Synthesis of aryl ethers from benzoates through carboxylate-directed C-H-activating alkoxylation with concomitant protodecarboxylation

Article abstract of DOI:10.1002/anie.201208755

One in, one out: In the presence of a copper/silver bimetallic catalyst system, aromatic carboxylate salts undergo ortho C-H alkoxylation with concomitant loss of the carboxylate directing group in a protodecarboxylation step (see scheme, FG=functional group). This process provides a convenient synthetic access to the important class of aromatic ethers from widely available carboxylic acids. Copyright

Full text of DOI:10.1002/anie.201208755

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Chemie
DOI: 10.1002/anie.201208755
À
C H Activation
Synthesis of Aryl Ethers from Benzoates through Carboxylate-
À
Directed C H-Activating Alkoxylation with Concomitant
Protodecarboxylation**
Sukalyan Bhadra, Wojciech I. Dzik, and Lukas J. Gooßen*
The aryl ether functionality is a common motif in many
classes of biologically active compounds and functional
materials.[1] Traditional methods to attach ether groups to
aromatic rings require either activated substrates (SNAr) or
harsh reaction conditions (Ullmann-type condensation), and
are therefore not ideal for the late-stage functionalization of
complex molecules.[2]
substrate combinations, the decarboxylative alkoxylation was
found to proceed regiospecifically at the ipso position. Only
for one substrate, we observed the formation of a by-product
À
in which the C O bond had formed in the ortho position of
the extruded carboxylic group. This by-product, although
obtained in only modest yield and using stoichiometric
amounts of silver and copper, pointed to the principal
feasibility of a second, complementary pathway for alkox-
ylations with decarboxylation, namely the carboxylate-
In the last two decades, the need for reliable and atom-
economic methods to access aryl ethers started a rapid
À
À
development of new methodologies for C O bond formation.
directed ortho alkoxylation of benzoates through C H
Important advances include copper- and palladium-catalyzed
couplings of aryl halides with alcohols or phenols,[3] and
copper-mediated oxidative coupling of aryl borates with
phenols.[4] These methods depend on the availability of
starting materials with the desired substitution pattern,
activation,[18,19] followed by protodecarboxylation.[20] If fur-
ther developed, such a reaction concept could give access to
aryl ethers with a complementary product range to decar-
boxylative ipso alkoxylations: meta-substituted aryl ethers
would arise from para- or ortho-substituted benzoates, and
para-substituted aryl ethers from meta-substituted benzoates
(Scheme 1).
À
a limitation that may be overcome with C H activating
alkoxylation processes.[5] Several protocols for palladium-
catalyzed regioselective alkoxylations of arenes were recently
reported with the use of pyridyl,[6] N-methoxyimine,[7] N-
methoxybenzamide,[8] cyano,[9] and anilide[10] directing groups.
Copper-based systems have been used for similar trans-
formations,[11] including oxygen atom transfer[12] or catalytic
acetoxylation reactions,[13] but to the best of our knowledge,
À
no copper-catalyzed C H activating alkoxylations of arenes
have been reported to date. However, the possibility of
directed copper-mediated aryl ether formation is supported
by two experimental findings: Yu et al. observed the coupling
of 2-phenylpyridine with 4-cyanophenol,[13a] and Ribas and
Stahl found that methoxylation of a benzene ring within
a macrocyclic ligand scaffold takes place in the presence of
copper.[14]
Scheme 1. Ipso- versus ortho-directed decarboxylative alkoxylation.
FG=functional group.
In our studies of decarboxylative coupling reactions,[15,16]
we recently disclosed a decarboxylative etherification in
The viability of a reaction concept that combines an ortho
alkoxylation with a concomitant decarboxylation is further
supported by the documented ability of aromatic carboxylate
À
which a C O bond is formed at the original position of the
carboxylate group of benzoates.[17] For a broad range of
groups to direct other types of C H functionalization into
À
their ortho position,[21] for example, as described by Yu
[*] Dr. S. Bhadra,[+] Dr. W. I. Dzik,[+] Prof. Dr. L. J. Gooßen
FB Chemie-Organische Chemie
et al.[22] Furthermore, Satoh, Miura, and Larrosa and their
À
respective co-workers described C H arylations of aromatic
Technische Universitꢀt Kaiserslautern
Erwin-Schrçdinger-Str. Geb. 54, 67663 Kaiserslautern (Germany)
E-mail: goossen@chemie.uni-kl.de
carboxylic acids with subsequent removal of the carboxylate
directing group by in situ protodecarboxylation.[23]
Our mechanistic outline for an efficient decarboxylative
ortho alkoxylation process of benzoates is shown in Scheme 2.
