Efficient anodic and direct phenol-arene C,C cross-coupling: The benign role of water or methanol

Article abstract of DOI:10.1021/ja211005g

C,C cross-coupling reactions for the synthesis of nonsymmetrical biaryls represent one of the most significant transformations in contemporary organic chemistry. A variety of useful synthetic methods have been developed in recent decades, since nonsymmetrical biaryls play an evident role in natural product synthesis, as ligand systems in homogeneous catalysis and materials science. Transformation of simple arenes by direct C,H activation belongs to the cutting-edge strategies for creating biaryls; in particular the 2-fold C,H activation is of significant interest. However, in most examples very costly noble metal catalysts, ligand systems, and significant amount of waste-producing oxidants are required. Electrochemical procedures are considered as inherently "green" methods, because only electrons are required and therefore, no reagent waste is produced. Here, we report a metal-free electrochemical method for cross-coupling between phenols and arenes using boron-doped diamond (BDD) anodes in fluorinated media. Our sustainable approach requires no leaving functionalities. Employing water or methanol as mediator represents the key improvement for achieving nonsymmetrical biaryls with superb selectivity and synthetic attractive yields.

Full text of DOI:10.1021/ja211005g

Article
pubs.acs.org/JACS
Efficient Anodic and Direct Phenol-Arene C,C Cross-Coupling: The
Benign Role of Water or Methanol
Axel Kirste,‡ Bernd Elsler,† Gregor Schnakenburg,§ and Siegfried R. Waldvogel*,†
†Institute for Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55128 Mainz, Germany
§
‡Kekule
́
Institute for Organic Chemistry and Biochemistry and Institute for Inorganic Chemistry, University of Bonn,
Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany
S
* Supporting Information
ABSTRACT: C,C cross-coupling reactions for the synthesis of
nonsymmetrical biaryls represent one of the most significant tran-
sformations in contemporary organic chemistry. A variety of useful
synthetic methods have been developed in recent decades, since
nonsymmetrical biaryls play an evident role in natural product
synthesis, as ligand systems in homogeneous catalysis and materials
science. Transformation of simple arenes by direct C,H activation
belongs to the cutting-edge strategies for creating biaryls; in particular the 2-fold C,H activation is of significant interest.
However, in most examples very costly noble metal catalysts, ligand systems, and significant amount of waste-producing oxidants
are required. Electrochemical procedures are considered as inherently “green” methods, because only electrons are required and
therefore, no reagent waste is produced. Here, we report a metal-free electrochemical method for cross-coupling between
phenols and arenes using boron-doped diamond (BDD) anodes in fluorinated media. Our sustainable approach requires no
leaving functionalities. Employing water or methanol as mediator represents the key improvement for achieving nonsymmetrical
biaryls with superb selectivity and synthetic attractive yields.
Because of the high overpotential for oxygen evolution in
aqueous and alcoholic media, highly reactive hydroxyl or
alkoxyl radicals21 are directly and efficiently formed. These
enormously aggressive spin centers are employed for waste-
water treatment or constructive synthesis.22 In general, the oxida-
tive power usually leads to a complete electrochemical
mineralization ending with CO2. Fluorinated alcohols like
1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) turned out to be the
key for stabilizing these intermediates and minimizing
mineralization of the substrates.23,24 This particular solvent
has experienced significant attention in organic synthesis since
its use affected several improvements in organic synthesis.25
Combining both materials, BDD and HFIP, we developed an
anodic biphenol synthesis26 to provide starting material for
supercapacitor applications.27 Although the yields were
moderate to low, the first anodic phenol−arene cross-coupling
was demonstrated.28
INTRODUCTION
■
Nonsymmetrical biaryls play an important role in organic
chemistry in natural product synthesis,1 molecular catalysis,2
and materials science.3 C,C cross-coupling reactions represent
highly versatile transformations for their synthesis.4 Typically,
such coupling reactions require leaving functionalities and
complex catalysts often based on toxic transition metals, such
as palladium.5,6 Selective functionalization of arenes by direct
C,H activation consequently belongs to the hot topics in con-
temporary organic synthesis. A variety of efficient protocols for
this modern strategy were recently developed, allowing a
catalytic approach to the active transition metal species in the
presence of only one activated coupling partner.7−10 C,H acti-
vation of one reaction partner can also be achieved by an
organocatalytic pathway.11 Considerably more atom economy
results in the direct oxidative coupling of arenes.12,13 The
absence of leaving functionalities creates a cutting-edge concept.
