Active Molybdenum-Based Anode for Dehydrogenative Coupling Reactions

Article abstract of DOI:10.1002/anie.201712718

A new and powerful active anode system that can be operated in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) has been discovered. In HFIP the molybdenum anode forms a compact, conductive, and electroactive layer of higher-valent molybdenum species. This system can replace powerful but stoichiometrically required Mo^V reagents for the dehydrogenative coupling of aryls. This electrolytic reaction is more sustainable and allows the conversion of a broad scope of activated arenes.

Full text of DOI:10.1002/anie.201712718

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Novel Active Molybdenum-Based Anode for Dehydrogenative
Coupling Reactions
Authors: Siegfried R Waldvogel, Sebastian B Beil, Timo Müller,
Sydney Sillart, Peter Franzmann, Alexander Bomm, Michael
Holtkamp, Uwe Karst, and Wolfgang Schade
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201712718
Angew. Chem. 10.1002/ange.201712718
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Angewandte Chemie International Edition
10.1002/anie.201712718
COMMUNICATION
Novel Active Molybdenum-Based Anode for Dehydrogenative
Coupling Reactions
[
a,b]
[a]
[a]
[a]
[c]
Sebastian B. Beil,
Timo Müller, Sydney B. Sillart, Peter Franzmann, Alexander Bomm,
[
d]
[d]
[c]
[a,b]
Michael Holtkamp, Uwe Karst, Wolfgang Schade, and Siegfried R. Waldvogel*
[
8]
Abstract: A new and powerful active anode system that can be
operated in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) was found.
The molybdenum anode forms in HFIP a compact, conductive, and
electroactive layer of higher-valent molybdenum species. This
system can be used to replace powerful but stoichiometrically
required Mo(V) reagents for dehydrogenative coupling of aryls. The
performance of this electrolytic protocol is more sustainable and
allows the conversion of a broad scope of activated arenes.
biological waste flows. Due to the limited solubility of organic
molecules in aqueous media, alcohols are more versatile
solvents for anodic conversions. In particular, fluorinated
alcohols, such as 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), offer
[9]
a broad applicability and are known to stabilize radicals. Such
intermediates are of significant importance in the
dehydrogenative coupling reaction employing Mo(V) reagents.
By a partial ligand exchange of HFIP and chloride, an improved
reagent could be established.
electrochemical generation of an Mo(HFIP)
[
10]
[
11]
We anticipated an
reagent, which will
5
Active electrodes exhibit on their surface electrocatalytically
active species, which represent immobilized mediators. Ideally,
a compact and electrically conductive reagent is formed which is
either dissolve in the electrolyte or remain at the electrode to
form an active surface layer (Scheme 1).
[1]
regenerated in-situ. This does not only facilitate an electro-
conversion, but also represents a redox-filter since only the
electrochemical potential of this redox-active layer is relevant
[1]
and the experimental set-up is tremendously simplified. Within
the multitude of electrochemical transformations, only a few
metals are known to perform as active anodes in protic media.
Most commonly, nickel and lead systems perform well with their
active and conductive surfaces, but represent harmful or
[2]
environmentally less benign metals. In the Simons process Ni
electrodes and anhydrous hydrogen fluoride are utilized for
Scheme 1: Comparison between stoichiometric Mo(V) and electrocatalytic
dehydrogenative coupling reactions.
[
3]
electrochemical fluorination.
conditions Ni forms NiOOH as an active and stable surface.
This anodic system is versatile example for many
transformations and even suitable for the selective degradation
Furthermore, under alkaline
[4]
a
This approach is safer, since no highly reactive reagents are
used and high amounts of waste are avoided. In addition, this
method promises to be easily scalable combined with an
[5]
of bio-based and complex mixtures such as lignin. By using the
different alloys of nickel or cobalt the performance in alkaline
[12]
inherently safe protocol.
