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Letter
DDQ-Catalyzed Direct C(sp3)–H Amination of Alkylhet-eroarenes: Synthesis of Biheteroarenes under Aerobic and Metal-free Conditions Chunlan Song, Xin Dong, Hong Yi, Chien-Wei Chiang, and Aiwen Lei ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04434 • Publication Date (Web): 03 Feb 2018 Downloaded from http://pubs.acs.org on February 4, 2018
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ACS Catalysis
DDQ-Catalyzed Direct C(sp3)–H Amination of Alkylheteroarenes: Synthesis of Biheteroarenes under Aerobic and Metal-free Conditions Chunlan Song†, Xin Dong†, Hong Yi†, Chien-Wei Chiang*† and Aiwen Lei*†‡ †
College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS), Wuhan University, Wuhan 430072, P. R. China ‡
National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang 330022, P. R. China
ABSTRACT: A strategy for the oxidative Csp3-H/N-H cross-coupling is presented. This reaction successfully utilizes DDQ and TBN as the co-catalysts to construct the biomedical applicable biheteroarenes under aerobic conditions. Notably, this amination reaction is successful with a wide range of alkylheteroarenes and could be used as a functionalization tactic for pharmaceutical research and other areas. Furthermore, preliminary mechanistic studies indicate that the C-N bond formation proceeds through the nucleophilic attack of azole to the carbon cation. KEYWORDS: C(sp3)-H amination, C(sp3)-H/N-H cross-coupling, metal-free, biheteroarenes, alkylheterarenes, azoles
Biheteroarenes, possess a privileged structural cores in organic functional materials, ligands, and biological activities.1 The construction of biheteroarenes through the C–H activation with two heteroaromatic units establishd one of the most important subjects in organic chemistry.2 Notably, thiophene, an important molecular scaffold, is frequently found in numerous natural products and functional materials.3 Moreover, aminated alkylthiophenes by utilizing azoles as the amination reagents are often employed as the building blocks in pharmaceuticals, such as glucagon receptor antagonists, inhibitors of KV 1.5 and inhibitors of ITK kinase (Scheme 1).4 Therefore, to discover a sustainable way for the preparation of these valuable aminated alkylthiophenes is important in organic synthesis.
since it avoided the prefunctionalization steps of the substrates.5 Nevertheless, due to the high bond energy and low acidity, direct amination to accomplish the C(sp3)–H to C–N bond formation regioselectively is still a challenge.6 Recently, some C(sp3)-H bond amination methods have been reported as attractive ways, such as transition-metal catalysis,7 photo-catalysis,8 electrocatalysis,9 iodine-catalysis.10 In contrast, developing metal-free, cheaper and catalytic amount of reagents’ systems are of great importance. On the other hand, the amination products of alkylheteroarenes were useful for the biological and pharmaceutical applications, such as aminated alkylthiophenes, alkylfurans and alkylindoles. Unfortunately, the above catalysis methods lack of the aminated reactions of these alkylheteroarenes. Our strategy successfully utilized DDQ and TBN as the co-catalysts to construct the biomedical applicable biheteroarenes under aerobic condition with low cost and high efficiency (Scheme 2). H N N
X
Y
DDQ, TBN, O2
X
N N Y
X= S, O, N Alkylheteroarenes
Scheme 1. Important molecules containing aminated alkylthiophenes
Scheme 2. Strategy for amination of Csp3–H bonds
Over the past decade, C(sp2)-H/N-H direct crosscoupling has been developed as one of the most straightforward ways for the construction of C-N bonds
2,3-Dichloro-5,6-dicyano-p-benzoquinone (DDQ) is known for facilitating a number of C-H bond transformations, relying on its one-electron reduction
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Y= N, CH
up to 99% yield
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potential and hydrogen abstraction capacity.11 In order to achieve the C(sp3)-H amination of alkylheteroarenes, we assumed that the reaction could be triggered by hydrogen atom transfer (HAT) between alkylheteroarene 1 and DDQ, and then generated the alkyl radical (I). Subsequent oxidation led to the alkyl cation (II), which reacted with pyrazole 2 to furnish the corresponding amination product 3. Meanwhile, DDQH- could be protonated to form the hydroquinone DDQH2. According to previous works of Hu and our group, DDQH2 could be oxidized to DDQ by NO2, which was generated from tertbutyl nitrite (TBN) (Scheme 3).12
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using biphenyl as an internal standard. N.D. implies that no d product could be detected by GC-MS and GC. A nitrogen balloon was used.
