Electro-oxidative C(sp3)-H Amination of Azoles via Intermolecular Oxidative C(sp3)-H/N-H Cross-Coupling

Article abstract of DOI:10.1021/acscatal.7b03551

A method for electrooxidative C(sp3)-H amination via intermolecular oxidative C(sp3)-H/N-H cross-coupling has been developed under metal- and oxidant-free conditions. The C(sp3)-H bonds adjacent to oxygen, nitrogen, and sulfur atoms could all react smoothly with various amines to give the corresponding products with moderate to good yields (30-93%). In addition, the C(sp3)-H bonds of benzylic and allylic are also tolerated in this reaction. A preliminary mechanistic study indicates that the C-H cleavage of tetrahydrofuran is probably not involved in the rate-determining step.

Full text of DOI:10.1021/acscatal.7b03551

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Article
Electrooxidative C(sp3)-H Amination of Azoles via
Intermolecular Oxidative C(sp3)-H/N-H Cross-Coupling
Jiwei Wu, Yi Zhou, Yuchen Zhou, Chien-Wei Chiang, and Aiwen Lei
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03551 • Publication Date (Web): 26 Oct 2017
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Jiwei Wu†, Yi Zhou†, Yuchen Zhou†, Chien-Wei Chiang† and Aiwen Lei*†‡
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† The Institute for Advanced Studies (IAS), college of Chemistry and Molecular Sciences, Wuhan University, Wuhan,
430072, P. R. China.
‡ State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,Chinese Acad e-
my of Sciences, Lanzhou 730000, P. R. China.
ABSTRACT: A method for electrooxidative C(sp3)-H amination via intermolecular oxidative C(sp3)-H/N-H cross-coupling has
been developed under metal- and oxidant-free conditions. The C(sp3)-H bonds adjacent to oxygen, nitrogen and sulfur atom could
all react smoothly with various amines to give corresponding products with moderate to good yields (30-93%). In addition, the
C(sp3)-H bonds of benzylic and allylic are also tolerated in this reaction. A preliminary mechanistic study indicates that the C-H
cleavage oftetrahydrofuranis probably notinvolved in the rate-determining step.
KEYWORDS: Electrooxidative, C(sp3)−H Amination, C(sp3)−H/N−H Cross-Coupling, dehydrogenation, External oxidant-free,
Azoles
Nitrogen-containing compounds as an important class of
compounds have been widely found in pharmaceutical agents,
natural products and functional materials.1 Consequently,
seeking efficient strategies for the facile construction of car-
bon-nitrogen bonds has been one of the major research topics
in synthetic chemistry.2 Over the past few years, direct catalyt-
ic C–H amination as one of the most efficient, atom- and step-
economical ways to construct C-N has gained more and more
attention in this field.3 Although, great progress has been made
in direct C(sp2)-H amination,4 the methods fordirect C(sp3)-H
amination5 were still rare and of great challenges due to the
relatively inert properties of C(sp3)−H bonds. In recent years,
our group and other groups have reported some methods for
C(sp3)-H/N-H amination under transition-metal-catalyzed
such as Cu,6 Ni,7 Fe8 and Ru9 etc. or metal-free conditions.10
Despite the advances achieved, some limitations still exist
such as tedious work-up procedures, poor selectivity, harsh
reaction conditionsandthe use ofexcess externaloxidants.
In recent years, electrochemical oxidation11 as a versatile
and environmentally friendly synthetic method has been used
as an alternative tool for construct new chemical bonds via
direct C–H bond activation.12 Recently, electrochemical oxida-
tion direct C-H amination13 has been studied. However, there
is no precedent for electrooxidative inert C(sp3)-H/N-H cross-
coupling.
Although several approaches for construct C(sp3)-N by
transition-metal-catalyzed have been achieved,6-9 there is still a
common problem that metal catalysts are easily coordinated
with amines (Scheme 1, a). As a result,the metal catalysts
Table 1. Optimization of the reactionconditions.a
Entry
Variation from thestandard conditions
Yield of
3a (%)b
92
1
none
2
2.5 h instead of 3 h
82
77
83
74
49
80
41
92
86
79
n.d.
3
LiClO4 instead of nBu4NBF4
nBu4NPF6 instead of nBu4NBF4
60 oC instead of 80 oC
RT instead of 80 oC
4
5
6
7
1 mL 2a instead of 2 mL
8
1equiv. 2a instead of 2 mL
n
9
0.2 mmol Bu4NBF4 instead of 2 mmol
n
10
11
12
0.1 mmol Bu4NBF4 instead of 2 mmol
air instead of N2
no electric current
a Reaction conditions: Pt anode, Pt cathode, constant current = 12 mA,
n
1a (0.3 mmol), 2a (2.0 mL), Bu4NBF4 (2 mmol), in 8.0 mL CH3CN
b
at 80 oC under N2 for 3 h. Yields shown are of isolated products. RT
= room temperature. n.d. = not detected.
