Electro-oxidative C(sp2)-H/O-H cross-dehydrogenative coupling of phenols and tertiary anilines for diaryl ether formation

Article abstract of DOI:10.1039/d1cy00186h

The formation of diaryl ethers is generally achievedviatransition metal catalyzed etherification reactions (Ullmann, Chan-Lam, Buchwald-Hartwig) with prefunctionalized aryl halide substrates at elevated temperatures. Herein, we report a protocol for electrochemical C(sp²)-H/O-H cross-dehydrogenative coupling of phenols and tertiary anilines to synthesize diaryl ethers. The C(sp²) H/O-H coupling product can be obtained under metal- and oxidant-free conditions at room temperature in moderate to excellent yield (up to 83% yield) with high regioselectivity (>99% forpara-substitution) and with a broad substrate scope (22 examples).

Full text of DOI:10.1039/d1cy00186h

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Electro-oxidative C(sp2)–H/O–H cross-
dehydrogenative coupling of phenols and tertiary
Cite this: DOI: 10.1039/d1cy00186h
anilines for diaryl ether formation†
Hongyang Tang, Simon Smolders,
Yun Li,
*
*
Dirk De Vos
and Jannick Vercammen
The formation of diaryl ethers is generally achieved via transition metal catalyzed etherification reactions
(Ullmann, Chan–Lam, Buchwald–Hartwig) with prefunctionalized aryl halide substrates at elevated
temperatures. Herein, we report a protocol for electrochemical C(sp2)–H/O–H cross-dehydrogenative
coupling of phenols and tertiary anilines to synthesize diaryl ethers. The C(sp2) H/O–H coupling product
can be obtained under metal- and oxidant-free conditions at room temperature in moderate to excellent
yield (up to 83% yield) with high regioselectivity (>99% for para-substitution) and with a broad substrate
scope (22 examples).
Received 1st February 2021,
Accepted 25th April 2021
DOI: 10.1039/d1cy00186h
rsc.li/catalysis
proceed with H2 evolution, eliminating the need for oxidants,
Introduction
ligands
and
increased
temperature.7
The
direct
Diaryl ether scaffolds are important structural motifs in the
field of organic synthesis due to the prevalence of C(sp2)–O
bonds in intermediates of natural products and in several
prominent pharmaceuticals (Fig. 1).1 Therefore, the
development of an effective diaryl ether synthesis method has
great research significance and extensive application value.
Previous research focused on the transition-metal mediated
phenol etherification with prefunctionalized arenes (aryl
dehydrogenative coupling of phenols with arenes could result
in an optimal step- and atom-economy.8 However, this
approach remains largely unexplored due to an often
unpredictable, poor chemoselectivity between
C
or
O-arylation; C–O coupling was only accidentally observed in a
few studies involving the synthesis of asymmetric biaryls.9
Furthermore, the few studies that did report O-arylation
suffer from a low yield and limited substrate scope. König
et al. obtained diaryl ethers in the photocatalytic, ruthenium-
catalyzed coupling of electron-rich arenes with phenols (yield
up to 44%, selectivity of 51%).10 A mechanistic rationale was
postulated in which one arene (e.g., trimethoxybenzene) is
oxidized selectively to the corresponding radical cation,
which is attacked by a poorly oxidizable nucleophile (e.g.
phenol). The group of Waldvogel observed C–O cross-coupled
compounds as side products in the electrochemical coupling
of trimethoxyphenol with a phenol derivative (1 example,
yield of 12%, selectivity of 25%) and in the coupling of
naphthylamines with phenolic substrates (3 examples, yield
up to 28%).11 They proposed that the absence of steric
hindrance would be essential for diverting the selectivity
halides or pseudohalides), which not only generate
a
stoichiometric amount of halide salt waste but also require
elevated temperatures (Scheme 1a).2,3 Biaryl ethers can also
be obtained from benzoate esters via a palladium or nickel
mediated decarboxylation (Scheme 1b).4 Some alternative
prefunctionalized substrates were developed, such as aryl
boronic acids (Chan–Lam coupling) and more recently, Ritter
et al. managed to form aryl sulfonium salts, which could be
converted into C–O bonds using photoredox catalysis.5,6
Although a number of pioneering processes were developed
in the synthesis of diaryl ethers, some limitations still remain,
such as the requirement of pre-functionalized arenes,
transition metals and often cost-intensive ligands, while
cheaper, unmodified arenes remain elusive reactants.