TBN catalysis system, the desired product 3a could be obtained in 99% yield (Table 1, Entry 1). When other quinones such as benzoquinone and tetra-chlorobenzoquinone were used as the catalyst, no desired product could be observed (Table 1, Entry 2-3). Due to the lower redox potential, benzoquinone and tetra-chlorobenzoquinone could not oxidize the substrate.13 As we tried to decrease the loading of 2-ethylthiophene or catalyst, the yield of this transformation showed slightly decrease (Table 1, Entry 4-5). Other common oxidants, such as tert-butyl hydroperoxide, diacetoxyiodobenzene, sodium periodate and potassium persulfate couldn’t promote this reaction (Table 1, entries 6-9). Control experiments showed that no desired product could be detected without DDQ (Table 1, Entry 10). Besides, only a small amount of product could be observed in the absence of TBN since DDQ could not be regenerated (Table 1, Entry 11). With the optimized protocols in hand, the substrate Table 2. Substrate scope of alkylheteroarenes a
Scheme 3. Our strategy for DDQ-catalyzed direct C(sp3)–H amination of alkylheteroarenes
R1
N
X
R2
R1
X N
3 mL DCE, r.t., 12 h
Y
1 1.8 eq.
We began to choose 2-ethylthiophene and 4-chloro-1Hpyrazole as the substrates to perform the C–N formation reaction. Fortunately, with the combination of DDQ and
20 mol% DDQ, 20 mol%TBN, air
H N
R2
N Y 3
2 0.3 mmol
O S
S
S
N
N
N
N
N
Table 1. Conditions screening a
N
Cl
Cl
3a 84%
Cl 3c 77%b
3b 84%
OAc
COOMe
S
S N
S
N
N
N
2N
Cl
Entry
1
none
2
benzoquinone instead of DDQ
99
tetra-chloro-benzoquinone instead of DDQ
4
15 mol% DDQ, 15 mol% TBN were used
5
1.5 eq. thiophene was used
6d
0.3 mmol TBHP was used as the oxidant
N.D.
0.3 mmol PhI(OAc)2 was used as the oxidant 0.3 mmol NaIO4 was used as the oxidant
N.D.
0.3 mmol K2S2O8 was used as the oxidant
N.D.
10
no DDQ
N.D.
11
no TBN
15
d
8d 9
d
3e 79%b
Cl
S
S
N
N.D. c
3
7
a
Yield (%)b
Variation from the standard conditions
N
N.D.
Cl 3h 63%b
3g 99%
1
N
2N
Cl
73
S
N
N
N
N 3f 55%b (N1:N2= 3.2:1)
Cl
3d 90%b
1
N 3i 86%b (N1:N2= 1:1)
83 S 1N
O
2N
N
N.D.
Standard conditions: reactions were performed with 1a (0.54 mmol), 2a (0.3 mmol), DDQ (20 mol %, 0.06 mmol), and TBN (20 mol %, 0.06 mmol) in 3 mL 1,2-dichloroethane b with an air balloon for 12 h. GC yields were determined by
O
1N
N
2N
N
N
3j 71%b (N1:N2= 1.2:1)
Cl
3k 96%b (N1:N2= 1:1.5)
3l 64%
N N O N N Cl 3m 41%
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N N
N N
Cl MeO
3n 99%
3o 90%
Cl
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Standard conditions as shown in table 1. Isolated yields b o were shown. 80 C.
