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Novel Substrates for Palladium-Catalyzed Coupling Reactions of Arenes.

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A. Zapf
Cross-Coupling Reactions
Novel Substrates for Palladium-Catalyzed Coupling
Reactions of Arenes
Alexander Zapf*
carbonic acids · cross-coupling · homogeneous
catalysis · ketones · palladium
Dedicated to Professor Dr. Gnther Oehme
on the occasion of his 65th birthday
renes and heteroarenes are ubiquitous substructures in biologically
active agents and new materials. Thus, functionalization (“refinement”) of simple arene precursors is still of major importance for
preparative organic chemistry. During the last 20 years, especially
transition-metal-catalyzed cross-coupling reactions of aryl halides and
triflates have given arene chemistry new impetus. The first industrial
applications were realized a few years ago. Quite recently, carbonic
acid derivatives such as anhydrides and esters have added to the scope
of substrates for these coupling reactions. Some recent developments in
this area are presented in this Minireview.
1. Introduction
Palladium-catalyzed coupling reactions allow a number of
elegant derivatizations of aryl halides and similar aryl-X
compounds.[1] Although these reactions have been known
since the 1970s, there are currently only few examples of the
industrial application of these reactions.[2] However, based on
a considerable improvement of the catalyst systems since the
mid 1990s, the prospects for this type of coupling chemistry in
industry are much better today. Especially because of the high
costs of palladium, the first applications were in the syntheses
of relatively expensive drugs and fine chemicals.[2] A number
of catalyst systems are now available that allow reactions of
(in most cases) the most economical and readily available aryl
halides, that is, the aryl chlorides, with high turnover
numbers.[3] Other disadvantages have to be accepted to gain
high catalytic activity and productivity: highly active catalysts
are often based on costly and air-sensitive phosphane ligands.
Furthermore, additives (e.g., cesium salts) are essential in
many cases; these additives as well as the processing and
disposal of their waste products come at a high price. Besides
the development of new catalyst systems (e.g., catalysts that
are based on N-heterocyclic carbenes[4] as ligands instead of
[*] Dr. A. Zapf
Leibniz-Institut f2r Organische Katalyse
an der Universit6t Rostock e.V. (IfOK)
Buchbinderstrasse 5–6, 18 055 Rostock (Germany)
Fax: (+ 49) 381-46693-24
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
phosphanes and that require simple or
no additives), the search for novel
substrates for the cross-coupling reactions has been the focus of much
attention. The development of such
substrates is covered in this Minireview. Functionalization reactions of arenes by C H activation
will not be discussed, as important progress in this area has
already been summarized.[5]
2. Carbonic Acids, Anhydrides, and Esters
Carbonic acid chlorides are highly reactive species. It was
shown in the 1970s for the first time that they also react with
palladium(0) species and can therefore be employed in
coupling reactions (Scheme 1).[6] The carbonyl group is either
retained in the products (e.g. in cross-coupling reactions with
organotin compounds as demonstrated by Stille and coworkers[7a]) or it is eliminated as carbon monoxide after
Scheme 1. Palladium-catalyzed coupling reactions of aryl carbonic acid
derivatives (for X = Cl, see references [6–8]). M = SnR23, BR22, ZnX, Cu,
AlR12, PbR13, SiR13.
