Decarboxylative etherification of aromatic carboxylic acids

Article abstract of DOI:10.1021/ja304539j

Decarboxylative Chan-Evans-Lam-type couplings are presented as a new strategy for the regiospecific construction of diaryl and alkyl aryl ethers starting from easily available aromatic carboxylic acids. They allow converting various aromatic carboxylate salts into the corresponding aryl ethers by reaction with alkyl orthosilicates or aryl borates, under aerobic conditions in the presence of silver carbonate as the decarboxylation catalyst and copper acetate as the cross-coupling catalyst.

Full text of DOI:10.1021/ja304539j

Communication
pubs.acs.org/JACS
Decarboxylative Etherification of Aromatic Carboxylic Acids
Sukalyan Bhadra, Wojciech I. Dzik, and Lukas J. Goossen*
Department of Organic Chemistry, Technische Universitat Kaiserslautern, Erwin-Schrodinger-Str. Geb. 54, 67663 Kaiserslautern,
̈
̈
Germany
S
* Supporting Information
studies by Stahl et al. reveal that this reaction may also be used
to access alkyl aryl ethers.9 It is a valuable alternative to the
Buchwald−Hartwig coupling because it draws on a different
starting material pool, although the use of boronic acids renders
it rather costly.
ABSTRACT: Decarboxylative Chan−Evans−Lam-type
couplings are presented as a new strategy for the
regiospecific construction of diaryl and alkyl aryl ethers
starting from easily available aromatic carboxylic acids.
They allow converting various aromatic carboxylate salts
into the corresponding aryl ethers by reaction with alkyl
orthosilicates or aryl borates, under aerobic conditions in
the presence of silver carbonate as the decarboxylation
catalyst and copper acetate as the cross-coupling catalyst.
Within recent years, a rapidly growing number of catalytic
coupling reactions have been reported that make use of
carboxylic acids as substrates.10 The advantage of using
carboxylates as leaving groups for regiospecific couplings is
that aromatic carboxylic acids are available in great structural
diversity. Since they are accessed via pathways that differ
substantially from those leading to aryl halides or organo-
metallic reagents, their substitution patterns are often
complementary. In this context, catalytic decarboxylative
coupling reactions have evolved into an efficient methodology
for regiospecific C−C and C−heteroatom bond formation,
allowing the expedient synthesis, e.g., of vinyl arenes,11
biaryls,12−14 aryl ketones,15 thioethers,16 haloarenes,17 phos-
phine oxides,18 ynamides,19 or allyl compounds.20
The use of carboxylic acids in Chan−Evans−Lam-type
couplings would be of substantial synthetic interest in view of
the lower price and better availability of the starting materials.21
We reasoned that decarboxylative Chan−Evans−Lam processes
may be possible via the mechanistic concept outlined in
Scheme 2. In the proposed decarboxylation cycle (Scheme 2,
he selective installation of alkoxy substituents at specific
T
positions of aromatic rings is a synthetic transformation of
fundamental importance, given the abundance of the aryl ether
moiety in biologically active molecules and functional
materials.1,2 Traditional methods for the synthesis of aryl
ethers via sp2 C−O bond formation, e.g., nucleophilic aromatic
substitutions of haloarenes or the Ullmann coupling of phenols
with aryl halides, require rather harsh conditions unsuitable for
the late-stage derivatization of functionalized molecules.3
Two reaction concepts have recently emerged that allow
forming aromatic C−O bonds under mild conditions (Scheme
1). The Buchwald−Hartwig approach consists of coupling aryl
Scheme 1. Strategies for C(aryl)−O Bond Formation
Scheme 2. Proposed Mechanism for a Decarboxylative
Etherification Reaction
halides with phenols or alcohols in the presence of a palladium
catalyst and base.4 Especially in the case of alcohols that can
undergo β-hydride elimination at palladium catalysts, e.g.,
ethanol, highly complex ligand systems are necessary to achieve
good yields.5 Copper catalysts have a lower tendency to
promote β-hydride elimination, but their activity extends only
to costly aryl iodides and some activated aryl bromides.6 The
other strategy is the Chan−Evans−Lam coupling of boronic
acids and phenols in the presence of amine base and copper(II)
acetate, which is mostly used in stoichiometric amounts in
combination with air or oxygen as the oxidant.7,8 While
publications focusing on preparative-scale reactions describe its
application only to the synthesis of diaryl ethers, mechanistic
left), a transition-metal carboxylate forms via salt exchange and
extrudes CO2 to give an arylmetal species C. A transmetalation
step connects this cycle with a Chan−Evans−Lam cycle in
analogy to that proposed by Stahl et al. for the alkoxylation of
Received: May 10, 2012
Published: June 8, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
arylboronic esters.9 The arylcopper(II) species D thus formed
is oxidized to the CuIII species E, which reacts with the O-
nucleophile and reductively eliminates the aryl ether product F.