Unfortunately, most of the elaborated examples require costly
catalyst systems and produce large quantities of reagent waste.
Electroorganic synthesis has turned out to be an attractive
alternative in the preparation of organic compounds,14 in
particular when C,C coupling reactions are involved.15,16 Since
only electrons serve as reagent and carbon electrodes are re-
produced sustainably, electrochemistry complies with the terms
of “green chemistry”.17 Particular attention was given to the
novel electrode material boron-doped diamond (BDD), which
is easily available by chemical vapor deposition methods.18 Due
to their unique electrochemical properties,19 BDD electrodes
open up new synthetic pathways in electroorganic synthesis.20
Here, we report the specific role of water or methanol being
the key additive with respect to selectivity, scope, and
minimizing overoxidation of the desired products.
EXPERIMENTAL DETAILS
■
An experimental setup is shown in Figure 2 in the Supporting
Information. A solution of phenol component (0.005 mol),
arene component (0.015 mol), and N-methyl-N,N,N-triethy-
lammonium methylsulfate (Et3NMe·O3SOMe, 0.68 g, 0.003 mol)
Received: December 6, 2011
Published: January 16, 2012
© 2012 American Chemical Society
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in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP, 27 mL) and
methanol (6 mL) was transferred into an undivided electrolysis
cell equipped with a BDD anode and a nickel cathode. At
50 °C, a constant current electrolysis with a current density of
2.8 mA/cm2 was performed. After 965 C (2 F/mol of phenol),
the electrolysis was stopped and the solvent mixture was recovered
in vacuo (50 °C, 150 mbar). Nonconverted starting materials
were recovered by short-path distillation (120−145 °C, 10−3
mbar), and purification of the crude products by column
chromatography (SiO2, ethyl acetate/cyclohexane) afforded the
coupling products. They were washed with cold n-heptane
(0 °C) and dried in high vacuum to afford the corresponding
nonsymmetrical biaryls as almost colorless solids.
traces of water and methanol. Thus, a systematic variation with
well-defined amounts of water or methanol in the electrolysis
was carried out (Table 1, entries 2−12). For easier workup on a
preparative scale, 1ab was acylated. With water as the additive,
best results were obtained in the presence of 9 vol % H2O; the
cross-coupling product 1ab was formed with pronounced
selectivity with a product ratio of 85:1 compared to
homocoupling product 1bb. The isolated yield for 1d of 67%
is synthetically attractive (Table 1, entry 4). For methanol as
additive, the use of 18 vol % was superior and 1d was isolated in
69% yield with a slightly improved selectivity up to 90:1 (Table
1, entry 10). It is noteworthy that this corresponds to almost
the same molar amount of additive.
In order to minimize the excess of arene component 1b, the
ratio of these substrates was studied. When 3 equiv of 1b was
subjected to electrolysis, the mixed biaryl 1d was the only
detectable dehydrodimer, which was obtained in almost 70%
isolated yield (Table 1, entry 13). The use of these conditions also
leads to decreased oligomer formation due to overoxidation.
The superior properties of HFIP on the anodic phenol−
arene cross-coupling reaction is underscored by the comparison
of other common fluorinated solvents like 1,1,1-trifluoroetha-
nol (TFE), 2,2,3,3,4,4,5,5-octafluoropentanol (OFP), and
trifluoroacetic acid (TFA, Table 2). The latter is known for
RESULTS
■
Since the mechanistic rationale for previous studies was not
clear, the phenol−arene cross-coupling of phenol 1a and
arene 1b on BDD anodes was carried out in highly pure HFIP.
