The C-H activated controlled mono- And di-olefination of arenes in ionic liquids at room temperature

Article abstract of DOI:10.1039/c9ra09736h

In this study, controlled mono and di-olefination of arenes was first realized at room temperature via the C-H bond activation in ionic liquids, probably due to the positive effects of ionic liquids. It is an energy-saving routes in industrial production without the need for heating equipment. Different catalysts were screened, and it was found that [Ru(p-cymene)Cl₂]₂ generated mono-olefinated products predominantly while [Cp?RhCl₂]₂ selectively gave di-olefinated products. These catalysts ([BMIM]NTf₂ and [BMIM]PF₆) as green and recyclable reaction media are highly efficient under mild conditions. This reaction process can avoid any volatile and environmentally toxic organic solvents, and is much safer without the need for pressure-tight equipment. A wide substrate scope with good yields and satisfactory selectivity was achieved. The reactions can be scaled up to gram-scale. Furthermore, an expensive rhodium/ruthenium catalytic system was recycled for at least 6 times with consistently high catalytic activity, which was economical and environmental friendly from an industrial point of view. According to the mechanistic study, the C-H bond cleavage was probably achieved via the concerted metalation-deprotonation. This technique can be applied in the synthesis of various valuable unsaturated aromatic compounds and shows a great potential for industrial production.

Full text of DOI:10.1039/c9ra09736h

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The C–H activated controlled mono- and di-
olefination of arenes in ionic liquids at room
temperature†
Cite this: RSC Adv., 2020, 10, 3203
b
a
*
Kaifeng Du
and Tian Yao
In this study, controlled mono and di-olefination of arenes was first realized at room temperature via the
C–H bond activation in ionic liquids, probably due to the positive effects of ionic liquids. It is an energy-
saving routes in industrial production without the need for heating equipment. Different catalysts were
screened, and it was found that [Ru(p-cymene)Cl2]2 generated mono-olefinated products predominantly
while [Cp*RhCl2]2 selectively gave di-olefinated products. These catalysts ([BMIM]NTf2 and [BMIM]PF6) as
green and recyclable reaction media are highly efficient under mild conditions. This reaction process can
avoid any volatile and environmentally toxic organic solvents, and is much safer without the need for
pressure-tight equipment. A wide substrate scope with good yields and satisfactory selectivity was
achieved. The reactions can be scaled up to gram-scale. Furthermore, an expensive rhodium/ruthenium
catalytic system was recycled for at least 6 times with consistently high catalytic activity, which was
economical and environmental friendly from an industrial point of view. According to the mechanistic
study, the C–H bond cleavage was probably achieved via the concerted metalation–deprotonation. This
technique can be applied in the synthesis of various valuable unsaturated aromatic compounds and
shows a great potential for industrial production.
Received 21st November 2019
Accepted 29th December 2019
DOI: 10.1039/c9ra09736h
rsc.li/rsc-advances
catalyzed by Rh, Ru or Pd.5 Although this strategy reduces costs
signicantly, the catalytic conditions are tough in most cases,
Introduction
and toxic organic solvents can cause serious environmental
pollution. Some other researchers have found some ways to
achieve the recycling of these noble transition-metals without
the loss in their content. Nano-functional materials or metal-
immobilized macromolecules6 are most popular because they
can be reused and recycled to lower the overall cost and reduce
environmental contamination. However, unavoidable harsh
catalytic conditions as well as tedious multiple steps for the
synthesis of metal-loaded functional materials are still needed.
Thus, it is highly desirable to develop new green catalytic
systems with easy accessibility to solve these problems.
