Reagent- and Metal-Free Anodic C?C Cross-Coupling of Aniline Derivatives

Article abstract of DOI:10.1002/anie.201612613

The dehydrogenative cross-coupling of aniline derivatives to 2,2′-diaminobiaryls is reported. The oxidation is carried out electrochemically, which avoids the use of metals and reagents. A large variety of biphenyldiamines were thus prepared. The best results were obtained when glassy carbon was used as the anode material. The electrosynthetic reaction is easily performed in an undivided cell at slightly elevated temperature. In addition, common amine protecting groups based on carboxylic acids were employed that can be selectively removed under mild conditions after the cross-coupling, which provides quick and efficient access to important building blocks featuring free amine moieties.

Full text of DOI:10.1002/anie.201612613

Angewandte
Communications
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International Edition: DOI: 10.1002/anie.201612613
German Edition: DOI: 10.1002/ange.201612613
Electrosynthesis Very Important Paper
À
Reagent- and Metal-Free Anodic C C Cross-Coupling of Aniline
Derivatives
Lara Schulz, Mathias Enders, Bernd Elsler, Dieter Schollmeyer, Katrin M. Dyballa,
Robert Franke, and Siegfried R. Waldvogel*
Abstract: The dehydrogenative cross-coupling of aniline
derivatives to 2,2’-diaminobiaryls is reported. The oxidation
is carried out electrochemically, which avoids the use of metals
and reagents. A large variety of biphenyldiamines were thus
prepared. The best results were obtained when glassy carbon
was used as the anode material. The electrosynthetic reaction is
easily performed in an undivided cell at slightly elevated
temperature. In addition, common amine protecting groups
based on carboxylic acids were employed that can be
selectively removed under mild conditions after the cross-
coupling, which provides quick and efficient access to impor-
tant building blocks featuring free amine moieties.
amounts of an acid provide symmetric 2,2’-diaminobiaryls in
good yields while the synthesis of nonsymmetric derivatives is
still challenging.[9] Furthermore, several steps are required for
synthesizing those starting materials.[9,10] Applying these
strategies requires much effort and complicated reaction
conditions. Such approaches often lead to complex reaction
mixtures and low yields of the desired products. In contrast,
À
oxidative coupling reactions by direct C H activation are of
great interest as a sustainable method.[11,12] Recently, an
oxidative conversion of diaryl amines in the presence of
a highly fluorinated iron phthalocyanine catalyst was reported
[13]
À
À
that enabled C C as well as N N bond formation.
In particular, electrochemical techniques are remarkably
advanced in terms of avoiding the formation of waste
products because no leaving groups or oxidants are
required.[14] The generation of reactive intermediates by
electrochemical means is a highly attractive pathway in
terms of atom economy and cost efficiency.[15] In our group,
anodic phenol–phenol[12,16,17] and phenol–arene cross-cou-
plings have been developed.[18] The best results for the direct
oxidative cross-coupling of aryl compounds were attained in
stabilizing media such as 1,1,1,3,3,3-hexafluoro-2-propanol
(HFIP), which is frequently used in reactions that involve
hypervalent iodine.[19,20] This unique solvent can form strong
hydrogen bonds,[21] which results in a significantly different
solvation of the individual coupling partners, and the oxida-
tion potential is thus decoupled from its nucleophilicity.[22]
Herein, we present a selective method for the synthesis of
nonsymmetric 2,2’-diaminobiaryls by anodic cross-coupling
(Scheme 1). The electrochemical synthesis of nonsymmetric
2,2’-diaminobiaryls is very easy to conduct as only a simple
two-electrode arrangement in an undivided beaker-type cell is
required. In addition, the electrolysis is performed in a con-
stant-current mode, which immensely simplifies the equip-
ment needed. To efficiently find suitable coupling partners,
several screening experiments were performed.[23] Usually,
component A has a lower oxidation potential than aniline B,
and is therefore preferentially oxidized at the anode. To
statistically favor the cross-coupling over the homocoupling
product, the aniline with the higher oxidation potential, that
N
onsymmetric biaryls are very important structural motifs
in natural products and in catalysis.[1] In particular, 1,1’-
binaphthyl-2,2’-diamine (BINAM) is a promising ligand
system and has already been studied in detail. Asymmetric
Michael additions,[2] indole N arylation reactions,[3] as well as
hydrogenations of ketones and olefins[4] have been realized
using BINAM or derivatives thereof as a ligand in transition-
