Mn-Mediated Electrochemical Trifluoromethylation/C(sp2)-H Functionalization Cascade for the Synthesis of Azaheterocycles

Article abstract of DOI:10.1021/acs.orglett.8b04010

A general electrohemical strategy for the combined trifluoromethylation/C(sp²)-H functionalization using Langlois' reagent as the CF₃ source under oxidant-free conditions was developed. Using Mn salts as the redox mediator, this method provides an efficient and sustainable means to access a variety of functionalized heterocycles bearing a CF₃ moiety. Detailed mechanistic studies are consistent with the formation of CF₃-bound high oxidation state Mn species, suggesting a transition-metal-mediated CF₃ transfer mechanism for this trifluoromethylation/C(sp²)-H functionalization process.

Full text of DOI:10.1021/acs.orglett.8b04010

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Mn-Mediated Electrochemical Trifluoromethylation/C(sp2)−H
Functionalization Cascade for the Synthesis of Azaheterocycles
Zhenxing Zhang,† Lei Zhang,† Yang Cao,† Feng Li,‡ Guangcan Bai,‡ Guoquan Liu,‡ Yang Yang,§
and Fanyang Mo*,†,⊥
‡
†Department of Energy and Resources Engineering, College of Engineering and State Key Laboratory of Natural and Biomimetic
Drugs, School of Pharmaceutical Sciences, Peking University, Beijing 100871, China
§Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: A general electrohemical strategy for the
combined trifluoromethylation/C(sp2)−H functionalization
using Langlois’ reagent as the CF3 source under oxidant-free
conditions was developed. Using Mn salts as the redox
mediator, this method provides an efficient and sustainable
means to access a variety of functionalized heterocycles
bearing a CF3 moiety. Detailed mechanistic studies are
consistent with the formation of CF3-bound high oxidation
state Mn species, suggesting a transition-metal-mediated CF3 transfer mechanism for this trifluoromethylation/C(sp2)−H
functionalization process.
Scheme 1. Conventional and Electrochemical Approaches
to the Generation of Trifluoromethyl Radical
wing to the prevalence of the trifluoromethyl group in
1
2
Opharmaceuticals, agrochemicals, and functional materi-
als,3 the development of efficient and sustainable methods for
the introduction of the trifluoromethyl group into organic
molecules has attracted much attention.4 Among various
means to prepare trifluoromethylated compounds, trifluor-
omethylation based on a radical mechanism has been
extensively studied and proven highly useful.4d Two strategies
are commonly used to generate the trifluoromethyl radical,
namely, the chemical oxidation of nucleophilic trifluorome-
thylation reagents (e.g., Langlois’ reagent and Baran’s reagent)
and the single electron reduction of electrophilic trifluor-
omethylation reagents (e.g., triflyl chloride, CF3I and Togni’s
reagent) (Scheme 1). Despite their synthetic utility, the use of
stoichiometric quantities of chemical oxidants or reductants
inevitably generates environmentally hazardous waste and
greatly reduces the functional group compatibility of these
methods.
To address these limitations, organic electrochemistry
represents a very attractive alternative to conventional redox
transformations, wherein the substrate or reagent can directly
undergo redox processes without the use of stoichiometric
chemical oxidants/reductants.5 As a part of our program
directed toward the development of novel electrochemical
methods for organic synthesis,6 we recently became interested
in using the bench stable and highly inexpensive Langlois’
reagent (CF3SO2Na) for radical trifluoromethylation under
electrochemical conditions. In this approach, cathodic hydro-
gen evolution balances the oxidative part of the redox cycle,
thus making the overall transformation oxidant-free. Addtion-
ally, the release of reactive intermediate (CF3·) can be easily
regulated by electric current, allowing the reaction to proceed
in a controlled maner. Moreover, the use of a transition metal
redox mediator can benefit the reaction by enabling the key
steps of the aimed transformation being conducted homoge-
neously. Despite these desirable features, electrochemical
trifluoromethylation is still rare and has only been applied in
a few examples, including the C−H functionalization of
Received: December 16, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b04010
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Letter
electron-rich arenes7 and heterocycles8 and olefin difunction-
(entry 5). Other polar and water miscible solvents, such as
DMA and MeCN, were less efficient (entries 6 and 7). It was
found that the use of an acid additive such as H3PO4 can
significantly improve the rate of proton reduction on the Pt
electrode, and the yield of 2a was much lower in the absence of
H3PO4 (entry 8). Replacing H3PO4 with a weaker acid, HOAc,
resulted in a similar yield (entry 9). No reaction occurred in
the absence of externally applied potential (entry 10). Notably,
we were able to avoid the use of electrolytes for this
transformation, thus further fulfilling the requirement of
green and sustainable chemistry.