scope of this amination method was screened next (Table 2). Firstly, alkyl group in linear, alkenyl-containing, and ester-containing alkylthiophenes could all be transferred into aminated products as shown in Table 2. Amination of linear system such as methyl and electron-donating groups gave the corresponding products with good efficiency, as illustrated by 3b-3d. Alkenyl substituent could be tolerated in this transformation in 79% yield (3e). Notably, the electron-deficient alkylthiophene also afforded the desired product in slightly diminished yield (3f). Methyl- or chloro- substituted alkylthiophenes were reached as well (3g-3h). Additionally, the amination product of 2ethyl-5-methylthiophene showed high selectivity between methyl and ethyl group (3g). Moreover, 2ethylbenzo[b]thiophene and 3-ethylthiophene also could be converted to the products (3i-3j). What’s more, other alkylheteroarenes such as furans and indoles were tolerated with good efficiency (3k-3o). This synthesis pathway could deliver an alternative option to synthesize the important molecules which contain the thiophenes, furans or indoles.
our delight, the current catalytic system was suitable for a wide range of nitrogen sources. Multifarious benzotriazoles were well transferred into the corresponding products with two or three isomers, including different electron-donating or electron-withdrawing substituted benzotriazoles (6a-6l). 5-Methyl-benzotriazole and 5-tertbutylbenzotriazole gave the desired products in good yields (6b-6c). Furthermore, the halide substituted products of benzotriazoles (6d-6f) could provide a handle of further synthetic manipulations and no effect on halogen group could be observed. A series of strong electrondeficient substituted benzotriazoles such as trifluoromethyl, ester, nitro and cyanide groups were performed under the standard conditions with up to 99% yield (6g-6j). Then the presence of α-substituted or di-substituted benzotriazoles were also well-tolerated (6k-6l). It was worth noting that 4-phenyltriazole was suitable in this system, achieving the C(sp3)-H amination product in good yield (6m). What’s more, pyrazol and it’s derivatives also had good reactivities and afforded the desired products in high yields (6n-6s). Table 4. Scope of nitrogen sources a 20 mol% DDQ, 20 mol%TBN, air
H N
By utilizing this protocol, alkylarenes could also provide good efficiencies (Table 3). Moderate to good yields were obtained with ethyl benzene derivatives (5a-5b). 2Ethylnaphthalene and diphenylmethane were given the desired products in good yields (5c-5d). Cyclical substrates could be reached as well such as fluorene and indane (5e-5f).
N
S
Y 2 0.3 mmol
6 1N
1 N
R= F, 6d 89% (N1:N2:N3= 1.7:1.3:1)
1N 2N 3N
R R= Cl, 6e 83% (N1:N2:N3= 1:1:1)
1N
1N
COOMe
3N
6c 75% (N1:N2:N3= 1.6:1.2:1) 1N
N
1N
Cl
2N
N
6k 84% (N1:N2:N3= 5.3:1:5)
N
Cl
6l 79% (N1:N2= 1:1.9) R= H, 6o 50% c
N Ph b
6n 83% a
N
N
6g 97% (N1:N2:N3= 1.3:1.7;1)
1N
CN
3N
6i 99% (N1:N2:N3= 1.7:1.6:1)
1 N 2N 3N
CF3
2N
2N
3
6h 90% (N1:N2:N3= 1:1.1:1.1)
Consequently, nitrogen-containing heteroarenes were also explored for the amination reactions (Table 4). To
NO2
2N
Ph
3N
3 R= Br, 6f 99% (N1:N2:N3= 1.3:1:1)
2N
Bu
2N
3N 6b 70% (N1:N2:N3= 1.6:1.3:1)
N
t
1N
2N
2N
Table 3. Substrate scope of alkylarenes a
Standard conditions as shown in table 1. Isolated yields b o c were shown. 80 C. Reaction was performed with 4 (0.9 mmol), 2 (0.3 mmol), DDQ (30 mol %, 0.09 mmol), and TBN (30 mol %, 0.09 mmol) in 3 mL 1,2-dichloroethane with an o air-balloon at 100 C for 24 h.
N N Y
1a 1.8 eq.
