Palladium-Catalyzed Electrochemical C-H Alkylation of Arenes

Article abstract of DOI:10.1021/acs.organomet.8b00550

Palladium-catalyzed electrochemical C-H functionalization reactions have emerged as attractive tools for organic synthesis. This process offers an alternative to conventional methods that require harsh chemical oxidants. However, this electrolysis requires divided cells to avoid catalyst deactivation by cathodic reduction. Herein, we report the first example of palladium-catalyzed electrochemical C-H alkylation of arenes using undivided electrochemical cells in water, thereby providing a practical solution for the introduction of alkyl groups into arenes.

Full text of DOI:10.1021/acs.organomet.8b00550

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Palladium-Catalyzed Electrochemical C−H Alkylation of Arenes
Qi-Liang Yang,†,‡ Chuan-Zeng Li,‡,§ Liang-Wei Zhang,‡ Yu-Yan Li,*,§ Xiaofeng Tong,† Xin-Yan Wu,*,†
and Tian-Sheng Mei*,‡
†Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China
University of Science and Technology, 130 Meilong Road, Shanghai 200237, People’s Republic of China
‡State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, People’s Republic of China
§Department of Medicinal Chemistry, China Pharmaceutical University, Nanjing 210009, People’s Republic of China
S
* Supporting Information
ABSTRACT: Palladium-catalyzed electrochemical C−H
functionalization reactions have emerged as attractive tools
for organic synthesis. This process offers an alternative to
conventional methods that require harsh chemical oxidants.
However, this electrolysis requires divided cells to avoid
catalyst deactivation by cathodic reduction. Herein, we report
the first example of palladium-catalyzed electrochemical C−H
alkylation of arenes using undivided electrochemical cells in water, thereby providing a practical solution for the introduction of
alkyl groups into arenes.
uring the past decades, transition-metal-catalyzed site-
Dselective C−H functionalization reactions have devel-
oped into promising transformations for the construction of
carbon−carbon (C−C) bonds.1 Palladium-catalyzed C−H
cross-coupling reactions, in particular, have received significant
attention.2 In 2006, Yu and co-workers elegantly demonstrated
the first example of Pd-catalyzed aryl C−H cross-couplings
with alkyl tin and alkyl boron reagents (Scheme 1a).3 With this
seminal report as inspiration, many examples of Pd-catalyzed
aryl C−H couplings with organoboron reagents have been
developed.4,5 While these transformations hold great promise
in expedient construction of C−C bonds in organic synthesis,
the need for a stoichiometric amount of a transition-metal
chemical oxidant constitutes a practical disadvantage in these
systems.6 Thus, the development of novel oxidation systems is
in high demand.7
Transition-metal-catalyzed electrochemical C−H bond
functionalization has emerged as an attractive tool for organic
synthesis, since it avoids the use of dangerous and toxic
chemical oxidants, thereby eliminating side reactions and
byproducts.8−10 Inspired by seminal work from Amatore,
Jutand, and Kakiuchi,11 our group has developed Pd-catalyzed
electrochemical C(sp3)−H and C(sp2)−H oxygenation.12 In
addition, we recently demonstrated Pd-catalyzed electro-
chemical aryl C−H methylation with MeBF3K (Scheme
1b).13 Unfortunately, this alkylation is mainly limited to
methylation.14 The use of divided electrochemical cells is also
problematic, since it complicates the setup.15 Herein, we report
Pd-catalyzed electrochemical C−H alkylation of arenes and
alkyl boron reagents using an undivided cell in water (Scheme
1c).