Aromatic olens are important chemical intermediates and
widely applied in the synthesis of pharmaceutical intermedi-
ates, natural products, and functional materials.7 The olena-
tion of unreactive aryl C–H bonds catalyzed by transition metals
is among the most signicant chemical transformations in
organic syntheses.8 It has drawn considerable attention through
these years for a single reaction step and a few side reactions
compared with conventional methods.9 Most achievements in
the olenation of arene via the C–H activation have been
accomplished by using noble metal catalyst systems, such as
ruthenium,10 rhodium,11 palladium,12 cobalt,5b and iridium,13
with high temperature and organic solvents. These catalysts
generally show high reactivity and broad substrate scope. A
systematic comparative study on the catalytic activities of
Recently, transition metal-catalyzed C–H activation has
emerged as one of the most atom-economical approach for the
formation of C–C bonds and has made signicant progress in
the eld of synthetic chemistry.1 It is straightforward, highly
efficient and can avoid multi-step pre-activation with a rare
production of unwanted by-products.2 The establishment of
a wide range of new C–C, C–N, C–O, C–P bonds is assisted by
suitable applications of Lewis-basic directing groups.3 Most of
these transition metal catalysts, such as Pd, Ir, Ru and Rh, are
expensive, earth-scarce, require harsh reaction conditions and
environmentally toxic organic solvents. These drawbacks
greatly limit their applications in the synthesis of functional
organic compounds.4 Some researchers explored to nd inex-
pensive earth-abundant transition metals to replace them as
alternative catalysts. For example, Ackermann, Yoshikai and
other groups discovered co-catalyzed C–H activation and
applied in various chemical reactions, which were only
aKey Laboratory of Drug-Targeting and Drug Delivery System of the Education Ministry
and Sichuan Province, West China School of Pharmacy, Sichuan University, Chengdu,
610041, P. R. China. E-mail: yaotian90@scu.edu.cn
bDepartment of Pharmaceutical
& Biological Engineering, School of Chemical
Engineering, Sichuan University, Chengdu 610065, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra09736h
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different catalysts on the olenation of arenes is important and on.16,20 The employment of ILs in C–H activation is still rare and
necessary.
Lately, the controllably selective catalysis of the mono- and
needs to be expanded.
Herein, we report the rst catalyst-controlled selective mono
di-olenation via C–H activation has been of great interest. For and di-olenation of arenes via C–H activation at room
example, N. Umeda reported the selective mono and di- temperature. The notable features of our methodology include:
vinylation of 1-phenylpyrazoles by controlling the Cu(OAc)2 (a) the selective mono- and di-olenation of arenes was realized
amount.14 Selective olenation was also controlled by changing at room temperature for the rst time, which was energy-saving
the solvent, catalyst or substrate.15 However, all these expensive in industrial production without the need for heating equip-
catalytic systems could be used only once and the reaction ment. (b) The catalysts play a key role in controlling the mono-
condition is harsh (Scheme 1a). Recently, our group realized the and di-olenation, where [Ru(p-cymene)Cl2]2 gave mono-
temperature-controlled mono- and di-olenation of arenes with olenation products predominantly, while [Cp*RhCl2]2 affor-
good yield, excellent selectivity and satisfactory recyclability.16 ded the diolenation products with high selectivity. (c) Instead
However, the generation of di-olenated products required of an organic solvent, [BMIM]NTf2 and [BMIM]PF6 served as
a high temperature, which was energy-consuming and unsafe in excellent solvents to allow the recycling of the noble metal
an industrial production process. To the best of our knowledge, catalytic system for at least 6 times, which was economical and
a protocol for the selective synthesis of mono- and di-olenated environmental friendly from an industrial point of view, and the
products at room temperature with full control is yet to be process was much safer without the need of pressure-tight
developed.
equipment. (d) The reaction tolerated a broad substrate with
A solvent is generally used in large amounts in most reac- satisfactory yields and excellent selectivity even in gram-scale
tions and plays a key role in an organic synthesis. Many volatile (Scheme 1b).
and environmentally toxic organic solvents are commonly
applied in the C–H olenation. In recent years, many
researchers have paid attention to the replacement of these
Results and discussion
harmful solvents with an eco-friendly medium to meet the Our initial experiment was performed with 2-phenylpyridine
requirement of green chemistry. Water and polyethylene glycol (1a) and styrene (2a) in the ionic liquid-[BMIM]NTf2 at room
are extensively studied and successfully applied in some temperature. Different types of catalysts were investigated (entry
chemical transformations.17 However, their applications are 1–9) at the beginning. A catalyst with a pentamethylcyclo-
signicantly restricted by the low solubility of starting pentadienyl (Cp*) ligand tended to have catalytic activity.
