C-to N-Center Remote Heteroaryl Migration via Electrochemical Initiation of N Radical by Organic Catalyst

Article abstract of DOI:10.1021/acs.orglett.9b04141

Herein an exogenous oxidant- A nd metal-free electrochemical heteroaryl migration triggered by N radicals to construct new N-C bonds was developed. This methodology features a high atom economy and utilization rate of energy, and it is insensitive to water and air. Moreover, a user-friendly undivided cell was employed. The use of an organic catalyst makes it more efficient, green, and practical.

Full text of DOI:10.1021/acs.orglett.9b04141

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Letter
C- to N‑Center Remote Heteroaryl Migration via Electrochemical
Initiation of N Radical by Organic Catalyst
Chengkou Liu, Qiang Jiang, Yang Lin, Zheng Fang, and Kai Guo*
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ABSTRACT: Herein an exogenous oxidant- and metal-free electro-
chemical heteroaryl migration triggered by N radicals to construct new
N−C bonds was developed. This methodology features a high atom
economy and utilization rate of energy, and it is insensitive to water and
air. Moreover, a user-friendly undivided cell was employed. The use of an
organic catalyst makes it more efficient, green, and practical.
a
Scheme 1. Remote (Hetero)aryl Migrations
he intramolecular migration reaction, a powerful tool for
Tthe construction of unexpected and highly complex
molecular scaffolds with abundant bioactivities, has attracted
increasing attention.1 The classical Smiles rearrangement
supplied a particularly appealing methodology to construct a
new C−C or C-hetero bond via an intramolecular nucleophilic
substitution reaction featuring a key spirocyclic intermediate I
(Scheme 1a,2 I). On the basis of the mechanism of the
spirocyclic intermediate and the resurgence of interests in
radical chemistry, many radical-based rearrangement reactions
have been developed and have gained increased attention.3,4
Radical reactions usually exhibit higher selectivity and better
functional-group compatibility under neutral and mild
conditions.3 Lately, Zhu5 and many other research groups6
have succeeded in the heteroarylation of unactivated alkenes
via heteroaryl ipso-migration with the formation of a carbonyl
group (Scheme 1, b1). The radical addition of a C−C triple
bond or double bond initiates the reactions above. Moreover,
3
Zhu developed a C(sp )−H bond heteroarylation through
heteroaryl migration based on the 1,5-HAT (HAT = hydrogen
atom transfer) mediated by alkoxyl radicals (Scheme 1, b27).
Nevertheless, to the best of our knowledge, the remote
heteroaryl migration initiated by N-centered radical is rare. It
leads to C−N bond formation to construct abundant N-
containing compounds, which are prevalent in marketed drugs,
material industries, and natural products.8
a
(a) Classical Smiles rearrangement, X = S, SO, SO2, O, CO2, YH =
OH, NHR, SH, CH2R, CONHR. (b) Radical (hetero)aryl migration
driven by the formation of carbonyl. (c) Heteroaryl migration via
electrochemical initiation of N-centered radicals by organic catalyst
(this work).
Therefore, the development of novel, sustainable, and
efficient methodologies to assemble unprecedented N-
containing compounds is still in high demand. However,
attributing to the difficulty in homolysis of the N−H bond, the
lack of practical and green radical initiation methods makes N
radical chemistry still underexplored. Traditionally, the N-
centered radical can be obtained under the costly oxidant of
hypervalent iodine reagents9 by the direct homolysis of the N−
Received: November 19, 2019
https://dx.doi.org/10.1021/acs.orglett.9b04141
© XXXX American Chemical Society
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A

