Rare-Earth Y(OTf)3 Catalyzed Coupling Reaction of Ethers with Azaarenes

Article abstract of DOI:10.1021/acs.orglett.9b02763

Rare-earth catalysis has become a hot topic in the field of catalytic organic reaction. Chain ethers mostly have lower reactivity and lower boiling points which limited their reaction scope. Herein, we found a rare-earth Y(OTf)₃ can catalyze th

Full text of DOI:10.1021/acs.orglett.9b02763

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rare-Earth Y(OTf)3 Catalyzed Coupling Reaction of Ethers with
Azaarenes
Yue-Hua Wu,† Nai-Xing Wang,*,† Tong Zhang,† Lei-Yang Zhang,† Xue-Wang Gao,† Bao-Cai Xu,*,‡
Yalan Xing,*,§ and Jian-Yi Chi∥
†Technical Institute of Physics and Chemistry & University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing,
100190, China
‡School of Food and Chemical Engineering, Beijing Technology and Business University, Beijing 100048, China
§Department of Chemistry, William Paterson University of New Jersey, Wayne, New Jersey 07470, United States
∥Baotou Rare Earth Research and Development Center, Chinese Academy of Sciences, Baotou, 014010, China
S
* Supporting Information
ABSTRACT: Rare-earth catalysis has become a hot topic in the
field of catalytic organic reaction. Chain ethers mostly have lower
reactivity and lower boiling points which limited their reaction
scope. Herein, we found a rare-earth Y(OTf)3 can catalyze the
coupling reaction of ethers especially chain ethers and thioethers
with azaarenes. This protocol features simple operations, a broad
substrate scope (31 examples), moderate to good yields (up to
85%), and atom economy.
Scheme 1. C(sp3)−H Functionalization of Ethers
ransition metal catalyzed organic reactions have been
1−3 Rare-earth catalysis has
Twidely studied for a long time.
become a hot topic in the field of catalytic organic reaction.4−6
Ethers are important moieties in a great number of biologically
active compounds and pharmaceuticals.7−11 Simple ethers,
such as diethyl ether, and tetrahydrofuran, are bulk readily
available chemicals which are often used as solvents for many
reactions.12−14 In the past few years, ethers were used as
substrates in many α-C(sp3)−H bond functionalization
reactions via oxidative coupling processes.15−20 Zhang21
reported the bifunctionalization of styrenes with cyclic ethers,
and Li22 reported the 1,2-alkylarylation of styrenes with cyclic
ethers and indoles. Only cyclic ethers are used in those reports.
Therefore, α-C(sp3)−H bond functionalized ethers, especially
chain ethers, remain a challenge.23
Recently, our group has successfully developed several
strategies of C(sp3)−H functionalization of ethers (Scheme.
1). We previously reported Ni- and Mn-catalyzed decarbox-
ylative cross-coupling of α,β-unsaturated carboxylic acids with
cyclic ethers (Scheme. 1A).24 When Ni(OAc)2·4H2O was used
as a catalyst, oxyalkylation was achieved. and Mn(OAc)2·4H2O
promoted the alkenylation. We also disclosed the C(sp3)−H
bond functionalization of chain ethers with α,β-unsaturated
carboxylic acids (Scheme 1B).25 Additionally, we developed a
Cu(OAc)2·H2O mediated functionalization of the unactivated
C(sp3)−H bond of chain ethers with styrenes using tert-butyl
hydroperoxide (TBHP) as a radical initiator (Scheme. 1C).26
Cross-dehydrogenative coupling (CDC) has become one of
the most straightforward strategies in C−H bond functional-
ization without the need for prefunctionalization of reac-
tants.23,27−30 Li and co-workers published a CDC reaction of
pyridines and cycloalkanes, which was known as a Minisci-type
reaction.31−35 Although the alkylation products were obtained
in moderate yields, most pyridines as substrates afford almost
equal amounts of mono- and bis-alkylation products, and only
bis-alkylation products were obtained in some cases, which
showed low regioselectivity.36 The substrates were limited to
Received: August 5, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02763
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
cycloalkanes, and the detailed mechanism was not investigated.
Later, Lei and co-workers developed a selectfluor promoted
CDC reaction of pyridines with cycloalkanes.37 Another
alkylation of pyridine derivatives using K2S2O8 was disclosed
by Barriault and co-workers.38 However, this method lacks
regioselectivity. Coupling of ethers with azaarenes is important
from a practical point of view. Herein, we would like to
describe the coupling reaction of ethers and azaarenes.
Table 1. Optimization of the Reaction Conditions of
Coupling Reaction of the Cyclic Ethers
a
radical
initiator
(equiv)
b
additive
(equiv)
temp time yield
Results and Discussion
entry cat. (mol %)
(°C)
(h)
(%)
We began our study of the coupling reaction of the chain
ethers with azaarenes. The optimization investigation was
performed using 2-methylpyridine and diethyl ether as model
substrates, and the results are shown in Table S1. However, the
yield was up to 54%. Then more reactive 1,4-dioxane and 2-
phenylpyridine were used as the substrates for the optimization
studies, and the results are shown in Table 1.