compounds and metal catalysts. Considering this, it is prefer- Cp*Co(CO)I2, [Cp*CoCl2]2, and [Cp*Co(MeCN)3][SbF6]2 were
able to nd an excellent medium to allow the C–H olenation to effective under these conditions, but only less than 30%
perform smoothly under mild conditions and reuse the metal conversion was achieved. The above successes encouraged us to
catalyst. Ionic liquids (ILs), a class of non-molecular solvents at explore more transition-metal complexes. To our delight,
room temperature,18 which generally consist of an organic [Cp*Rh(MeCN)3][SbF6]2 and Cp*Rh(OAc)2 demonstrated nearly
cation and a weak nucleophilic anion, have drawn widespread 50% conversion of the starting compound. Surprisingly,
attention of researchers due to their superior properties such as [Cp*RhCl2]2 could achieve 97% conversion of 1a and di-
negligible volatility, thermal and chemical stability, wide liquid olenated (3a0) product was obtained with an isolated yield of
range, non-ammability, and good solubility.19 They oen 91%. When [Ru(p-cymene)Cl2]2 was employed as the catalyst,
replace hazardous organic solvents and serve as an environ- 87% isolated yield of the mono-olenated product was ob-
mental friendly, non-volatile and recyclable reaction medium. tained. RhCl3, [Cp*IrCl2]2, Pd(OAc)2, RuCl3 and Ru3(CO)12 could
Furthermore, they are much safer in the high-temperature or not afford any products. As a result, selective mono- and di-
high-pressure synthetic processes. Up to now, ILs have been olenation was achieved at room temperature controlled by
successfully employed in the Suzuki reaction, cross-coupling the catalyst. Inspired by the preliminary success, we screened
reaction, Heck reaction, C–H activated olenation and so several other reaction parameters, and the results are listed in
Table 1. In the [Ru(p-cymene)Cl2]2 catalyzed reaction systems,
different amounts of 2a were then investigated (entry 10, 13 and
14). 1 equimolar of 2a reduced the conversion of 1a to 66%,
while 2 equiv. of 2a could increase the conversion of 1a up to
89% and afforded satisfactory yield of mono-olenated product
3a (85%) with very few di-olenated products (3a0). Cu(OAc)2
served as an oxidant in this catalytic system and 2 equiv. of
Cu(OAc)2 or more could afford a good yield of the mono-
olenated product (entry 10 and 13). To save the starting
compound, 2a and Cu(OAc)2 were both determined as 2 equiv. It
demonstrated that selectivity was not controlled by the amount
of the starting compound. Further, the effects of different
additives were explored (entry 15–18). Various species of Ag salts
Scheme 1 C–H activated olefination of arene.
3204 | RSC Adv., 2020, 10, 3203–3211
were screened and all of them had satisfactory performances
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except that no reaction occurred when an Ag salt was not added. reaction performances were evaluated with different ILs,
It revealed that the Ag salt as an additive was essential in the organic solvents and water (entry 19–27). Traditional organic
C–H activated olenation. The anion of the additive is not solvents such as toluene, CH2Cl2, t-AmOH, HFIP and DMF
important and has little impact on the reaction efficiency. Thus, could not make the reaction happen. When reaction was per-
Ag+ might be crucial in the catalytic process. Furthermore, formed in water, no transformation occurred either. Four
Table 1 Optimization of the reaction conditionsa,h
2a
Additive (0.1
Temp.
(ꢀC)
Entry Catalyst (0.05 equiv.)
(equiv.) Oxidant (equiv.) equiv.)
Solvent
Yieldb (%) 3a/3a0 Conversion of 1ac (%)
1
2
3
4
5
6
7
8
Cp*Co(CO)I2
[Cp*CoCl2]2
[Cp*Co(MeCN)3][SbF6]2
[Cp*Rh(MeCN)3][SbF6]2
RhCl3
[Cp*RhCl2]2
Cp*Rh(OAc)2
[Cp*IrCl2]2
Pd(OAc)2
4
4
4
4
4
4
4
4
4
4
4
4
2
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
4
2
4
4
4
4
4
4
4
4
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgSbF6
Ag2SO4
AgOAc
—
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
8/4
15
30
23
49
—
97
47
—
—
92
—
—
89
66
87
88
85
—
24
82
32
—
—
—
—
—
—
89
91
90
87
72
61
97
98
97
98
90
41
46
92
14/13
10/11
18/29
—/—
5/91
8/37
—/—
—/—
87/3
—/—
—/—
85/3
61/3
83/3
82/5
80/3
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28d
29
30
31e
32
33
34f
35g
36
37
38
39
40
41e
[Ru(p-cymene)Cl2]2
RuCl3
Ru3(CO)12
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Ru(p-cymene)Cl2]2
[Cp*RhCl2]2
—/—
15/7
76/4
[BMIM]BF4
[BMIM]PF6
[BMIM]OTf
H2O
Toluene
t-AmOH
CH2Cl2
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
19/10
—/—
—/—
—/—
—/—
—/—
—/—
84/3
87/3
86/3
35/7
16/55
27/31
5/90
5/92
4/92
DMF
HFIP
[BMIM]NTf2 r.t.