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H bond. Furthermore, the homolysis of the N−O,10 N−N,11
N−X,12 or N−S13 smoothly leads to the corresponding N
radical. However, employing the expensive oxidants or the
preactivated amino precursors restricts the wide application of
these methods. Recently, the groundbreaking development of
photoredox catalysis has supplied new strategies for the
generation of the N radical from direct N−H compounds.14
Nevertheless, a complex and costly photocatalyst is necessary
under the photoredox-initiated radical reaction.
a
Table 1. Optimization of Reaction Conditions
b
entry
variation from “standard conditions”
yield (%)
Electrochemistry, an environment-friendly and practical
synthesis tool employing traceless electrons as “reagents” to
activate numerous small molecules by a redox process avoiding
exogenous oxidants, has attracted increasing attention.15
Lately, Xu and have Lei developed several electrochemical
cyclization reactions initiated by N-centered radical via the
homolysis of the N−H bond of amides without any additional
oxidants.16 However, to date, N−C bond formation reactions
promoted by a N-centered radical initiated by electrochemistry
remain underdeveloped due to lack of applicative reaction
types. Herein we reported a remote heteroaryl migration to
access to 2-aminophenyl benzothiazoles via the electro-
chemical initiation of N-centered radicals under metal-free
and oxidant-free conditions with good H2O and air tolerance
(Scheme 1c), and an organic catalyst was employed to give
higher productivity.
1
2
3
4
5
6
7
8
none
93
trace
28
16
27
18
44; 42; 86
42; 47; 51
55; 14; 17
35
0
trace
trace
84
42
with TEMPO as catalyst
with ferrocene as catalyst
with 1,1′-dimethylferrocene as catalyst
with cyanoferrocene as catalyst
with ethylferrocene as catalyst
2-4 as catalyst
TCE or EtOH or HFIP instead of TFE
nBu4NPF6 or Bu4NOAc or Bu4NI as electrolyte
n
n
9
10
11
12
13
14
15
16
17
no electrolyte
Pt (+)/Pt (−)
RVC (+)/Pt (−)
C rod (+)/Pt (−)
C fiber (+)/Pt (−)
carbon cloth (+)/carbon cloth (−)
30 or 50 °C
18; 39
89; 87; 82
c
Initially, N-(2-(1-(benzo[d]thiazol-2-yl)-1-hydroxyethyl)ph-
enyl)-4-methylbenzenesulfonamide 1a was chosen as the
model substrate to explore the reaction conditions for the
envisioned remote heteroaryl migration via the electrochemical
initiation of N-centered radical (Table 1). After considerable
screening of the reaction parameters, the desired migration
product 2a was generated in excellent yield (93%) after the
consumption of 1.990 F/mol of charge in a user-friendly
undivided cell equipped with a carbon cloth anode and a
platinum plate cathode using a mixed solvent of ACN/TFE
((6.5:3.5) mL) under a constant current of 8 mA at 70 °C
(entry 1), and the organic catalyst 1 was essential for this
reaction. In theory, 1.996 F/mol of electric energy is necessary.
Thus this proposal features an extremely high utilization rate of
energy. When TEMPO was used as the catalyst, only a trace
amount of the desired product was detected (entry 2), and
using commercially available ferrocene or substituted ferrocene
led to a dramatic decrease in the yield (entries 3−6).
Interestingly, poor catalytic performance was observed when
10-(substituted phenyl)-10H-phenothiazine was used as the
redox catalyst (entry 7). A dramatic yield reduction was
obtained when TFE was replaced by TCE, EtOH, or HFIP
2 V; 3 V; 4 V
Control Experiments
no current
no catalyst
18
19
0
d
35, 87
a
Reaction conditions: 1a (0.3 mmol), catalyst (0.015 mmol, 5 mol
n
%), Bu4NBF4 (0.3 mmol), solvent (ACN/TFE = 6.5 mL/3.5 mL),
70 °C, 2 h, 1.990 F/mol, undivided cell, carbon cloth anode (40 mm
× 20 mm), platinum plate cathode (20 mm × 20 mm), constant
current = 8 mA. ACN: acetonitrile, TFE: 2,2,2-trifluoroethanol,
TEMPO: 2,2,6,6-tetramethylpiperidino-oxy, TCE: trichloroethanol,
b
c
d
HFIP: 1,1,1,3,3,3-hexafluoro-2-propanol. Isolated yield. 1.5 h. 3.5
h.
the electrolysis time to 3.5 h with 3.482 F/mol electricity
consumed (entry 19).
With the optimized electrochemical remote heteroaryl
migration to construct a new C−N bond obtained, a series
of experiments were carried out to investigate the reaction
scope by varying R1, HetAr, and R2 (Scheme 2). To our
delight, an excellent yield could be obtained when the phenyl
ring A was substituted with an electron-rich or an electron-
deficient group (2a−l).
This methodology was compatible with halogens including
F, Cl, Br, and I, which could be used to further the
functionalization (2d−2g, 2j, 2k), and the structure of 2f
was confirmed by X-ray crystallographic analysis (Figure S3,
CCDC = 1955548). Moreover, this reaction did not seem to
be affected by the positions of the substituents on the A ring
(2e, 2j, 2k). When A rings were replaced by heteroaryl or alkyl
