Highly selective electrosynthesis of biphenols on graphite electrodes in fluorinated media

Article abstract of DOI:10.1002/chem.201102182

The direct and selective phenol coupling reaction that provides biphenols still represents a challenge in organic synthesis. The recently developed electrosynthesis on boron-doped diamond anodes with fluorinated additives was developed further to allow the application to less-expensive electrodes and fluorinated media. This advanced protocol allows the highly selective anodic phenol coupling reaction on graphite with a broad scope.

Full text of DOI:10.1002/chem.201102182

DOI: 10.1002/chem.201102182
Highly Selective Electrosynthesis of Biphenols on Graphite Electrodes in
Fluorinated Media
Axel Kirste,[b] Shotaro Hayashi,[d] Gregor Schnakenburg,[c] Itamar M. Malkowsky,[f]
Florian Stecker,[f] Andreas Fischer,[f] Toshio Fuchigami,[e] and Siegfried R. Waldvogel*[a]
Abstract: The direct and selective phenol coupling reaction that provides biphe-
nols still represents a challenge in organic synthesis. The recently developed elec-
trosynthesis on boron-doped diamond anodes with fluorinated additives was devel-
À
Keywords: C C coupling · electro-
chemistry · fluorinated compounds ·
oped further to allow the application to less-expensive electrodes and fluorinated
oxidation · phenols
media. This advanced protocol allows the highly selective anodic phenol coupling
reaction on graphite with a broad scope.
Introduction
tecture a reliable and sustainable synthesis is challenging.
Direct oxidation of methyl-substituted phenols by conven-
tional oxidation methods or anodic treatment mostly results
in a complex mixture of products—where the desired biphe-
nol represents only a minor component.[7] Purification of
such multicomponent mixtures is costly and time consuming.
In many fields of organic chemistry biphenols represent a
common structural motif, in particular in natural product
synthesis[1] and in technical applications.[2] In homogeneous
catalysis biphenols have an outstanding role as ligands or
starting materials thereof. Some methylated biphenols, for
example, 2,2’-dihydroxy-3,3’,5,5’-tetramethylbiphenyl (2), are
important as backbone compounds for powerful ligand sys-
tems.[3,4,5] Furthermore, recent investigations have revealed
that biphenoxyborates-based electrolytes exhibit an out-
standing electrochemical stability. Thus, they were applied
as novel electrolyte systems to super capacitor cells.[6]
À
À
In addition to C C bond formation, C O coupling with sub-
sequent rearrangements and integration of additional
phenol units takes place leading to unique polycyclic scaf-
folds.[8] This allows molecular diversity within very few steps
by overoxidation (Scheme 1).[9]
Because of the high significance of methylated biphenols,
selective and efficient synthetic methods are required for
their preparation. Unfortunately, despite their simple archi-
[a] Prof. Dr. S. R. Waldvogel
Scheme 1. Product distribution for the oxidation reaction of simple phe-
nols.
Johannes Gutenberg-Universitꢀt Mainz
Institut fꢁr Organische Chemie
Duesbergweg 10-14, 55128 Mainz (Germany)
Fax : (+49)6131-39-26777
Oxidative treatment of phenols by inorganic oxidants, for
example, peroxodisulfate, or transition-metal cations in high
oxidation states, for example, Fe3+, V5+, Cr6+, or Mn7+
gives rise to excessive reagent waste. Electrosynthetic meth-
ods provide an outstanding atom economic and efficient ap-
proach because only electrons are shifted. In this case, no
leaving functionalities are necessary and only hydrogen
atoms are sacrificed. Consequently, excessive reagent waste
could be avoided. Moreover, a large variety of useful elec-
E-mail: waldvogel@uni-mainz.de
[b] A. Kirste
Rheinische Friedrich-Wilhelms-Universitꢀt Bonn
Kekulꢂ-Institut fꢁr Organische Chemie und Biochemie
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
[c] Dr. G. Schnakenburg
Rheinische Friedrich-Wilhelms-Universitꢀt Bonn
Rçntgenstruktur-Abteilung, Institut fꢁr Anorganische Chemie
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
[d] Dr. S. Hayashi
National Defense Academy, Department of Applied Chemistry
1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-9696 (Japan)
À
trochemical protocols for C C bond formation has been de-
veloped in the past.[10,11] To overcome the challenges of un-
desired phenol-coupling pathways an electrochemical proto-
col based on a boron template strategy was successfully es-
tablished.[12] Because a direct synthesis would allow a prepa-
rative short-cut, we focused on this practical one-step
procedure. Unfortunately, only very few examples for the
selective anodic biphenol synthesis using common electrode
[e] Prof. Dr. T. Fuchigami
Tokyo Institute of Technology, Department of Electronic Chemistry
Midori Ku, Yokohama, Kanagawa 226-8502 (Japan)
[f] Dr. I. M. Malkowsky, Dr. F. Stecker, Dr. A. Fischer
BASF SE, GCI/E-M311, 67056 Ludwigshafen (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102182.
