Reductive arylation of aliphatic and aromatic aldehydes with cyanoarenes by electrolysis for the synthesis of alcohols

Article abstract of DOI:10.1021/acs.orglett.1c00920

An electroreductive arylation reaction of aliphatic and aromatic aldehydes as well as ketones with electro-deficient (hetero)arenes is described. A variety of cyano(hetero)arenes and carbonyl compounds, especially aliphatic aldehydes, have been examined, providing secondary and tertiary alcohols in moderate to good yields. Mechanistic studies, including cyclic voltammetry (CV), electron paramagnetic resonance (EPR), and divided-cell experiments, support the generation of aliphatic ketyl radicals and persistent heteroaryl radical anions via cathodic reduction followed by radical-radical cross-coupling.

Full text of DOI:10.1021/acs.orglett.1c00920

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Letter
Reductive Arylation of Aliphatic and Aromatic Aldehydes with
Cyanoarenes by Electrolysis for the Synthesis of Alcohols
Xiao Zhang, Chao Yang,* Han Gao, Lei Wang, Lin Guo, and Wujiong Xia*
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ABSTRACT: An electroreductive arylation reaction of aliphatic and aromatic aldehydes as well as ketones with electro-deficient
(hetero)arenes is described. A variety of cyano(hetero)arenes and carbonyl compounds, especially aliphatic aldehydes, have been
examined, providing secondary and tertiary alcohols in moderate to good yields. Mechanistic studies, including cyclic voltammetry
(CV), electron paramagnetic resonance (EPR), and divided-cell experiments, support the generation of aliphatic ketyl radicals and
persistent heteroaryl radical anions via cathodic reduction followed by radical−radical cross-coupling.
arbonyl compounds, including aldehydes and ketones,
are pervasive structures that exist in numerous bio-
highly negative redox potential (e.g., E1/2(3-methylbutanal) =
−2.24 V vs SCE), and therefore it requires a strongly reducing
agent;12 (2) the lifetime of the crucial alkyl-substituted ketyl
radical intermediate is not sufficiently long.10 A very recent
breakthrough was made by Baran and co-workers, who
successfully developed an electroreductive reaction of aliphatic
ketones with unactivated olefins.13 As part of our ongoing
interest in developing viable protocols for novel electro-
chemical transformations,14 we were interested in exploring an
electroreductive reaction of aliphatic aldehydes coupled with a
highly valuable functionality, for example, a heteroarene. From
a synthetic point of view, the ability to generate heterocycle-
containing alcohols would be highly rewarding, as heterocycles
are quite prevalent in biological and small molecular drugs.2,15
Herein, we report that using aliphatic/aromatic aldehydes and
ketones with electron-deficient (hetero)arenes allows reductive
arylation reaction to proceed simply by electrolysis, affording
secondary and tertiary alcohols in good to moderate yields.16
The desired transformation was proposed to proceed through a
radical−radical cross-coupling pathway (Scheme 1c).
C
logically active molecules, pharmaceuticals, and functional
materials.1,2 The conversion of carbonyl compounds to
alcohols represents a key and fundamental transformation in
organic chemistry.3 Although the traditional nucleophilic
addition of aldehydes and ketones with a variety of
organometallic reagents has been successfully applied in
synthetic chemistry (Scheme 1a),4 the requirement of extra
preparation steps, harsh reaction conditions, and low func-
tional group compatibility restricted its broad utilization. With
the rapid development of photochemistry and electro-
chemistry, which is regarded as an environmentally benign
and orthogonal tool for redox transformations,5,6 impressive
progress has recently been achieved for converting carbonyl
compounds to alcohols through a photoinduced7 or electro-
chemical pathway8 (Scheme 1b). In these two strategies,
carbonyl compounds are reduced to generate stabilized ketyl
radicals,9 which can be either coupled selectively with
persistent radicals or trapped by unsaturated compounds to
deliver alcohols as products. It is worth noting that, for most of
the cases, these strategies are only applied to aromatic
aldehydes and ketones,10 leading to benzylic alcohols as
major products.
