C–N Coupling of Azoles or Imides with Carbocations Generated by Electrochemical Oxidation

Article abstract of DOI:10.1002/ejoc.201900714

An approach for electrochemistry-enabled intermolecular oxidative C–N coupling reactions of azoles and imides with 1,3,5-cycloheptatriene and xanthenes is disclosed. The reaction proceeds via the anodic oxidation of 1,3,5-cycloheptatriene or xanthenes to

Full text of DOI:10.1002/ejoc.201900714

A Journal of
Accepted Article
Title: C–N Bond Couplings of Azoles and Imides with Tropylium and
Xanthene Cations Generated by Electrochemical Oxidation
Authors: Xiaoqing Shao, Lifang Tian, and Yahui Wang
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900714
Link to VoR: http://dx.doi.org/10.1002/ejoc.201900714
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10.1002/ejoc.201900714
European Journal of Organic Chemistry
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C–N Bond Couplings of Azoles and Imides with Tropylium and
Xanthene Cations Generated by Electrochemical Oxidation
Xiaoqing Shao+, Lifang Tian+, and Yahui Wang*
Dedicated to Professor Antonio M. Echavarren on the occasion of his 64th birthday
Abstract:
An
approach
for
electrochemistry-enabled
electrochemical oxidation,[8-10] that proceed via tropylium and
xanthene cations as key intermediates (Scheme 1c).
intermolecular oxidative C–N coupling reactions of azoles and
imides with 1,3,5-cycloheptatriene and xanthenes is disclosed.
The reaction proceeds via the anodic oxidation of 1,3,5-
cycloheptatriene or xanthenes to generate the tropylium ion or
xanthene cations, followed by a nucleophilic attack of azoles and
imides to form C–N bond.
Introduction
Cross dehydrogenative couplings (CDC) of C–H/N–H are one of
the most efficient C–N bond formation reactions in organic
chemistry,[1] avoiding the prefunctionalization of substrates.
Electrochemical oxidation[2] has been introduced as an elegant
alternative to external chemical oxidants used in the formation of
C–N bonds.[3] Mechanistically, these anodic C–N bond
constructions occur by the generation of a nitrogen-centered
radical, which is subsequently trapped by a carbon-based
substrate to form a C–N bond. These reactions perform well when
using intramolecular trapping strategies, normally employing
unsaturated C–C bonds, thus delivering five or six-membered
nitrogen-containing heterocycles (Scheme 1a).[4] In contrast,
carbocation intermediates are seldom involved in C–N bond
formation reaction in electrochemical transformations.[3h,3i]
Scheme 1. Cross-dehydrogenative couplings of C–H/N–H bond by
electrochemical oxidation.
Usually, carbocations generated at the anode are very reactive
species, referred to as ‘hot carbocations’.[5] They easily undergo
reactions with mildly nucleophilic solvents (e.g., CH3CN, MeOH,
etc.), rearrangement, or elimination to form alkenes. Accordingly,
in order to facilitate the formation of C–N bonds, the carbocation
intermediates should be well stabilized and long-lived.[6] For
example, α-heteroatoms, such as N, O and S, are generally
required to stabilize the carbocations generated by
electrochemical oxidation (Scheme 1b).[2l] Therefore, the strategy
of forming C–N bond from carbocation intermediates generated
by electrochemical oxidation is still under development. Herein,
we report an intermolecular oxidative C–N coupling reactions of
azoles and imides with 1,3,5-cycloheptatriene[7] and xanthenes by
We decided to choose 1,3,5-cycloheptatriene/xanthene and
azoles/imides to exemplify the feasibility of this new strategy due
to their natural merits: 1) by either chemical or electrochemical
methods, 1,3,5-cycloheptatriene is known to be easily oxidized
into cycloheptatrienyl cation,[11] tropylium, which is aromatic,
stable and can react with many nucleophiles to form a covalent
bond; 2) xanthenes are important motifs and are reported to be
oxidized under electrochemical conditions to form stabilized
diarylcarbenium ions;[10] 3) azoles and imides are important
moieties in organic chemistry[12,13] and normally serve as good
nucleophiles.