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FULL PAPER
materials such as platinum or graphite exist. The electrooxi-
dation of vanillin and eugenol on platinum in basic media
into the corresponding natural occurring biphenols has been
reported.[13,14] Owing to the high costs of platinum, carbon
electrode materials are definitely more attractive for techni-
cal applications. Unfortunately, the conversion of simple me-
thylated phenols like para-cresol or 2,4-dimethylphenol (1)
lead to the formation of the corresponding Pummererꢄs
ketone derivative 3 and not to the biphenol.[15] Significant
progress was achieved by the use of boron-doped diamond
(BDD) electrodes. Their unique electrochemical properties
open up new synthetic pathways because oxyl radicals are
easily formed in aqueous media. Through this approach a
direct and selective electrosynthesis of 2 to 2,4-dimethylphe-
nol (1) was developed.[16] However, the substrate scope was
limited to only a few phenols. With the addition of fluorinat-
ed alcohols, for example, 1,1,1,3,3,3-hexafluoroisopropanol
(10), the method became more general.[17] As fluorinated al-
cohols such as 10 tremendously enhance the lifetime of
highly reactive spin centers,[18] this modified electrolysis
allows the conversion of various electron-rich and halogen-
ated phenols in a selective manner.[19] The stabilizing effect
might be attributed to the trapping of intermediate oxyl spe-
cies in a hydrogen-bonding network.[20] The non-nucleophilic
and redox stable properties of 10 are often exploited in oxi-
dative conversions.[21] Moreover, in this way the nonsymmet-
rical biphenol 30 was obtained exclusively.[22] This discovery
opened up the pathway for the first anodic phenol–arene
cross-coupling reaction, which enables the selective electro-
synthesis of various nonsymmetrical biaryls. Therefore, the
strategy belongs to the current cutting-edge approaches of
modern arylation methods.[22] As the technical application of
BDD electrodes is still challenging and the technical use of
highly fluorinated alcohols like 10 is nonpracticable, this
strategy has to be transferred to more simple and less costly
electrode/electrolyte systems. Herein, we report the success-
ful modification and application of the electrolysis protocol
using readily available graphite electrodes. A technical elec-
troorganic synthesis is mostly realized with graphite electro-
des.[23] A variety of additives were tested in respect to selec-
tivity, yield, and scope of substrates.
Scheme 2. Selective electrosynthesis of biphenol 2 on graphite.
volume of 30 mL, where the phenolic substrate (52 wt%)
served as the major component. For stabilizing reactive in-
termediates and minimizing overoxidation of the biphenols,
fluorinated solvents were added only in small quantities (4–
9 mL).
The ionic liquids 5–7, which are based upon quaternary
ammonia salts, were employed as supporting electrolytes be-
cause they were the most appealing in tests based on past
studies (Scheme 3).[25] To prevent degradation, electrolysis
Scheme 3. Ionic liquids as supporting electrolytes in anodic phenol cou-
pling reactions. Pyr=pyrrolidine.
was stopped before complete conversion of the phenolic
starting material. Advantageously, the desired biphenols
were obtained by simple and practical purification steps.