Our study began by the investigation of the reaction
between cyclohexanecarbaldehyde (1a) and 4-cyanopyridine
Received: March 17, 2021
Published: April 16, 2021
Until now, aliphatic carbonyl compounds have merely been
employed for the synthesis of alcohols via a ketyl radical
intermediate without the assistance of strongly reducing agents
(such as SmI2) or dissolving metals (such as Na, K in liquid
ammonia).11 Challenges still exist as two major difficulties limit
its generalization: (1) aliphatic carbonyl compounds exhibit a
https://doi.org/10.1021/acs.orglett.1c00920
© 2021 American Chemical Society
Org. Lett. 2021, 23, 3472−3476
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Letter
solvent of DMF/MeOH in the reaction system. Due to the
important role of nBu4NBF4 in the electrolysis, some other
ammonium salts as the supporting electrolytes were further
evaluated, such as Et4NCl and nBu4NClO4, but lower yields
were obtained (entries 6 and 7). A slight erosion in yield was
observed when using platinum plate as both electrodes (entry
8), while a moderate yield was observed with a graphite rod
anode and a platinum plate cathode (entry 9). Furthermore,
the use of other additives was explored but provided no
improvement over DABCO, and the reaction in the absence of
DABCO resulted in much lower yields of 3a (entries 10 and
11). As anticipated, control experiments revealed that electric
current was critical for the desired transformation (entry 12).
In addition, the yield was slightly reduced when the reaction
was performed under a nitrogen atmosphere (entry 13).
Encouraged by our results, we turned our attention to
exploring the generality of our reaction with aldehydes and
ketones. As shown from the results compiled in Scheme 2, the
reaction scope of aliphatic aldehydes was first examined with 4-
cyanopyridine (2a) as a coupling partner under the optimized
conditions. Gratifyingly, the method was applicable to a wide
range of aliphatic aldehydes with various substitution patterns.
Cyclic aldehydes with different ring sizes were all converted
into the corresponding secondary alcohols 3a−3e in good to
moderate yields. Substrates bearing acyclic hydrocarbons also
worked well (3f−3u). The chemoselectivity profile of our
method is illustrated by the fact that an alkene (3b, 3p, 3r, and
3u), ester (3c), alcohol (3q), or furan (3o), among others,
could all be well accommodated. Aromatic aldehydes could
also be employed as the reaction partner, and again formation
of the corresponding alcohols was observed. A variety of
functionalized aryl aldehydes were all converted into the
corresponding alcohols in good to high yields (3v−3ii),
including substrates containing methoxy (3x, 3cc, 3ee, and
3hh), fluorine (3z), and thiol ether (3aa), regardless of
whether the substituents are in the ortho-, para-, or meta-
position. Notably, heteroaromatic aldehyde, such as furan-2-
carbaldehyde, was also effectively transformed into the
corresponding product 3ii. Additionally, the scope with respect
to ketones as the coupling partner was tested. A series of
acetophenone derivatives readily underwent the electro-
chemical arylation reaction with 2a to furnish the correspond-
ing tertiary alcohols in moderate to good yields (3ii−3pp).
The scope with respect to analogues of 4-cyanopyridine (2a)
as the aromatic partner was further explored (Scheme 3).
Cyano-substituted pyridine compounds, including 3-methyliso-
nicotinonitrile, 2,6-dimethylisonicotinonitrile, and 3-fluoro-
isonicotinonitrile, underwent electrochemical arylation reac-
tion with aliphatic aldehyde 1a to furnish the corresponding
pyridine-containing alcohols in moderate yields (4a−4c).
Cyano-substituted pyrazine is also a suitable substrate in this
electrochemical arylation, providing 4d in 76% yield. More-
over, direct arylation of benzaldehyde with methyl, tert-butyl,
and hydroxyl-substituted isonicotinonitrile derivatives (4f−
4h), as well as cyano-substituted pyrazine (4i) and quinoline
(4j), proceeded smoothly under the standard conditions.
