Catalyst-Free, Direct Electrochemical Tri- and Difluoroalkylation/Cyclization: Access to Functionalized Oxindoles and Quinolinones

Article abstract of DOI:10.1021/acs.orglett.9b00361

The catalyst-free electrochemical di- and trifluoromethylation/cyclization of N-substituted acrylamides was realized under external oxidant-free conditions. The strategy provides expedient access to fluoroalkylated oxindoles and 3,4-dihydroquinolin-2(1H)-ones with ample scope and broad functional group tolerance by mild, direct electrolysis of sodium sulfinates in an undivided cell. Detailed mechanistic studies provided strong support for a SET-based reaction manifold.

Full text of DOI:10.1021/acs.orglett.9b00361

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Catalyst-Free, Direct Electrochemical Tri- and Difluoroalkylation/
Cyclization: Access to Functionalized Oxindoles and Quinolinones
,
†
†
†
†
†
†
Zhixiong Ruan,* Zhixing Huang, Zhongnan Xu, Guangquan Mo, Xu Tian, Xi-Yong Yu,
,
‡
and Lutz Ackermann*
†Key Laboratory of Molecular Target & Clinical Pharmacology and the State Key Laboratory of Respiratory Disease, School of
Pharmaceutical Sciences & the Fifth Affiliated Hospital, Guangzhou Medical University, Guangzhou 511436, P.R. China
‡
̈
Institut fur Organische und Biomolekulare Chemie, Georg-August-Universitat, Tammannstraße 2, 37077 Gottingen, Germany
̈ ̈
*
S Supporting Information
ABSTRACT: The catalyst-free electrochemical di- and trifluoromethylation/cyclization of N-substituted acrylamides was
realized under external oxidant-free conditions. The strategy provides expedient access to fluoroalkylated oxindoles and 3,4-
dihydroquinolin-2(1H)-ones with ample scope and broad functional group tolerance by mild, direct electrolysis of sodium
sulfinates in an undivided cell. Detailed mechanistic studies provided strong support for a SET-based reaction manifold.
he installation of fluorinated substituents into organic
compounds uniquely affects their lipophilicity, electro-
Scheme 1. Electrochemical Fluoroalkylation/Cyclization
T
negativity, solubility, metabolic stability, and bioavailability,
among others. For instance, fluorine can be found in
1
approximately 20% of all marketed drugs and 30% of all
2
agrochemicals. As a consequence, methodologies for the facile
synthesis of fluorinated molecules continue to be in strong
3
demand. Oxindoles and quinolinones are privileged hetero-
cyclic scaffolds found in diverse bioactive natural products and
4
pharmaceuticals. Thus, extensive research efforts have been
devoted to the development of methods that provide access to
5
functionalized oxindoles and quinolinones. Among these
strategies, oxidative trifluoromethylation of alkenes in N-
arylacrylamides through trifluoromethyl radical cyclization is
one of the most attractive synthetic routes for the construction
5
d,6
of fluoroalkylated oxindoles or quinolinones,
featuring the
7
inexpensive, solid, and stable Langlois’s reagent CF SO Na.
3
2
However, despite these indisputable advances, the radical
3l,8
trifluoromethylation largely require transition-metal catalysts,
primarily stoichiometric quantities of toxic and cost-intensive
9
13
copper(II) or silver(I) salts, and strong chemical oxidants, such
electrochemical transformation, we have now uncovered
5
f,6c,g
as reactive hypervalent iodine (PhI(OAc) ),
tert-butyl
robust and mild reaction conditions for direct, catalyst-free
electrochemical fluoroalkylation/cyclization of alkenes, on
which we report herein (Scheme 1b). Notable features of our
findings include (a) catalyst-free, direct electrolysis of sulfinate
salts for radical formation, (b) effective tandem fluoroalkyla-
tion/cyclization being devoid of (photo)redox or metal catalyst
and chemical oxidants, (c) exceedingly mild reactions at 23 °C
2
6b,f
6a,e
peroxide (TBHP), (NH ) S O , or K S O , that lead to
4
2
2
8
2
2
8
undesired side products (Scheme 1a). In recent years,
electrochemistry has been established as an increasingly
10
powerful tool for molecular synthesis. In this context, Baran
and co-workers have recently developed a metal-free radical C−
H trifluoromethylation of heteroarenes under electrochemical
1
1
condition. Moreover, very recently, Zeng reported a bromide-
catalyzed indirect electrolysis of CF SO Na to trifluoromethy-
lated oxindoles. In sharp contrast, within our program on
Received: January 28, 2019
3
2
1
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00361
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
under ambient air, and (d) ample scope toward di- and
trifluoromethylated oxindoles and 3,4-dihydroquinolin-2(1H)-
ones.