Surprisingly, the desired cross-coupling product 1ab was
only formed only as a minor product (Table 1, entry 1).
Table 1. Influence of Water and Methanol on the Anodic
a
Phenol−Arene Cross-Coupling Reaction
Table 2. Solvent Effects in the Anodic Phenol−Arene Cross-
a
Coupling Reaction
b
entry
fluorinated solvent
yield, %
CE,
%
selectivity 1ab:1bb
b
c
entry ratio 1a:1b HFIP/ROH, mL/mL
yield,
%
ratio 1ab: 1bb
c
1
2
3
4
HFIP
TFE
OFP
TFA
69 (1ab)
33 (1ab)
23 (1ab)
49 (1d)
69
>100:1
1
1: 5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:7
1:3
1:2
30/-
21 (1d)
47 (1d)
68 (1d)
67 (1d)
41 (1ab)
56 (1d)
66 (1c)
64 (1d)
66 (1d)
69 (1d)
55 (1d)
58 (1d)
69 (1d)
55 (1d)
1:3
1.5:1
8:1
33
23
49
85:1
2
32/1 (H2O)
31/2 (H2O)
30/3 (H2O)
28/5 (H2O)
31/2 (MeOH)
30/3 (MeOH)
29/4 (MeOH)
28/5 (MeOH)
27/6 (MeOH)
26/7 (MeOH)
27/6 (MeOH)
27/6 (MeOH)
27/6 (MeOH)
c
>100:1
3
1:1
4
85:1
a
Electrolysis conditions: 50 °C, BDD anode, constant current (j = 2.8
d
5
>100:1
mA/cm2), undivided cell, 2 F/mol of phenol, ratio phenol/arene = 1:3,
0.68 g of Et3NMe O3SOMe, 27 mL of fluorinated solvent, 6 mL of
MeOH. Current efficiency. 1bb was not detected by GC.
6
2:1
6:1
7
b
c
8
11:1
10:1
90:1
22:1
45:1
9
its ability to stabilize radical cations.23,24 Although TFE and
OFP provided the cross-coupling product 1ab with remarkable
selectivity, the yields dropped dramatically (Table 2, entries 2
and 3). In the case of TFA, an inferior yield for 1ab with no
selectivity was observed.
The properties of the fluorinated electrolyte, such as HFIP,
turned out to be more essential than the nature of the elec-
trode material. Usually, the electrode material represents one
of the most important parameters in electrochemistry, since
different reaction courses are often determined. Interestingly,
the investigated reaction of phenol 1a and arene 1b gave con-
gruent results when performed on different standard electrode
10
11
12
13
14
d
>100:1
45:1
a
Electrolysis conditions: 50 °C, constant current (j = 2.8 mA/cm2),
BDD anode, nickel cathode, undivided cell, 2 F/mol of phenol, 0.68 g
Et3NMe O3SOMe. Isolated yield of cross-coupling derivative.
Determined by GC. Homocoupling product 1bb was not detected.
b
c
d
Employment of recycled HFIP improved the situation
tremendously. H NMR data of such recycled HFIP indicated
1
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materials like platinum, graphite, or glassy carbon (Table 3).
However, BDD is the superior electrode material and provides
by far the best results.
prevented or strongly suppressed. In a few cases, regioisomers
of mixed biaryls are found (3ab, 4ab, 17ab, and 18ab). In these
cases the coupling is not limited to the position ortho to the
OH group (11ab) but also occurs in the position para to it
(12ab). This is because the aromatic coupling partner should
exhibit electron-releasing groups such as methoxy moieties.