After electrolysis, abundant starting material was recovered
by short-path distillation. Subsequent column chromatogra-
phy of the crude product on silica gel or crystallization from
isopropanol/water mixtures yielded the biphenols (see the
Supporting Information). The results of the optimization
studies are summarized in Table 1. Initial reactions were car-
ried out with 1,1,1,3,3,3-hexafluoroisopropanol (10, HFIP)
as an additive and 5 as the supporting electrolyte. When
high current densities were chosen, biphenol 2 was isolated
in yields of 50–60% (entries 1 and 2), but also a significant
amount of polymeric by-product was observed. In addition,
the inner cell resistance has increased significantly during
electrolysis.
A partial conversion of 1 with the application of 0.77 F
per mol and a constant current of 10 mAcmÀ2 revealed the
best result with respect to yield and minimizing side reac-
tions (entry 3). Lowering the current density (entry 4) or the
quantity of 10 resulted in no improvement (entry 5). When
a 3–5 fold quantity of supporting electrolyte was employed
the applied cell voltage decreased to 50%, but the yield of
biphenol 2 did not improve (entries 6 and 7). Switching to
ionic liquid 6 as the supporting electrolyte, led to similar re-
sults (entry 8). As 10 is still relatively costly an alternative
additive was required. In anodic oxidation of arenes, tri-
fluoroacetic acid (19; TFA) is a common electrolyte compo-
nent. In previous studies of analogous phenol coupling on
boron-doped diamond, 19 proved to be dramatically inferior
as an additive.[16] However, different results were observed
Results and Discussion
When performing a constant-current electrolysis various pa-
rameters can be adjusted to control the electrolysis. The
most important are: electrode material, current density,
amount of applied electricity, and the composition of the re-
spective electrolyte system. Our experience from previous
investigations has shown that it is useful to establish suitable
electrolysis conditions first. The oxidative coupling reaction
of 2,4-dimethylphenol (2) on graphite anodes served as a
model system, because this electrooxidation was intensively
investigated in the recent past (Scheme 2).[24] In the course
of the study several electrolyte compositions were tested.
The electrolyses were carried out in an undivided cell with a
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S. R. Waldvogel et al.
Table 1. Screening for general conditions in the selective synthesis of biphenol on graphite electrodes by constant current electrolysis of 1 in an undivid-
ed cell.
Fluorinated
Supporting
Electricity[a]
Umax [V] j [mAcm-2][b] T [8C] Yield of 2 [%][c,d] CE [%][e] Recovered 1 [%][f]
(isolated)
additive (wt%) electrolyte (wt%) [F per mol of phenol]
AHCTUNGTRENNUNG
1
2
3
4
5
6
7
8
9
10 (45)
10 (45)
10 (45)
10 (45)
10 (28)
10 (43)
10 (41)
10 (45)
19 (45)
5 (3)
5 (3)
5 (3)
5 (3)
5 (4)
5 (9)
5 (14)
6 (3)
5 (3)
5 (3)
7 (3)
1.00
0.77
0.77
0.77
0.77
0.77
0.77
0.77
1.00
0.77
0.77
27
22
17
13
21
8
10
17
14
18
20
20
20
10
5
10
10
10
10
10
10
10
30
30
30
30
30
30
30
30
50
30
30
66 (60)
51
66 (55)
49 (49)
49 (49)
47
53
49
50
35
36
35
38
43
41
55
43
19
25
42
44
44
43
40
39
19
37
35
53
54 (51)
51 (45)
67 (64)
– (51)
10 19 (45)
11 19 (45)
[a] In the range of 0.77–1.00 F per mol of phenol applied electricity; the product precipitated affecting an increased cell resistance. Consequently, the cell
voltage abruptly rose. [b] j=current density [c] Yield based on recovered starting material. [d] Yields of 2 were estimated by gas chromatography using
pentadecane as an internal standard. Yield of isolated product is given in parenthesis. [e] Current efficiency (CE) is defined as the quotient of electricity
consumed for the synthesis of 2 by total applied electricity. [f] Unchanged starting phenol 1 was recovered by short-path distillation. CE=current effi-
ciency,.
in this study: Elevated temperature lowered the cell resist-
ance, but it seemed to be unfavorable with respect to yield
(entry 9). When the electrolysis was performed at 308C, the
yield of isolated 2 was slightly improved with an acceptable
current density (entry 10). Use of the more common imida-
zolium-based ionic liquid 7 did not ameliorate the result
(entry 11). A major challenge in the direct oxidation of 2,4-
dimethylphenol (1) is the control of product selectivity. Con-
sequently, the effect of additives on the product distribution
was studied in more detail. As mentioned above, biphenol 2,
Pummererꢄs ketone derivative 3, and some pentacyclic scaf-
folds were the major products.