Scheme 2. Catalyst-Free Electrochemical
Trifluoromethylation
Our studies were initiated by probing various reaction
conditions for the envisioned electrochemical trifluoromethyla-
tion/cyclization of N-arylacrylamide 1a with CF SO Na (2a)
3
2
under constant current electrolysis conditions (see Table 1).
a
Table 1. Optimization of Reaction Conditions
b
entry
solvent
electrolyte
yield (%)
1
2
3
4
5
6
7
8
9
EtOH
19
<5
54
44
43
67
74
63
53
54
0
H O
2
MeCN
MeCN/H O (1/1)
MeCN/H O (1/2)
MeCN/H O (2/1)
MeCN/H O (2/1)
MeCN/H O (2/1)
MeCN/H O (2/1)
MeCN/H O (2/1)
MeCN/H O (2/1)
MeCN/H O (2/1)
MeCN/H O (2/1)
MeCN/H O (2/1)
2
2
2
Et NClO
4
2
4
n-Bu NBF
4 4
2
n-Bu NOAc
2
4
10
11
12
13
14
n-Bu NClO
2
4
4
n-Bu NI
2
4
n-Bu NHSO
70
68
0
2
4
4
c
Et NClO
2
4
4
d
Et NClO
2
4
4
a
Reaction conditions: undivided cell, RVC anode, Pt cathode,
constant current = 4.0 mA, 1a (0.5 mmol), 2a (1.5 mmol), electrolyte
b
(
1.0 mmol), solvent (5.0 mL), under air, 23 °C, 14 h. Yield of
c
isolated products. Zn(SO CF ) (0.75 mmol) instead of NaSO CF .
2
3
2
2
3
d
No electricity.
The difluoromethyl group represents a key structural motif as
lipophilic hydrogen-bond donor as well as an isostere for OH or
14
SH groups. Therefore, we turned our attention to the
electrochemical difluoromethylation with differently substituted
N-arylacrylamides 1 and sodium difluoromethanesulfinate (2b)
(Scheme 3). Hence, the catalyst-free electrochemical difluor-
omethylation/cyclization of activated alkenes was tolerant of
various electrophilic functional substituents, such as chloro,
bromo, and ester groups. It is noteworthy that the azaindole 1k
was also fully tolerated and furnished the desired difluoromethy-
lated oxindole 4k.
Preliminary experimentation highlighted a mixture of CH CN
3
and H O (v/v 2:1) as the optimal solvent for the
2
trifluoromethylation/cyclization of alkene 1a (entries 1−6).
Among a set of representative electrolytes (entries 7−12),
Et NClO salt provided the optimal results and the desired
4
4
product 3a was obtained in 74% yield (entry 7); however, other
salts such as n-Bu NI failed to deliver the functionalized
4
oxindole (entry 11). Notably, Zn(SO CF ) as the trifluor-
2
3 2
omethylation source gave a comparable efficacy (entry 13). The
robustness of the scalable electrocatalysis was reflected by the
outstanding performance under an atmosphere of ambient air.
Control experiments verified the essential nature of the
electricity (entry 14).
The electrochemical fluoroalkylation/cyclization regime was
not restricted to N-arylacrylamides 1. Indeed, N-arylcinnama-
mides 5 were successfully converted into 3,4-dihydroquinolin-
2(1H)-ones 6 under slightly modified reaction conditions
15
(Scheme 4).
With the optimal reaction conditions in hand, we next
explored its versatility with a set of representative N-
arylacrylamides 1 (Scheme 2). Thus, the electrochemical
trifluoromethylation/cyclization manifold proved amenable to
both electron-rich and electron-deficient substituents. There-
fore, a variety of synthetically useful electrophilic functional
groups were fully tolerated, including chloro, bromo, cyano, and
ester substituents, which should prove invaluable for further late-
stage manipulation. The electrochemical difunctionalization of
acrylamides 1 smoothly proceeded with various N-substituents.