It consists of a silver/copper-mediated oxidative ortho alkox-
ylation cycle of the benzoate linked with a decarboxylation
cycle. In the alkoxylation cycle, a CuII benzoate species (c),
Homepage: http://www.chemie.uni-kl.de/goossen
[+] These authors contributed equally to this work.
[**] We thank the DFG (SFB/TRR-88, “3MET”) and the Alexander von
Humboldt Foundation (fellowship to W.I.D.) for financial support,
R. Bashirbayeva for technical assistance, and Prof. Dr. F. W.
Patureau for advice.
À
which is formed by salt metathesis, undergoes C H activation
in the presence of another CuII salt, leading to a CuIII–aryl
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208755.
species (d), as established by Ribas and Stahl.[14] Transfer of
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a silver-mediated decarboxylation of ortho alkoxy-
benzoates. In the presence of stoichiometric amounts
of both Cu(OAc)2 and Ag2CO3 and using tetrame-
thoxysilane, the alkoxide source of choice for ipso
alkoxylations, the desired product 3aa was indeed
obtained, while the ipso product was not detected
(entry 1).
The key modification to achieve higher yields was
the use of trimethylborate, whereas other methoxide
sources were not effective (entry 2). Reducing the
loading of copper(II) acetate to 25 mol% improved
the yield, while the competing esterification to 4a was
slowed down (entries 2 and 3). Performing the
reaction under O2 atmosphere and lowering the
reaction temperature to 1408C further improved the
yield to 84% (entries 4 and 5).
Scheme 2. Suggested mechanism for a decarboxylative alkoxylation.
Additional studies confirmed that copper acetate
and silver carbonate are the most effective metal
precursors (see the Supporting Information). Control experi-
ments showed that without either Ag2CO3 or Cu(OAc)2, the
reaction does not proceed (entries 6 and 7). In agreement
with the proposed mechanism, the presence of silver carbon-
ate was crucial, not only for the decarboxylation, but also for
the methoxylation process. Neither ortho-alkoxylated anisic
acid nor anisole were formed in its absence.
an alkoxide to the CuIII center and reductive alkoxyarene
elimination give rise to a CuI ortho alkoxybenzoate. The
carboxylate ion is likely to be directly transferred to the silver
co-catalyst (f,g), and the catalytic cycle is closed by reox-
idation of the copper center (a). The choice of silver as
a decarboxylation co-catalyst is decisive. In contrast to
copper, silver effectively promotes the decarboxylation
specifically of ortho-alkoxybenzoic acids already at 1208C,
while it does not decarboxylate benzoic acids without s-
electron-withdrawing groups in the ortho position.[24] As long
as the substrates do not contain such groups, competing
decarboxylative ipso substitution would thus effectively be
suppressed. In contrast, a silver ortho-alkoxybenzoate would,
once it is formed, swiftly decarboxylate, thereby precluding
double alkoxylation.
We started our search for a suitable protocol by evaluating
alkoxide sources as well as alkoxylation and decarboxylation
catalysts, using the methoxylation of potassium 4-anisate as
a model reaction (Tables 1 and S1 in the Supporting
Information). The reaction temperature was set to 1508C,
well below the temperature required for a copper-mediated
protodecarboxylation of this substrate, but sufficient for
With an effective catalyst system in hand, we next
investigated the scope of the new transformation. As shown
by the examples in Scheme 3, both for primary and secondary
alkoxides, the alkyl aryl ethers were obtained in good yields
(3aa–3ag). Chiral alkoxides were transferred with retention
of configuration (3ah). The reaction is also broadly applicable
with regard to the aromatic carboxylic acids. Benzoates with
electron-donating or electron-withdrawing groups were
smoothly converted, and a broad range of functional groups,
including keto, cyano, nitro, sulfonyl, N-heterocyclic, and
even bromo substituents are tolerated. When starting from
meta-substituted benzoates, the coupling occurs selectively in
the position para to the substituent (3ka–3na), with only one
notable exception: for 3-nitro-4-methoxybenzoate the alkox-
ylation takes place in ortho position (3oa), presumably
because of chelate assistance by the nitro group. Meta-
substituted ethers are obtained from para-substituted carbox-
ylates (3ci–3ea and 3ga–3jb) and from ortho-alkyl benzoates
(3pa, 3qi). The anthraquinone 3ra and flurenone 3sa are not
known to be accessible through Ullmann-type couplings.
Competing decarboxylative ipso alkoxylation was only
observed for substrates with s-electron-withdrawing substitu-
ents, for example, alkoxy groups in ortho position to the
carboxylate group. Neither double alkoxylation nor non-
decarboxylative alkoxylation was observed, confirming the
high efficiency of the decarboxylation step following alkox-
ylation.