Scheme 1. Pd-Catalyzed C−H Alkylation
Initially, we chose 2-(o-tolyl)pyridine (1a) and potassium
trifluoromethylborate (MeBF3K) as reaction partners and
probed various reaction conditions for the electrochemical C−
H alkylation in an undivided cell. After extensive optimization,
we found that a 70% isolated yield of desired product (2a)
could be obtained under constant-current electrolysis at 1.0
mA in the presence of 10 mol % Pd(OAc)2 and 2 equivalents
Special Issue: Organometallic Electrochemistry: Redox Catalysis
Going the Smart Way
Received: August 1, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00550
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Communication
a
of MeBF3K in a TFE/AcOH/H2O solution (2 mL/2 mL/0.5
mL) (Table 1, entry 1). Decreasing or increasing the electric
Scheme 2. Evaluation of Arylpyridines
a
Table 1. Reaction Optimization with Substrate 1a
b
entry
variation from standard conditions
yield (%)
c
1
2
3
4
5
6
none
75 (70)
67
63
34
32
0.5 mA instead of 1 mA
1.5 mA instead of 1 mA
40 °C instead of 60 °C
80 °C instead of 60 °C
TFE/AcOH = 2.25 mL/2.25 mL instead of
TFE/AcOH/H2O = 2 mL/2 mL/0.5 mL
65
7
8
TFE/H2O = 2.25 mL/2.25 mL instead of TFE/AcOH/ 52
H2O = 2 mL/2 mL/0.5 mL
AcOH/H2O = 2.25 mL/2.25 mL instead of
TFE/AcOH/H2O = 2 mL/2 mL/0.5 mL
64
9
10
no Pd(OAc)2
no electric current
nr
nr
a
Standard conditions: 1a (0.3 mmol), MeBF3K (2.0 equiv),
Pd(OAc)2 (10 mol %), and TFE/AcOH/H2O (2 mL/2 mL/0.5
mL), in an undivided cell with two platinum electrodes (each 1.5 ×
1.0 cm2), 60 °C, 1.0 mA, 18 h, 2.2 F mol−1. TFE = trifluoroethanol.
a
b
Standard conditions: 1 (0.3 mmol), MeBF3K (2.0 equiv), Pd(OAc)2
(10 mol %), and TFE/AcOH/H2O (2 mL/2 mL/0.5 mL), in an
undivided cell with two platinum electrodes (each 1.5 × 1.0 cm2), 60
°C, 1.0 mA, 18−36 h.
The yield was determined by GC analysis with tridecane as the
c
internal standard. Isolated yield in parentheses.
current results in lower yields (entries 2 and 3). Various
temperatures were examined, and 60 °C afforded the best yield
(entries 4 and 5). Evaluating different solvents revealed that
the combination TFE/AcOH/H2O (2 mL/2 mL/0.5 mL) was
optimal (entries 6−8). Finally, control experiments indicated
that Pd(OAc)2 and electricity are essential for this reaction
(entries 9 and 10).
a
Scheme 3. Evaluation of Alkyltrifluoroborates
With the optimized reaction conditions in hand, the scope of
arylpyridines was investigated to test the generality and
limitations of the reaction. As shown in Scheme 2, arenes
substituted with various functional groups such as alkyl, ether,
fluoro, and trifluoromethyl groups were well tolerated under
the standard reaction conditions (2a−o). In general, substrates
with electron-rich (Me, Et, i-Pr, t-Bu, OMe) substituents
reacted particularly well (2i−m). A strongly electron with-
drawing group, such as CF3, afforded a lower yield as a result of
lower conversion (2o). This alkylation proved sensitive toward
steric encumbrance near the targeted C−H bond: the less
hindered ortho position was preferentially alkylated in the case
of substrates bearing a meta substitutent (2g,p). A dialkylated
product was normally formed in the case of substrates bearing
a para substituent. To our delight, the presence of a methyl
group in the 3-position of a pyridine ring ensures the
monoselectivity (2h−o), presumably due to increased steric
encumbrance for the monoalkylated products.
Encouraged by the feasibility of Pd-catalyzed electro-
chemical methylation using MeBF3K as a coupling partner,
we examined the reactivity of a series of alkyl boron reagents
(Scheme 3). To our satisfaction, potassium trifluoroethylbo-
rate (EtBF3K) and potassium trifluorobutylborate (n-BuBF3K)
are viable alkylation reagents, providing the corresponding
alkylation in moderate yields (3a−d,f−h). However, potassium
trifluorophenethylborate (PhEtBF3K) results in a lower yield
as a result of lower conversion (3e).
a
Standard conditions: 1 (0.3 mmol), MeBF3K (2.0 equiv), Pd(OAc)2
(10 mol %), and TFE/AcOH/H2O (2 mL/2 mL/0.5 mL), in an
undivided cell with two platinum electrodes (each 1.5 × 1.0 cm2), 60
°C, 1.0 mA, 18−36 h.