metal catalysis. Moreover, new ligands for contrast agents in
magnetic resonance imaging also feature 2,2’-diaminobiaryls
as structural motifs.[5] However, access to these substrates by
classical synthetic means is limited because anilines easily
undergo oxidative polymerization to polyaniline, which is also
known as aniline black.[6] Symmetric 2,2’-diaminobiaryls are
accessible by copper-catalyzed Ullmann reactions, for exam-
ple.[7] Direct oxidative cross-couplings with stoichiometric
amounts of inorganic reagents such as CuII salts have only
been reported for naphthylamines and provided the desired
compounds in rather poor yields.[8] Sigmatropic rearrange-
ments of diaryl hydrazines in the presence of catalytic
[*] L. Schulz, M. Enders, Dr. B. Elsler, Dr. D. Schollmeyer,
Prof. Dr. S. R. Waldvogel
Institut fꢀr Organische Chemie
Johannes Gutenberg-Universitꢁt Mainz
Duesbergweg 10–14, 55128 Mainz (Germany)
E-mail: waldvogel@uni-mainz.de
Homepage: http://www.chemie.uni-mainz.de/OC/AK-Waldvogel/
Dr. K. M. Dyballa, Prof. Dr. R. Franke
Evonik Performance Materials GmbH
Paul-Baumann-Strasse 1, 45772 Marl (Germany)
Prof. Dr. R. Franke
Lehrstuhl fꢀr Theoretische Chemie
Ruhr-Universitꢁt Bochum
44780 Bochum (Germany)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201612613.
À
Scheme 1. Direct anodic C C cross-coupling of protected aniline
derivatives. PG=protecting group.
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is, B, was applied in slight excess. It was essential to stabilize
the reactive radical intermediates during the electrolysis to
avoid undesired oligomerization or even mineralization. The
favorable effects of HFIP and protic additives such as
methanol or water on yield and selectivity were recently
described.[12,18,22] Unfortunately, simple aniline derivatives are
prone to overoxidation because of their electron-rich nature
and the resulting low oxidation potentials. Polymerization is
a serious problem in anodic reactions of anilines.[24] Organo-
catalytic reactions currently use nitrogen protecting groups to
prevent the formation of undesired side products.[19] N-Mesyl-
protected anilines were converted in good yields in a hyper-
valent iodine mediated coupling but the desired amino-
biphenyls were only obtained after waste-intensive and time-
consuming deprotection.[19] Recently, MuÇiz and co-workers
used a hypervalent iodine reagent in a diamination reaction
generating tosyl-protected diamines.[25] However, in our work,
various easily removable protecting groups were used for the
aniline derivatives (Figure 1).
oxidation. The oxidation potential can be increased to 1.39 V
or even 1.57 V, respectively, through the installation of an
acetyl or trifluoroacetyl group (pKa(acetanilide) = 18.8,
pKa(trifluoroacetanilide) = 12.6 in DMSO).[26] Owing to the ability
of the amide functional group to participate in hydrogen
bonding as a donor as well as an acceptor, the protected
anilines experience strong solvation by HFIP. CV investiga-
tions of differently protected 3,4-dimethoxyanilines revealed
a clear shift of the oxidation potentials to lower values with an
increasing concentration of methanol in HFIP. Methanol acts
as a weak base when added to the electrolyte and thus
weakens the solvation of the amides. The oxidation potentials
of anilines can be adjusted to some extent by using different
protecting groups (Figure 2). This benefits the anodic cross-
coupling of anilides as there seems to be an ideal difference
for the oxidation potentials of the coupling partners.[22] In
general, CV helps to indicate the correct stoichiometry for the
anodic cross-coupling as component A should exhibit a lower
oxidation potential than component B and thus be preferably
oxidized at the anode to initiate the cross-coupling sequence
(Scheme 1).
Usually, the electrode material has a crucial influence on
electrochemical conversions as different reaction pathways
and adsorption characteristics often result. However, in this
system, the electrolyte system was found to be the key.
Interestingly, the investigated reaction of anilides 1 and 2
(Scheme 2) gave consistent results when performed at differ-
ent electrode materials, such as platinum, boron-doped
diamond, graphite, or glassy carbon (Table 1).
Figure 1. Common and easily removable carbonyl-based protecting
groups for anilines.
Amides and carbamates proved to be suitable groups for
N protection and exhibited sufficient stability during the
electrolysis. In addition, cyclic voltammetry (CV) studies
indicated that the protecting groups have a significant influ-
ence on the oxidation potential (which correlates with the pKa
values of the anilides)[26] as well as on the potential interaction
between substrate and solvent by hydrogen bonding
(Figure 2).
