Electrochemical Semipinacol Rearrangements of Allylic Alcohols: Construction of All-Carbon Quaternary Stereocenters

Article abstract of DOI:10.1021/acs.orglett.9b00263

The first examples of electrochemical trifluoromethylation and sulfonylation/semipinacol rearrangements of allylic alcohols were developed using cheap and stable RSO₂Na (R = CF₃, Ph) as reagents. Various β-trifluoromethyl and sulfonated ketones were obtained in moderate to excellent yields. This strategy provides a facile, direct, and complementary approach to construct all-carbon quaternary stereocenters. In addition, the reaction has the advantages of being chemical oxidant-free and metal-free and has safe and mild reaction conditions.

Full text of DOI:10.1021/acs.orglett.9b00263

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Electrochemical Semipinacol Rearrangements of Allylic Alcohols:
Construction of All-Carbon Quaternary Stereocenters
Jun-Chen Kang, Yong-Qiang Tu, Jia-Wei Dong, Chao Chen, Jia Zhou, Tong-Mei Ding, Jian-Tao Zai,
Zhi-Min Chen,* and Shu-Yu Zhang*
Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs & School of Chemistry and Chemical Engineering, Key
Laboratory for Thin Film and Microfabrication of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, P.R.
China
S
* Supporting Information
ABSTRACT: The first examples of electrochemical trifluoromethylation
and sulfonylation/semipinacol rearrangements of allylic alcohols were
developed using cheap and stable RSO2Na (R = CF3, Ph) as reagents.
Various β-trifluoromethyl and sulfonated ketones were obtained in
moderate to excellent yields. This strategy provides a facile, direct, and
complementary approach to construct all-carbon quaternary stereocenters.
In addition, the reaction has the advantages of being chemical oxidant-free
and metal-free and has safe and mild reaction conditions.
ll-carbon quaternary stereocenters are common structural
motifs in many bioactive natural products and pharma-
as catalysts.7 In 2014, our group developed Mn(III)-mediated
radical azidation/1,2-rearrangement of allylic silyl ethers using
a stoichiometric amount of Mn(OAc)3 as an oxidant.8 Despite
these advances, metal-free and environmentally friendly
radical-initiated 1,2-rearrangements for the construction of
all-carbon quaternary stereocenters are still lacking and are in
high demand.
A
ceuticals (Figure 1).1 Accordingly, the construction of all-
Electrochemical organic synthesis, an efficient and environ-
mentally friendly synthetic tool, has attracted significant
attention. It has a unique characteristic of directly using
electric current as the oxidant, so it can allow investigators to
avoid the use of stoichiometric chemical oxidants in an
electrical environment. During the past decade, the resurgence
of organic electrochemistry has promoted the progress of
radical chemistry and enabled the development of novel redox
organic transformations.9 Numerous synthetic methods for C−
H functionalization have been reported (Scheme 1a).10
Recently, Moeller, Lin, Lei, and other groups have made
great efforts to explore organic electrochemistry as a new
approach for alkene functionalization, avoiding the use of
oxidants or expensive transition-metal catalysts.11 The strategy
for the electrochemical alkene functionalization is to assist the
anode in releasing an electrophilic radical (e.g., halogen radical,
azide radical, trifluoromethyl radical, and thiyl radical)
followed by addition to the CC bond with the generation
of an alkyl radical adduct. For example, Lin’s group reported
the diazidation and dichlorination of alkenes using manganese
Figure 1. Natural products and pharmaceuticals containing all-carbon
quaternary stereocenters.
carbon quaternary centers has received special attention, and a
number of efforts to develop synthetic methods have been
made.2 Among them, the electrophile-promoted semipinacol
rearrangement of allylic alcohols has become a commonly
applied and remarkable strategy for the construction of
carbonyl compounds with α-quaternary carbon centers.3
A
series of methodologies have been developed and successfully
applied in the synthesis of natural products and medical
molecules.4 Recently, radical-initiated 1,2-rearrangements for
the formation of all-carbon quaternary stereocenters were also
explored, which further extended the application range of
rearrangement reactions.5 In 2013, Wu’s and our group
independently developed copper-catalyzed trifluoromethyla-
tion/1,2-rearrangements of allylic alcohols using hypervalent
Togni’s reagent.6 In 2014, Toste and co-workers described an
arylation/1,2-rearrangement reaction of alkenyl cycloalkanols
with aryl diazonium salts using Ru(bpy)3(PF6)2 and Ph3PAuCl
Received: January 22, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00263
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Letter
(Langlois reagent), which is a stable, inexpensive, and practical
CF3 source that has previously been used successfully in
electrochemistry.15,11e Herein, we present the development of
successful metal-free electrochemical trifluoromethylation and
sulfonylation/semipinacol rearrangements of allylic alcohols
under mild reaction conditions.