Organic electrochemistry is a green synthetic tool that
employs traceless electricity to promote redox reactions.
Electricity-powered dehydrogenative cross-coupling reactions
Centre for Membrane Separations, Adsorption, Catalysis and Spectroscopy for
Sustainable Solutions (cMACS), KU Leuven, Celestijnenlaan 200F, 3001 Leuven,
Belgium. E-mail: dirk.devos@kuleuven.be, jannick.vercammen@kuleuven.be
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cy00186h
Fig. 1 Prominent fine chemicals that contain the diaryl ether moiety.
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obtained in an excellent yield of 78% after optimization of
the reaction conditions (Table 1). Quite surprisingly, no C–C
cross-coupling product was observed.13 The only observed
side-product was the methylene bridged dimer of 2a, namely
4,4′-methylene-bis(N,N-dimethylaniline)
(4a),
which
is
reported to form via a 2-electron oxidation of 2a.14
Currents lower than the optimal 5 mA did not significantly
decrease the yield, while a higher current of 10 mA afforded a
slightly lower yield (entries 2 and 3). As a supporting electrolyte,
nBu4NBF4 gave the best yield, although a similar reactivity was
obtained with other electrolytes (entries
combination of hexafluoroisopropanol
4
and 5).
A
(HFIP) and
Scheme 1 Diaryl ether formation.
dichloromethane (DCM) in a ratio of 6 : 4, respectively, was
shown to be optimal, as significantly lower yields were
obtained in the neat solvents (entries 6 and 7), and no product
was observed in acetonitrile (entry 8). The use of a graphite rod
anode significantly decreased the formation of 3a (entry 9),
which could be attributed to the lower surface area of this
electrode. Changing the Ni cathode for Pt shows a moderately
lower yield, indicating the cathode material can influence the
reaction outcome, probably by promoting side-reactions (entry
10). Higher or lower temperatures resulted in lower yields
(entries 11 and 12). A slightly decreased reaction yield was
obtained when the inert nitrogen atmosphere was replaced by
air (entry 13). No desired product was observed when the
reaction was run under air and without electricity (entry 14).
from C–C to C–O cross-coupling products.12 In this study, we
provide an electrochemical, additive-free method to couple
anilines with phenolic substrates via O-arylation with high
yields and selectivities (Scheme 1c).
Results and discussion
We investigated the electrochemical coupling between the
sterically unhindered 4-tert-butylphenol (1a) and N,N-
dimethylaniline (2a), which is quite susceptible to oxidation.
Our hypothesis was that the absence of steric hindrance
around both the phenolic moiety of 1a as well as the
activated para-position of 2a, together with the low redox
potential of 2a, would result in an improved selectivity for
the C–O cross-coupled product. An undivided cell equipped
with a carbon felt anode and a nickel cathode was employed
(ESI,† Fig. S3). The desired diaryl ether product 3a was
Kinetic study
In a kinetic study, the evolution of the formation of the
product (3a) and the side product 4,4′-methylene-bis(N,N-
dimethylaniline) (4a) was followed (Fig. S3†). Upon supplying
Table 1 Optimization of reaction conditionsa
Entry
Variation from standard conditions
Yieldb [%]
1
2
3
4
None
78b, 70c
71
56
38–50
67
1 mA, 42 h
10 mA, 2.5 h
Et4NBF4, nBu4NPF6, nBu4NClO4, LiClO4
5
1 eq. nBu4NBF4
6
7
DCM
HFIP
9 (48)
36
8
9
MeCN
N.R.
15
45 (24)
68
33 (7)
69
N.R.
