recent years, ILs have also been employed as solvents
for transition-metal-catalyzed C−H activation reactions.11 Recently,
we have described an oxidative cross-coupling reaction for C−C
bond formation promoted by IL firstly.12 To the best of our
knowledge, up to now, studies of using the classical heterocyclic IL-
catalyzed C−H activation reactions for C−N bond formations
reactions have not been reported. Herein we present a metal-free,
environment-friendly and highly selective benzylic C−H bond
amination protocol by using recycled and reused ionic liquid 1-
Recently, as an emerging field in C−H activation, the cross-
dehydrogenative coupling (CDC) reaction in realizing C−N bonds
have become a highly attractive coupling strategy due to its atom-
economic manner.6 One typical example is the oxidative coupling of
azoles with benzyl compounds via cleavage of nonactivated sp3 C−H
bonds. For instance, in early 2011, Qiu’s group established FeCl2-
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butylpyridinium iodide ([Bpy]I) as a catalyst and TBHP as an oxidant, MeCN, H2O, PhCl and DMF, and the reactions did not provide
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which potentially provides a facile entry to substituted and higher yields (Table 1, entries 11−15). In DaOdId: i1t0io.1n0,39lo/Cw5eOrBin0g07t8h1eJ
functionalized N-alkylation azoles.
amount of the [Bpy]I resulted in a lower yield of the product (Table
The investigation started with the coupling reaction of p-xylene 1, entry 16). Increasing the amount of [Bpy]I achieved similar
(1a) and benzotriazole (2a) as a mode system (Table 1). In the reaction efficiency (Table 1, entry 17). Finally, when the reaction
presence of 10 mol% [Bpy]I, 3.0 equiv of TBHP, the reaction of 1a temperature and time were changed, the yields of 3a were lower
and 2a at 75 oC for 12
h
gave 1-(4-methylbenzyl)-1H- than that obtained at 75 oC for 12 h (Table 1, entries 18−21).
benzo[d][1,2,3]triazole 3a in 77% yield (Table 1, entry 1).
With the optimized conditions established, the scope of the
Encouraged by this result, further optimization of the reaction substrates with benzylic C−H bonds were investigated (Table 2). It
conditions was carried out. When the reaction was catalyzed by can be seen that various benzyl compounds were found to be
other ionic liquids such as [BPy]Br and [BPy]Cl, gave unsatisfactory compatible under standard reaction conditions (Table 2). Toluenes
results (Table 1, entries 2 and 3). The other oxidants instead of
Table 2. Reactions of benzotriazole with various benzylic compoundsa
TBHP were also screened (Table 1, entries 4−10), the yields
decreased dramatically and TBHP was found to be the optimal
oxidant. Next, a variety of solvents were tested, including DCE,
Table 1. Reaction optimizationa
Entry
Ionic liquid (mol%)
Oxidantb
Solvent
Yieldc (%)
1
2
[BPy]I (10)
[BPy]Br (10)
[BPy]Cl (10)
[BPy]I (10)
[BPy]I (10)
[BPy]I (10)
[BPy]I(10)
[BPy]I (10)
[BPy]I (10)
[BPy]I (10)
[BPy]I (10)
[BPy]I (10)
[BPy]I (10)
[BPy]I (10)
[BPy]I (10)
[BPy]I (5)
TBHP
TBHP
TBHP
DTBP
BPO
neat
neat
neat
neat
neat
neat
neat
neat
neat
neat
DCE
77
trace
N.R.d
N.R.
N.R.
N.R.
22
3
4
5
6
Oxone
m-CPBA
PhI(OAc)2
H2O2
7
8
N.R.
N.R.
N.R.
55
9
10
11
12
13
14
15
16
17
18e
19f
20g
21h
DDQ
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
MeCN
H2O
45
62
PhCl
DMF
neat
neat
neat
neat
neat
neat
54
trace
58
[BPy]I (20)
[BPy]I (10)
[BPy]I (10)
[BPy]I (10)
[BPy]I (10)
78
59
69
59
76
aReaction conditions: 1a (1.5 mmol), 2a (0.3 mmol), oxidant (3.0 equiv), 75 oC, 12
h. bTBHP: tert-butyl hydroperoxide 70% in water, DTBP: di-tert-butyl peroxide,
BPO: benzoyl peroxide, Oxone: potassium peroxomonosulfate, m-CPBA: m-
chloroperoxybenzoic acid, H2O2 30% in water, DDQ: 2,3-dichloro-5,6-
dicyanobenzoquinone. cIsolated yield. dNot reaction. eReaction temperature: 60
oC. fReaction temperature: 90 oC. gReaction time: 6 h. gReaction time: 18 h.
aReaction conditions: 1 (1.5 mmol), 2a (0.3 mmol), [Bpy]I (10 mol%), TBHP (3.0
equiv), 75 oC, 12 h. Isolated yield.