[
6]
media can be modulated. With PbO
2
electrodes (on Pb),
By serendipity, we found that using molybdenum as anode and
platinum as inert cathode in a HFIP electrolyte already proved its
utility in dehydrogenative coupling reactions. The effect of this
two-electrode galvanostatic arrangement was monitored by the
conversion of 4-methyl veratrole (1) to the respective
usually a significant concentration of sulfuric acid is required for
[
7]
the stability of the anode.
Nevertheless, also alkaline
conditions are reported in the context of the degradation of
dehydrodimer 2. By applying a constant current density of
[
a]
Sebastian B. Beil, Timo Müller, Sydney B. Sillart, Peter Franzmann,
Prof. Dr. Siegfried R. Waldvogel
2
7.5 mA/cm
,
different electrolyte systems were investigated
(
Table 1). With regards to the coordination ability of the anions,
Institute of Organic Chemistry
Johannes Gutenberg University Mainz
Duesbergweg 10-14, 55128 Mainz (Germany)
E-mail: waldvogel@uni-mainz.de
Sebastian B. Beil, Prof. Dr. Siegfried R. Waldvogel
Material Science in Mainz (MAINZ)
less coordinating and more stable anions turned out to be more
beneficial (Table 1, Entries 1-5). Tetrabutylammonium hexa-
fluorophosphate gave the highest conversion of 85%, which
could neither be improved by altering the solvent nor by
application of different cathode materials (Table 1, Entries 6-9).
Recently, we found that the counter electrode can have a
significant impact onto the electro-organic transformation since
the hydrogen evolution is promoted and other cathodic
[
[
[
b]
c]
d]
Graduate School of Excellence
Staudingerweg 9, 55128 Mainz (Germany)
Alexander Bomm, Prof. Dr. Wolfgang Schade
Fraunhofer Heinrich-Hertz-Institut
Abteilung Faseroptische Sensorsysteme
Am Stollen 19H, 38640 Goslar
[13]
processes suppressed.
Due to the inexpensive nature of
Michael Holtkamp, Prof. Dr. Uwe Karst
Institute of Inorganic and Analytical Chemistry
University of Münster
graphite, this material was further applied as cathode. By the
use of 3.0 F per mole of electricity the best conversion of 95%
was achieved (Table 1, Entry 10). The addition of methanol or
water to the electrolyte system resulted in a strong decrease in
conversion (see SI). Using the optimized reaction conditions
Corrensstr. 30, 48149 Münster
Supporting information for this article is given.
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COMMUNICATION
Table 1: Optimization of the electrolysis conditions to 2.
or even no conversion and are undesirable (Table 2, Entries 5-7).
Group 5 and 6 elements located near molybdenum in the
periodic table were also investigated with regards to their
applicability. Of the group 5 elements only vanadium gave 4 in a
moderate yield of 57%, whereas niobium and tantalum only
resulted in decomposition of the substrate (Table 2,
Entries 8-10). This finding corresponds to an expected cross-
relation between molybdenum and vanadium. Interestingly,
chromium gave product 4 in a high conversion and a reasonable
yield, however, during the electrolysis tremendous corrosion
took place. Besides the dissolution of chromium, it results in the
formation of a thick green film at the anode. In addition, this
fouling suppresses the conductivity almost completely (Table 2,
Entry 11). The heavier congener tungsten gave poor conversion
and isolated yield of 4 and is non-favored due to the challenging
machinability of tungsten (Table 2, Entry 12).
[
a]
Entry
Solvent
HFIP
HFIP
HFIP
HFIP
HFIP
NR
4
X
Cathode
Q (F)
2.1
2.1
2.1
2.1
2.1
2.1
2.1
2 (GC-%)
1
2
3
4
5
6
7
NEt
4
Cl
Br
OAc
Pt
Pt
Pt
Pt
Pt
Pt
Pt
46
9
NBu
4
NBu
4
16
49
85
54
57
NBu
NBu
NBu
NBu
4
4
4
4
BF
PF
PF
PF
4
6
6
6
CH
3
CN
Table 2: Investigation of anode material onto the yield of 4.