[BMIM]NTf2 60
[BMIM]NTf2 80
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 r.t.
[BMIM]NTf2 60
[BMIM]NTf2 80
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
5/92
7/81
11/27
13/31
7/29
[BMIM]PF6
[BMIM]BF4
[BMIM]OTf
r.t.
r.t.
r.t.
[Cp*RhCl2]2
[BMIM]NTf2 r.t.
a
Reaction conditions: 1a (0.2 mmol), 2a (certain equivalent), catalyst (0.01 mmol), Cu(OAc)2 (oxidant) and additive (0.02 mmol) in a solvent (0.6 mL)
b
were stirred under argon at a certain temperature for 24 h in a sealed tube. Yield of the product isolated aer the preparative thin layer
chromatography. Conversion based on the yield of the recovered 1a. [Ru(p-cymene)Cl2]2 (0.10 equiv.). Reaction performed under air for
c
d
e
f
g
h
24 h. [Cp*RhCl2]2 (0.10 equiv.). [Cp*RhCl2]2 (0.15 equiv.). Cp* ¼ pentamethylcyclopentadienyl; HFIP ¼ hexauoroisopropanol; t-AmOH ¼
tertiary amyl alcohol; r.t. ¼ room temperature.
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Table 2 Comparison of the catalytic activity of different catalystsa,b,c
different ILs were investigated, and all of them were effective.
[BMIM]BF4 and [BMIM]OTf had a low conversion of 1a with
poor selectivity, while [BMIM]NTf2 and [BMIM]PF6 had more
than 80% conversion of 1a with satisfactory selectivity. This
phenomenon is probably because [BMIM]NTf2 and [BMIM]PF6
were much more hydrophobic than the other ILs. They could
better dissolve the transition-metal complex and the hydro-
phobic starting compounds. Thus, sufficient interactions of
starting compounds with catalysts could make the reaction
happen more easily. 0.1 equiv. amount of the catalyst could not
improve the yield obviously (entry 28), so 0.05 equiv. amount of
the catalyst was enough. Moreover, increasing the reaction
temperature to 80 ꢀC had no positive effect on the conversion of
1a, and the yield of 3a investigated (entry 29 and 30). Subse-
quently, the experiment was conducted under air (entry 31).
Only 35% yield of mono-olenated product was afforded and
most of the starting compound was converted to impurity. So,
the argon atmosphere was essential. In the [Cp*RhCl2]2 cata-
lyzed reaction systems, when 2a or Cu(OAc)2 was less than 4
equiv. in amount, the conversion of 1a decreased sharply and
selectivities were poor with both mono-(3a) and di-olenated
(3a0) products generated (entry 32 and 33). 4 equiv. of 2a and
Cu(OAc)2 could afford excellent yield of di-olenated product
(91%). Increasing the catalyst amount or the reaction temper-
ature had no positive effect (entry 34–37). Employing [BMIM]
BF4 or [BMIM]OTf as the reaction solvent could make the
conversion of 1a higher than 40% but with poor selectivities.
[BMIM]NTf2 and [BMIM]PF6 also demonstrated excellent
performances with the conversion of 1a higher 90% (entry 38–
40). Similar to the [Ru(p-cymene)Cl2]2 catalytic system, argon
atmosphere was necessary for the reaction. Accordingly, the
reaction conditions of selective mono-olenation was opti-
mized as follows: 1a (0.2 mmol), 2a (0.4 mmol), [Ru(p-cymene)
Cl2]2 (0.01 mmol), Cu(OAc)2 (0.4 mmol) and AgNTf2 (0.02 mmol)
in [BMIM]NTf2 or [BMIM]PF6 (0.6 mL) under argon at room
temperature for 24 h in a sealed tube. The reaction conditions of
the selective di-olenation was optimized as follows: 1a (0.2
mmol), 2a (0.8 mmol), [Cp*RhCl2]2 (0.01 mmol), Cu(OAc)2 (0.8
mmol) and AgNTf2 (0.02 mmol) in [BMIM]NTf2 or [BMIM]PF6
(0.6 mL) under argon at room temperature for 24 h in a sealed
tube.