Scheme 1.Amine functionalization.
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Table 2. Substrate scope of electrooxidative C(sp3)−H
Table 3. Substrate scope of electrooxidative C(sp3)−H
amination of 1H-benzotriazole with other C(sp3)-H
sources.a
amination of azoles with tetrahydrofuran.a
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a Reaction conditions: Pt anode, Pt cathode, constant current = 12 mA,
1a (0.3 mmol), 2 (10 equiv.), nBu4NBF4 (0.2 mmol), in 10.0 mL
o
CH3CN at 80 C under N2 for 3 h. Yields shown are of isolated prod-
ucts. b 2v (2 equiv.). c 2w (6 equiv.), CH3CN/CF3CH2OH = 8/2 mL.
a Reaction conditions: Pt anode, Pt cathode, constant current = 12 mA,
n
o
1 (0.3 mmol), 2a (2.0 mL), Bu4NBF4 (0.2 mmol), in 8.0 mL CH3CN
product reduced when the temperature decreased to 60 C or
o
b
at 80 C under N2 for 3 h. Yields shown are of isolated products. 4 h.
c 5 h.
room temperature (Table 1, entries 5-6). However, a good
yield of 3a was still observed when equivalent of 2a was used
(Table 1, entries 7-8). Through an investigation of the amount
of electrolyte we found that the yield of the target product
didn’t reduce even the amount of electrolyte decreased to 0.2
mmol (Table 1, entries 9-10). Notably, a 79% yield of 3a was
still observed under atmospheric conditions (Table 1, entry 11).
No desired product could be obtained without an electric cur-
rent in this reaction (Table 1,entry 12).
With the optimal reaction conditions established, reactions
of various azoles (1) combined with tetrahydrofuran (2a) were
explored and results are summarized in table 2. Initially, we
tested the reactivity ofsubstituted 1H-benzotriazole. In general,
both electron-donating and electron-withdrawing substituents
on the benzene ring of 1H-benzotriazole were well-tolerated in
this reaction (Table 2, 3b-3e). 5-Phenyl-2H-tetrazole could
also successfully react with THF to give the desired product in
good yield (Table 2, 3f). For 4-phenyl-1H-1,2,3-triazole, reac-
tions also proceeded smoothly with THF to deliver the desired
products in good to excellent yields, in which p-OMe (3h) and
p-Cl (3i) groups on the benzene ring of 4-phenyl-1H-1,2,3-
triazole could be well tolerated. 3-Phenyl-1H-pyrazoles were
also suitable substrates, although longer reaction time (4-5 h)
was required to give good yields (Table 2, 3j-3l). Notably, 3,5-
diphenyl-1H-pyrazole (Table 2, 3m) proved to be good sub-
strates under the current conditions. In addition, a 73% yield
was obtained by employing 1H-indazole as the reactant (Table
2, 3n).
deactivation that dramatically limit the scope of these reac-
tions. However, electrochemical oxidation can provide con-
venient access to reactive radical species via single electron
transfer and subsequent hydrogen transfer process on the sur-
face of the electrode (Scheme 1, b).12g,13d,14 This provides a
new choice for amine functionalization. Therefore, we envi-
sioned the goal of construction of C(sp3)-N bond via electro-
chemical oxidation process could be achieved. Herein, we
disclose a new strategy for the formation of C-N bond through
electrooxidative dehydrogenative C(sp3)-H/N-H cross cou-
pling undermetal- and oxidant-free conditions.
We initially chose 1H-benzotriazole (1a) and tetrahydrofu-
ran (2a) as the model substrates to test the reaction and select-
ed data are listed in Table 1 and Table S1. To our delight, 1a
and 2a reacted efficiently to afford the desired product (3a) in
92% yield by using a constant current of 12 mA in an undivid-
o
ed three-necked bottle, at 80 C, for 3 h (Table 1, entry 1).
Notably, 4a, which was produced in previous works,9, 10d, 10e
was not observed in our reaction. Encouraged by the excellent
selectivity, we further optimized the reaction conditions. The
yield of 3a reduced when the reaction time was 2.5 h (Table 1,
entry 2). The influence of the electrolyte was also studied.
n
When other electrolytes, such as LiClO4 and Bu4NPF6 were
n
used instead of Bu4NBF4, the yield of desired product was
reduced (Table 1, entries 3-4). Then the reaction temperature
was studied andresultsshowedthatthe yield ofthe target
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Notably, when this reaction run on gram scale, the desired
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product 3a could still be obtained in 64% yield, highlighting
the potentialapplication ofthis method(Scheme 2).
In order to elucidate the reaction mechanism, some addi-
tional experiments were carried out. First, the cyclic voltam-
metry (CV) experiments on both reactants have been carried
out and showed that the oxidation peaks of 1a and 2a in ace-
tonitrile were 2.00 V and 2.54 V (Figure S1.)15 respectively.