1
Sc(OTf)3
(5.0)
Y(OTf)3
(5.0)
Ce(OTf)3
(5.0)
Cu(OAc)2·
H2O (5.0)
Y(OTf)3
(5.0)
DTBP
DBU (1.5)
DBU (1.5)
DBU (1.5)
DBU (1.5)
120
12
33
(2.0)
2
DTBP
(2.0)
DTBP
(2.0)
DTBP
(2.0)
DTBP
(2.0)
DTBP
(2.0)
TBHP
(2.0)
DTBP
(2.0)
DTBP
(2.0)
DTBP
(2.0)
DTBP
(2.0)
DTBP
(1.0)
DTBP
(3.0)
DTBP
(3.0)
DTBP
(3.0)
DTBP
(3.0)
DTBP
(3.0)
120
120
120
120
120
120
120
120
120
120
120
120
130
110
90
12
12
12
12
12
12
8
36
3
26
4
trace
35
5
After screening metal catalysts (entries 1−4), 3ap was
achieved in 36% yield (entry 2). Only trace amount of product
was obtained when Cu(OAc)2·H2O was used as the catalyst
(entry 4). The results showed that Y(OTf)3 was the most
powerful catalyst. To our delight, the reaction could also run
without DBU (entry 5). When no Y(OTf)3 catalyst was added,
trace amount of product was obtained, which showed that the
rare-earth catalyst is essential for this reaction (entry 6). The
rare-earth yttrium ion belongs to a hard Lewis acid, which
exhibits strong ability to coordinate with compounds
containing a N or an O atom. We speculated that, in this
case, the azaarenes and ethers can be pulled together by the
yttrium ion to react more smoothly. Subsequently, we found
that TBHP was less effective (entries 7). By screening the
reaction time, the results showed that prolonging the reaction
time could further increase the yield of 3ap to 57% (entries 8,
9). Next, we studied the catalyst loading of Y(OTf)3 and found
that reducing or increasing the amount of the catalyst did not
affect the reaction yield significantly (entries 10, 11). However,
changing the amount of DTBP has a great impact on the yield
of the reaction. When 3.0 equiv of DTBP were used, the yield
of 3ap was further improved to 65% (entries 12, 13). Then,
when the reaction was carried out at 90 °C, the yield of 3ap
dropped dramatically (entries 14−16). We also tried a
photocatalytic reaction. 3ap was obtained with Na2-eosin Y
as a photosensitizer under blue light at room temperature
(entries 17, 18). It is worth noting that when the reaction was
performed at gram scale, 3ap was obtained in 57% yield (entry
19). When using 2,6-dimethylpyridine instead of 2-phenyl-
pyridine, the corresponding product was obtained in 85% yield
(entry 20).
6
trace
trace
31
7
Y(OTf)3
(5.0)
Y(OTf)3
(5.0)
Y(OTf)3
(5.0)
Y(OTf)3
(3.0)
Y(OTf)3
(1.0)
Y(OTf)3
(3.0)
Y(OTf)3
(3.0)
Y(OTf)3
(3.0)
Y(OTf)3
(3.0)
Y(OTf)3
(3.0)
Y(OTf)3
(3.0)
Y(OTf)3
(3.0)
Y(OTf)3
(3.0)
Y(OTf)3
(3.0)
8
9
24
24
24
24
24
24
24
24
12
12
24
24
57
10
11
12
13
14
15
16
17
18
19
20
56
58
34
65
58
32
trace
trace
n.r.
57
c
Na2-eosin Y
Acid red 94
30
c
DTBP
(3.0)
DTBP
(3.0)
DTBP
(3.0)
30
d
e
120
120
85
a
Reaction conditions: 2-phenylpyridine 1b (0.2 mmol, 1.0 equiv),
1,4-dioxane 2b (0.25 mL) in sealed tube. Isolated yields. Under blue
light. Gram scale (10 mmol scale). Using 2,6-dimethylpyridine
instead of 2-phenylpyridine.
b
c
d
e
With the optimal conditions in hand, the substrate scope
was investigated (Figure 1). First, a series of ethers were
investigated. Chain ethers, such as diethyl ether and dibutyl
ether, methy butyl ether, methy tert-butyl ether, and anisole,
underwent the reaction smoothly and produced the corre-
sponding products 3aa−3af in moderate yields. Compared 3aa
with 3ab, we found that the reactivity of 2-phenylpyridine was
lower than 2-methylpyridine. In addition, when 1,2-diethoxy-
ethane was used as the starting material, 3ag and 3ah were
obtained in 33% and 14%, respectively, which showed that the
steric effect was the major factor. And due to the large steric
hindrance, diisopropyl ether only gave a trace amount of the
product (3ai).
When the 2-methyltetrahydrofuran reacted with 2-phenyl-
pyridine, the coupling product was obtained exclusively at the
C5 of the 2-methyltetrahydrofuran (3am). Since there are two
chiral centers in 3am, two diastereomers were generated (d.r. =
5:3; see Supporting Information). Using 1,3-dioxolane as
substrate, 3an and 3ao were obtained in 18% and 55%,
respectively. The reason might be that the hydrogen on the C2
of the 1,3-dioxolane is more easily removed to generate the
radical (3ao).