a
The reaction conditions: 1x (0.2 mmol), 2x (0.8 mmol), catalyst (0.01
mmol), Cu(OAc)2 (0.8 mmol) and AgNTf2 (0.02 mmol) in [BMIM]NTf2
(0.8 mL) were stirred under argon at r.t. for 24 h in a sealed tube.
b
c
Isolated yield. The catalysts are: A ([Cp*RhCl2]2); B (Cp*Rh(OAc)2);
([Cp*Rh(MeCN)3][SbF6]2); ([Cp*CoCl2]2); ([Cp*Co(MeCN)3]
C
D
E
[SbF6]2); F ([Ru(p-cymene)Cl2]2).
products, so ethyl acrylate had lower reactivity than styrene.
Consequently, [Cp*RhCl2]2 and [Ru(p-cymene)Cl2]2 were chosen
to further explore the substrate scope of selective mono- and di-
olenation.
To examine the effects of ILs on the reaction activity, we
compared the reaction scope of different ILs in the [Cp*RhCl2]2
catalytic system. As listed in Table 3, six different kinds of
substrates were investigated. To our delight, applying these four
ILs as reaction media could generally give the corresponding
products in moderate to good yields for all substrates.
Surprisingly, for all substrates, [BMIM]NTf2 and [BMIM]PF6
could afford excellent yield of the product with satisfactory
selectivity compared with [BMIM]BF4 and [BMIM]OTf. Theo-
retically, [BMIM]NTf2 and [BMIM]PF6 were much more hydro-
phobic than another two ILs. They could better dissolve the
transition-metal complex and the hydrophobic starting
compounds. As a result, the interactions of starting compounds
with catalysts were more sufficient, and the conversion of
starting compounds was higher. Then, [BMIM]NTf2 and [BMIM]
PF6 were applied to further explore the substrate scope of
selective mono- and di-olenation.
Based on the above results, we then compared the reaction
scope of different transition-metal catalysts under the optimal
conditions. As shown in Table 2, different substituted
substrates at different positions of the arenes and substrates
with different directing groups were studied in detail. Both
electron-donating and electron-withdrawing groups were
selected as the substituent. In addition, 2-(naphthalen-2-yl)
pyridine as well as ethyl acrylate were also explored. It is
obvious that all 6 catalysts were effective for this kind of reac-
tion. Cp*Rh(OAc)2, [Cp*Rh(MeCN)3][SbF6]2, [Cp*CoCl2]2 and
[Cp*Co(MeCN)3][SbF6]2 were able to make reactions happen,
but the catalytic activities were not satisfactory and the selec-
tivity of the products were poor. On the contrary, [Cp*RhCl2]2
and [Ru(p-cymene)Cl2]2 revealed excellent catalytic activities
and wonderful selectivity for all kinds of substrates. In partic-
ular, the yield of 3r or 3r0 was relatively lower than that of other
With the optimized reaction conditions in hand, we inves-
tigated the substrate scope for the selective mono-olenation of
different arenes via C–H activation catalyzed by [Ru(p-cymene)
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Table 3 Comparison of the reaction activity with different ionic Table 4 Reaction scope of the selective mono-olefination of different
liquidsa,b,c
arenesa,b,c
a
The reaction conditions: 1x (0.2 mmol), 2x (0.8 mmol), [Cp*RhCl2]2
(0.01 mmol), Cu(OAc)2 (0.8 mmol) and AgNTf2 (0.02 mmol) in
a certain ionic liquid (0.8 mL) were stirred under argon at r.t. for 24 h
b
c
in a sealed tube. Isolated yield. The ionic liquids are: A ([BMIM]
NTf2); B ([BMIM]PF6); C ([BMIM]BF4); D ([BMIM]OTf).
a
The reaction conditions: 1a (0.25 mmol), 2a (0.50 mmol), [Ru(p-
cymene)Cl2]2 (0.0125 mmol), Cu(OAc)2 (0.50 mmol) and AgNTf2 (0.025
mmol) in [BMIM]NTf2 (0.8 mL) were stirred under argon at r.t. for
24 h in a sealed tube. The reaction conditions: 1a (0.25 mmol), 2a
Cl2]2. Both [BMIM]NTf2 and [BMIM]PF6 were employed for the
substrate scope expansion. As shown in Table 4, various
substituted 2-aryl-pyridine could afford mono-olenated prod-
ucts in high yields with ideal selectivity at room temperature.