CH
2
Cl
2
[
b]
glassy
carbon
8
9
HFIP
NBu
4
PF
6
2.1
85
85
HFIP
HFIP
NBu
4
4
PF
6
6
graphite
graphite
2.1
3.0
[
c]
10
NBu
PF
95
[
a]
[a]
[b]
Entry
Anode
Mo
3 (GC-%)
4 (GC-%)
Yield (%)
[
a] GC conversion determined by referencing to hexadecane as internal
1
2
-
-
100
91
75
81
standard. [b] HFIP: 1,1,1,3,3,3-hexafluoroisopropanol. [c] Isolated yield: 67%.
[
c]
BDD
Pt
(
Table 1, Entry 10), product 2 could be obtained in 67% isolated
yield. We found by ICP-OES measurements, that only
.740.01 mg/g molybdenum is found upon electrolysis in the
3
4
5
6
7
8
9
13
1
84
90
-
69
72
-
0
Au
Cu
Al
final mixture (for details see SI). This clearly corresponds to a
very low amount of 0.53 mol% molybdenum which is dissolved
during electrolysis. We attribute this to a slight and undesired
corrosion. The very low Mo concentration and the effect of
surface make an active species in the electrolyte unlikely.
93
92
99
-
1
5
Fe
V
-
-
To further prove the molybdenum-effect of this anode material a
variety of other metals or commonly used anode materials were
tested. Therefore, 4-bromo veratrole (3) was used as test
substrate, since dehalogenation might occur as a common side
79
-
57
-
Nb
Ta
Cr
W
87
88
29
43
10
1
-
[
14]
reaction.
Besides the isolated yields of the respective
[
d]
dehydrodimer 4 also the GC data for the conversion are
provided. For the active molybdenum anode an isolated yield of
11
60
38
80
12
26
7
5% was obtained (Table 2, Entry 1). Commonly used materials
[
15]
like Pt, Au, and boron-doped diamond (BDD)
provided the
[
a] Absolute GC integral values; if 3 and 4 do not sum to 100% by-products are
product in high yields of up to 81%, but are significantly more
expensive materials and therefore less attractive for simple
laboratory applications and upscaling, respectively (Table 2,
Entries 2-4). In addition, these materials gave 4 in high yields,
but significantly lower purity, because unselective coupling
processes occur. Such by-products have to be removed by
expensive and/or time-consuming purification protocols.
Electroanalytical studies reveal that the first oxidation peak of 3
directly correlates with the yield of 4, which is visible in cyclic
voltammetry (see SI).
formed. [b] Isolated yield. [c] BDD: boron-doped diamond. [d] Strong corrosion.
Subsequently, we focused onto the synthetic application of this
improved methodology using molybdenum as active anode
material. Initially, we subjected anisole derivatives 5 to this
electrolytic protocol. The para-coupled dimers 6 were obtained
in moderate to high yields of up to 67% (Table 3). The chloro-
and bromo-substituted derivatives 6a-g resulted in the highest
yields and only fluoro and iodo substituents gave lower yields. In
general, substrates involving the heavier halogens (Br or I)
exhibited a decreased solubility and therefore a trend to lower
conversion due to product precipitation at the anode. Beside
halogenated substrates, also alkylated ones were suitable, but
provided the products 6h-i in moderate yields. When the 4-
Since commonly CuCl
2
,
AlCl
3
and FeCl
3
are used
stoichiometrically in reagent-mediated dehydrogenative coupling
[16]
reactions,
we also investigated Cu, Al, and Fe as anode
materials. All of them performed this transformation with a poor
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COMMUNICATION
Table 3: Scope of dehydrogenative coupling reactions of anisole derivatives
10e in a moderate yield of 39%. This involves a second
dehydrogenation process. Triphenylene 10f was isolated in an
excellent yield of 82%.
[
a]
5.
Newly formed bond indicated in bold.
