Catalytic Decarboxylative Cross-Coupling of Aryl Chlorides and Benzoates without Activating ortho Substituents

Article abstract of DOI:10.1002/anie.201505843

The restriction of decarboxylative cross-coupling reactions to ortho-substituted or heterocyclic carboxylate substrates was overcome by holistic optimization of a bimetallic Cu/Pd catalyst system. The combination of a CuI/Me₄phen decarboxylation catalyst and a [(MeCN)₄Pd](OTf)₂/XPhos cross-coupling catalyst enables the synthesis of biaryls from inexpensive aryl chlorides and potassium benzoates regardless of their substitution pattern. Lifting the restriction: A combination of a CuI/Me₄phen decarboxylation catalyst and a [(MeCN)₄Pd](OTf)₂/XPhos cross-coupling catalyst enables the synthesis of biaryls from inexpensive aryl chlorides and potassium benzoates regardless of their substitution pattern (see scheme; FG=functional group). This approach lifts the restriction of decarboxylative cross-coupling reactions to ortho-substituted or heterocyclic carboxylate substrates.

Full text of DOI:10.1002/anie.201505843

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International Edition: DOI: 10.1002/anie.201505843
German Edition: DOI: 10.1002/ange.201505843
Decarboxylative Cross-Coupling
Catalytic Decarboxylative Cross-Coupling of Aryl Chlorides and
Benzoates without Activating ortho Substituents
Jie Tang, Agostino Biafora, and Lukas J. Goossen*
Abstract: The restriction of decarboxylative cross-coupling
reactions to ortho-substituted or heterocyclic carboxylate
substrates was overcome by holistic optimization of a bimetallic
Cu/Pd catalyst system. The combination of a CuI/Me4phen
decarboxylation catalyst and a [(MeCN)4Pd](OTf)2/XPhos
cross-coupling catalyst enables the synthesis of biaryls from
inexpensive aryl chlorides and potassium benzoates regardless
of their substitution pattern.
T
he past decade has witnessed tremendous progress in the
Scheme 1. Scope and limitations of known decarboxylative coupling
reactions. FG=functional group.
development of decarboxylative cross-coupling reactions.[1]
The key advantage of this reaction concept is that it is based
on inexpensive carboxylate salts as the source of carbon
nucleophiles instead of preformed organometallic reagents.
Various synthetic transformations have been developed based
on this strategy, including decarboxylative Heck-type vinyl-
ations,[2] redox-neutral cross-coupling reactions,[3] allyla-
tions,[1a,4] oxidative coupling reactions,[5] C H arylations and
À
acylations,[6] and addition reactions.[7] Whereas aromatic
carboxylates are usually coupled through two-electron steps,
one-electron steps predominate in the activation of aliphatic
carboxylates.[8] A particularly topical development in this
field is the implementation of photoredox catalysts that
permit decarboxylatively coupling, for example, of aliphatic
carboxylic and amino acids.[9]
Scheme 2. Decarboxylative cross-coupling reaction. M=Cu, Ag;
R=(hetero)aryl, vinyl, acyl; R’=(hetero)aryl; alkenyl.
The discovery of bimetallic Cu/Pd or Ag/Pd catalysts has
enabled the efficient coupling of various aromatic and
heteroaromatic carboxylates with aryl electrophiles.[3b, 10]
Initially, the reaction concept appeared to be limited to
certain heterocyclic and mono- or di-ortho-substituted car-
boxylates. In protodecarboxylations, improved catalysts soon
allowed the energetic barrier to decarboxylation to be
surmounted, even for meta- and para-substituted ben-
zoates.[11] Unfortunately, the addition of halide salts, as they
form in cross-coupling reactions with aryl halides, completely
suppresses protodecarboxylations of such non-activated ben-
zoates (Scheme 1).[3b] Based on the mechanistic outline
depicted in Scheme 2, it was initially hypothesized that the
thermodynamically disfavored salt metathesis between
copper or silver halides (A) and the potassium carboxylate
(B) proceeds only when aided by coordinating groups in the
ortho position. This theory appeared to be supported by the
successful coupling of non-ortho-substituted carboxylates
with carbon electrophiles with noncoordinating sulfonate
leaving groups,[10d,e] for which the salt metathesis step (I)
should be favorable. Unfortunately, the price and availability
of these electrophiles somewhat limit the practical utility of
these procedures.