alization.9
Oxindole is an important structural motif in numerous
natural products and bioactive compounds, and trifluorome-
thylated oxindoles and their analogs hold great promise for
application in medicinal chemistry.10 Previous synthetic routes
toward these molecules all required the use of stoichiometric
quantities of chemical oxidants or reductants to generate the
key trifluoromethyl radical species.11 Herein, we report a
general Mn-mediated electrochemical trifluoromethylation for
the construction of a variety of trifluoromethyl-containing
oxindoles and related heterocycles to bypass the use of
stoichiometric chemical oxidants. Notably, a variety of
substrates, including those previously unexplored ones, are
readily accommodated by this electrochemical process,
affording a diverse range of trifluoromethylated heterocycles
bearing an all-carbon quaternary center in excellent yield.12
Using N-methyl-N-phenylmethacrylamide (1a) as the model
substrate, we commenced our study on the electrochemical
trifluoromethylation/C(sp2)−H functionalization using Lan-
glois reagent as the CF3 source. The use of a catalytic amount
of redox active transition-metal species was found to be critical
for this transformation, and manganese salts proved to be
optimal for this process.13 After extensive optimization, we
found that using MnBr2 as the mediator, H3PO4 as the
sacrificial oxidant, and Pt meshes as the electrodes with a
constant electric current of 10 mA for 6 h, the desired
trifluoromethyl-substituted oxindole product 2a formed in 67%
yield (Table 1, entry 1). Control experiment in the absence of
With the optimized reaction conditions in hand, we next
surveyed the substrate scope of this trifluoromethylation/
C(sp2)−H functionalization process (Scheme 2). A wide range
Scheme 2. Oxidant-Free Electrochemical
Trifluoromethylation-Initiated Radical Oxidative
a
Annulation towards Oxindoles
a
Table 1. Reaction Development and Optimization
b
entry
deviation from the standard conditions
yield %
1
2
3
4
5
6
7
8
9
10
none
no Mn salt
69 (67)
12
54
trace
trace
53
58
27
66
0
MnSO4 instead of MnBr2·4H2O
NiBr2 instead of MnBr2·4H2O
room temperature
DMA instead of 1,4-dioxane
MeCN instead of 1,4-dioxane
no H3PO4
a
Reaction conditions: 1 (0.3 mmol), MnBr2 (0.06 mmol, 0.2 equiv),
AcOH instead of H3PO4
no electricity
1,4-dioxane 3 mL, H2O 0.3 mL, Pt gauze electrodes (1 cm × 1 cm, 52
meshes) were applied. cc, constant current. 1.5 equiv CF3SO2Na was
added at the beginning. After 3 h, another 1.5 equiv was added.
a
Reaction conditions: 0.3 mmol scale, 1,4-dioxane 3 mL, H2O 0.3
mL, Pt gauze electrodes (1 cm × 1 cm, 52 meshes) were applied. cc,
constant current. 1.5 equiv of CF3SO2Na was added at the beginning.
b
Isolated yields were given. isolated yields from ref 11f were given.
Reaction conditions of ref 11f: 1 (0.25 mmol), CF3SO2Na (0.75
mmol), K2S2O8 (2 equiv), CH3CN/H2O (2.5 mL, 4/1), 80 °C, 12−
36 h, under N2.
b
After 3 h, another 1.5 equiv of CF3SO2Na was then added. Yields
were determined by 1H NMR spectroscopic analysis using 1,3,5-
trimethoxybenzene as the internal standard. Isolated yield are shown
in parentheses.
of N-arylmethacrylamide derivatives bearing different sub-
stituents on aryl ring was found to be excellent substrates.
Substrates bearing electron-withdrawing (2c−2i) or electron-
donating groups (2b and 2j) afforded the desired products.
Notably, this anodic oxidative process tolerated a variety of
functional groups, such as an ester (2i), halides (2d−2g), a
nitrile (2h), and a free alcohol (2o). Substrates with different
protecting groups on the nitrogen atom (1k−1o) were also
successfully transformed to the corresponding oxindole
manganese salt only provided 12% yield of 2a (entry 2),
thereby demonstrating the pivotal role of the manganase
catalyst. We found that MnSO4 also catalyzed this trifluor-
omethylation/C(sp2)−H functionalization reaction, albeit in
lower yield (entry 3). In contrast, other transition metal salts
such as NiBr2 were found to be ineffective (entry 4).