Since the operating voltage ranged from 1.90 to 2.23 V, only
1a was possible oxidated under the standard conditions. Se-
cond, radical-trapping experiment was conducted to clarify
whether this reaction involved radical process (Scheme 3, a).
When, 2 equiv. of TEMPO (2, 2, 6, 6-tetramethyl-1-
piperidinyloxy) or BHT (2,4-di-tert-butyl-4-methylphenol)
was added into the reaction system, the desired product 3a was
totally suppressed. Additionally, 5a was observed in 76%
yield. These mechanistic observations implicated that this
reaction probably proceeds by a radical pathway. Then the
kinetic isotopic effect (KIE) was studied. When two parallel
reactions were carried out with tetrahydrofuran and d8-
tetrahydrofuran as the substrates respectively to determine the
kinetic isotopic effect, a kH/kD = 1.07 was obtained (Scheme
3, b). We further did an intermolecular competitive reaction of
tetrahydrofuran and d8-tetrahydrofuran with 1a and a KIE
value of kH/kD = 1.22 was observed (Scheme 3, c)16. These
results suggest that the C−H cleavage of tetrahydrofuran was
probably not involved in the rate-determining step. Last but
not least, H2 gas bubbles were clearly observed and detected
by GC at cathode in the scare-upreaction.
On the basis of the above results and literature re-
ports,12g,13d,14 a tentative mechanismwas proposed as shown in
scheme S117. First, nitrogen radical A was formed by the an-
ode oxidant through single electron transfer and subsequent
hydrogen transfer process. Then nitrogen radical A reacts with
2a to afford the cross-coupling product 3a. Meanwhile, ca-
thodic reduction of proton-hydrogen leads to the formation of
hydrogengas.
In conclusion, we have developed a simple and efficient
method for electrooxidative C(sp3)−H amination of azoles via
intermolecular oxidative C(sp3)−H/N−H cross-coupling under
metal- and oxidant-free conditions. The C(sp3)-H bonds adja-
cent to oxygen, nitrogen and sulfur atomcan all react smooth-
ly with various amines to give corresponding products with
moderate to good yields (30-93%). In addition, the C(sp3)-H
bonds of benzylic and allylic are also tolerated in this reaction.
Notably, this reaction can be run on gram scale, giving the
desired product in 64% yield. The reaction features mild reac-
tion conditions, wide range of substrates, good to excellent
yields and an easy work-up procedure, which demonstrate the
potential application of this protocol in synthetic chemistry.
Detailed mechanistic studies of this methodology are currently
ongoing in ourlaboratory.
Scheme 2.Gram scale synthesis.
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Scheme 3.Additional experiments.
Furthermore, we examined the scope of functionalized ether
substrates and other C(sp3)-H sources 2 using 1a as the cou-
pling partner and the results are listed in table 3. Initially,
functionalized ether substrates such as tetrahydropyran, iso-
chroman and butyl ether were employed to react with 1H-
benzotriazole under standard conditions. Although the amount
of ethers was reduced to 10 equiv., the corresponding products
were obtained in moderate to excellent yields (Table 3, 3o-3q).
Then other C(sp3)-H sources were examined in this reaction.
To our delight, N-Methyl pyrrolidone showed a high reactivity
and 3r as the only product was obtained in 85% yield. Fur-
thermore, the C(sp3)-H bonds adjacent to sulfur could also be
reacted with1H-benzotriazole smoothly (Table 3, 3s). Delight-
fully, 1,4-thioxane reacted smoothly with 1H-benzotriazole to
give the 3t as the only product. Notably, N-formylmorpholine
could also react with the 1H-benzotriazole to form the target
products (Table 3, 3u) in good yields but with poor selectivity.
It is noteworthy to emphasize that the benzylic C(sp3)−H and
allylic C(sp3)-H bonds could be also tolerated in this reaction
give the corresponding products in moderate yields (Table 3,
3v and 3w).
This material is available free of charge via the Internet at
http://pubs.acs.org.” For instructions on what should be included
in the Supporting Information as well as how to prepare this mate-
rial for publication, refer to the journal’s Instructions for Authors.
* corresponding author: aiwenlei@whu.edu.cn.
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The manuscript was written through contributions of all authors.
All authors have given approvalto the final version of the manu-
script.
The authors declare no competing financial interest.
9
This work was supported by the National Natural Science Foun-
dation of China (21390400, 21520102003, 21272180, 21302148),
the Hubei Province Natural Science Foundation of China
(2013CFA081), the Research Fund for the Doctoral Program of
Higher Education of China (20120141130002), and the Ministry
of Science and Technology of China (2012YQ120060). The Pro-
gram of Introducing Talents of Discipline to Universities of China
(111 Program) is also appreciated.
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