Upon oxidation of 1 on a platinum anode in basic media
[7]
À
the C O coupling product 3 dominates. By switching to
Scheme 4. Major products obtained by anodic treatment of 1 on graphite.
BDD anodes and neutral water-containing electrolytes the
selectivity inverted towards the biphenol 2.[15] Applying a
variety of additives (8–20) to electrolysis of 1 on graphite
anodes for the selective formation of biphenol 2 turned out
to be suitable for the synthesis of 3. However, a new by-
product was observed that could not yet be clearly assigned.
Mass spectrometry measurements indicated the formation
of a dehydrotrimer 4 (Scheme 4). The X-ray crystal struc-
ture analysis of a single crystal could not unequivocally
assign the molecular structure. Analysis of the NMR spectra
indicated the presence of two isomers. Atropic isomers
could be excluded because temperature dependent spectros-
copy did not change the number of signals. By GC no
method could be established to separate these isomers. De-
rivatization with propionic acid anhydride confirmed the
presence of two regioisomers 4a and 4b, as NMR analysis
of the product clearly showed two different, but very similar
compounds. Products 4a and 4b are formed by overoxida-
tion of the targeted biphenol 2. The biphenol 2 served as
the substrate for a terphenol coupling reaction. The terphe-
nols only differ in the connectivity on the central benzene
moiety.
fluorinated alcohol 1,1,1-trifluoroethanol (8, TFE) the bi-
phenol product 2 was obtained in good yield and selectivity
(entry 1). A longer fluorinated carbon chain, for example, in
1H,1H,5H-octafluoropentanol 9, led to decreased yield and
selectivity (entry 2). Use of hexafluoroisopropanol (10) im-
proved the formation of biphenol 2 significantly. Thus, the
selectivity of 2 versus 4 was determined as 17:1. Ketone 3
was only detected in trace amounts (<1%; entry 3). Al-
though replacing a CF3 moiety by a phenyl group was ex-
pected to show similar results,[17] only moderate yield and
selectivity were found when 11 (entry 4) was employed. In-
stallation of an additional fluorine group on the phenyl
moiety (entry 5) or a CF3 substituent gave comparable re-
sults (entry 6). Although the use of enol 14 led to the exclu-
sive formation of the desired biphenol 2, it was formed in
20% yield with only moderate efficiency (entry 7). In the
case of methane sulfonic acid (16) similar data were ob-
tained (entry 9). Slight improvement was observed by the
addition of acetic acid (17; entry 10) or its trichloro ana-
logue 18 (entry 11). However, the acidic nature of the addi-
tive did not seem to be crucial. The application of fluorinat-
The results of the screening for additives are summarized
in Table 2. By performing the electrolysis with the simplest
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Electrosynthesis of Biphenols
FULL PAPER
Table 2. Influence of additives on the product distribution in the electrol-
ysis of 1.[a]
Additive
Yield of
CE
[%]
Product
2 [%][b]
ratio 2/3/4[c]
1
2
8
9
48
38
32
27
13:<1[d]:1
8:<1[d]:1
3
4
10
11
55
21
49
14
17:<1[d]:1
4:–:1
5
12
15
14
8:–:1
Scheme 5. Scope of the electrochemical synthesis of biphenols.
6
7
13
14
6
5
8
6:3:1
–
served. Remarkably, several halogenated biphenols were
formed upon direct oxidation of the corresponding phenols.