Unsubstituted 2-phenylpyridine 1a gave an 85% isolated yield of
3a in [BMIM]NTf2 and a 79% yield of 3a in [BMIM]PF6,
respectively. Electron donating group substituted 2-aryl-
pyridine, such as 40-Me, 40-t-Bu, 40-OCH3, 40-Ph and 50-Me
derivatives, had slightly increased yields of 3b, 3c, 3d, 3e and 3i
for both ILs reaction systems, respectively. Although electron-
withdrawing group substituted 2-aryl-pyridine (CF3 or CN)
have a relatively lower conversion of the substrate and lower
yield of products, these groups were able to exhibit ne
performances. As for halogens (F, Cl, and Br), they were satis-
factorily tolerated irrespective of their position (para- or meta-),
and obtained 82% yield of 3f, 84% yield of 3g, 86% yield of 3h,
83% yield of 3j in [BMIM]NTf2, and could achieve 81% yield of
3f, 78% yield of 3g, 80% yield of 3h, 79% yield of 3j in [BMIM]
PF6. Interestingly, 3q had an excellent yield probably due to the
increased conjugation of the naphthalene ring. Different types
of olens were also reacted with 1a. Similarly, Me, t-Bu and
halogen-substituted styrene had excellent performances, and
NO2 substituted styrene only had about 60% yield of the mono-
olenated product in both ILs. Surprisingly, ethyl acrylate and
unreactive 1-octene even vinylcyclohexane could react with 1a
with ideal selectivity. Moreover, altering the directing group to
N-methylamide, N-methoxylamide and oxime ether could also
selectively afford the mono-olenated products in ne yields.
Finally, ortho-substituted 2-phenylpyridine could satisfacto-
rily give the mono-olenated product. This mono-olenated
strategy demonstrated wide substrate adaptability, good reac-
tivity and high selectivity. Both [BMIM]NTf2 and [BMIM]PF6
b
(0.50 mmol), [Ru(p-cymene)Cl2]2 (0.0125 mmol), Cu(OAc)2 (0.50 mmol)
and AgNTf2 (0.025 mmol) in [BMIM]PF6 (0.8 mL) were stirred under
c
argon at r.t. for 24 h in a sealed tube. Isolated yield.
could serve as an excellent reaction medium, and [BMIM]NTf2
had slightly better performances than [BMIM]PF6 for all
substrates.
Subsequently, we further explored the substrate scope of the
selective di-olenation of different arenes via C–H activation
catalyzed by [Cp*RhCl2]2. Both [BMIM]NTf2 and [BMIM]PF6
were employed for the substrate scope expansion. As demon-
strated in Table 5, most of the substituted 2-aryl-pyridine could
selectively give di-olenated products in high yields at room
temperature and had similar electronic effects as above.
Exceptionally, the NO2-substituted substrate only gave mono-
olenated product at this condition, probably because the
extremely strong electron-withdrawing effects deactivate the
aromatic ring towards the second olenation. Notably, meta-
substituted 2-aryl-pyridine had a relatively lower yield of di-
olenated products compared to para-substituted substrates.
It was probably because the steric hindrance of the meta-
substituted group interferes with the formation of the di-
olenated product. Ethyl acrylate performed well to selectively
give a di-olenated product with good yield. Then, 2-phenyl-
pyridine was subjected to react with unreactive 1-octene and
vinylcyclohexane, and mono-olenated products were predom-
inantly afforded. It was possibly due to the low reactivity of the
unconjugated olens. When applying the naphthalene ring to
increase conjugation, 3q exhibited an ideal yield of di-olenated
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Table 5 Reaction scope of the selective di-olefination of different
arenesa,b,c
Scheme 2 Gram-scale mono- and di-olefination of 1a.
investigate the recyclability of these catalytic systems. The
reactions of 2-phenylpyridine with styrene were selected as
a model for the reusability evaluation. The recyclability perfor-
mances are shown in Fig. 1. Aer the completion of the reac-
tion, the mixture was extracted with an equivoluminal amount
of diethyl ether 4 times. The upper diethyl ether layers, con-
taining the product were combined and evaporated, and then
directly used for purication simply through a preparative thin
layer chromatography to get the pure product. The lower layer
(reaction system) was further treated by vacuum evaporation to
completely remove the miscible diethyl ether. Then, this cata-
lytic system was added with new starting materials and used for
the next cycle. It was found that the desired products were ob-
tained in high yields with only a slight drop aer 6 cycles for all
four different catalytic systems. These results strongly indicate
that [BMIM]NTf2 and [BMIM]PF6 were indeed excellent recy-
clable reaction media for the selective mono- and di-olenation.