Graphite as anode
Pt as cathode
40 °C
0 °C
Under air
10
11
12
13
14
Without electricity, under air
a
Reaction conditions: carbon felt as anode and nickel plate as cathode (10 mm × 10 mm × 1 mm), constant current at 5 mA, 1a (0.3 mmol), 2a
(1 equiv., 0.3 mmol), nBu4NBF4 (0.5 equiv., 0.15 mmol), HFIP/DCM (6 : 4, 6 ml), room temperature, nitrogen, 5 h (3.1 F mol−1). Yield
determined by GC analysis using octane as the external standard. Recovery of unreacted 1a shown within parentheses. Isolated yield. HFIP =
b
c
hexafluoroisopropanol, DCM = dichloromethane.
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2.5 F mol−1, the 3a product yield continuously increased,
while that of 4a decreased. Supplying 3–3.5 F mol−1 is the
optimum for obtaining the highest yield of 3a, with minimal
formation of 4a. When the homo-coupling of either 1a or 2a
was attempted, no reaction was seen with 1a, and a limited
yield of 4a was observed when the reaction was operated with
only 2a. When the reaction was conducted with the phenol
1a and the side-product 4a under standard conditions,
similar results with significant formation of the product (3a)
were obtained (Section S4.2 in ESI†).14 This suggests the
instability of 4a under the reaction conditions, and its
possible conversion with 1a to the product 3a.
conditions (Table 2). Different substituents on the aromatic
ring of phenol were assessed first. As shown in Table 2,
electron-donating groups, such as tert-butyl, phenyl, methyl,
ethyl, propyl and cumyl were tolerated, providing the
corresponding products in 37–83% yields (3aa–3af).
Especially, the yields with the cumyl, bulky tert-butyl and
phenyl groups as substituents were 75%, 78% and 83%,
respectively. Interestingly, all halide substituents, which can
be used as functional handle in subsequent transformations,
afforded the desired products in 47–67% yields (3ag–3aj).
Electron-withdrawing groups, such as halides (3ah–3aj), are
preferred over electron-donating substituents, such as an
acetamido, methoxy or phenoxy substituents (3ak–3am).
Although with reduced yields, meta-, ortho- and multi-
substituted phenolics were also tolerated in this
transformation (3an–3at). The scope with respect to the
tertiary aniline coupling partner was next examined (Table 2).
Substrate scope
We next explored the substrate scope of this electrochemical
C(sp2)–H/O–H cross coupling with the optimal reaction
Table 2 Scope of the electrochemical aryl C–H activation for diaryl ether formationa
3aa, 78% (70%)
3ab, 83% (59%)
3ac, 47% (38%)
3af, 75% (67%)
3ai, 61% (44%)
3al, 27% (21%)
3ad, 49% (40%)
3ae, 37% (32%)
3ag, 47% (52%)b
3ah, 67% (58%)
3aj, 56% (43%)
3ak, 65% (64%)
3am, 22% (20%)
3an, 56% (53%)c
3ao, 50% (47%)
3ar, 31% (22%)
3ba, 18% (12%)
3ap, 39% (31%)c
3aq, 20% (18%)
3at, 77% (71%)d
3as, 20% (17%)
3ca, 18% (11%)
a
Reaction conditions: carbon felt as anode and nickel plate as cathode (10 mm × 10 mm × 1 mm), constant current of 5 mA, 1 (0.3 mmol), 2 (1
equiv., 0.3 mmol), nBu4NBF4 (0.5 equiv., 0.15 mmol), HFIP/DCM (6 : 4, 6 ml), room temperature, nitrogen, 5 h (3.1 F mol−1), isolated yields in
parentheses. Reaction for 3.5 h (2.2 F mol−1). Reaction for 6.5 h (4.0 F mol−1). 1a (0.6 mmol), 2a (0.6 mmol), nBu4NBF4 (0.3 mmol),
b
c
d
constant current of 10 mA.
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upon the use of our optimal solvent mixture (HFIP/DCM)
compared to more conventional solvent mixtures (MeCN/
MeOH), while the tert-butyl group of 1a further increases the
oxidation potential by approximately 1.0 V compared to a
methoxy substituent in 4-methoxyphenol (see ESI†). When we
tested the CV of a mixture of both substrates (1a + 2a) under
standard conditions, the potential of the oxidation peaks did
not change significantly. Subsequently, reactions were
conducted at a constant cell potential to control the oxidation
of the substrates. The reaction was run at a cell potential of
1.00 V, where only 2a could be oxidized, and at a cell
potential of 2.75 V where both 1a and 2a could be oxidized
(Fig. 2). In these electrocatalytic reactions, 65% and 48%
yields were obtained, respectively, proving that the selective
activation of 2a is beneficial for the yield.