Results and Discussion
[*]
X. Shao, Dr. L. Tian, Prof. Dr. Y. Wang
Institute of Advanced Synthesis (IAS), School of Chemistry and
Molecular Engineering (SCME), Jiangsu National Synergetic
Innovation Center for Advanced Materials (SICAM), Nanjing Tech
University
30 South Puzhu Road, Nanjing, 211816 (China)
E-mail: ias_yhwang@njtech.edu.cn
Guided by this hypothesis, we investigated the model reaction of
1,3,5-cycloheptatriene with phthalimide. As shown in Table 1, with
tetra-nbutylammonium tetrafluoroborate as supporting electrolyte,
acetonitrile as solvent, graphite (+) / nickel (–) as electrodes, the
desired product (3a) was obtained in 30% yield (entry 1). Tetra-
nbutylammonium perchlorate was found to be the most effective
supporting electrolyte (entry 4). When Pt (+) / Pt (–) were used as
electrodes, the yield decreased to 30% (entry 5). With HOAc,
[+]
These authors contributed equally to this work.
Supporting information for this article can be found under:
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10.1002/ejoc.201900714
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COMMUNICATION
NaOAc or ferrocene as additives, the yield was not improved
(entries 6-8). We found the duration of electrolysis is a critical
factor (entry 9, 10). At 1.7 hours, 89% yield was observed, while
after 1.9 hours, the yield dropped to 40%. This is attributed to the
further oxidation of the product, a derivative of cycloheptatriene,
after depletion of starting 1,3,5-cycloheptatriene. In addition, all
these reactions were conducted under air, and no obvious
difference in yield was observed under a nitrogen atmosphere.
3d and 3e, leaving the untouched bromo which can be potentially
further functionalized. Similarly, 1,2,4-triazole also coupled well
with 1,3,5-cycloheptatriene during electrolysis, providing 3f in
80% yield. Benzotriazole reacted well under standard conditions,
albeit, a N1 and N2 (8:1, thermodynamic controlled) mixture of
substituted cycloheptatrienes were obtained after purification due
to the fast, reversible dissociation of 3g at room temperature.[15]
Table 1. Optimization of reaction conditions for generating 3a.
Entry[a]
Supporting
electrolyte
Additive
Time (h)
Yield (%)[b]
1
2
nBu4NBF4
nEt4NPF6
-
1 h
(1.3 F/mol)
30
37
55
70
30
31
30
20
89
40
-
1 h
(1.3 F/mol)
3
nBu4NPF6
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
nBu4NClO4
-
1 h
(1.3 F/mol)
4
-
1 h
(1.3 F/mol)
Scheme 2. Scope of the C-N bond formation of 1,3,5-cycloheptatriene with
imides and azoles by electrochemical oxidation. Reaction conditions: 1 (0.3
mmol), 2a (0.6 mmol), nBu4NClO4 (0.025 M), 8 mL MeCN, 23 °C, under air,
constant current (7 mA), 3 h. Isolated yields after column chromatography were
reported. [a] The reaction was performed for 1.5 h.
5[c]
-
1 h
(1.3 F/mol)
6
HOAc[d]
1 h
(1.3 F/mol)
Xanthene derivatives are an important class of organic
compounds and known to be oxidized to form stabilized
cations.[10] We assumed xanthene derivatives could be applied to
react with imides and azoles under electrochemical conditions to
further demonstrate our strategy. As expected, the desired C-N
coupled products were produced in good to excellent yields. As
shown in Scheme 3, not only imides were good N sources (5a,
5b), but also azoles, such as pyrazoles (5c, 5d), 1H-indazole (5e),
benzimidazole (5f), triazoles (5g-5i) reacted well with xanthene
under standard conditions. Furthermore, 9H-thioxanthene also
worked with imides and azoles to provide 5j-5m in good yield
under slightly modified conditions, in which 1.2 mL of CH2Cl2 was
added as a co-solvent to dissolve 9H-thioxanthene. Benzyl
acridine-10(9H)-carboxylate reacted smoothly under standard
conditions to give 5n-5p in good yield.