It appeared plausible that Cu(OAc)2, the cross-coupling catalyst
used in standard Chan−Evans−Lam reactions, should be active
also in this reaction variant. For the decarboxylation cycle, the
use of AgI catalysts as used by Myers, Becht, Larrosa, and
ourselves11a,13,22 appeared to be more promising than of CuI
phenanthroline complexes,12b−d because the latter would
probably be converted to CuII under the strongly oxidizing
conditions, thereby losing their activity.
In order to verify this mechanistic hypothesis, we investigated
whether potassium 2-nitrobenzoate can be converted into 2-
ethoxy-nitrobenzene by reaction with ethoxide derivatives in
the presence of Cu(OAc)2 as the Chan−Evans−Lam catalyst
and Ag2CO3 as both the decarboxylation catalyst and
stoichiometric oxidant (Table 1).
copper acetate are the optimal catalyst precursors (see Table
S1, Supporting Information (SI)). Control experiments
revealed that the reaction is effective only in the presence of
both catalyst metals. Using copper acetate alone, the yield
dropped to 24%, and large amounts of side products were
observed. With silver carbonate alone, the reaction did not
proceed at all.
Having thus found an effective protocol for the decarbox-
ylative etherification,25 we next investigated its scope with
regard to the O-nucleophile. As can be seen from Table 2,
a
Table 2. Scope of the Decarboxylative Etherification
a
Table 1. Optimization of the Reaction Conditions
yield (%)
b
entry
MOEt
additive
temp (°C)
3ab
4a
5a
1
2
3
4
EtOH
−
−
−
−
O2
120
120
120
120
120
145
120
120
145
145
0
0
62
tr.
tr.
13
15
tr.
62
tr.
23
tr.
0
0
NaOEt
B(OEt)3
Si(OEt)4
Si(OEt)4
Si(OEt)4
Si(OEt)4
Si(OEt)4
Si(OEt)4
Si(OEt)4
52
72
68
81
0
0
5
c
5
4
c
6
O2
0
c
7
BQ
K2S2O8
O2
0
c
8
d
0
0
9
24
0
14
0
c,e
10
O2
a
Reaction conditions: 0.3 mmol potassium 2-nitrobenzoate, 1.2 equiv
MOEt, 1 equiv Ag2CO3, 1 equiv Cu(OAc)2, 2 mL solvent, 18 h. Yields
b
determined by GC using n-tetradecane as internal standard. 1 bar O2
c
or 1 equiv para-benzoquinone (BQ) or 1 equiv K2S2O8. 25 mol %
d
e
Ag2CO3. Without Ag2CO3. Without Cu(OAc)2.
When ethanol was employed as the O-nucleophile in the
presence of stoichiometric amounts of silver carbonate and
copper acetate at 120 °C in DMF, the formation of
nitrobenzene (4a) indicated that decarboxylation takes place
but that the resulting aryl−metal intermediate undergoes
protonolysis by the alcoholic OH group (Table 1, entry 1).
For metal alkoxides, no reaction was observed (entry 2).
However, we were pleased to see that the desired product
formed in reasonable yields when using borate23 or silicate
esters5c,24 as O-nucleophiles. Only small quantities of nitro-
benzene and 2,2′-dinitro-1,1′-biphenyl (5a) were observed as
side products (entries 3,4). The amount of silver carbonate
could be reduced to substoichiometric amounts when perform-
ing the reaction under an oxygen atmosphere (entry 5). By
increasing the reaction temperature to 145 °C, the yield could
be improved to 81% using 25 mol % Ag2CO3 (entry 6). This is
an excellent result for the introduction of an alkoxy group that
is prone to competing β-hydride elimination reaction.5
a
Reaction conditions: 1.00 mmol benzoic acid potassium salt, 1.20
mmol Si(OR)4, 0.25 mmol Ag2CO3, 1.00 mmol Cu(OAc)2, O2
b
balloon, 5 mL DMF, 145 °C, 18 h. 5.00 mmol Si(OMe)4 was
c
d
used. 1.20 mmol B(OPh)3 was used. 19F NMR yield based on
CF3CH2OH as an internal standard.