Fig.
2 Cyclic voltammogram on a glassy carbon anode (3 mm
X-Band electron paramagnetic resonance (EPR) spectra of
electrolyzed solutions of 1a and/or 2a were recorded to obtain
information on the identity of any radical species under
reaction conditions (Fig. 3). No signal was observed for 1a,
while for 2a a signal was observed corresponding to the
radical cation of 2a (g = 2.0058, AN = 7.46 G, AH = 6.79 G,
AH–Ar = 5.63 G).16 When we tested a solution of both 1a and
2a, a different EPR signature was observed; the spectrum was
successfully fitted to that of the cationic radical of 3a (g =
2.007, AN = 9.51 G, AH = 9.78 G, AH–Ar-ortho = 4.77 G).17
According to these results, it can be concluded that only N,N-
dimethylaniline is oxidized under the reaction conditions;
the cation readily combines with 1a to obtain the C–O
coupled product.
diameter), a platinum cathode and ferrocene reference electrode at
0.1 V s−1 under nitrogen (see ESI† for detailed description).
N-Methyl-N-phenylaniline was tolerated and afforded the
corresponding diaryl ether in moderate yield (3ba). N,N-
Dimethylaniline with a bromine group at the meta-position of
the benzene ring also afforded the desired product (3ca).
Mechanistic study
To understand the mechanism of the electrochemical C–O
coupling reaction, cyclic voltammetry (CV) experiments were
carried out to probe the reactivity of the substrates (Fig. 2).
The
oxidation
of
N,N-dimethylaniline
(2a)
is
A possible mechanism for the electrochemical C(sp2)–H/
O–H cross-dehydrogenative coupling is proposed in
Scheme 2. First, the tertiary aniline moiety in the starting
thermodynamically favoured over that of 4-tert-butylphenol
(1a), with an oxidation peak at 0.81 V vs. ferrocene for 2a,
compared to 1.66 V vs. ferrocene for 1a. The measured
oxidation potential of 1a was considerably higher compared
to literature values.15 Further CV experiments showed an
increase in oxidation potential of approximately 0.8 V for 1a
material is anodically oxidized to furnish
a cationic
intermediate I; after this electrochemical umpolung,
I
undergoes nucleophilic attack by phenol. Concomitant
cathodic reduction of in situ generated protons can release
hydrogen gas during the reaction process. A key role is
ascribed to the use of HFIP as solvent, as its high dielectric
constant and low nucleophilicity stabilize the generated
radicals and cations, thereby avoiding side-reactions, such as
radical recombination or nucleophilic addition of 1a with
HFIP. Finally, its hydrogen-bond donating ability facilitates
Fig.
3 Electron paramagnetic resonance (EPR) spectra after
electrolysis in HFIP/DCM for 1 hour with a CF anode and a Pt cathode
Scheme
2 Proposed mechanism for the dehydrogenative C–O
(see ESI† for detailed description).
coupling of tertiary anilines and phenolics.
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hydrogen transfer from the coupling reagents to the
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Conclusions
In conclusion, we have developed an electrochemical protocol
for the formation of diaryl ethers by direct C–H aryloxylation
of tertiary anilines with free phenolics. From a mechanistic
standpoint, the key advance was a careful selection of
coupling partners together with the use of a HFIP/DCM
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aniline coupling partner, which was experimentally verified
by means of EPR and CV measurements. Such a mechanistic
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Author contributions
Under supervision of D. D. V. and J. V., H. T. was responsible
for the design and analysis of the experiments. S. S.
performed the EPR measurements. Y. L. was responsible for
the CV experiments. All authors have given approval to the
final version of the manuscript.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This work was funded by grants from the China Scholarship
Council (CSC, grant number: 201806070154 for H. T.), and
the Flemish government (FWO, grant number: 1S17620N for
J. V. and 12X7319N for Y. L.; VLAIO for S. S., EoS BioFACT,
CASAS Methusalem program for D. D. V.).
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