Subsequently, an array of the pyridine moiety bearing
electron-donating groups such as 2-methyl, 2,3-dimethyl, 2,3-
cyclopenteno, 2,5-dimethyl, and 2,6-dimethyl groups were
found to provide the desired products 3aq−3au in favorable
yields. Due to the only reactive site at C4 of the 2,6-
Cyclic ethers such as tetrahydropyran and tetrahydrofuran
reacted smoothly to give the desired products (3aj−3al).
B
DOI: 10.1021/acs.orglett.9b02763
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Mechanism Studies
that α-C(sp3)−H bond cleavage of ether is involved in the
rate-determining step (Scheme. 2B).
Based on the mechanistic studies, a plausible reaction
mechanism was proposed (Scheme 3). For example, in the
Scheme 3. Plausible Reaction Mechanism
Figure 1. Substrate scope for the reaction of azaarenes and ethers.a
a Reaction conditions (for the chain ethers): 1 (0.2 mmol, 1.0 equiv),
2 (0.25 mL) Y(OTf)3 (5.0 mol %), DTBP (3.0 equiv), in sealed tube
at 120 °C for 24 h; (for the cyclic ethers and thioethers): 1 (0.2
mmol, 1.0 equiv), 2 (0.25 mL) Y(OTf)3 (3.0 mol %), DTBP (3.0
equiv), in sealed tube at 120 °C for 24 h, unless otherwise noted.
dimethylpyridine, 3au was obtained in 85% yield. And the
pyridine moiety bearing an electron-withdrawing group such as
2-acetylpyridine and dimethylpyridine-3,5-dicarboxylate could
give corresponding products 3av and 3aw in lower yield.
Furthermore, pyridine, quinoline, isoquinoline, and benzothia-
zole smoothly underwent the reaction and achieved good
yields (3ax−3ba).
At last, we studied the thioethers such as dimethylsulfane,
diethylsulfane, and tetrahydrothiophene to react with quino-
line, the corresponding coupling products 3bb−3be were
obtained in moderate yield. It is worth noting that the products
of thioethers are still sulfides, which are not further oxidized to
sulfoxides and sulfones.
case of the reaction of diethyl ether and 2-methylpyridine,
under heat, the DTBP decomposed into two tert-butylperoxyl
radicals. Then, the tert-butylperoxyl radical turned the ether
into a carbon-centered radical. The ether radical then
combined with 2-methylpyridine to give intermediate A. The
intermediate A was further oxidized by DTBP to generate the
product.
Since the reaction is a free radical reaction, stereoselectivity
control of the reaction meets a great challenge. First, chiral
ligands and catalysts such as BINOL, BINAP, L-proline, and
cyclohexanediamine were added to the reaction system.
However, none of these reactions gave any stereoselectivity.
Subsequently, three methods were tried to slow down the
reaction rate (see Supporting Information): (a) the reaction
temperature was dropped from 120 to 90 °C; (b) 0.2 equiv of
TEMPO was added to the reaction system to capture some of
the newly generated free radicals; (c) the reaction was carried
out under photocatalyzed conditions at room temperature. No
positive result was achieved. Although the approaches we used
gave no stereoselectivity, chemists interested in the stereo-
selectivity control of radical reactions could learn from the
strategies that we provided.
To gain further insight into the reaction mechanism,
mechanistic studies were designed and conducted (Scheme
2). Initially, 3.0 equiv of radical scavenger 2,2,6,6-tetramethyl-
1-piperidinyloxyl (TEMPO) were added to the reaction system
under the standard conditions. It was found that the
transformation was completely inhibited, and the adduct of
TEMPO and radical generated from 1,4-dioxane was detected
by HRMS-ESI. Ethers can be transformed into acetal in the
presence of TBHP. And the acetal can further generate an
oxocarbenium to react with other nucleophiles.39 However, the
acetal and corresponding cyclic oxocarbenium were not
detected by HRMS. These results suggested that the present
reaction should involve a free radical process (Scheme. 1A).
Then the deuterated intermolecular competing kinetics isotope
experiment was performed using deuterated THF as substrate.
Conclusions
In summary, we have developed a rare-earth Y(OTf)3
catalyzed coupling reaction of ethers, including chain ethers,
cyclic ethers, and thioethers, with azaarenes to generate
corresponding alkylation products in moderate to high yields
1
By integrating H NMR, the KH/KD = 3.17, which suggested
C
DOI: 10.1021/acs.orglett.9b02763
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02763.
■
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: nxwang@mail.ipc.ac.cn.
*E-mail: xubac@163.com.
*E-mail: xingy@wpunj.edu.
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C(sp3)−H Oxidative Coupling of Glycine and Peptide Derivatives.
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ORCID
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Oxidative Povarov Reactions between N-Alkyl N-Methylanilines and
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Nai-Xing Wang: 0000-0001-9520-3254
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Financial support for this research was provided by National
Natural Science Foundation of China (21676003) and
National Key R&D Program of China (2017YFB0308700).
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