Electron-rich anilines, such as 3,4-dimethoxyaniline, have
low oxidation potentials (832 mV in pure HFIP; pKa(anilinium)
=
4.6 in DMSO)[26] and are therefore prone to anodic over-
Scheme 2. The influence of methanol and different electrode materials
on the anodic cross-coupling of anilines. 2-Acetamidonaphthalene (1)
was used as component A, compound 2 as component B.
Considering the selectivity in HFIP, all electrode materials
gave similar results but the best yields were achieved on
graphite and platinum. The addition of methanol to the
solvent led to significantly improved selectivity, and the cross-
coupling product was exclusively obtained on glassy carbon
(Table 1, entry 2). In addition, significantly less oligomeriza-
tion was observed on glassy carbon than on the other
electrode materials. This is important for two reasons: First,
it simplifies the work-up tremendously, and second, non-
converted starting materials can be easily recovered. Fur-
thermore, glassy carbon shows excellent chemical stability
even after long-term use whereas graphite undergoes signifi-
Figure 2. Changes in the oxidation potentials of various protected 3,4-
dimethoxyanilines with increasing MeOH concentration in HFIP. WE:
glassy carbon electrode tip, 2 mm diameter; CE: glassy carbon rod;
RE: Ag/AgCl in saturated LiCl/EtOH; solvent: HFIP with 0–27% v/v
MeOH; supporting electrolyte: 0.09m Bu3NMeO3SOMe.
2
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Table 1: Influence of the electrode material on the aniline–aniline cross-
coupling.[a]
Entry[b] Electrode material
Additive Yield [%][c]
3ab/3bb[c]
1
2
3
4
5
6
7
8
glassy carbon
glassy carbon
boron-doped diamond
boron-doped diamond MeOH
graphite
graphite
platinum
platinum
–
51
20
48
34
6:1
>100:1[d]
3:1
MeOH
–
34:1
6:1
28:1
7:1
–
60[e]
28[e]
60[e]
29[e]
MeOH
–
MeOH
29:1
[a] 2-Acetamidonaphthalene 1 was used as component A, compound 2
as component B. [b] Electrolysis conditions: 508C, constant current
(j=5.2 mAcmÀ2), undivided screening cell, Q=2 F (aniline A), solvent:
HFIP or HFIP/MeOH (18% v/v), supporting electrolyte: 0.09m
Bu3NMeO3SOMe, A/B=1:2. [c] Determined by GC analysis. [d] The
homocoupling product 3bb was not detected. [e] Increased formation of
oligomeric side products.
cant erosion. When taking into account the selectivity,
electrochemical stability, and cost, glassy carbon is the
electrode material of choice.
À
During this aniline cross-coupling, the new C C bond is
formed exclusively ortho to the amine moiety, and 2,2’-
diaminobiaryl derivatives are obtained. Aniline derivatives
with electron-releasing moieties such as methoxy groups in
À
the para position to the newly formed C C bond seem to
particularly favor the coupling reaction. The scope of
protected, nonsymmetric 2,2’-diaminobiaryl derivatives
obtained by electrolysis is shown in Figure 3. It should be
noted that none of the protecting groups was removed during
electrolysis.
Oligomerization is still the predominant side reaction
decreasing the yields of the cross-coupling products. Never-
theless, N-Boc-protected 3-methoxy-4-methylaniline pro-
vided the mixed 2,2’-diaminobiaryl derivatives 4, 5, 6, and 7
in good yields of 40 to 51%. The highest yield of 74% was
achieved for the coupling of N-(3,4-dimethoxyphenyl)aceta-
mide with N-(3-methoxy-4-methylphenyl)benzamide (8). In
the cross-coupling of N-(3,4-dimethoxyphenyl)acetamide
with a benzodioxole derivative, the yield was slightly inferior
(38%, 12). 2-Acetamidonaphthalene (1) was also cross-
coupled with trifluoroacetamide 2 to provide 3 in 59%
yield. Accordingly, anilines with electron-releasing or alkyl
moieties in the 3- and 4-position turned out to be suitable
coupling partners. The coupling of anilines with chloro
substituents was also possible (56%, 10 and 51%, 11). The
amount of unconverted component A was about 2–24%.
Increasing the applied charge could, in some cases, improve
the yields.
During the initial optimization of the electrolysis con-
ditions, the influence of temperature on the yields of the
cross-coupling products was evaluated (Table 2). These
studies revealed that an electrolysis temperature of 308C
seems to be advantageous (entries 3 and 6).