6a 86% (N1:N2= 1:1.7)
a
S
3 mL DCE, r.t., 12 h
N
R= Br, 6p 90% R= I, 6q 80%
R
R= NO2, 6r 80%
6j 99% (N1:N2:N3= 1.4:1.3:1)
1 N 2N N 3
Ph 6m 69% (N1:N2:N3= 3.3:2.6:1) 1 2N
N I
6s 62% (N1:N2= 2:1)
Standard conditions as shown in table 1. Isolated yields were shown. The ratio of the isomer was determined by NMR. b Reaction was performed with 1a (0.6 mmol), 2 (0.3 mmol), DDQ (30 mol %, 0.09 mmol), and TBN (30 mol %, 0.09 mmol) in 3 mL 1,2-dichloroethane with an air-balloon for 24 c h. Reactions were performed with a solution of the 1a (0.54 mmol), DDQ (20 mol%, 0.06 mmol) and tert-butyl nitrite (20 mol%, 0.06 mmol) in 3 mL 1,2-dichloroethane with an airballoon for 12 h. Pyrazole (0.3 mmol) was dissolved in 0.2 ml
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ACS Catalysis DCE, then the solution was added to the reaction system for four times when the reaction performed for 0, 2, 5, 8 h.
To further explore the utilities of this method for constructing aminated alkylthiophene, we tried to expand the reaction to gram scale. Reaction selectivity and yield could still be well-obtained (Figure 1). Obviously, this DDQ catalyzed oxidative system showed a great potential according to the demand for metal-free, atom-economical and sustainable chemistry.
Figure 1. Gram scale reaction Figure 2. Mechanism studies for DDQ-catalyzed direct C(sp3)–H amination of alkylheteroarenes, (a) radical-inhibiting experiments; (b) nucleophile trapping experiments. a 0.225
-3
-1 -1 per (mol L min )
0.200
Initial rate x 10
As demonstrated, aminated alkylthiophenes, as synthesized by the catalytic method reported herein, was utilized in the synthesis of glucagon receptor antagonists (Scheme 4). The coupling of compound A with B in presence of 30 mol% of DDQ and TBN produced aminated alkylthiophene C in 85% yield. Then compound C underwent acetylation with nBuLi and DMF to give the acetylbiheterocyclic compound D. A two-step manipulation involving the oxidation and esteration furnished compound E, a precursor of glucagon receptor antagonists in good yield.
0.175 0.150 0.125 0.100 0.075 0.050 0.025 0.000 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
-1
[2-Ethylthiophene] per mol L b 0.40 0.35 -1
Initial rate x 10 per (mol L min )
-1
0.30 0.25 0.20
-3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Scheme 4. The synthesis of Glucagon Receptor Antagonists
0.15 0.10 0.05 0.00 0.00
In addition, to get insight into the reaction process, the radical-inhibition studies were carried out. By adding 2 equiv. of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) or 2,6-di-tert-butyl-4-methylphenol (BHT) as the radical scavengers, the reaction processes were completely suppressed, which revealed that a radical process was involved in this reaction (Figure 2a). Moreover, methanol and acetic acid were introduced to trap the in-situ generated alkyl cation. As a result, the alkoxide-bound adducts were obtained, indicating that this transformation proceeded through a cation procedure (Figure 2b). Furthermore, kinetic studies of this reaction by detecting the initial reaction rate with different loading of 2ethylthiophene and DDQ demonstrated that the substrate and DDQ both followed the first-order dependencies (Figure 3).
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
-1
[DDQ] per mol L
Figure 3. Kinetic plots of the reactions with different concentrations of (a) 2-ethylthiophene 1a and (b) DDQ catalyst. In conclusion, we have developed a direct C(sp3)–H amination of alkylheteroarenes by introducing a catalytic amount of DDQ and electron mediator TBN under aerobic conditions. Broad alkylheteroarenes could be selectively transformed into biheteroarenes in excellent yields, with H2O as the only byproduct. Utilizing the aerobic and metal-free strategy, a precursor of glucagon receptor antagonists was able to be synthesized with good efficiency. Additionally, in comparison with previous reports, this protocol can successfully diminish the usage of metals and equivalent oxidants.
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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ***. General information, experimental procedure, mechanism studies, detailed descriptions for products, copies of product, references and NMR spectra (PDF).
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. *E-mail:
[email protected].
ORCID Aiwen Lei: 0000-0001-8417-3061 Chien-Wei Chiang: 0000-0001-9399-8284
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21390402, 21520102003). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated.
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