2 | J. Name., 2012, 00, 1-3
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with electron-donating groups, such as methyl and methoxyl smoothly to generate the desired coupling product with good yield
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reacted smoothly and obtained the corresponding N-alkylated (Table 3, 4a). Subsequently, the coupDlinOgI: 10.1039/C5OB00781J
of a variety of
azoles in good yields (Table 2, 3a−3e). Gratifyingly, it was found that benzimidazoles and diphenylmethane were tested (Table 3, 4b−4h).
toluene reacted well with benzotriazole in 61% yield (Table 2, 3f). It should be noted that benzimidazoles with electron-donating
Benzylic compounds beared electron-with-drawing groups, such as groups as well as electron-withdrawing groups ran well in this
fluoro, chloro, bromo and iodo on the benzene ring and moderate reaction (Table 3, 4c−4h). 5-Methylbenzimidazole and 5,6-
yields of the corresponding products were afforded (Table 2, 3g−3l). dimethylbenzimidazole generated the corresponding N-alkylation
It should be mentioned that the process also tolerated 2- azoles 4c and 4d with moderate yields. The oxidative coupling of
substituted toluenes (Table 2, 3c, 3l). Additionally, 3-substituted diphenylmethane and benzimidazoles bearing an electron-
toluenes were converted into the desired products in moderate to withdrawing groups at the 5 position also proceeded well (Table 3,
good yields (Table 2, 3b, 3d, 3k). Direct N-alkylation of 2a with 4e−4f). Moreover, 2-substituted benzimidazoles gave the desired
ethylbenzene, cumene and diarylmethanes were successful in this products under the optimal conditions in 48% and 43% yields,
protocol (Table 2, 3m−3p). The triphenylmethane which did not respectively (Table 3, 4g and 4h). Unfortunately, poor
give the expected product was probably caused by steric effect. To regioselectivities were observed in this transformation that 5-
our delight, the less activated indane and 1,2,3,4- substituted benzotriazoles or benzimidazoles gave a mixture of
tetrahydronaphthalene reacted with benzotriazole leading to C−N regioisomers (see the Supporting Information). In addition,
coupling products in good yields (Table 2, 3q and 3r). To further imidazole did not give the desired product.
showcase the potential of this methodology, methyl substituted
To demonstrate the synthetic utility of our methodology, an IL-
heteroarenes were submitted to the optimized conditions. To our catalyzed scale-up reaction was performed. The oxidative coupling
surprise, 2-methylquinoline and 2-methylthiophene were reaction between diphenylmethane and benzotriazole was easily
compatible with this transformation, leading to products in 72% performed under standard reaction conditions to furnish the
and 36% yields, respectively (Table 2, 3t and 3u). Notably, N- desired product 3o in 82% isolated yield (Scheme 2).
alkylation azoles were also highly selective, affording only mono-
amination products.
To further define the scope of the synthesis of N-alkylated azoles,
a
series of azoles were applied in this process (Table 3).
Benzotriazoles such as 5-chlorobenzotriazole were found to react
Table 3. Reactions of diphenylmethane with various azolesa
Scheme 2 gram-scale oxidative coupling reaction between diphenylmethane and
benzotriazole.
An important goal in modern catalyst design is the
development of recyclable, inexpensive, environmentally
friendly, and efficient catalysts. Thus, the reusability of the IL
as catalyst was studied. The recycling reactions were
performed with the oxidative coupling between
diphenylmethane and benzotriazole. After the reaction was
complete, water and ethyl acetate were added, and then the
[Bpy]I was separated in the aqueous phase and reused after
drying in vacuo. At least eight consecutive runs of reaction
were carried out in the same condition and the results showed
that ionic liquid catalyst [Bpy]I was recyclable without any
significant loss of activity (Figure 1).
85%
83%
80%
79%
79%
78%
80
60
40
20
0
78%
78%
77%
aReaction conditions: 1o (1.5 mmol), 2 (0.3 mmol), [Bpy]I (10 mol%), TBHP (3.0
equiv), 75 oC, 12 h. Isolated yield. bSeparable isomeric products, 4a-1/4a-2 = 1 : 1.
cInseparable isomeric products. dSeparable isomeric products, 4e-1/4e-2 = 1 : 1.
eSeparable isomeric products, 4f-1/4f-2 = 1 : 1.
0
1
2
3
4
5
6
7
8
Run
Figure 1 Recycling reactions.
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In order to elucidate the reaction mechanism, control recyclable and reused for eight cycles without showing any
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experiments were carried out. When 1 equiv. TEMPO (2,2,6,6- significant loss of catalytic activityD.OI:T10h.i1s039/fCac5OileB007a8n1dJ
tetramethylpiperidine-N-oxyl) as a radical scavenger was environmentally friendly protocol has opened up the
added to the reaction mixture under the optimal conditions, application field of ionic liquids catalyzed C-H bond activation.
the yield of 3a was decreased to trace. When the amount of
TEMPO was increased to 5 equiv., the desired product 3a was
not afforded. The benzyl radical intermediate was trapped and
Acknowledgements
the oxyamination product was isolated in 55% yield. The
results indicated the reaction involves a radical process.
According to the above results as well as the known
literatures7-8,13, a plausible mechanism is proposed (Scheme 3).
The authors are grateful to NSFC (21162025 and 21262035),
the Program for Outstanding Youth Science and Technology
Innovation Talents Training in Xinjiang Uygur Autonomous
Region (2014721004), Natural Science Foundation of Xinjiang
University (BS110133) and Key Laboratory of Oil & Gas Fine
Initially, [BPy]I is oxidized by TBHP to form the {[Bpy]+[IO]-}
A
or {[Bpy]+[IO2]-}
C−H bond is induced by
benzyl radical is liable to be oxidized by active iodine species
or to form the benzyl cation , with a hydroxide ion being
released. After that, the benzotriazole is deprotonated by the
hydroxide ion to generate the anionic specie . Finally, the
nucleophilic reaction of specie with the benzyl cation
forms the desired product 3a (Scheme 3, path A). In addition,
benzyl radical react with azole to give the N-benzyl azole
radical anion , followed by a SET process by losing an electron
B
species. Subsequently, homolysis of a benzyl
or and give a benzyl radical . The
Chemicals, Ministry of Education
&
Xinjiang Uyghur
A
B
C
Autonomous Region (XJDX0908-2013-1 and XJDX0908-2013-2)
for financial support of this research.
A
B
D
E
Notes and references
E
D
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