groups, no detrimental effect on the yield was observed (2m−
q). Although electron-deficient R1 led to lower reactivity,
complete substrate conversion could be achieved by extending
the electrolysis time, with good yield obtained (2i, 2m, 2o).
Excellent yields were produced when the benzothiazole ring
was replaced by 6-methoxy-1,3-benzothiazole, thiazole, or
imidazole (2r−t). Furthermore, replacing R2 with a linear
alkyl, unsubstituted, or substituted phenyl group did not blunt
this reaction (2u−w). To our delight, 81% isolated yield was
n
(entry 8), and Bu4NBF4 was the best choice compared with
nBu4NPF6, nBu4NOAc, and nBu4NI (entry 9). Only 35% of the
desired product was obtained in the absence of an electrolyte
(entry 10). Replacing carbon cloth with Pt, RVC, or C rod led
to nearly no corresponding product generated (entries 11−
13). Using C fiber instead of carbon cloth resulted in a slight
decrease in the yield, with 84% yield obtained (entry 14)
When carbon cloths were used as the anode and cathode,
only 42% product yield was produced (entry 15). The decrease
in temperature led to a dramatic yield reduction (entry 16).
No dramatic yield reduction was observed when this reaction
was performed under a constant voltage (entry 17). The
control experiment indicated that the electric current was
indispensable in this transformation (entry 18). Only 35%
yield was produced in the absence of a catalyst (entry 19).
Interestingly, the yield could be boosted to 87% by increasing
B
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To gain insight into the reaction mechanism, radical
trapping and cyclic voltammetry experiments were carried
out (Scheme 3b,c). Only a trace amount of the desired product
was detected in the presence of BHT (butylated hydrox-
ytoluene, Scheme 3b), and the corresponding 1b radical was
trapped, which was confirmed by MS (Figure S1). Moreover,
cyclic voltammetry (CV) experiments were investigated to get
more information (Scheme 3c). No obvious oxidation peak of
1a was observed (black, Scheme 3c). It was noteworthy that
Cat. 1 featured two obvious oxidation peaks at 0.84 and 1.65 V
(red, Scheme 3c).
Scheme 2. Substrate Scope of Electrochemical Remote
Heteroaryl Migration Initiated by N-Centered Radicals
a
Similar oxidation peaks can be found in the presence of 1a
and Cat. 1 (blue, Scheme 3c). The results above indicated that
Cat. 1 was oxidized first at the anode, which was likely to
conduce to the single electron oxidation of 1a.
On the basis of the results above and previous reports,17
a
possible reaction process of electrochemical C- to N-center
remote heteroaryl migration was proposed, as shown in
Scheme 4.
Scheme 4. Proposed Mechanism
a
b
Reaction conditions: Table 1, entry 1; isolated yields are shown. 3.5
c
h. 4 h.
obtained when the reaction was scaled up to the gram scale in
a user-friendly undivided cell (Scheme 3a).
Scheme 3. Gram-Scale Experiment and Mechanistic Studies
The TFE anion was produced at the cathode, which led to
the generation of the N-anion intermediate 1-1. Meanwhile,
Cat. 1 was oxidized at the anode to a stable radical cation
intermediate Cat. 1+,18 and the corresponding N-radical
intermediate 1-2 was obtained via a single-electron oxidation
by Cat. 1+, which was transformed into spiro radical
intermediate 1-3.16b Then, C-radical intermediate 1-4 was
formed via a carbon-to-nitrogen heteroaryl migration with
C(sp )−C(sp ) cleavage, which was oxidized by Cat. 1+ to form
the desired product.
3
2
In summary, an electrochemical heteroaryl migration
triggered by N radicals was demonstrated, which supplied an
alternative strategy to construct new N−C bonds besides
Ullmann coupling. An organocatalyst was used to avoid the
transition-metal complex, and exogenous oxidants, which
C
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04141.
General experimental procedures, characterization, and
copies of NMR spectra (PDF)
Accession Codes
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CCDC 1955548 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Kai Guo − Nanjing Tech University, Nanjing, China;
orcid.org/0000-0002-0013-3263; Email: guok@
njtech.edu.cn
Other Authors
Chengkou Liu − Nanjing Tech University, Nanjing,
China
Qiang Jiang − Nanjing Tech University, Nanjing, China
Yang Lin − Nanjing Tech University, Nanjing, China
Zheng Fang − Nanjing Tech University, Nanjing, China
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research was supported by the National Key Research and
Development Program of China (2016YFB0301501), the
National Science Foundation of China (grant no. 21776130),
and the Jiangsu Synergetic Innovation Center for Advanced
Bio-Manufacture (nos. X1821 and X1802).
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