To enable the later use of such protected 2,2’-diamino-
biaryls in organocatalysts, ligands, or functionalized materi-
als,[27] it is vital to efficiently deprotect the amino moiety. The
applied protecting groups are based on common carboxylic
Figure 3. Scope of protected nonsymmetric 2,2’-diaminobiaryls. Yields
of isolated products are given. [a] Electrolysis conditions: 508C,
constant current (j=5.2 mAcmÀ2), glassy carbon anode, glassy carbon
cathode, undivided beaker-type cell, Q=2 F (aniline A), solvent: HFIP/
MeOH (18% v/v), supporting electrolyte: 0.09m Bu3NMeO3SOMe,
A/B=1:2. [b] Electrolysis conditions as in [a], solvent: HFIP.
acids and are thus easily removed by standard procedures.[28]
By employing orthogonal protecting groups, the two amino
groups can be independently deprotected, which allows for
further chemo- and regioselective transformations (Table 3).
These partially protected cross-coupling products had pre-
viously not been described and can be considered as
congeners to the recently reported partially protected biphe-
nol derivatives.[16]
A Boc group could be selectively removed with water
without the need for additional acid,[29] whereas the acetyl and
benzoyl groups in substrates 4 and 6, respectively, remained
unaffected (Table 3, entries 1 and 2). A trifluoroacetyl group
could be selectively removed with potassium carbonate in the
presence of an acetamide (entry 3). Both protecting groups in
3, an acetyl and a trifluoroacetyl moiety, could be simulta-
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Table 2: Temperature dependence of the anodic cross-coupling of
In conclusion, an innovative and facile method to obtain
aniline derivatives.[a]
À
2,2’-diaminobiaryls by electrochemical C C cross-coupling
Yield [%][b]
has been established. The scope includes differently substi-
tuted and protected aniline derivatives. The HFIP solvent
effect is essential for the high selectivity while no leaving
groups are required. This highly fluorinated alcohol allows
the oxidation potential to be decoupled from the nucleophi-
licity of the coupling partners. The protecting groups are
based on common carboxylic acids and therefore easily
removable. When a set of different protecting groups is
applied, they can be selectively removed, and thus liberate the
respective amino groups, with different nucleophiles. The use
of electric current to drive this conversion leads to an
exceptionally sustainable metal- and reagent-free reaction
sequence. Moreover, the electrolysis protocol is very easy to
conduct because an undivided cell and a two-electrode
arrangement are employed.
Entry
Product
T [8C]
1
2
3
4
50
40
30
20
49
46
59
49
5
6
7
50
30
20
51
59
54
[a] Electrolysis conditions: 20–508C, constant current (j=5.2 mAcmÀ2),
glassy carbon anode, glassy carbon cathode, undivided beaker-type cell,
Q=2 F (aniline A), HFIP/MeOH (18% v/v), supporting electrolyte:
0.09m Bu3NMeO3SOMe, A/B=1:2. [b] Yields of isolated products.
[c] Acetyl- and benzoyl-protected 3,4-dimethoxyaniline was used as
component A for the synthesis of 4 and 6, respectively.
Acknowledgments
S.R.W. thanks the DFG (Wa1278/14-1) for funding. We highly
appreciate
support
by
BMBF-EPSYLON
(FKZ
12XP2016D).
Table 3: Partial and complete deprotection of differently protected
2,2’-diaminobiaryl derivatives.
Conflict of interest
The authors declare no conflict of interest.
Entry Substrate Removed PG
Product
Yield [%]
À
Keywords: biaryls · C H activation · cross-coupling ·
electrochemistry · protecting groups
1
2
4
6
Boc[a]
98[d]
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4
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Manuscript received: December 29, 2016
Final Article published: && &&, &&&&
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Communications
Electrosynthesis
L. Schulz, M. Enders, B. Elsler,
D. Schollmeyer, K. M. Dyballa, R. Franke,
S. R. Waldvogel*
&&&& — &&&&
À
Reagent- and Metal-Free Anodic C C
Cross-Coupling of Aniline Derivatives
Cross-couplings under electrochemical
conditions selectively provide protected
2,2’-diaminobiaryl derivatives. For clean
conversion, 1,1,1,3,3,3-hexafluoro-2-
propanol (HFIP) and glassy carbon
should be used as the electrolyte and
anode material, respectively. Easily
removable protecting groups enable the
stepwise and selective liberation of the
amine functional groups.
6
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
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