Table 1: Optimization of the catalyst system and reaction conditions.[a]
Entry
MOMe
Cu
Additive
T
3aa
4a
[equiv]
[8C]
[%]
[%]
1
2
3
4
Si(OMe)4
B(OMe)3
B(OMe)3
B(OMe)3
B(OMe)3
B(OMe)3
B(OMe)3
1
1
0.25
0.25
0.25
0
–
–
–
O2
O2
O2
O2
150
150
150
150
140
140
140
12
51
73
83
84
0
0
35
12
8
trace
trace
0
The synthesis of the borate ester reagent can be combined
with the decarboxylative alkoxylation in a one-pot procedure
(Scheme 4). The alcohol is first treated with an equivalent
amount of pinacol borane, and when gas evolution ceases, the
carboxylate salt and catalyst are added and the decarbox-
ylative coupling is performed as usual. This variant is
5
6[b]
7[b,c]
0.25
0
[a] Reaction conditions: 0.3 mmol potassium 4-methoxybenzoate,
5 equiv MOMe, Cu(OAc)2, 1 equiv Ag2CO3, 2 mL DMF, 36 h. Yields
determined by GC using n-tetradecane as internal standard. [b] Near-
quantitative recovery of 1a. [c] No Ag2CO3.
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limiting, which is consistent with the proposed mechanism,
but less so with other conceivable reaction pathways, that is,
single-electron-transfer (SET) mechanisms or an attack of
copper-coordinated oxide or peroxide onto the arene
ring.[12,25]
The intermediacy of phenols as well as the involvement of
acetoxylation pathways were excluded, because no ether-
ification of the phenoxy group was observed when potassium
salicylate, 3-methoxyphenol, potassium 3-methoxyphenolate,
or potassium acetyl salicylate were subjected to the reaction
À
conditions (Scheme 5). This observation confirms that the C
O bond is formed between the alkoxide and a metalated
arene and not through an attack of (per)oxo–copper species
onto the arene ring.[12]
Scheme 5. Evidence against the formation of (acetyl)phenols as inter-
mediates.
In conclusion, a regiospecific ortho alkoxylation of
aromatic carboxylates with concomitant decarboxylation has
been developed. It gives access to the important substrate
class of aromatic ethers from widely available carboxylic
acids. This process, in which the carboxylate substituent serves
as a cleavable directing group,[26] represents a rare example of
an aromatic substitution reaction in which the original
substitution pattern is altered in a defined way. Ongoing
work is directed toward the development of reaction variants
that start directly from alcohols.
Scheme 3. Scope of the directed etherification of aromatic carboxy-
lates. Reaction conditions: 1.00 mmol 1a–t, 1.20 mmol 2a–i,
0.25 mmol Cu(OAc)2, 1.00 mmol Ag2CO3, 1 atm. O2, 5 mL DMF,
1408C, 36 h. [a] With 5 mmol B(OR)3. [b] With 10% methylester.
[c] With 12% methylester. [d] With 19% quinoline.
Experimental Section
General procedure for the aryl ether synthesis: A 70 mL Schlenk tube
was charged with the potassium carboxylate (1a–t; 1.00 mmol),
copper(II) acetate (0.25 mmol), silver carbonate (1 mmol), and the
trialkylborate (2a–i; 1.2 mmol). Anhydrous DMF (5 mL) was added,
and the mixture was stirred at 1408C for 36 h under an oxygen
atmosphere. After cooling, the reaction mixture was diluted with
diethyl ether (20 mL) and washed with saturated aqueous sodium
bicarbonate (20 mL). The aqueous layer was extracted with diethyl
ether (3 ꢀ 20 mL), the organic layers were washed with water, 5n HCl,
and brine, dried over MgSO4, filtered, and concentrated in vacuum.
The residue was purified by column chromatography on SiO2 with an
n-hexane/ethyl acetate gradient to give the corresponding alkyl aryl
ether.
Scheme 4. Alkoxylation with a trialkoxyborate that was generated
in situ.
particularly advantageous if the alcohol is too precious to be
used in excess.
Received: October 31, 2012
Published online: January 31, 2013
A series of experiments were performed to shed more
light on the reaction mechanism. In the reaction of potassium
benzoate with trimethylborate, a kinetic isotope effect of 2.8
was observed (see Scheme S1 in the Supporting Information).
À
À
À
Keywords: aryl ethers · C C activation · C H activation · C
O coupling · decarboxylations
.
À
This suggests that the breaking of the C H bond is rate-
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