Next, we investigated the selective mono- and di-olenation
in gram-scale with 2-phenylpyridine and styrene as an
a
The reaction conditions: 1a (0.25 mmol), 2a (1.00 mmol), [Cp*RhCl2]2
(0.0125 mmol), Cu(OAc)2 (1.00 mmol) and AgNTf2 (0.025 mmol) in
[BMIM]NTf2 (0.8 mL) were stirred under argon at r.t. for 24 h in
a sealed tube. The reaction conditions: 1a (0.25 mmol), 2a (1.00
mmol), [Cp*RhCl2]2 (0.0125 mmol), Cu(OAc)2 (1.00 mmol) and AgNTf2
(0.025 mmol) in [BMIM]PF6 (0.8 mL) were stirred under argon at r.t.
for 24 h in a sealed tube. c Isolated yield.
b
products. Moreover, changing the directing group to N-meth-
ylamide and N-methoxylamide could also selectively afford the
di-olenated products in satisfactory yields. However, less
reactive aryl oxime ether only gave mono-olenated products at
this condition. It can be concluded that aryl pyridine and aryl
amides have a much higher reactivity than aryl oxime ether
towards the second olenation. The ortho-substituted 2-phe-
nylpyridine had higher yields of mono-olenated products
compared with the Ru catalytic system. This di-olenation
strategy also showed wide substrate adaptability, good reac-
tivity and high selectivity. As a result, the selective mono- and di-
olenation of arenes was achieved by controlling the catalyst at
room temperature. [BMIM]NTf2 and [BMIM]PF6 both served as
ideal reaction solvents, and [BMIM]NTf2 performed slightly
better than [BMIM]PF6 for all substrates.
Selective olenation was also realized in organic solvents
with high yield of mono- and di-olenated products. However,
high reaction temperatures and environmentally toxic organic
solvents were required.15a,b Zhang15c realized high yield of
selective olenation in water, but a high temperature of more
than 378 K was required for the reaction. Compared with the
previous literatures, this study reported comparably high yield
of mono- and di-olenated products with ionic liquids as
a green solvent at room temperature.
Fig. 1 Recycling performances of the reaction of 2-phenylpyridine
with styrene at room temperature: isolated yield of di-olefinated
product in the [Cp*RhCl2]2/[BMIM]NTf2 catalytic system; isolated
yield of di-olefinated product in the [Cp*RhCl2]2/[BMIM]PF6 catalytic
system; isolated yield of mono-olefinated product in the [Ru(p-
cymene)Cl2]2/[BMIM]NTf2 catalytic system; isolated yield of mono-
olefinated product in the [Ru(p-cymene)Cl2]2/[BMIM]PF6 catalytic
system.
Recyclability could reduce the cost of noble metal and avoid
environmental contamination. Therefore, it is very important to
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Scheme 3 Mechanistic study.
example. Scheme 2 demonstrates that satisfactory yields were [Cp*RhCl2]2 system were deuterated aer a 4 h reaction, and it
obtained at both circumstances. Therefore, this method is suggested that the C–H bond activation was reversible.
economical, green and shows a great potential in industrial Furthermore, the intermolecular competition experiments were
production.
conducted between 1b and 1k for both catalytic systems. The
To further gain a mechanistic understanding of the reaction, products were separately obtained with isolated yields of 3b/3k
a series of experiments were performed, and the results are ¼ 63%/37% for the [Ru(p-cymene)Cl2]2 system and 3b0/3k0 ¼
demonstrated in Scheme 3. First, the H/D exchange experi- 81%/19% for the [Cp*RhCl2]2 system. The results reveal that the
ments were performed by treating 1a with CD3OD in [BMIM] electron-rich substrate had much higher reactivity over the
NTf2. The 1H NMR spectra revealed that 43% ortho C–H for the electron-decient substrate and is in accordance with previous
[Ru(p-cymene)Cl2]2 system and 37% ortho C–H for the literatures.