The use of α-phenyl-substituted acrylamide slightly decreased
the yield of 3o. Furthermore, the reaction could also be
performed in the presence of a tetrahydroquinoline moiety 1l,
affording tricyclic oxindole 3l in 68% yield.
In consideration of the remarkable performance of the
catalyst-free electro-oxidative fluoroalkylation, we became
intrigued to delineate its mode of action. To this end, cyclic
voltammetry experiments were performed. Cyclic voltammetric
analysis revealed key mechanistic insights into the electro-
chemical functionalization (Figure 1). The voltammogram
disclosed that anodic oxidation of the sulfonate anions 2a or 2b,
followed by an irreversible reaction, occurred at a potential of ca.
1.4 or 1.1 V (vs Ag/AgCl), respectively, well below the oxidation
potential of the substrate 1a (1.8 V), indicating a direct
oxidation of the N-arylacrylamide unlikely to be operative.
In addition, BHT (7, 2,4-di-tert-butyl-4-methylphenol) was
selected as a typical radical trapping agent because its oxidation
occurred at lower potential than for the substrate 1a but at a
15
B
DOI: 10.1021/acs.orglett.9b00361
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Direct Electrochemical Difluoromethylation
considerably higher potential than for the triflinate 2a. Thus, the
trifluoromethylation/cyclization of substrate 1a was fully
suppressed upon the addition of the radical scavenger BHT
with a trace of desired product formed, suggestive of a single-
electron transfer (SET) process. Gratifyingly, evidence for the
radical intermediate 9 (Scheme 5) could be provided by the
15
isolation and full characterization of the BHT adduct 8.
Scheme 5. Proposed Mechanism
a
b
MeCN (5 mL). MeCN (5 mL), 2 mA, 50 °C, 6 h.
Based on our studies, a plausible mechanistic scenario was
proposed for the catalyst-free, electrochemical fluoroalkylation/
cyclization in Scheme 5. The anodic oxidation of the sulfinate
anion produces the corresponding radical that decomposes to
Scheme 4. Catalyst-Free Electrochemical Di- and
Trifluoromethylation of N-Arylcinnamamides 5
16
the CF radical in a desulfurative manner. Attack of the CF3
3
radical to the alkene, in the vicinity of the anode, results in
intermediate 9, which is further intramolecularly cyclized to the
intermediate 10. Finally, further aromatization affords the
corresponding product 3 under anodic oxidation conditions.
Hydrogen cations are concurrently reduced on the cathode,
releasing molecular hydrogen.
In summary, we have developed a mild and efficient method
for the preparation of biologically meaningful functionalized
oxindoles or 3,4-dihydroquinolin-2(1H)-ones by user-friendly,
electrochemical fluoroalkylation/cyclization of N-substituted
acrylamides under catalyst- and chemical-oxidant-free con-
ditions. The robust catalyst-free electrochemical di/trifluor-
omethylation/cyclization proved broadly applicable toward
various N-aryacrylamides as well as N-heterocyclic acrylamides.
Detailed mechanistic studies provided strong support for a SET-
based reaction manifold in the direct electrosynthesis.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b00361.
1
Experimental procedures, characterization data, and H
1
3
and C NMR spectra for all products (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Figure 1. Cyclic voltammetry studies.
*
*
E-mail: zruan@gzhmu.edu.cn.
E-mail: lutz.ackermann@chemie.uni-goettingen.de.
C
DOI: 10.1021/acs.orglett.9b00361
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
ORCID
Letter
2014, 50, 936−938. (g) Zhang, L.; Li, Z.; Liu, Z.-Q. Org. Lett. 2014, 16,
3
(
3
688−3691.
Lutz Ackermann: 0000-0001-7034-8772
Notes
7) (a) Langlois, B. R.; Laurent, E.; Roidot, N. Tetrahedron Lett. 1991,
2, 7525−7528. (b) Langlois, B. R.; Laurent, E.; Roidot, N. Tetrahedron
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The authors declare no competing financial interest.
(
1
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ACKNOWLEDGMENTS
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9) (a) Prakash, G. K.; Ganesh, S. K.; Jones, J. P.; Kulkarni, A.;
Support by the Guangzhou Education Bureau University
Scientific Research Project (201831845), the Talent Fund for
High-Level University Construction of Guangzhou
B185006009007), the National Natural Science Foundation
of China (No. U1601227 to X.-Y.Y.), and the Science and
Technology Programs of Guangdong Province (No.
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Org. Lett. XXXX, XXX, XXX−XXX