In order to gain further insight into this electrochemical C−
H alkylation reaction, palladacycle 4 was prepared according to
a literature report.16 Catalytic reactions suggested that this
palladacyle is a viable intermediate for C−H coupling reactions
(Scheme 4). Furthermore, a cyclic voltammogram (CV) of
palladacyle 4 in CH3CN reveals one oxidation peak at 0.94 V
versus Ag/AgCl and one irreversible reduction peak at 0.77 V
(curve d, Figure 1), both of which are significantly lower than
the onset potential for the oxidation of substrate 1a (curve c,
1.70 V) and MeBF3K (curve b, 1.38 V). A catalytic current and
the disappearance of the reductive wave were observed when
B
DOI: 10.1021/acs.organomet.8b00550
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Communication
Scheme 4. Catalytic Reaction with Palladacycle 4
Scheme 5. Kinetic Isotope Effect Studies
Scheme 6. Proposed Catalytic Cycle
Figure 1. Cyclic voltammograms recorded on a platinum electrode
(area 0.03 cm2) at 100 mV s−1: (a) in MeCN containing 0.1 M n-
Bu4NPF6; (b) in solution (a) after addition of 5 mM MeBF3K; (c) in
solution (a) after addition of 5 mM 1a; (d) in solution (a) after
addition of 5 mM palladacycle 4; (e) in solution (d) after addition of
20 mM MeBF3K.
undivided cell in water. This method offers an alternative to
conventional organic synthesis which requires strong chemical
oxidants. In addition, this protocol represents an environ-
mentally friendly tool for the use of an electrochemical
strategy. Investigations aimed at understanding the reaction
mechanism and alkylation of C(sp3)−H bonds are currently
underway in our laboratory.
MeBF3K was added into a solution of palladacyle 4 (curve e),
suggesting that methylation takes place smoothly in the
Pd(III) or Pd(IV) catalyst center.17
In addition, a kinetic isotope effect (KIE) value of 2.0 was
observed in the intermolecular kH/kD value (Scheme 5a).
Futhermore, a similar kinetic isotope effect was detected in
parallel experiments (kH/kD = 2) (Scheme 5b). This indicates
that the C−H cleavage occurs during the rate-determining
step.18
On the basis of the above experimental results, a plausible
mechanism is proposed for the Pd-catalyzed electerochemical
C−H alkylation (Scheme 6). Initially, the nitrogen atom in
substrate 1a coordinates with the palladium catalyst to
generate complex 5. Next, C−H activation takes place to
give palladacyle 4, which is the rate-determining step. Then
complex 4 could be oxidized by anodic oxidation in the
presence of potassium trifluoromethylborate to generate
Pd(III) or Pd(IV) species 6,7 which could afford methylated
product 2a and regenerate Pd(II) species upon reductive
elimination. However, at this stage, we could not rule out the
possibility that this alkylation undergoes Pd(II)/Pd(0)
catalysis.2,7
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00550.
■
S
1
Experimental details, characterization data, and H and
13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for Y.-Y.L.: yylicpu@163.com.
*E-mail for X.-Y.W: xinyanwu@ecust.edu.cn.
*E-mail for T.-S.M.: mei7900@sioc.ac.cn.
■
ORCID
Tian-Sheng Mei: 0000-0002-4985-1071
In summary, we have developed a protocol for palladium-
Notes
catalyzed C(sp2)−H alkylation via anodic oxidation using a
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.organomet.8b00550
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Communication
oborates in conjunction with a MnIII oxidant under mild conditions.
ACKNOWLEDGMENTS
■
Tetrahedron 2013, 69, 5580. (d) Romero-Revilla, J. A.; García-Rubia,
This work was financially supported by the Strategic Priority
Research Program of the Chinese Academy of Sciences (Grant
XDB20000000), “1000-Youth Talents Plan”, NSF of China
(Grants 21572245, 21772222, 21772220), and S&TCSM of
Shanghai (Grants 17JC1401200, 18JC1415600).
́
́
́
́
A.; Arrayas, R. G.; Fernandez-Iban
̃
ez, M. A.; Carretero, J. C.
Palladium-catalyzed coupling of arene C−H bonds with methyl-and
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