7
NaOAc[e]
1 h
(1.3 F/mol)
8
Ferrocene[f]
1 h
(1.3 F/mol)
9
-
-
1.7 h
(2.2 F/mol)
10
1.9 h
(2.5 F/mol)
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), supporting
electrolyte (0.025 M), 8 mL MeCN, 23 °C, under air. [b] Yield determined
by 1H NMR analysis of the crude reaction mixture using 1,3,5-
trimethoxybenzene as an internal standard. [c] Pt (+) / Pt (–) were used as
electrodes. [d] 0.4 mmol HOAc. [e] 0.4 mmol NaOAc, MeCN/H2O = 8 mL:
1mL. [f] 5 mol%.
With the efficient conditions for obtaining product 3a established,
we explored the substrate scope of this transformation (Scheme
2). Phthalimide, succinimide and maleimide were all found to
effectively react with 1,3,5-cycloheptatriene under standard
conditions to give the desired C-N coupling products (3a-3c),
although 3c was obtained in slightly lower yield probably due to
the polymerization of maleimide during electrolysis.[14] 4-
Bromopyrazole and benzimidazole also worked smoothly to give
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10.1002/ejoc.201900714
European Journal of Organic Chemistry
COMMUNICATION
carbocations. Based on these experimental results and the
literature support, we believe 1,3,5-cycloheptatriene and
xanthene are most likely been oxidized to their respective cations
at the anode of our electrochemical reactions.
For benzotriazole (see Figure 1 and S4 ), succinimide (see Figure
1 and S5) and phthalimide (see Figure 1 and S6), several obvious
reduction peaks were seen from -1.5 V to -3.3 V, with one
exception of a weak oxidation peak of benzotriazole at 1.7 V with
its small peak current (only 8% of oxidation peak current of
cycloheptatriene). Furthermore, in order to obtain more redox
information for the electrochemical reactions, we also conducted
the CV experiments of the mixture solutions of cycloheptatriene /
xanthene and imines (see Figure S7-11) in acetonitrile. The
recorded CV curves were mostly consistent with the combination
of their individual spectrums. These results also pointed out that
in these mixture solutions, the cycloheptatriene / xanthene will be
oxidized during the electrolysis process.
0.0006
0.0004
0.0002
0.0000
-0.0002
1-Background
2-Cycloheptatriene
3-Xanthene
-0.0004
4-Benzotriazole
5-Succinimide
6-Phthalimide
-0.0006
-0.0008
-0.0010
Scheme 3. Scope of the C-N bond formation of xanthenes with imides and
azoles by electrochemical oxidation. Reaction conditions: 1 (0.3 mmol), 4 (0.45
mmol), nBu4NClO4 (0.025 M), 8 mL MeCN, 23 °C, under air, constant current (7
mA), 3 h. Isolated yields after column chromatography were reported. [a] The
reaction was performed for 1.5 h. [b] CH2Cl2 (1.2 mL) was added as a co-solvent,
9H-thioxanthene (0.9 mmol). [c] Benzyl acridine-10(9H)-carboxylate (0.9 mmol).