Scheme 1. Design of Electrochemical Semipinacol
Rearrangement
Through cyclic voltammetry studies, 1a demonstrates a
weak oxidation peak at Ep/2 = 2.1 V vs SCE compared to
CF3SO2Na Ep/2 = 1.4 V vs SCE, suggesting that 1a is stable
enough to carry out a trifluoromethylation/semipinacol
rearrangement reaction (for details, see the SI). Accordingly,
allylic alcohol 1a was chosen as the model substrate. We first
identified the process for 1a, which involved constant-current
electrolysis using a carbon anode and a Pt plate cathode, in an
undivided cell containing a mixed electrolyte solution of
CF3SO2Na and LiClO4 in acetonitrile and acetic acid (10:1) at
room temperature. To our delight, the desired product 2a was
produced in 40% yield (Table 1, entry 1). To improve the
a
Table 1. Optimization studies.
CF3SO2Na
species or a radical scavenger to trap the radical adduct.11c−h
Additionally, intramolecular trapping of the radical adduct by
an aromatic ring is another pathway for heterocycle synthesis,
and elegant work in this area has been reported by Zeng and
Lei (Scheme 1b).12 However, alkyl radicals can also be
overoxidized anodically to afford cations, followed by a
nucleophilic addition with oxygen-/nitrogen-related nucleo-
phile trapping.13 To the best of our knowledge, the
electrochemical semipinacol rearrangement of allylic alcohols
to synthesize α-quaternary carbon centers of β-functionalized
ketones is unexplored.
Our long-standing interest and experience with semipinacol
rearrangements14 led us to hypothesize that the 1,2-alkyl/aryl
rearrangement process is also likely to be involved in the
electrochemical pathway (Scheme 1c). However, to success-
fully achieve an electrochemical semipinacol rearrangement,
the following three main challenges should be carefully
considered: (1) the identification of suitable allylic alcohol
substrate and radical source, since some allylic alcohol
substrates are easily decomposed under high oxidant potential;
(2) as mentioned above, the alkyl radical intermediate may be
trapped through different pathways, and if it is trapped through
a radical migration process, the diastereoselectivity of the
products could be problematic; and (3) because the migration
process is slower than oxygen-trapping or deprotonation,
which will yield byproducts 4aa or 4ab, undesired processes or
products must be avoided. We envisioned that using 1-(1-
phenylvinyl)cyclobutan-1-ol (1a) as the substrate, since it was
stable under electrochemical conditions, and radical adduct 3a
might be easily oxidized to carbocation, which subsequently
would undergo a cation 1,2-rearrangement process. Mean-
while, the migration ability of cyclobutanol, which has been
proven, was prior to that of acyclic alkyl or aryl groups, which
could contribute to avoiding the undesired products 4aa or
4ab. Additionally, considering the importance of compounds
containing trifluoromethyl groups, we first chose CF3SO2Na
b
entry (+)/(−)
solvent
(equiv)
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
C/Pt
Pt/Pt
C/Pt
C/Pt
C/Pt
C/Pt
MeCN/AcOH = 10:1
MeCN/TFE = 10:1
MeCN/HFIP = 10:1
MeCN/MeOH = 10:1
MeCN/H2O = 10:1
MeCN/H2O = 10:1
MeCN/H2O = 10:1
MeCN/H2O = 10:1
MeCN/H2O = 2:1
MeCN/H2O = 2:1
MeCN/H2O = 2:1
MeCN/H2O = 2:1
MeCN/H2O = 2:1
MeCN/H2O = 2:1
3
3
3
3
3
2.5
2
1.5
2
2
40
48
48
53
55
71
c
87
73
d
e
91 (85)
21
35
85
76
0
f
2
2
2
2
g
h
i
a
Reaction conditions: allylic alcohol 1a (0.4 mmol), LiClO4 (3.0
equiv) in MeCN (8 mL) and cosolvent in an undivided cell with
b
carbon anode and Pt plate cathode, 15 mA, 2 h. Yield was
c
determined by crude 19F-NMR. 10% of 4aa was found by crude 19F-
d
NMR. Anode potential starts from 1.42 V vs SCE, Ecell range from
e
f
g
2.4 to 4 V. Isolated yield. 10 mA for 3 h. 3.0 equiv of TBAPF6 as
h
electrolyte. The reaction was conducted at a constant potential 1.45
i
V vs SCE. No electricity.
yield, we first screened the cosolvent, and the yield was
enhanced to 55% when a combination of acetonitrile and water
(10:1) was used (entry 5). It was determined that the amount
of CF3SO2Na was an important factor in this reaction (entries
6−8). We observed competitive trifluoromethylation of the
arenes when 3.0 equiv of CF3SO2Na was used (see the SI).