Therefore, coupling of such substrate combinations turned out
to be favorable (1ab, 11ab, and 12ab). In addition, alkyl groups
Table 3. Influence of Anode Material on the Phenol−Arene
a
Cross-Coupling Reaction
b
c
entry
anode material
yield,
%
CE, %
ratio 1ab:1bb
d
1
2
3
4
BDD
69
58
59
57
69
58
59
57
>100:1
t
such as methyl, propyl (9ab), and butyl (10ab) proved to be
graphite
50:1
beneficial as well. Application of more electricity than 2 F/mol
strongly promotes the formation of oligomers and a significant
coloring of the electrolyte.28
glassy carbon
platinum
100:1
d
>100:1
a
Electrolysis conditions: 50 °C, constant current (j = 2.8 mA/cm2),
Polycyclic aromatic moieties that are prone to overoxidation,
such as naphthalene (6ab, 7ab, 8ab, and 16d) or anthracene
(5ab) fragments, can also be incorporated. In contrast to other
arene cross-coupling methods, wherein halogen8−11 atoms or
OH13,29 groups were sacrificed, such moieties were conserved
(14ab, 15ab) since no leaving functionality is required.
Remarkably, the scope of the anodic phenol−arene cross-
coupling reaction, proceeding under acidic conditions due to
the 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP), was expanded to
amino arenes (8ab). If phenolic components are subjected with
activated positions being blocked, C,C-coupled aryl cyclo-
hexadienones (17ab−20ab) are furnished. In fact, this
demonstrates the formation of C(sp2)−C(sp3) cross-coupling
products by anodic C,H activation. Their molecular structures
were successfully confirmed by X-ray analysis of suitable single
nickel cathode, undivided cell, 2 F/mol of phenol, 0.68 g of
b
Et3NMe O3SOMe. Isolated yield of cross-coupling derivative.
c
d
Determined by GC. Homocoupling product 1bb was not detected.
To elucidate the scope of the electrolysis protocol, it was
applied to the substrates shown in Table 4. Compared to the
initial conditions of the electrosynthesis,28 the mixed biaryls
6ab, 7ab, 13ab, and 15ab are formed with significantly im-
proved selectivity with respect to the homocoupling products
1bb, 6bb, and 7bb. For comparison, the previous data are given
in parentheses. The isolated yields are the same or even better.
Then the scope was broadened since several new substrates
could be applied (Table 5). The transformation proceeded in
most cases with high chemo- and regioselectivity. The com-
petitive formation of homocoupling products bb was either
a
Table 4. Application of Improved Protocol
a
Electrolysis conditions: 50 °C, BDD anode, constant current (j = 2.8 mA/cm2), undivided cell, 2 F/mol of phenol, ratio phenol/arene = 1:3, 0.68 g
of Et3NMe O3SOMe, 27 mL of HFIP, 6 mL of MeOH. bNonoptimized electrolysis conditions in pure HFIP; ratio phenol/arene = 1:10; values are
taken from ref 3. bb was not detected by GC. Current efficiency.
c
d
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a
Table 5. Anodic Cross-Coupling Reaction of Various Phenols with Different Arenes in the Presence of HFIP/MeOH
a
Electrolysis conditions: 50 °C, BDD anode, constant current (j = 2.8 mA/cm2), undivided cell, 2 F/mol of phenol, ratio phenol/arene = 1:3, 0.68 g
b
c
of Et3NMe·O3SOMe, 27 mL of HFIP, 6 mL of MeOH. CE, current efficiency. bb was not detected by GC. Isomers could not be separated by
several techniques. For a better separation, the crude product was derivatized.
d
crystals (see Figure 1 and Supporting Information). Thus, this
electrolysis opens up access to an additional attractive substance
family that plays an important role in natural product synthesis.30
confirmed by the result displayed in Table 1, entry 1. The selectivity
was reversed with far inferior yield. Obviously, by employment
of water or methanol, a synthetically attractive access to cross-
coupling product 1ab was established. Almost identical
oxidation potentials for 1a and 1b28 exclude the control of
selectivity by electrochemical potential. Thus, a chemo-
selectivity by stabilization of an oxyl intermediate is obvious.