In general, the formation of six-membered rings is favored over
five-membered cycles, since dibenzothiophene 10g and
carbazole 10h were obtained in poor yields. Benzothiophene
functionalized spirocyclic product 10i gave a slightly higher yield
of 31%. By this method, also seven- and eight-membered rings
(
10j and 10k) are accessible in high yields of 67% and 59%,
respectively. Interestingly, micro-structured molybdenum, which
was prepared by laser-grafting (see SI for details), resulted in an
additionally increased yield of biaryl 8b and phenanthrene 10a
compared to the planar anode (Figure 3). Due to a larger anodic
area and therefore, an increased active electrode an excellent
yield of 83% was obtained in both cases.
Figure 3: Micro-structured molybdenum resulted in an increased yield of 8b
and 10a.
[
a] Isolated yields.
position was blocked, ortho-coupling was observed in
reasonable yield (6j).
In conclusion, we found an innovative active anodic system. The
protocol is extremely simple to conduct by applying molybdenum
In particular, tetra-halogenated products 6a-f, which were
accessible by this method, are valuable platform molecules for
graphene substructures and can be applied to further
functionalization by simultaneous or orthogonal cross-coupling
Table 4: Scope of dehydrogenative coupling reactions of veratrole substrates
[a]
7.
Newly formed bond indicated in bold.
[
17]
functionalization.
In addition to anisole substrates 5, veratrole derivatives 7 gave
higher yields in this established electrochemical active
molybdenum anode system (Table 4). Alkylated, halogenated,
and methoxylated derivatives 7a-f were coupled in excellent
isolated yields of up to 87% (Table 4). Also up-scaling to a
5
mmol scale gave 8b in 69% yield (1.49 g, 3.45 mmol). Again
fluoro substituted substrate resulted in significantly diminished
yield (8d). In addition, para-dimethoxy arenes could be applied,
in a yield of 54% (8g) and 15% (8h) respectively. Excellent
yields were achieved for bromo- and iodo-functionalized biaryls
8b and 8c demonstrating the superior performance of this
methodology compared to classical methods, which employ
graphene oxide, transition metal catalysts or hypervalent
[
18]
iodine(III) reagents.
In our ongoing study we subjected a variety of substrates to
cyclization reactions, which are known to be converted by
[
19]
classical MoCl
5
-mediated reactions.
Phenanthrenes were
synthesized in high yields of up to 80% (Table 5, 10a-d) and
even free carboxylic acids do not interfere in the course of the
reaction (10c). This is of particular interest, since molybdenum
chloride reagents do not tolerate free carboxylic acids. Ethyl-
tethered biaryls reacted directly to the respective phenanthrene
[a] Isolated yields.
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COMMUNICATION
[
[
1]
2]
O. Hammerich, B. Speiser, 5th ed.; Taylor & Francis Group: 2016; pp
as anode and HFIP as protic solvent. The active anode
characteristic is unique for molybdenum and can be even
enhanced by structured anode surfaces. A broad scope of
substrates was successfully converted demonstrating the utility
of this system for electro-organic synthesis. This active anode
system tolerates all halo substituents, which can be used for
further functionalization since dehalogenation was never
observed. This procedure can easily compete with classical
Mo(V)-mediated conversions, but is more environmentally
friendly since large amounts of reagent waste are avoided.
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2
Keywords: Molybdenum • arene coupling • electrochemistry •
oxidative coupling • anode
4
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Electrically driven and with a
comparable reactivity as Mo(V)
reagents in the dehydrogenative
coupling is achieved by the novel
active anode system based on
S. B. Beil, T. Müller, S. B. Sillart, P.
Franzmann, A. Bomm, M. Holtkamp,
U. Karst, W. Schade, S. R.
Waldvogel*
1,1,1,3,3,3-hexafluoroisopropanol
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and molybdenum.
Novel Active Molybdenum-Based
Anode Material for Oxidative
Coupling Reactions
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