Therefore, the yield increased to 87% when the amount of
CF3SO2Na was decreased to 2.0 equiv. Meanwhile, 4aa14f was
also produced with a yield of approximately 10% under the
conditions. Interestingly, after the ratio of H2O to acetonitrile
was increased, we were surprised to find that the byproduct
4aa could be suppressed and the yield further improved to
B
DOI: 10.1021/acs.orglett.9b00263
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
91%, which suggested that increasing the polarity of the solvent
was able to promote the migration process (entry 9). It should
be noted that the electricity is 2.8 F and Faraday efficiency is
71% under the conditions (entry 9). In comparison, use of the
Pt anode or lowering the current dramatically decreased the
formation of 2a (entries 10 and 11). In addition, using
tetrabutylammonium hexafluorophosphate (TBAPF6) as an
electrolyte instead of LiClO4 afforded a similar yield (entry
12). The constant potential experiment was also conducted
and gave the desired product with 76% yield (entry 13). The
control experiments demonstrated that electricity was crucial
for this reaction (entry 14). As a result, the reaction conditions
of entry 9 were selected as the optimal procedure, and the
isolated yield was 85%.
Scheme 2. Substrate Scope of the Electrochemical Tandem
Trifluoromethylation/Semipinacol Rearrangement
a
With optimized conditions in hand, the reaction scope was
investigated with a variety of styrene derivatives. Overall, the
desired all-carbon α-quaternary centers of β-trifluoromethyl
ketones were obtained with moderate to good yields (Scheme
2). Compared with the model substrate 1a, the ortho-
substitution (1b) led to an evidently decreased yield, while
electron-deficient substituents or a methyl group in the meta-
or para-position did not significantly affect the yields (2c−g).
It is worth mentioning that although competing reactions may
occur on aryl rings when the substrates with electron-rich
substituents were used, by controlling the reaction time and
properly lowering the current, we could also obtain the
corresponding products (2h−j) in moderate yields. However,
the electron-withdrawing substituents did not work well. The
indene-type substrates (1l,m) were also tolerant, which
afforded the corresponding products (2l,m) with spiro
structures in moderate yields and moderate to good
diastereoselectivities. However, an increasing amount of
CF3SO2Na was needed for the full conversion of those
substrates. The relative configuration of product 2l was
determined by X-ray crystallography, and thus, 2m was
assigned by analogy. Cyclopentanol and 9-fluorenol were also
tested in the reaction and gave the desired products (2n and
2o, respectively) in reasonable yields. It should be noted that,
except for the case of ring expansion, 1,2-aryl migration was
also accessible under the standard conditions. Allylic alcohol
1q, which is a nonactivated alkene, afforded product 2q in
moderate yield. In contrast, we determined that the desired
product 2p was obtained only in 33% yield along with an
epoxide byproduct 4pa in the reaction of substrate 1p. Finally,
in order to evaluate the synthetic utility of this reaction, a gram
scale (5 mmol) trifluoromethylation/rearrangement of sub-
strate 1a was performed under the standard electrochemical
reaction conditions, and the desired product 2a was obtained
in 65% yield.
Encouraged by the above studies, we assumed that more
versatile transformations could be realized via semipinacol
rearrangement of allylic alcohols under electrochemical
oxidation conditions. Organic sulfone compounds are very
important synthetic intermediates and are also widespread in
many medical molecules and functional materials.16 Although
some traditional methods have been documented, the
development of metal-free,17 efficient, and eco-friendly
synthetic methods for the preparation of sulfone-containing
compounds is still highly desirable.18 We noted that aryl
sulfinate was also readily oxidized into sulfonyl radicals under
electrochemical conditions. Aryl sulfonyl radicals may be
trapped by alkenes prior to SO2 extrusion. Therefore, we began
to study the electrochemical sulfonylation/semipinacol rear-
a
Allylic alcohol (0.4 mmol), CF3SO2Na (2.0 equiv), and LiClO4 (3.0
equiv) in MeCN/H2O = 8:4 mL in an undivided cell with carbon
anode and Pt plate cathode under constant current 15 mA for 2 h. 12
mA for 2.2 h. 2.5 equiv of CF3SO2Na was added. 20 mA and 3.0
equiv of CF3SO2Na were added.