Overcoming the restriction to ortho-substituted benzoates
for aryl halide substrates remained a prime target in the
development of decarboxylative arylations, because meta-
and para-substituted biaryls are otherwise difficult to access
from inexpensive precursors, whereas ortho-substituted biar-
yls can be synthesized in increasing diversity through ortho-
[*] J. Tang, A. Biafora, Prof. Dr. L. J. Goossen
FB Chemie-Organische Chemie
À
C H arylation.
In-depth mechanistic studies confirmed the unfavorable
position of the upstream salt metathesis equilibrium (I), but
surprisingly revealed that it is almost unaffected by the
substitution pattern of the benzoates.[12] Instead, the presence
of s-withdrawing groups in the ortho position enables
decarboxylative coupling reactions by reducing their rate-
determining energy span by 4–8 kcalmolÀ1 overall, thus
Technische Universität Kaiserslautern
Erwin-Schrçdinger-Strasse Geb. 54
67663 Kaiserslautern (Germany)
E-mail: goossen@chemie.uni-kl.de
Homepage: http://www.chemie.uni-kl.de/goossen
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505843.
13130
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Table 1: Optimization of the reaction conditions.[a]
making them just feasible at manageable reaction temper-
atures.[13] The transmetalation step (III) was found to have an
energy barrier in the same high range as the salt metathesis/
decarboxylation process. Improving the decarboxylation
catalysts by introducing ligands that strongly stabilize inter-
mediate D would facilitate its formation but reduce its
reactivity in step III, once again disabling the overall process.
Only a holistic optimization of all reaction steps together
might enable the desired decarboxylative cross-coupling of
non-activated benzoates with aryl halides using metal medi-
ators only in catalytic amounts.
For the development of a decarboxylative arylation of
benzoates without activating ortho substituents, we set the
focus on aryl chloride substrates, the most available and
inexpensive of the aryl halides. Thus, we started by inves-
tigating the protodecarboxylation of 3-nitrobenzoic acid. At
1908C, quantitative conversion was observed in the presence
of the standard Cu2O/1,10-phenanthroline catalyst, but when
1 equiv of potassium chloride was added, the yield dropped to
14%. The addition of other salts, for example, NaCl,
nBu4NCl, or CsCl, suppressed the protodecarboxylation to
a comparably strong extent, whereas it was unaffected by the
presence of potassium salts with weakly coordinating anions
such as KOTf. This confirms that it is mostly the anion and not
the cation that affects the decarboxylation step. However,
after in-depth optimization, a quantitative yield was finally
Entry [Cu] N ligand
[Pd]
P ligand
Yield
[%]
3ba
4
1[b]
2[b]
3[b]
4[b]
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Cu2O Me4phen
Cu2O phen
Cu2O Ph2phen
PdBr2
PdBr2
PdBr2
JohnPhos
JohnPhos
JohnPhos
JohnPhos
JohnPhos
JohnPhos
JohnPhos
JohnPhos
tBu3P·HBF4 17 28
PCy3
SPhos
DavePhos 17 25
XPhos
XPhos
XPhos
XPhos
XPhos
XPhos
15 70
4
5
0
60
72
18
Cu2O (MeO)2phen PdBr2
Cu2O Me4phen
CuBr Me4phen
CuCl Me4phen
PdBr2
PdBr2
PdBr2
PdBr2
PdBr2
PdBr2
PdBr2
PdBr2
16 40
12 42
14 38
19 30
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
CuI
Me4phen
Me4phen
Me4phen
Me4phen
Me4phen
Me4phen
Me4phen
Me4phen
Me4phen
Me4phen
Me4phen
Me4phen
Me4phen
8
28
14 34
PdBr2
PdI2
36 23
37 24
42 40
Pd(OAc)2
[Pd2(dba)3]
[Pd(allyl)2Cl2]
XPhos-Pd-G2
[(MeCN)4Pd](OTf)2 XPhos
[(MeCN)4Pd](OTf)2 XPhos
7
25
29 32
12 27
58 50
67 38
20[c] CuI
achieved using
a 3,4,7,8-tetramethyl-1,10-phenanthroline
[a] Reaction conditions: 1b (0.6 mmol, 1.2 equiv), 2a (0.5 mmol), Cu
source (10 mol%), N ligand (10 mol%), Pd source (2 mol%), P ligand
(5 mol%), 3 mL of solvent (quinoline/NMP=1:1), 1908C, 16 h, yields
determined by GC analysis using n-tetradecane as the internal standard
based on 2a, for abbreviations see Ref. [15]. [b] In quinoline. [c] 5 mL of
solvent.
