Cathode Material Determines Product Selectivity for Electrochemical C?H Functionalization of Biaryl Ketoximes

Article abstract of DOI:10.1002/anie.201809679

The synthesis of polycyclic N-heteroaromatic compounds and their corresponding N-oxides has been developed through electrochemical C?H functionalization of biaryl ketoximes. The oxime substrates undergo dehydrogenative cyclization when a Pt cathode is used, resulting in unprecedented access to a wide range of N-heteroaromatic N-oxides. The products of the electrosynthesis are switched to the deoxygenated N-heteroaromatics by employing a Pb cathode through sequential anode-promoted dehydrogenative cyclization and cathodic cleavage of the N?O bond in the initially formed N-oxide.

Full text of DOI:10.1002/anie.201809679

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Accepted Article
Title: Cathode Material Determines Product Selectivity for
Electrochemical C–H Functionalization of Biaryl Ketoximes
Authors: Huai-Bo Zhao, Pin Xu, Jinshuai Song, and Hai-Chao Xu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201809679
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10.1002/anie.201809679
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Cathode Material Determines Product Selectivity for
Electrochemical C–H Functionalization of Biaryl Ketoximes
Huai-Bo Zhao,† Pin Xu,† Jinshuai Song, Hai-Chao Xu*
Abstract: The synthesis of polycyclic N-heteroaromatic compounds
and their corresponding N-oxides has been developed through
electrochemical C–H functionalization of biaryl ketoximes. The oxime
substrates undergo dehydrogenative cyclization when a Pt cathode
is used, resulting in an unprecedented access to a wide range of N-
heteroaromatic N-oxides. The products of the electrosynthesis are
switched to the deoxygenated N-heteroaromatics by employing a Pb
cathode via sequential anode-promoted dehydrogenative cyclization
and cathodic cleavage of the N–O bond in the initially formed N-
oxide.
anodic oxidation and cathodic H2 evolution.[10,11] Although these
electrochemical methods are certainly of great synthetic value,
in most cases only one of the two electrochemical half-reactions
generates a product of interest. In contrast, paired electrolysis
that results in product formation on both electrodes would be
more energy efficient and industrially economical. Examples in
this direction are available but remain rare.[12] As excellent
examples which are also relevant to our current study, Shono
and more recently Waldvogel have reported the electrochemical
synthesis of aryl nitriles from aldoximes through sequential
anodic oxidation and cathodic reduction.[13]
Given the prevalence of N-doped polycyclic aromatic
structures in natural products and bioactive compounds,[14] we
report herein the synthesis of polycyclic N-heteroaromatics and
their corresponding N-oxides via iminoxyl radical-mediated
electrochemical C–H functionalization of biaryl ketoximes
(Scheme 1c). An interesting and useful finding in our study is
that the choice of cathode material played a key role in
determining the reaction outcome. Specifically, electrolysis of
the biaryl ketoxime using a Pt-based cathode resulted in its
dehydrogenative cyclization and furnished an N-heteroaromatic
The development of efficient and sustainable C–N bond forming
reactions has been a long-standing interest in the field of organic
synthesis due to the importance of nitrogen-containing organic
molecules for chemistry, biology and materials science.[1]
Recently, radical-mediated C–N bond construction has been
gaining traction because the relatively high reactivity of the
open-shell species provides opportunities for the development of
transformations that are difficult using the two-electron
manifold.[2] In this context, many research groups have
employed the easily available oximes and their derivatives as
radical precursors in radical C–N coupling reactions.[3] The
cleavage of the O–H bond in oxime produces a σ-type iminoxyl
radical with both a reactive oxygen and nitrogen atom.[4] Han
and coworkers have demonstrated that iminoxyl radicals can
cyclize with alkenes or alkynes to form 5-membered N-
heterocyclic compounds (Scheme 1a).[5] However, to the best of
our knowledge, similar cyclization reactions with the less
reactive arenes have only been mentioned in mechanistic
proposals.[6] On the other hand, the thermal or photochemical
cleavage of the relatively weak N–O bond in oxime ethers and
oxime esters generates an iminyl radical,[3,7] which is reactive
toward various π-systems including arenes to afford
phenanthridines and some other N-heteroaromatics (Scheme
1b).[8] Although these oxime derivatives are easily prepared, their
usage reduces the step and atom economy of the synthetic
method.
N-oxide.
In
contrast,
sequential
dehydrogenative
cyclization/cathodic deoxygenation occurred when a Pb cathode
was employed, leading to the formation of a deoxygenated N-
heteroaromatic compound.