b
c
d
rangement. To our delight, the reaction proceeded smoothly,
and the desired product 3a was obtained in 78% yield after
simply screening the reaction conditions. As shown in Scheme
3, numerous electron-withdrawing, electron-rich substituents
on the benzene ring of cycloalkanol-substituted styrene
derivatives were well-tolerated and afforded the corresponding
products (3b−k) in good yields. Subsequently, the 2-naphthyl-
substituted substrate was also subjected to the reaction and
gave product 3l in good yield. Of note, the reaction with
substrate 1m, derived from 1-indanone, was suitable and
afforded the product 3m with good yield and excellent
diastereoselectivity, suggesting that this reaction was a cation
1,2-migration process. The relative configuration of the
product 3m was also assigned by X-ray crystallography.
Finally, we were pleased to find that several sodium
benzenesulfonate salts were also able to promote this reaction
in moderate yields (3aa−ac).
C
DOI: 10.1021/acs.orglett.9b00263
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Organic Letters
Letter
Scheme 3. Substrate Scope of Electrochemical Tandem
Sulfonylation/Semipinacol Rearrangement
Scheme 4. Proposed Mechanistic Pathway
a
preparation of all-carbon α-quaternary centers of β-function-
alized ketones with moderate to excellent yields. The
transformations can be achieved via a 1,2-migration process.
The requirements of stoichiometric oxidants, transition-metal
catalysts, and harsh reaction conditions were avoided. Studies
of the application of these reactions and other electrochemical
semipinacol rearrangements are underway in our group and
will be reported in due course.
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.9b00263.
Experimental details, cyclic voltammetry experiment,
electricity, X-ray crystallography structure of compounds
2l and 3m, analytical data for new compounds, NMR
spectra of new compounds (PDF)
Accession Codes
CCDC 1880038−1880039 contain the supplementary crys-
tallographic 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.
a
Allylic alcohol (0.4 mmol), ArSO2Na (3.0 equiv), and LiClO4 (3.0
equiv) in MeCN/H2O = 5:5 mL in an undivided cell with a carbon
anode and Pt plate cathode under constant current of 10 mA for 3 h.
AUTHOR INFORMATION
Corresponding Authors
■
To gain mechanistic insight into the reaction, some
preliminary mechanistic experiments were performed for this
tandem trifluoromethylation/semipinacol rearrangement. After
the addition of TEMPO as a radical scavenger, the reaction
was inhibited. No desired product 2a was detected, but we did
detect a TEMPO-trapped CF3 adduct by GC−MS. Cyclic
voltammetry experiments showed the oxidation peak of
CF3SO2Na, indicating the initial CF3 radical. In addition,
during the solvent filtration, we found the formation of
methanol-trapped species when methanol was employed as
cosolvent, suggesting the involvement of a cation intermediate.
Based on the above mechanistic studies and previous
reports, we proposed a plausible mechanism for trifluorome-
thylation, as shown in Scheme 4. The SET oxidation process of
CF3SO2Na under anode oxidation would give the sulfonyl
radical followed by a fast extrusion of SO2 to generate a CF3
radical. Upon the addition of the CF3 radical to the CC
bond, the generated benzyl radical 5 would be further oxidized
to form cation 6. After ring expansion and deportation, 2a
would be formed. Predictably, the sulfonylation process is
similar.
*E-mail: chenzhimin221@sjtu.edu.cn.
*E-mail: zhangsy16@sjtu.edu.cn.
ORCID
Zhi-Min Chen: 0000-0002-6988-8955
Shu-Yu Zhang: 0000-0002-1811-4159
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the “Thousand Youth Talents
Plan”, NSFC (21672145, 51733007, 21702135, and
21871178), the Shuguang program (16SG10) from SEDF &
SMEC, STCSM (17JC1403700), and the Drug Innovation
Major Project of the Ministry of Science and Technology of
China (2018ZX09711001-005-002)
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D
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DOI: 10.1021/acs.orglett.9b00263
Org. Lett. XXXX, XXX, XXX−XXX