DOI: 10.1002/anie.200301681
Angew. Chem. Int. Ed. 2003, 42, 5394 –5399
Cross-Coupling Reactions
oxidative addition of the acid chloride to Pd0. The acyl–PdII
complex formed initially gives the corresponding aryl–PdII
complex, which undergoes aryl-X coupling reactions. Accordingly, Blaser and Spencer showed that olefins can be arylated
with aryl carbonic acid chlorides in a Heck-type reaction
(Blaser–Heck reaction).[7b]
It has also been known since the 1970s that nickel(0) can
insert into the C O bond of carbonic acid anhydrides[9] and
esters.[10] Although the basis for new coupling reactions was
laid early, it took about 20 years before it was demonstrated
that palladium(0) also inserts into anhydride and ester
groups[11] and that the resulting acyl complexes are suited
for coupling reactions.[12] Stephan, de Vries et al. first described the use of aromatic carbonic acid anhydrides instead of
aryl halides in the Heck reaction in 1998.[13] Under conditions
similar to those for classical Heck reactions, arylated olefins
were obtained in good yields in most cases [Eq. (1); NMP =
This protocol has some advantages over the Heck reaction
of aryl halides: no large amounts of waste salts are generated,
stoichiometric amounts of base to neutralize the liberated
acid are unnecessary, and the addition of phosphane ligands
for activation or at least stabilization of the palladium catalyst
is not required (in fact, they are even detrimental!). The role
of the phosphane ligands is played by sodium bromide, which
must be added in catalytic amounts; evidently, bromine–
palladium complexes are involved in the reaction. Carbon
monoxide, which is released by the coupling reaction, is
harmless on a laboratory scale and can easily be burned to
carbon dioxide in industrial processes. The carbonic acid that
is generated concomitantly can be reconverted into its
anhydride, so that the overall reaction produces comparatively little waste.
Alexander Zapf was born in Coburg, Germany, in 1970 and studied chemistry at the
Technische Universit#t M%nchen. He completed his PhD in 1998 in the group of M.
Beller on the topic of palladium-catalyzed
cross-coupling reactions of aryl chlorides. He
then joined the Leibniz-Institut f%r Organische Katalyse an der Universit#t Rostock
e.V. (IfOK), where he is in charge of the
“Aryl-X Activation” group. He was a fellow
of the Studienstiftung des deutschen Volkes
and the Max-Buchner-Forschungsstiftung.
Angew. Chem. Int. Ed. 2003, 42, 5394 –5399
Investigations into the oxidative addition of benzoic acid
to Pd0–phosphane complexes have shown that the resulting
acyl–PdII complexes are comparatively stable and have barely
any tendency to undergo CO elimination.[14] Under conditions
of decarbonylative Heck reaction (phosphane-free, but halide-containing reaction mixture),[13, 15–18] however, CO is
eliminated quickly—the formation of vinyl ketones resulting
from the insertion of the olefin into the acyl–Pd instead of
into the aryl–Pd bond has never been observed.
In the following years, several variants of this first
palladium-catalyzed coupling of carbonic acid anhydrides
were described. Ionic liquids can be used instead of NMP as
solvent, although a decrease in catalytic activity and productivity is observed.[15] Shmidt and Smirnov observed that the
use of lithium chloride instead of sodium bromide increases
catalyst activity and productivity, as chloride accelerates CO
elimination from the oxidative addition product more than
bromide does.[16]
A significant drawback of the olefination reaction with
anhydrides compared to the classical Heck reaction is that
few substituted aryl carbonic acid anhydrides are commercially available. Gooßen et al. reported a significant improvement of this method: They found conditions that allow the
coupling of free aryl and heteroaryl carbonic acids.[17] In the
presence of Boc2O, the free acids are converted in situ into the
corresponding mixed anhydrides, which can be coupled with
olefins [Eq. (2)].
Besides carbon monoxide and carbon dioxide, only tertbutanol is formed as waste product. Again, the addition of
lithium chloride is required to enhance CO elimination and
ligands must be added to stabilize the catalyst. Whereas
phosphanes reduce the activity of the system significantly,
amines (especially g-picoline) lead to active and comparatively productive catalysts. Besides extension of the substrate
scope, activation of carbonic acids in situ has the advantage
that all the applied acid equivalents are available for coupling,
that is, half does not function as a leaving group, which would
then either have to be discarded or reconverted into its
The same result, although in a different way, was obtained
by Myers and co-workers, who described the coupling of free
aryl carbonic acids with styrene or a,b-unsaturated carbonyl
compounds.[19] Under their conditions, Pd0 is not inserted into
an activated C O bond, but CO2 is eliminated from a PdII–
carboxylate complex to give an aryl–PdII complex, which is
required for the coupling reaction. After olefin insertion, b-
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
A. Zapf
hydride elimination and reductive elimination, a Pd0 species
results which must be reoxidized to PdII for the next catalytic
cycle with stoichiometric amounts of an oxidant (e.g., silver
carbonate). A further drawback of this interesting protocol is
the high catalyst concentration required (20 mol %).