The ever-increasing demand for safe and sustainable
synthetic methods has fueled a rapid growth of organic
electrochemistry, which employs traceless electrons as redox
reagents.[9] We and a few other research groups have achieved
oxidative C–N bond formation facilitated by the concurrence of
Scheme 1. Cyclization reactions of oximes and oxime derivatives.
[*]
H.-B. Zhao, P. Xu, Prof. Dr. H.-C. Xu
State Key Laboratory of Physical Chemistry of Solid Surfaces,
Innovative Collaboration Center of Chemistry for Energy Materials,
Key Laboratory of Chemical Biology of Fujian Province, and College
of Chemistry and Chemical Engineering, Xiamen University
Xiamen 361005 (P. R. China)
E-mail: haichao.xu@xmu.edu.cn
Web: http://chem.xmu.edu.cn/groupweb/hcxu/
Dr. J. Song
Our first task was to identify optimal conditions for the
cyclization of 1 (Tables 1, S1 and S2). Initial screening showed
that the best reaction system consisted of TEMPO (5 mol %)[15]
and KOH (0.5 equiv) in a mixed solvent of MeCN/H2O (1:1).
Notably, KOH fulfilled a dual role of base and supporting
electrolyte, which eliminated the need for additional salt. Under
the optimal conditions, the desired N-oxide 2 was obtained in 95%
yield after the consumption of 2.2 F mol−1 of charge (entry 1).
The electrosynthesis of 2 could also be conducted using a
Fujian Institute of Research on Structure of Matter
Chinese Academy of Sciences, Fuzhou 350002 (P.R. China)
These authors contributed equally to this work.
[†]
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201809679
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commercially available setup ElectroSyn 2.0 (entry 2). The yield
of 2 decreased dramatically in the absence of TEMPO (entry 3)
or KOH (entry 4). Interestingly, replacing the Pt cathode with
graphite (entry 5), stainless steel (entry 6), red brass (entry 7) or
Pb[13b] (entry 8) led to the formation of a mixture of 2 and the
corresponding deoxygenated product 3. Further optimization
using Pb cathode showed that the yield of 3 could be boosted to
95% by increasing the charge consumption to 5.5 F mol−1 (entry
9) and the reaction temperature to 80 °C (entry 10).[16] Notably,
the electrolysis of 1 could be conducted on a decagram scale to
afford 2 in 92% yield (entry 11), or 3 in 71% yield (entry 12).
all.[8,11a,17] Replacing Ar1 with another aromatic moiety, such as a
2-naphthalene- or a 3-thiophene substituent, exhibited no
detrimental effect on the regioselectivity of the reaction (25, 26).
A primary alcohol group was unaffected despite that TEMPO
was known to catalyze the electrochemical oxidation of alcohols
(7).[15a,b] The reaction was also broadly compatible with a variety
of functional groups at positions 8 and 9 of aryl ring Ar2,
including OCH2O (27), OMe (28, 32), Me (29, 33), F (30, 34) and
Cl (31, 35). The Me on the C-N double bond of the oxime moiety
could be substituted by an ethyl (36) or n-butyl (37) group.
Finally, phenylpyridine-based substrates were also well tolerated,
leading to the formation of
Table 1. Optimization of reaction conditions.[a]
Entry
Salt
(0.5 equiv)
Cathode
t
Charge
(F
Yield [%][b]
mol−1
)
2
3
1
1
KOH
KOH
Pt
Pt
RT
RT
RT
RT
RT
RT
2.2
2.2
2.2
2.2
2.2
2.2
95[c]
90
14
7
0
0
0
2[d]
3[e]
4
0
KOH
Pt
3
43
86
14
8
Na2SO4
KOH
Pt
0
5
graphite
47
85
29
6
6
KOH
stainless
steel
7
KOH
KOH
KOH
KOH
KOH
KOH
red brass
RT
RT
2.2
2.2
5.5
5.5
2.2
5.2
59
43
19
0
32
5
14
0
8
Pb
Pb
Pb
Pt
27
9
RT
54
10
11
12
80 °C
RT
95[c]
0
2, 92[c] (9.23 g)
3, 71[c] (6.62 g)
Pb
80 °C
[a] Reaction conditions: Undivided cell, 10 mA, RVC anode (j  0.16 mA cm−2),
cathode, 1 (0.3 mmol), H2O (5 mL), MeCN (5 mL), argon. [b] Yield determined
by 1H-NMR analysis using 1,3,5-trimethoxybenzene as the internal standard.
[c] Isolated yield. [d] Reaction using ElectroSyn 2.0. [e] No TEMPO.
Scheme 2. Substrate scope for the dehydrogenative cyclization. Reaction run
with 0.3 mmol of oxime for 1.7–2.7 h (2.1–3.4 F mol−1). [a] Isolated yield,
regioisomeric ratio in parenthesis. [b] Reaction at 80 °C. [c] Na2CO3 as base.