ligand in quinoline as the solvent (see the Supporting
Information). This result demonstrates that the decarboxyla-
tion barrier of non-activated carboxylates can be overcome
with customized catalysts, even when excess halide salt
reduces the availability of copper carboxylate intermediates.
Silver-based catalysts were ineffective under these con-
ditions. The proverbial stability of silver chloride shifts the
salt exchange equilibrium away from the silver carboxylates.
Furthermore, the influence of ortho substituents is partic-
ularly strong for silver, which catalyzes the decarboxylation of
2,6-dimethoxybenzoates under very mild conditions but is
inactive for non-ortho-substituted benzoates.[10a,14]
Encouraged by the protodecarboxylation results, we
searched for an effective decarboxylative cross-coupling
catalyst for the model reaction of potassium 3-nitrobenzoate
(1b) with 4-chlorotoluene (2a). Using a combination of the
optimal protodecarboxylation catalyst (Cu2O/Me4phen) with
a state-of-the-art cross-coupling system (PdBr2/JohnPhos),
only 15% yield of the desired product was detected (Table 1,
entry 1). The yield of 3ba was even lower with other
phenanthroline derivatives (entries 2–4). Remarkably, large
amounts of protodecarboxylation product 4 were formed.
This indicates that the decarboxylation step proceeds effi-
ciently and suggests that the transmetalation step has become
limiting. The ratio of 3ba to 4 improved when shifting to
a more polar solvent mixture of quinoline/NMP and to CuI as
the copper source (entries 5 and 8).
precursors with weakly coordinating counterions, that is,
[(MeCN)4Pd](OTf)2 (entry 19). Finally, the cross-coupling
catalyst was found to preserve its activity better at greater
dilution (entry 20). Control experiments showed that both
metals are essential for this transformation and that the yields
sharply decrease at lower temperatures (38% at 1808C, 30%
at 1708C, and 0% at 1508C; see the Supporting Information
for details).
By using the optimal catalyst, a mixture of CuI, Me4phen,
[(MeCN)4Pd](OTf)2, and XPhos in 1:1 quinoline/NMP, the
desired product 3ba was obtained in close to 70% yield after
16 h at 1908C, along with unreacted aryl chloride and the
protodecarboxylation product 4.
Under these conditions, a wide variety of benzoic acids
were coupled with the model substrate 4-chlorotoluene (2a)
in good yields (Table 2). The yields obtained for ortho-
substituted carboxylates are significantly higher than those
previously reported, thus confirming the superiority of the
new procedure. ortho-Methyl benzoate and ortho-phenyl
benzoate (3uf, 3vf), which have never before been converted
in decarboxylative coupling reactions, gave reasonable yields.