Additionally, when the reaction was carried out at room
temperature, only a trace amount of product was observed
B
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Letter
derivatives (2k−2o) in good yields (67−77%). Finally, this
reaction was also compatible with a fused ring system (2k).
Further investigations revealed that the current electro-
chemical trifluoromethylation/C(sp2)−H functionalization is a
widely applicable strategy and could be readily applied in the
synthesis of CF3-substituted indoline derivatives (Scheme 3)
Scheme 4. Oxidant-Free Electrochemical
Trifluoromethylation-initiated Radical Oxidative
Cyclization towards Hydroisoquinoline-1,3-diones
a
Scheme 3. Oxidant-Free Electrochemical
Trifluoromethylation-Initiated Radical Oxidative
a
Cyclization towards Indolines
a
Reaction conditions: 5 (0.3 mmol), MnBr2 (0.06 mmol, 0.2 equiv),
1,4-dioxane 3 mL, H2O 0.3 mL, Pt gauze electrodes (1 cm × 1 cm, 52
meshes) were applied. cc, constant current. 1.5 equiv CF3SO2Na was
added at the beginning. After 3 h, another 1.5 equiv was added.
b
Isolated yields were given. Isolated yields from ref 11i were given.
Reaction conditions of ref 11i: 5, CF3SO2Na (3.0 equiv), 1,2,3,5-
tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN, 2 mol %), N2
(with 0.5 mol % of O2), rt, c = 0.02 M, DCE, blue LEDs.
redox behavior of MnBr2, CF3SO2Na, 1a and their
combination in solution by cyclic voltammetry (Figure 1).
a
Reaction conditions: 3 (0.3 mmol), MnBr2 (0.06 mmol, 0.2 equiv),
1,4-dioxane 3 mL, H2O 0.3 mL, Pt gauze electrodes (1 cm × 1 cm, 52
meshes) were applied. cc, constant current. 1.5 equiv CF3SO2Na was
added at the beginning. After 3 h, another 1.5 equiv was added.
Isolated yields were given.
and CF3-containing hydroisoquinoline-1,3-dione derivatives
(Scheme 4). A variety of N-arylamide acrylates 3 and N-
methacryloyl-N-methylarylamides 5 could be converted to the
corresponding trifluoromethylated indoline and hydroisoqui-
noline-1,3-dione derivatives in good yields. Of special note, the
trifluoromethylation/C(sp2)−H functionalization of substrates
3 type of compounds has not been reported before.
To further demonstrate the utility of this electrochemical
protocol, a gram-scale synthesis of oxindole 2a was carried out
under air (eq 1). This reaction was performed with graphite as
the anode and platinum as the cathode, and furnished in 53%
isolated yield.
Figure 1. CVs of MnBr2, CF3SO2Na, and substrate 1a. Conditions: a
L-type glassy carbon working electrode, a saturated calomel electrode
(SCE) reference electrode, and a platinum wire counter electrode,
LiClO4 (0.10 M in MeCN), TFA (60 mM), 0.1 V/s with (a)
Background; (b) MnBr2·4H2O (2 mM); (c) CF3SO2Na (8 mM); (d)
MnBr2·4H2O (2 mM) and CF3SO2Na (8 mM); (e) 1a (4 mM); (f)
1a (4 mM), MnBr2·4H2O (2 mM) and CF3SO2Na (8 mM).
The TFA/MeCN solution of MnBr2 displays a quasi-reversible
anodic wave (line b) of 0.75 and 0.89 V and cathodic wave of
0.60 V versus saturated calomel electrode (SCE), which we
attributed to the MnII/MnIII redox couple of the bromide-
bound complex and the direct oxidation of free Br−.
CF3SO2Na shows an irreversible anodic wave at 0.71 V (line
c). Furthermore, the combination of MnBr2 and CF3SO2Na
exhibits a quasi-reversible anodic CV feature at 0.83 V (line d),
which we attributed to the MnII/MnIII redox couple of the
CF3-bound complex. The lower cathodic-to-anodic peak
current ratio (Ic/Ia = 0.56) reflects the instability and high
reactivity of MnIII−CF3 under these conditions. CV of 1a
shows a relatively higher oxidation wave larger than 1.5 V,
indicating its unlikely interacting with electrodes directly in the
To probe the mechanism of this electrochemical reaction,
we performed a series of polarographic and spectroscopic
analysis, including cyclic voltammetry (CV), 19F NMR, and
stoichiometric control experiment. First, we investigated the
C
DOI: 10.1021/acs.orglett.8b04010
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Letter
presence of Mn salt and Langlois’ reagent. Addition of 1a to a
mixture of MnBr2 and CF3SO2Na resulted in the appearance of
two irreversible anodic waves of 0.94 and 1.59 V (line f), which
corresponds to the formation of the putative MnIII−CF3 and
the single electron oxidation leading to the formation of the
final product (vide infra, Scheme 5).