Notably, electron-deficient arenes, such as 1,4-dicyanobenzene
and methyl 4-cyanobenzoate, were successfully employed,
giving 4k and 4l in 79% and 67% yields, respectively.
Scheme 1. Aldehyde/Ketone to Alcohol Conversion
(2a) in a simple undivided cell equipped with a platinum plate
anode and a graphite rod cathode as the working electrodes
(Table 1). After careful optimization of the reaction
conditions,17 a combination of nBu4NBF4, DABCO, and
DMF under 10 mA constant current electrolysis at room
temperature under air afforded secondary alcohol 3a in 75%
isolated yield. As shown in entries 2 to 5, unsatisfying results
were obtained when using MeCN, DCE, DMSO, or a mixed
a
Table 1. Optimization Studies
a
b
entry
variations from standard conditions
3a (%)
1
none
75
36
NR
62
NR
49
50
71
55
50
31
NR
68
2
MeCN as solvent
3
DCE as solvent
4
DMSO as solvent
5
6
7
DMF/MeOH (1:1) as solvent
Et4NCl instead of nBu4NBF4
nBu4NClO4 instead of nBu4NBF4
8
9
10
11
12
13
Pt(+)/Pt(−) instead of Pt(+)/C(−)
C(+)/Pt(−) instead of Pt(+)/C(−)
Et3N instead of DABCO
without DABCO
without constant current
under N2 atmosphere
a
Standard conditions: Pt plate anode (10 × 10 × 0.2 mm3), graphite
To further evaluate this electrochemical process, a series of
cyclic voltammetry (CV) were conducted, suggesting that both
aldehyde substrate and 2a are likely to undergo cathodic
reduction to generate the corresponding radical intermedi-
rod cathode (φ = 6 mm), 1a (2.4 mmol), 2a (0.4 mmol), DABCO
(1.2 mmol), nBu4NBF4 (0.1 M), anhydrous DMF (5.0 mL), constant
current = 10 mA under air at room temperature for 6 h (5.6 F/mol).
b
Isolated yield. NR: no reaction.
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Letter
a b
,
Scheme 2. Scope of Aliphatic, Aromatic Aldehydes, and Ketones
a
Reaction conditions: Pt plate anode (10 × 10 × 0.2, mm3), graphite rod cathode (φ = 6 mm), 1 (2.4 mmol), 2a (0.4 mmol), DABCO (1.2
b
mmol), nBu4NBF4 (0.1 M), anhydrous DMF (5.0 mL), constant current = 10 mA under air at room temperature for 6 h (5.6 F/mol). Isolated
yield.
a b
,
Scheme 3. Scope of Electron-Deficient (Hetero)arenes
Scheme 4. Mechanistic Study
a
Reaction conditions: Pt plate anode (10 × 10 × 0.2 mm3), graphite
rod cathode (φ = 6 mm), 1 (2.4 mmol), 2 (0.4 mmol), DABCO (1.2
mmol), nBu4NBF4 (0.1 M), anhydrous DMF (5.0 mL), constant
current = 10 mA under air at room temperature for 6 h (5.6 F/mol).
electrochemical arylation occurred surrounding the graphite
rod cathode and DABCO is only used as a sacrificial reducing
reagent rather than a PCET (proton-coupled electron transfer)
reagent. In order to gain more insight into the origination of
radical species in this electroreductive arylation, electron
paramagnetic resonance (EPR) measurement of valeraldehyde
(1g) was carried out in 0.1 M nBu4NBF4/DMF under constant
current for 20 min (Scheme 4b).17 Given that the lifetime of
the aliphatic ketyl radicals is not sufficiently long, no obvious
signal was detected in valeraldehyde. Instead, N-benzylidene-
b
c
Isolated yield. 1 (1.2 mmol), 2 (0.4 mmol).
ates.17 Subsequently, a divided-cell experiment was carried out
(Scheme 4a). DABCO was placed into the anode chamber. As
expected, the desired product 3a was detected in the cathode
chamber with 62% isolated yield, suggesting that the
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tert-butylamine N-oxide (PBN) was selected as a spin-trap and
introduced into the experiments.18 Through the comparison
with background experiment of PBN, a rapid growth of signal
was observed in the mixture of valeraldehyde and PBN,
showing a standard six-line spectrum (g = 2.0064, αN = 14.9 G,
αH = 2.93 G). These results strongly indicated that
valeraldehyde could be reduced at the cathode to generate
the crucial aliphatic ketyl radical species.