Electron-withdrawing substituents, such as nitro, cyano,
fluoride, trifluoromethyl, trifluoromethoxy, sulfonyl, and
sulfonamide may be present in any position of the aromatic
ring. The catalyst reaches its performance limit for 3-phenoxy
The key improvement in the overall yield was achieved by
optimizing the palladium co-catalyst. Among the electron-
rich, bulky phosphines known to activate aryl chlorides,
XPhos was by far the most effective (entry 13). Another
major improvement resulted from the use of palladium
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Table 2: Scope with regard to the aromatic carboxylates.[a]
Table 3: Scope with regard to the aryl chlorides.[a]
Product
Product
Product
Product
[a] Reaction conditions: 1b (0.6 mmol), 2b–o (0.5 mmol), CuI
(10 mol%), 3,4,7,8-tetramethyl-1,10-phenanthroline (10 mol%),
[(MeCN)4Pd](OTf)2 (2 mol%), XPhos (5 mol%), 5 mL of solvent, 1908C,
16 h. Yields of isolated products.
[a] Reaction conditions: 1a–v (0.6–0.75 mmol), 2a or 2 f (0.5 mmol), CuI
(10 mol%), 3,4,7,8-tetramethyl-1,10-phenanthroline (10 mol%),
[(MeCN)4Pd](OTf)2 (2 mol%), XPhos (5 mol%), 5 mL of solvent, 1908C,
16 h. Yields of isolated products. [b] 32 h. [c] GC yield. [d] 4-Chloroben-
zonitrile (2 f) used as coupling partner.
speed, and 3) the transmetalation is unaffected by the
substituent on the Pd-bound aryl group. The oxidative
addition equilibrium, which should lie more on the side of
the starting materials for electron-rich compared to electron-
poor substrates, increases the rate-determining energy span
for the former, so that the decarboxylative cross-coupling is
more selective for the latter.
benzoate (3oa). Substrates that were even more electron rich
gave unsatisfactory yields. However, the formation of proto-
decarboxylation by-products indicates that the decarboxyla-
tion step is not limiting for any of the substrates.
The scope with regard to the electrophilic coupling
partner was explored with potassium 3-nitrobenzoate (1b).
As shown in Table 3, various aryl chlorides with common
functionalities such as cyano, fluoro, trifluoromethyl, ether,
sulfonyl, and ketone groups, as well as heterocyclic deriva-
tives react smoothly. Notably, electron-rich derivatives gave
lower ratios of decarboxylative coupling to protodecarbox-
ylation than electron-poor substrates. A rationale for the
observation that the oxidative addition becomes the selectiv-
ity-determining step can be derived from the energy span
model, if one assumes that 1) the transmetalation proceeds
via the highest-energy transition state in the reaction profile,
2) the protodecarboxylation always proceeds with the same
In conclusion, a customized bimetallic PdII/CuI catalyst
system was found to enable the decarboxylative cross-
coupling of non-ortho-substituted aromatic carboxylates
with aryl chlorides. This confirms predictions by DFT studies
that the previously observed limitation to certain activated
carboxylates is not intrinsic. Since the decarboxylation step is
no longer limiting, further studies can now be directed
towards the development of a new generation of catalysts
with bridging ligands designed to facilitate the transmetala-
tion and allow catalytic turnover at strongly reduced temper-
atures.
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Keywords: aryl chlorides · benzoic acids · copper ·
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[15] dba = dibenzylideneacetone; NMP = N-methyl-2-pyrrolidone;
phen = 1,10-phenanthroline;
Me4phen = 3,4,7,8-tetramethyl-
1,10-phenanthroline; Ph2phen = 4,7-diphenyl-1,10-phenanthro-
line; (MeO)2phen = 4,7-dimethoxyl-1,10-phenanthroline; John-
Phos = 2-(di-tert-butylphosphino)biphenyl; SPhos = 2-dicyclo-
hexylphosphino-2’,6’-dimethoxybiphenyl; DavePhos = 2-dicy-
clohexylphosphino-2’-(N,N-dimethylamino)biphenyl; XPhos =
2-dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl; XPhos-
Pd-G2 = chloro(2-dicyclohexylphosphino-2’,4’,6’-triisopropyl-
1,1’-biphenyl)[2-(2’-amino-1,1’-biphenyl)]palladium(II); Ts = 4-
toluenesulfonyl; Tf = trifluoromethylsulfonyl.
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Received: June 25, 2015
Published online: September 4, 2015
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