potassium 2-nitrobenzoate was successfully coupled with both
linear (methyl, ethyl, propyl, butyl) and branched (isopropyl)
orthosilicates. Aromatic silicates are harder to access, so we
were pleased to find that the easily available triphenylborate is a
suitable reagent for the introduction of phenoxy groups (3af).
Further studies revealed that the new protocol is applicable to
the full spectrum of aromatic carboxylates that are known to
Further investigations revealed that oxygen is the most
effective oxidant (entries 5,7,8), DMF is the optimal solvent,
minimizing protodecarboxylation, and that silver carbonate and
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Journal of the American Chemical Society
Communication
undergo silver-catalyzed decarboxylations. Thus, aromatic
carboxylates with various electron-withdrawing and -donating
groups in the ortho position were successfully converted.
The particular suitability of this method for the synthesis of
ortho-functionalized aryl ethers makes it complementary to the
traditional Buchwald−Hartwig and Chan−Evans−Lam reac-
tions, for which only few examples of ortho-functionalized
products have been reported.
In order to probe whether it is indeed the silver that mediates
the decarboxylation and the copper that promotes the cross-
coupling, some mechanistic experiments were conducted
(Scheme 3). Preformed mesitylene silver (6),26 an arylsilver
valuable synthetic entry to aryl ethers starting from widely
available carboxylic acids. For carboxylates with a low tendency
to extrude carbon dioxide, an alternative pathway consisting of
ortho functionalization and protodecarboxylation has been
shown to be functional, so that the ether group is installed in
the ortho rather than the ipso position of the former
carboxylate group. These findings set the stage for the
development of a whole class of related decarboxylative C−
heteroatom bond-forming reactions, e.g., decarboxylative
aminations.
ASSOCIATED CONTENT
* Supporting Information
■
S
Scheme 3. Reaction of Arylsilver Species with Si(OMe)4 in
the Absence (top) and Presence (bottom) of Cu(OAc)2
Experiment details and spectral data for all compounds are
provided. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
goossen@chemie.uni-kl.de
■
Notes
The authors declare no competing financial interest.
species analogous to that which would form via decarboxylation
of a silver carboxylate, was heated to 145 °C with Cu(OAc)2
and Si(OMe)4 in DMF under oxygen. The formation of the
alkyl aryl ether 6a27 supports the proposed transmetalation
from silver to copper.28 In the absence of copper acetate no
alkyl aryl ether was formed, suggesting that the cross-coupling
can take place only at the copper.
ACKNOWLEDGMENTS
■
We thank Saltigo GmbH, the DFG (SFB/TRR-88, “3MET”),
BMBF (Green Talent Programme fellowship to S.B.), and
Alexander von Humboldt Foundation (fellowship to W.I.D.)
for financial support.
In all cases, the C−O bond formed regiospecifically in the
position of the carboxylate group with the remarkable exception
of compound 3oa, for which 29% of the regioisomer (3-
methoxyphenyl)(phenyl)methanone was observed as a side
product. We suspected that this product may have formed via a
carboxylate-directed11a,29 ortho methoxylation with C−H
activation,30 followed by protodecarboxylation, analogously to
a pathway described by Satoh, Miura, and Larrosa for C−C
bond-forming reactions.31 We went on to seek conditions
under which this reaction mode would dominate. Indeed, when
potassium 4-methoxybenzoate, which has a particularly low
tendency to decarboxylate, was heated in a microwave reactor
to 170 °C with B(OMe)3 in the presence of CuBr2 and
Ag2CO3, 1,3-dimethoxybenzene (7) was formed in reasonable
yield. Although significant catalyst development is still required
to bring this reaction mode to synthetic maturity, our finding
demonstrates that carboxylates can direct an alkoxylation not
only into the ipso but also into the ortho position in aromatic
rings. This opens up further opportunities for regioselective C−
O bond formation (Scheme 4).
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