We next investigated the substrate scope of the
electrochemical dehydrogenative cyclization reaction (Scheme
2). The benzene ring Ar1 of the biaryl oxime could be
functionalized with an electronically diverse range of
substituents at the position para or meta to the other phenyl
group Ar2 (4–24). When a meta-substituted substrate was used,
the annulation occurred in a regioselective manner at the site
para to the functional group on Ar1 due to mitigated steric
hindrance (13–18). In contrast, previously reported cyclizations
that proceeded through sterically less hindered iminyl radical
intermediates showed either reduced or no selectivity at
aromatic N-oxide products bearing free pyridyl nitrogen atoms
(38–41). These compounds are expected to be difficult for
traditional methods that rely on the oxidation of preformed N-
heteroaromatic compounds due to chemoselectivity issues. It
should be emphasized that the electrochemical cyclization of
oximes that form N-oxide products with low solubility in
MeCN/H2O (1:1) needed to be conducted at 80 °C to prevent
precipitation on the porous anode and the concomitant electrode
passivation.
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The dehydrative C–H functionalization reactions (Scheme 3)
showed similar substrate scope and regioselectivity as the
dehydrogenative cyclization reactions shown above. Exceptions
included the occurrence of debromination during the formation of
49 and the failure to form the thiophene-containing compound
63.
Scheme 4. Mechanistic studies and proposal.
The electrolysis of 81 bearing an alkene in the presence of 2
equiv of TEMPO led to the generation of 82 and a trace amount
of 83 (Scheme 4a), suggesting that the reaction with the phenyl
group to form a 6-membered ring was much more difficult than
the alternative 5-exo-trig cyclization with the alkene moiety.
These results also lent support to the involvement of an iminoxyl
radical intermediate. In addition, the cyclization of 1 into 2 could
−
also be promoted by using 3 equiv of TEMPO+BF4 , albeit at a
lower yield, but not by TEMPO (Scheme 4b). The yield reduction
was likely caused by the decomposition of the oxidant under the
basic conditions.
Scheme 3. Substrate scope for the dehydrative cyclization. Reaction run with
0.3 mmol of oxime for 4.0–5.2
h (5.0–6.5 F
mol−1). [a] Isolated yield,
regioisomeric ratio in parenthesis. [b] 43% of 46 was also obtained.
A
mechanism for the electrochemical cyclization of
ketoximes was proposed as follows (Scheme 4c). The
electrolysis begins with the anodic oxidation of TEMPO (Ep/2
0.51 V vs SCE) into TEMPO+, which then reacts with 1 (Ep/2
=
=
1.39 V vs SCE) to afford an iminoxyl radical (resonance
structures I and II).[19,20] N-Cyclization of the iminoxyl radical onto
the phenyl ring,[21] followed by re-aromatization, furnishes the N-
oxide 2. When a Pt cathode is employed, H2O is cathodically
reduced to generate H2 and HO−. Alternatively, the use of a Pb
cathode, which has a higher overpotential for H2 evolution,
would favor the formation of 3 due to the cathodic cleavage of
the N–O bond in 2. Since HO− is continuously generated at the
cathode, only a substoichiometric amount of KOH is needed.
In summary, we have demonstrated that the electrolysis of
easily available biaryl ketoximes can lead to the chemo- and
regioselective formation of polycyclic N-heteroaromatic N-oxides
or the corresponding deoxygenated N-heteroaromatics by
employing different cathode materials.
The deoxygenation of N-oxides generally requires Pd-
catalyzed hydrogenation or a stoichiometric amount of
a
chemical reductant.[18] Our electrochemical reaction provides a
highly efficient and clean method for deoxygenating N-oxides.
For example, aromatic N-oxides such as 2 and 79 could be
efficiently deoxygenated in the absence of TEMPO to afford the
desired compounds 3 and 80, respectively [Eq.s (1) and (2)].
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Acknowledgements
Financial
support
of
this
research
from
MOST
(2016YFA0204100), NSFC (21672178) and Fundamental
Research Funds for the Central Universities.
Keywords: electrochemistry • paired electrolysis • radicals •
oximes • heterocycles
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COMMUNICATION
Huai-Bo Zhao, Ping Xu, Jinshuai Song,
Hai-Chao Xu*
Page No. – Page No.
Cathode Material Determines Product
Selectivity for Electrochemical C–H
Functionalization of Biaryl Ketoximes
A divergent synthesis of polycyclic N-heteroaromatics and the corresponding N-
oxides through electrochemical C–H functionalization of biaryl ketoximes has been
developed. The product selectivity is controlled easily by the choice of the cathode
material.
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