-5
-4
-3
-2
-1
0
1
2
3
4
Potential / V (vs Ag/AgNO3)
Figure 1. Cyclic voltammograms recorded in 0.025 M Bu4NClO4-MeCN solution:
scan rate: 50 mV s-1; starting potential: 0V; glass carbon (3 mm diameter, Working
Electrode); platinum plate (Counter Electrode); Ag/AgNO3 (0.01 M AgNO3 in MeCN,
Reference Electrode)
In order to learn the oxidation / reduction potentials for the
substrates, the cyclic voltammetry (CV) experiments of 1,3,5-
cycloheptatriene, xanthene, benzotriazole and imides were
carried out in acetonitrile shown in Figure 1 (for details, see the
SI). For cycloheptatriene, its electrochemical behavior in
acetonitrile was investigated by Abraham[11d] with the aid of CV,
experiments at the rotating ring disk electrode (RRDE) and
controlled potential electrolysis. They proposed a mechanism of
the electrochemical oxidation proceeds through cycloheptatriene
radical cation dimerization: the dimer cations decompose under
deprotonation into one equivalent each of the tropylium cation and
the starting compound cycloheptatriene. In our CV experiment of
cycloheptatriene, an obvious oxidation potential peak was
observed at 1.3 V (see Figure 1 and S2). We attribute it to the
oxidation of cycloheptatriene in agreement with Abraham’s
precedent. For xanthene, three obvious oxidation peaks at 1.4 V,
1.7 V and 2.1 V were observed in acetonitrile (see Figure 1 and
S3). The Yoshida group has investigated the electrochemical
oxidation of diarylmethanes including xanthene.[10] They found the
carbocations can be trapped in situ by DMSO during the
electrochemical process, thus demonstrating the presence of
During the preparation of this manuscript, Li reported an
intermolecular anodic oxidative C-N bond formation of xanthenes
with azoles in the presence of 20 mol% of MsOH as an effective
promoter.[9b] Regarding the reaction mechanism, they found that
by adding radical trapping agents, either 2,6-di-tert-butyl-4-
methylphenol (BHT) or hydroquinone, the desired C-N coupling
was fully suppressed. Therefore, they proposed the process
occurred in the following pathway: alkyl and nitrogen radicals are
both generated by anodic oxidation of the xanthene and azole,
respectively, which is followed by a radical−radical combination to
give the final product. However, the radical-trapping experiments
do not unambiguously rule out alternative mechanistic pathways.
These results only demonstrated that a radical, serving as a key
intermediate, is generated in the reaction process. However, it
may not be this radical intermediate that directly precedes the final
product.
To obtain more solid experimental information, we designed
control experiments using H-type divided cell separated by an
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10.1002/ejoc.201900714
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AMI-7001 membrane (Table 2). The reactions were conducted
under the standard conditions, and both the divided anodic
chamber and cathodic chamber contained the same solution,
including substrates, solvent and supporting electrolyte. If the
reactive intermediates are generated at the anode, the final
product should be obtained in the anodic chamber and should
match the product obtained in an undivided cell. Indeed, in the
case of benzotriazole (1g) and xanthene (4a), the desired product
5i was produced in 22% yield in the anodic chamber, although the
efficiency of electrolysis is lower than that in the undivided cell. In
contrast, 5i was not detected in the cathodic chamber (entry 1,
Table 2). Similar results were obtained in the case of 1H-indazole
(1j) and xanthene (entry 2, Table 2). These experiments
demonstrate that the reaction of azoles with xanthene occur at the
anode. Furthermore, when commercially available tropylium
tetrafluoroborate (6a) was treated with benzotriazole (1g), the
desired product 3g was formed in 92% yield (entry 3, Table 2),
which is almost identical with the yield under electrolysis shown
in Scheme 2. This is consistent with our mechanistic hypothesis
that 1,3,5-cycloheptatriene is oxidized at the anode to tropylium
ion, which is then attacked by benzotriazole to form the C–N
coupling product.
[a] Reaction conditions: C(+)/Ni(-), 0.025 M nBu4NClO4, MeCN, 23 °C, under air.
[b] Yield determined by 1H NMR analysis of the crude reaction mixture using
1,4-dimethoxybenzene as an internal standard. [c] Each chamber contained 1g
(0.6 mmol), 4a (0.9 mmol), 16 mL MeCN, 7mA, 3.4 h. [d] Each chamber
contained 1j (0.6 mmol), 4a (0.9 mmol), 16 mL MeCN, 7mA, 3.2 h. [e] 1g (0.2
mmol), 6a (0.6 mmol), 8 mL MeCN, no electricity, 1 h. [f] Not detected.
Table 3. Comparing experiments of imides using H-type divided cell.