Fig. 2 Possible mechanism.
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Based on the above results and some previous related temperature for 24 h and monitored by TLC. Aerwards, the
reports,13,21 a possible mechanism has been proposed and mixture was extracted with diethyl ether and then the upper
demonstrated in Fig. 2. First, chloride ions in the catalyst phase (diethyl ether) was evaporated in vacuo. The residue was
combined with silver ion formed a precipitate. The Rh or Ru further puried via preparative thin layer chromatography by
catalyst dimer splits into monomers. The active catalytic species silica gel (developing solvent, acetone/petroleum ether ¼ 1 : 10
I for the Rh system (or I0 for Ru system) were formed by with several drops of triethylamine), affording the product 3a.
exchanging the ligand with Cu(OAc)2. Then, this active species
combined with 2-aryl-pyridine was used to activate the C–H General procedure for the synthesis of di-olenated product
bond through concerted metalation deprotonation to afford the (taking 3a0 as an example)
intermediate II or II0. Further, a molecule of acetic acid was
To a 15 mL sealed tube, 2-phenylpyridine 1a (38.75 mg, 0.25
mmol), styrene 2a (104 mg, 1.00 mmol), [Cp*RhCl2]2 (7.725 mg,
0.0125 mmol), AgNTf2 (9.7 mg, 0.025 mmol), Cu(OAc)2 (182 mg,
1.00 mmol), and [BMIM]NTf2 (0.8 mL) were added in sequence.
The mixture was stirred at room temperature for 24 h and
monitored by TLC. Aerwards, the mixture was extracted with
diethyl ether and then the upper phase (diethyl ether) was
eliminated to produce coordinate intermediate III or III0, fol-
lowed by further coordination with styrene to form IV or IV0.
Aer the b-H elimination, mono-olenated product was
produced.
Acetate ion was trapped to form the catalyst intermediate VI
or VI0. It was then oxidised by Cu(OAc)2 and eliminated
a molecule of AcOH to afford the original active catalytic
evaporated in vacuo. The residue was further puried via
species. In the Rh system, the di-olenated product was
preparative thin layer chromatography by silica gel (developing
generated by another catalytic cycle.
solvent, acetone/petroleum ether ¼ 1 : 10 with several drops of
triethylamine), affording the product 3a0.
Conclusions
Abbreviations
In summary, we reported the transition-metal controlled mono-
and di-olenation of arenes via the C–H activation in [BMIM]
NTf2 and [BMIM]PF6 at room temperature. This is the rst time
to realize selective mono- and di-olenation at room tempera-
ture, probably due to the positive effects of ionic liquids. It is an
energy-saving route in industrial production without the need
[BMIM]
NTf2
[BMIM]
PF6
3-Butyl-1-methyl-1H-imidazolium
bis((triuoromethyl)sulfonyl) amide
1-Butyl-3-methylimidazolium hexauorophosphate
of heating equipment. Aer the detailed screening of catalysts, [BMIM]
3-Butyl-1-methyl-1H-imidazolium tetrauoroborate
BF4
[BMIM]
OTf
it was found that the mono- and di-olenated products can be
selectively afforded by ruthenium and rhodium catalytic
systems, respectively. The reported reactions demonstrated
a broad substrate scope and gave the desired products in
excellent yields with satisfactory selectivity even in gram-scale.
In particular, the employment of non-volatile and chemically
stable ionic liquid was environmental friendly without the use
of organic solvents and made the catalytic process much safer
without the need of pressure-tight equipment. Furthermore, the
recyclability study revealed the consistent high activity of the
catalytic systems even aer recycling for 6 time. This advantage
saves costs signicantly and reduces environmental contami-
nation caused by transition metals from an industrial point of
view. The mechanistic study revealed that the C–H bond
cleavage was probably achieved via the concerted metalation–
deprotonation. This technique shows a great potential in the
industrial production of various valuable unsaturated aromatic
compounds.
3-Butyl-1-methyl-1H-imidazolium
triuoromethanesulfonate
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 81803486, 21676170), China
Postdoctoral Science Foundation (Grant No. 2018M633385) and
Sichuan Science and Technology Program (Grant No.
2019YJ0030).
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General procedure for the synthesis of mono-olenated
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