Consequently, water or methanol as mediator is proposed in
the initial step of the transformation (Scheme 1). Anodic oxida-
tion on BDD results in highly reactive hydroxyl or methoxyl
radicals. In the presence of HFIP, the reactivity is decreased due
to an enhanced lifetime23,24 of these spin centers. The stabiliz-
ing mode of action might be based on oxyl−HFIP interactions.
Because of its strong hydrogen-bond donor ability, HFIP forms
in solid state a helically arranged supramolecular network.31 In
DISCUSSION
■
Very recently, the first anodic phenol−arene cross-coupling
reaction was reported. The key to avoid a statistical product
distribution seemed to be the use of the fluorinated alcohol
HFIP, which was expected to act as a typical mediator. Cyclo-
voltametric measurements of HFIP electrolyte on BDD elec-
trodes revealed the largest electrochemical window for a protic
system of approximately 5.0 V.
Since the oxidation potentials of the employed phenols and
arenes were determined to about +1.0 V (vs Ag/AgCl), the
anticipated role as mediator of HFIP was excluded. This was
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originally was expected according to the oxidation potentials of
A and B, was successfully circumvented.
CONCLUSION
■
In summary, a novel and significantly improved electrochemical
C,C cross-coupling was reported. Advantageously, various non-
symmetrical biaryls are available in a metal-free and highly
selective transformation. Moreover, no additional leaving
functionalities are required for this conversion, which allows
the use of very simple starting materials. Since only electrons
are used as oxidants, no reagent waste is produced, based on
stoichiometric reagents. In addition to that, this new protocol
represents the first example of supramolecular taming of oxyl
species. This approach allows the synthetic use of highly
reactive intermediates, which usually cause complete substrate
degradation. Thus, this approach fulfills the requirements for
“green chemistry”. It represents a powerful and practicable
method for direct arylation by C,H functionalization in present-
day organic synthesis.
Figure 1. Molecular structures of (a) 19ab and (b) 7ab by X-ray
analysis.
ASSOCIATED CONTENT
* Supporting Information
■
S
analogy, a HFIP cluster (I) via hydrogen bonding in liquid state
is anticipated wherein oxyl intermediates are involved. This
supramolecular caging provides the stabilizing effect. Sub-
sequently, the encased spin centers attack the O−H group of
the phenol chemoselectively and abstract the corresponding
hydrogen atom, resulting in phenoxyl radicals II.
Such a reaction sequence is known as an electrochemical
Umpolung reaction.15,32 This indirect electrochemical formation
of phenoxyl radicals cannot be directly achieved under poten-
tiostatic control since phenol A and arene B exhibit very similar
oxidation potentials.28 Behaving as electrophiles, these radicals
II are trapped by electron-rich arene B, which acts as nucleophile.
The resulting tautomers III and IV undergo a second oxidation,
accomplishing the mixed biaryls AB after rearomatization.
Following this strategy, a statistical product mixture, which
Additional text, 13 figures, and two tables giving further results,
preparation of substrates, electrochemical setup, and analytical
data of products (PDF) and crystallographic data (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
waldvogel@uni-mainz.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Support by the SFB 813 Chemistry at Spin Centers (DFG) is
highly appreciated.
■
Scheme 1. Proposed Mechanism for Anodic Phenol−Arene C,C Cross-Coupling on Boron-Doped Diamond in the Presence of
a
HFIP
a
Water or methanol is oxidized at the anode, forming highly reactive spin centers (I). Encased in a H-bonding solvent cluster of HFIP, their lifetime
is dramatically enhanced. Chemoselective attack of phenol A furnishes phenoxyl radicals (II). Trapping them by excess arene B finally achieves cross-
coupling product AB after a second oxidation step of intermediates III and IV.
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Huitle, C. A., Eds.; Wiley: New York, 2011; pp 483−510.
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