We also conducted these control experiments using imides
reactants combined with 1,3,5-cycloheptatriene or xanthene
(Table 3). From the divided cell experiments of phthalimide (1a)
and 1,3,5-cycloheptatriene (2a), the desired product 3a was
obtained only in trace amount in both chambers (entry 1). In case
of succinimide (1b) and xanthene (4a), no product was detected
in both chambers. In addition, when phthalimide was treated with
tropylium tetrafluoroborate (6a), only 21% yield was obtained,
which is much lower than the yield (89% by NMR) under
electrolysis. In case of succinimide (1b), the desired product 3b
was not detected at all. Moreover, we found that product 3a was
obtained in 99% yield by mixing potassium phthalimide and
tropylium tetrafluoroborate in MeCN at room temperature. These
experimental results show that imides behave differently to azoles
in the reactions with 1,3,5-cycloheptatriene and xanthene under
electrochemical conditions. Based on the reduction peaks of
phthalimide’s CV (Figure 1), we think the more nucleophilic anion
is likely involved. The phthalimide anion may be generated by
either a direct cathodic reduction of phthalimide or deprotonation
by
a
base, which would be produced via cathodic
reduction.[3h,3i,13b,13c,16] Therefore, both electrodes are involved for
the reaction of imides with 1,3,5-cycloheptatriene and xanthene.
Table 2. Comparing experiments of azoles using H-type divided cell.
[a] Reaction conditions: C(+)/Ni(-), 0.025 M nBu4NClO4, MeCN, 23 °C, under air.
[b] Yield determined by 1H NMR analysis of the crude reaction mixture using
1,4-dimethoxybenzene as an internal standard. [c] Each chamber contained 1a
(0.6 mmol), 2a (1.8 mmol), 24 mL MeCN, 7mA, 3.4 h. [d] Each chamber
contained 1b (0.6 mmol), 4a (0.9 mmol), 16 mL MeCN, 7mA, 1.2 h. [e] 1a (0.2
mmol), 6a (0.6 mmol), 8 mL MeCN, no electricity, 1 h. [f] 1b (0.2 mmol), 6a (0.6
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1 h. [i] Not detected.
Based on these experimental results and the fact that xanthene /
cycloheptatriene or their radicals can be easily oxidized to
generate stabilized carbocations,[8a,10,11d] we believe the
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N bond forming reactions. A similar mechanism has also been
proposed in Zeng’s work on electrochemical C-N cross-coupling
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A
representative pathway of C-N bond formation of cycloheptatriene
and xanthenes with imides and azoles is shown in Scheme 4.
[3]
Scheme 4. Proposed mechanism for electrochemical C-N coupling.
Conclusions
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In summary, we report an electrochemistry-enabled
intermolecular oxidative C-N coupling reactions of azoles and
imides with 1,3,5-cycloheptatriene and xanthenes. Mechanistic
studies demonstrated the tropylium ion and xanthene cations are
generated by anodic oxidation and serve as the key intermediates
to form the C–N bond. These cationic intermediates are highly
electrophilic and are attacked directly by azoles to deliver the C–
N coupled products. In the case of imides, due to their low
nucleophilicity, cathodic reduction of imides to their anions is
proposed to be involved.
[5]
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applications in stimuli-responsive organic dyes: f) D. Josephine, M.
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Acknowledgements
We acknowledge financial support by the National Natural
Science Foundation of China (21702105), Natural Science
Foundation of Jiangsu Province, China (BK20170981), Nanjing
Tech University and SICAM Fellowship by Jiangsu National
Synergetic Innovation Center for Advanced Materials. We thank
Prof. Echavarren in ICIQ for valuable discussions and Prof.
McGonigal in Durham University for proof reading.
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Keywords: electrochemistry • carbon-nitrogen bond • anode •
oxidation • synthesis
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10.1002/ejoc.201900714
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10.1002/ejoc.201900714
European Journal of Organic Chemistry
COMMUNICATION
Layout 2:
Key Topic: Electrochemical Oxidation
COMMUNICATION
X. Shao, L. Tian, Y. Wang*
Page No. – Page No.
C–N Bond Couplings of Azoles and
Imides with Tropylium and Xanthene
Cations Generated by
Electrochemical Oxidation
1,3,5-Cycloheptatriene and xanthenes undergo anodic oxidation to generate the
tropylium ion and xanthene cations, followed by a nucleophilic attack of azoles and
imides to form C–N bonds.
This article is protected by copyright. All rights reserved.