Finally, we found that this electrochemical C−H trifluor-
omethylation method could be applied in the rapid
construction of complex ring systems. Under the same reaction
conditions as described above, trifluoromethlated product 8
featuring bicyclic ring systems could be conveniently
synthesized in 32% yield (eq 2). Further extension of this
electrochemical trifluoromethylation process to the preparation
of other complex polycyclic compounds is currently underway
in our laboratory.
Scheme 5. Stoichiometric Experiment
To gain more insight into the mechanism of this Mn-
mediated electrochemical trifluoromethylation, the electro-
chemical reaction of CF3SO2Na and MnII was studied with 19
F
NMR spectroscopy. In accord with literature, free CF3SO2Na
reagent shows a singlet peak at −87.5 ppm.14 When electric
current of 1.1 F/mol was applied, the 19F NMR signal of
CF3SO2Na disappeared, indicating its complete consumption
upon electrolysis. New peaks around −73.0 ppm appearance
indicated new CF3 species formation (Figure S4).
Moreover, we performed an electrically controlled stoichio-
metric experiment. We first applied electricity of 1.1 F/mol to
the dioxane/H2O solution of MnBr2 and CF3SO2Na and then
introduced substrate 1a. The mixture was allowed to react in
the absence of external electricity, and this two-step reaction
afforded the desired product (2a) in 11% isolated yield. The
result showed that the electrochemically generated high
oxidation state Mn species can function both as a CF3 carrier
and an oxidant for the trifluoromethylation/C(sp2)−H
functionalization.
In summary, we have developed a general Mn-mediated
electrochemical protocol for the trifluoromethylation/C(sp2)−
H functionalization process. This method provides a
sustainable means for the synthesis of a variety of
trifluoromethylated oxindoles and related heterocycles. We
expect that these efforts will guide the further development of
synthetically useful electrochemical organic transformations.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b04010.
Based on these mechanistic studies, a plausible mechanism
for the trifluoromethylation/C(sp2)−H functionalization is
proposed in Scheme 6. At the anode, the MnIII−CF3 species is
Experimental procedures, additional information, and
characterization data (PDF)
Scheme 6. Proposed Reaction Events on Both Electrodes
and in Bulk Solvent
Accession Codes
CCDC 1885350 and 1890462 contain 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: fmo@pku.edu.cn.
ORCID
Lei Zhang: 0000-0001-9031-4318
Guoquan Liu: 0000-0003-0680-4811
Yang Yang: 0000-0002-4956-2034
Fanyang Mo: 0000-0002-4140-3020
Present Address
⊥Jiangsu Weiming Environmental Protection S&T Co., ltd.
generated by anodic oxidation of MnII in the presence of
Langlois reagent. Subsequently, Mn-assisted delivery of CF3· to
the olefin moiety provides a transient carbon centered radical.
Further radical addition to the aromatic ring followed by either
anode or MnIII-mediated oxidation furnishes the final C(sp2)−
H functionalization product. At the cathode, reduction of
proton to dihydrogen takes place to balance the overall
transformation.
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.8b04010
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Letter
Qin, H.; Huang, M.; Hu, S.; Cai, S. Org. Biomol. Chem. 2018, 16,
6564.
(12) During the preparation of this work, a similar work was
reported by Zeng. Jiang, Y.-Y.; Dou, G.-Y.; Xu, K.; Zeng, C.-C. Org.
Chem. Front. 2018, 5, 2573.
ACKNOWLEDGMENTS
■
The project was supported by the Natural Science Foundation
of China (Grants 21772003). We also thank the “1000-Youth
Talents Plan” and Peking University for start-up funds.
(13) (a) Fu, N.; Sauer, G. S.; Lin, S. J. Am. Chem. Soc. 2017, 139,
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(14) Langlois, B. R.; Billard, T.; Mulatier, J.-C.; Yezeguelian, C. J.
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