Combining the above experimental results and literature
survey, a plausible mechanism for the electrochemical synthesis
of alcohols from aldehydes and ketones was proposed (Scheme
5). On one hand, carbonyl compound 1 undergoes a single
voltammetry and electron paramagnetic resonance data
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Chao Yang − State Key Lab of Urban Water Resource and
Environment, School of Science, Harbin Institute of
Technology (Shenzhen), Shenzhen 518055, China;
Email: xyyang@hit.edu.cn
Wujiong Xia − State Key Lab of Urban Water Resource and
Environment, School of Science, Harbin Institute of
Technology (Shenzhen), Shenzhen 518055, China; School of
Chemistry and Chemical Engineering, Henan Normal
University, Xinxiang, Henan 453007, China; orcid.org/
0000-0001-9396-9520; Email: xiawj@hit.edu.cn
Scheme 5. Proposed Reaction Mechanism
Authors
Xiao Zhang − State Key Lab of Urban Water Resource and
Environment, School of Science, Harbin Institute of
Technology (Shenzhen), Shenzhen 518055, China
Han Gao − State Key Lab of Urban Water Resource and
Environment, School of Science, Harbin Institute of
Technology (Shenzhen), Shenzhen 518055, China
Lei Wang − State Key Lab of Urban Water Resource and
Environment, School of Science, Harbin Institute of
Technology (Shenzhen), Shenzhen 518055, China
Lin Guo − State Key Lab of Urban Water Resource and
Environment, School of Science, Harbin Institute of
Technology (Shenzhen), Shenzhen 518055, China
electron transfer (SET) process by cathodic electrolysis to
furnish a crucial ketyl radical intermediate I. Cathodic
reduction of 4-cyanopyridine (2a) generates a persistent
radical anion II,19 which subsequently undergo a radical−
radical cross-coupling with ketyl radical I to produce anion
intermediate III. We speculate that the generation of a
transient radical I and a persistent radical II is more likely to
occur both at the cathode. These two free radicals have more
chance to contact each other at the same electrode surface
during the lifetime of transient ketyl radicals, making the
subsequent radical−radical cross-coupling possible to take
place.20 The final elimination of the nitrile anion occurs and
generates the desired coupling product 3. On the other hand,
additive DABCO serves as the major sacrificial reductant at the
anode surface,21 providing electrons for the simultaneous
cathodic reduction.
In summary, we have established an efficient electrochemical
strategy for the synthesis of secondary and tertiary alcohols. A
variety of aliphatic and aromatic aldehydes as well as ketones
can readily react with electron-deficient (hetero)arenes to
furnish heterocycle-containing alcohol derivatives. Mechanistic
studies reveal that both the crucial aliphatic ketyl radicals and
persistent heteroaryl radical anions can be generated from
substrates by cathodic electrolysis, and the desired products are
afforded through a radical−radical cross-coupling pathway.
The development of other type of electrochemical trans-
formations is currently underway in our laboratory.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00920
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Natural Science Foundation of China (No. 21672047), State
Key Laboratory of Urban Water Resource and Environment
(No. 2018DX02), The Science and Technology Plan of
Shenzhen (JCYJ20180306172044124) and Natural Science
Foundation of Guangdong (No. 2020A1515010564). W. X. is
grateful for the Talent Plan of the Pearl River in Guangdong,
Starting up fund from Shenzhen Government and the financial
support from Guangdong Province Covid-19 Pandemic
Control Research Fund (No.2020KZDZX1218). The project
was also supported by the Open Research Fund of the School
of Chemistry and Chemical Engineering, Henan Normal
University.
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The Supporting Information is available free of charge at
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Experimental details, characterization data, and copies of
NMR spectra for all novel compounds; cyclic
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