The electrolysis on graphite is particularly superior to the
transformation of halogenated phenols at boron-doped
anodes, for example, the conversion of 2-bromo-4-methyl-
phenol led to dibromobiphenol 22 in 76% yield. The elec-
trolysis of an electron-poor phenol, such as 2-fluoromethyl-
phenol still provided the corresponding biphenol 23. In the
conversion of chlorinated cresols both regioisomers para-
chloro- and ortho-chlorocresol was coupled and formed bi-
phenols 24 and 25, respectively.
20
8
9
15
16
trace[e]
6
–
4
–
15:2:1
10
11
12
13
17
18
19
20
20
14
10
53
43
6:–:1
7:–:1
16:–:1
7:–:1
18
64
Notably, electrolysis of 4-
chloro-2-methylphenol yielded
the dibenzofuran derivative 26
49[f]
as a by-product in 6% yield.
The molecular structure was
verified by X-ray crystal struc-
ture analysis of a suitable single
[a] General electrolysis conditions: graphite electrodes, constant current
j=10 mAcmÀ2
Q=0.77 F per mol of phenol 1, 308C, 3 wt% of
,
[Et3NMe] [O3SOMe] (5), 45 wt% of additive, and 52 wt% of phenol 1.
[b] Yield based on recovered starting material. [c] Product ration was es-
timated from gas chromatographic analysis of the crude product. [d] 3
was observed in quantities far below 1%. [e] A black, highly viscous oil
was formed containing only small quantities of 2. [f] Owing to low con-
ductivity the electrolysis was carried out at 508C.
crystal (see the Supporting In-
formation). No analogous by-products were detected upon
electrolysis of other phenols. These findings represent a sig-
nificant improvement of selectivity because in the past elec-
trolysis of halogenated phenols led to multicomponent prod-
uct mixtures only.[26] In the course of our studies, benzyl-
and tert-butyl-substituted substrates were tested. Upon elec-
trolysis according to the protocol, complex product mixtures
were obtained. The desired biphenolic species were ob-
served, but represented a minor component because random
transfer of tert-butyl or benzyl moieties was detected. Thus,
purification of these mixtures was pointless. These facts indi-
cate a reaction pathway that proceeds via a cationic species,
which can liberate cationic fragments thus leading to the ob-
served multicomponent mixture.
ed carboxylic acids, for example, TFA (19) turned out to be
beneficial for the electrolysis. In this way the formation of
ketone 3 was suppressed and biphenol 2 was selectively ob-
tained in 64% yield of isolated product (entry 12). In the
case of heptafluorobutyric acid (20) the conditions were not
improved any further. However, additive 20 is favorable for
conversions at higher temperature (entry 13). Fortunately,
the novel electrolysis protocol is not limited to substrate 1.
Thus, a variety of substituted phenols were subjected to the
optimized electrolysis conditions. A selection of biphenol
products is shown in Scheme 5. In all cases the ortho-ortho
biphenols 21–25 were obtained in a highly selective manner
or even exclusively. In this way the hexamethylated biphe-
nol 21 was accessible and isolated in 30% yield. As a result
of its electron-rich nature, the overoxidation of 21 is ob-
Subjecting 2-naphthol (27) to the elaborated protocol to
form BINOL (28) failed. As 27 is insoluble in TFA (19) and
the melting point of the substrate is 1238C, the reaction was
carried out at 1008C. Therefore, heptafluoroacetic acid (20)
was used instead of TFA. This modification of the protocol
enabled
a successful conversion (Scheme 6). However,
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S. R. Waldvogel et al.
Conclusion
The combination of readily available TFA and common
graphite electrodes is powerful electrode/electrolyte
a
system. For the TFA/graphite anode system a similar reac-
tivity profile as for boron-doped diamond was found. The
scope of the phenol coupling is strikingly similar. The proto-
col is suitable for a range of electron-rich and halogenated
phenols. Moreover, even homocoupling reaction of 4-meth-
ylguaiacol resulting in a nonsymmetrical biphenol was possi-
ble. This inexpensive approach can be used instead of the
boron-doped diamond electrodes in the synthesis of biphe-
nol using electroorganic methods. Because graphite is the
preferred technical electrode material, a scale-up for this
electroorganic method is viable. The protocol seems to be
an outstanding tool for electroorganic conversions and will
be applied to other transformations, which are typically per-
formed on boron-doped diamond, and reported in due
course.
Scheme 6. Electrosynthesis of BINOL (28).
avoiding the overoxidation of BINOL is, in this, case chal-
lenging. The BINOL product can be synthesized electro-
chemically by a TEMPO-mediated process.[27]
As recently reported,[21] an unexpected coupling product
was discovered for the anodic transformation of 4-methyl-
guaiacol (29) on boron-doped diamond in hexafluoroisopro-
panol 10. These conditions provide the ortho-meta homo-
coupling product 30 exclusively. Consequently, the electroly-
sis of guaiacol 29 was carried out according to the elaborat-
ed protocol at graphite anodes using TFA as the additive
(Scheme 7).
Experimental Section
General procedure for electrolysis of phenols on graphite electrodes: A
solution of phenol derivative (52 wt%), [Et3NMe] [O3SOMe] (5, 3 wt%),
and additive (45 wt%) was transferred into a undivided standard elec-
trolysis cell equipped with two graphite electrodes. At 308C a galvano-
static electrolysis with a current density of 10 mAcmÀ2 was performed.
After complete reaction (0.77–1.00 Fpermol of phenol) the electrolysis
was stopped, the electrolyte transferred into a flask by methanol (in the
case of acidic additives) or toluene (in the case of alcoholic additives)
and the solution was concentrated in vacuo. Solvents were recovered by
distillation or column chromatography. Unchanged starting material was
recovered by short-path distillation (1408C (oil bath temperature),
10À2 mbar).
Scheme 7. Electrooxidation of guaiacol 29 to give the nonsymmetrical bi-
phenol 30.
With the less expensive electrode/electrolyte system the
nonsymmetrical biphenol 30 was the only coupling product
and was isolated in approximately 55% yield. Different
electrolyses were performed by applying various amounts of
electric current (Table 3). The best combination with respect
Modified procedure for electrolysis of 2-naphthol (27) on graphite elec-
trodes: A solution of 2-naphthol (27, 22 wt%, 10 g, 0.07 mol), [Et3NMe]
[O3SOMe] (5, 7 wt%, 3 g, 0.012 mol), and heptafluorobutyric acid (20,
71 wt%, 20 mL) was transferred into a undivided standard electrolysis
cell equipped with two graphite electrodes. At 1008C, a galvanostatic
electrolysis with a current density of 10 mAcmÀ2 was performed. After
complete reaction (0.77 Fpermol of phenol) the electrolysis was stopped,
the electrolyte transferred into a flask by methanol and the solution was
concentrated in vacuo.
Table 3. Electrolysis of 29 by applying various amounts of electricity.[a]
Electricity
[F per mol of 29]
Yield of
CE
[%]
Recovered
30 [%][b]
29 [%][c]
1
2
3
4
0.77
0.9
1.0
55
57
54
35
45
31
36
13
31
49
33
21
For workup and further synthetic procedures as well as analytical details
see the Supporting Information.
2.0
[a] General electrolysis conditions: graphite electrodes, constant current
j=10 mAcmÀ2, 308C, 3 wt% of [Et3NMe] [O3SOMe] (5), 45 wt% of ad-
ditive, and 52 wt% of phenol 29. [b] Yield based on recovered starting
material. [c] Unchanged starting phenol 29 was recovered by short-path
distillation.
Acknowledgements
The support by BASF SE and the SFB 813 Chemistry at Spin Centers
(DFG) is highly appreciated. S.H. and T.F. thank the Global COE Pro-
gram (Tokyo Institute of Technology) for financial support.
to product yield and current efficiency was achieved after
application of 0.77 F of electricity per mol of 29 (entry 1).
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yields, but owing to overoxidation processes the current effi-
ciencies were lower (entries 2 and 3). When passing 2 F of
electricity per mol of 29, both the yield and the current effi-
ciency dropped dramatically. The electrolysis resulted in a
deep brown viscous reaction mixture (entry 4).
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FULL PAPER
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Received: July 15, 2011
Published online: November 23, 2011
Chem. Eur. J. 2011, 17, 14164 – 14169
ꢃ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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