Activation of C O bonds of carbonic acids for oxidative
addition to Pd0 can be realized not only by the formation of
anhydrides but also by esterification [Eq. (3)]. If electrondeficient phenols are employed as the alcohol-derived portion
of the esters (p-nitrophenol and pentafluorophenol are suited
best), the coupling reaction with olefins occurs under
conditions similar to those for mixed anhydrides formed
in situ (see [Eq. (2)]).[18] The liberated phenol can be acylated
again, so that only carbon monoxide is released as stoichiometric side product.
Activated carbonic acids can be used not only in Hecktype reactions, but also in coupling reactions with other types
of nucleophiles. In contrast to olefination reactions under CO
elimination, the carbonyl group is retained in the products.
The palladium-catalyzed coupling of trifluoroacetic anhydride with aryl tri-n-butylstannane to yield trifluoromethyl
ketones was already described in 1995.[20] However, this Stilletype reaction was developed further. On the other hand the
Suzuki-analogous coupling with aryl or alkyl boronic acids
was elaborated to a widely applicable method by the groups
of Gooßen and Yamamoto. The first coupling reactions with
aryl boronic acids led to the syntheses of trifluoromethyl
ketones.[21] Phenyl trifluoroacetate was treated with various
aryl boronic acids in the presence of palladium(ii) acetate and
tri-n-butylphosphane under comparatively mild conditions
[Eq. (4)].
For the coupling of more-electron-rich, nonfluorinated
carbonic acids, activation by esterification with simple
alcohols is not sufficient. In this case, more-reactive anhydrides have to be applied. Accordingly, substituted benzoic
anhydrides react smoothly with aryl boronic acids to give the
corresponding unsymmetrical diaryl ketones.[22, 23] [Pd(PPh3)4]
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
is a good catalyst in refluxing THF or dioxane at 80 8C,[23] but,
depending on the steric and electronic properties of the
anhydrides, other phosphanes such as tris(p-methoxyphenyl)phosphane or tricyclohexylphosphane lead to improved
results.[22] Aliphatic carbonic anhydrides (e.g., acetic anhydride) can also be coupled smoothly, but sterically congested
substrates such as pivalic anhydride hardly react. Mixed
anhydrides of pivalic acid and different aryl carbonic acids
therefore react with Pd0 to give aryl carbonyl–Pdii–pivaloate
complexes selectively, so that activation of free benzoic acids
in situ with pivalic anhydride is possible [Eq. (5)].[22, 24] Under
these conditions, the addition of a small amount of water was
found to enhance the reaction rate strongly.
The use of pivalic anhydride for carbonic acid activation
sometimes makes the isolation of the coupling products more
difficult, because 2 equivalents of pivalic acid are formed in
the reaction. An activating agent that leaves just readily
separable residues would thus be of some advantage. Di-(Nsuccinimidyl) carbonate[25a] as well as dimethyl dicarbonate[24, 25b, 26] fulfill this requirement. The former coupling reagent
is successful in the presence of electron-rich trialkyl phosphanes and a base such as sodium carbonate. By activating the
acid, carbon dioxide and N-hydroxysuccinimide are generated and can be removed simply by washing with water.
Coupling reactions with dimethyl dicarbonate are best
performed in the presence of tris(p-methoxyphenyl)phosphane and small amounts of water at 20–50 8C.[25b] In the
absence of water, reaction temperatures of 80 8C are required
to ensure efficient coupling.[24, 26] Besides carbon dioxide,
methanol is formed as a by-product, but does not disrupt the
workup of the reaction.
Benzoic acid can also be coupled with phenylacetylene
under similar reaction conditions to yield the corresponding
alkynyl ketone.[24, 26] So far, only this single example of a
Sonogashira-type reaction has been described, but more
should follow.
Angew. Chem. Int. Ed. 2003, 42, 5394 –5399
Cross-Coupling Reactions
3. Sulfonium Salts, Thiol Esters, and Thioethers
A new type of coupling reagent for palladium-catalyzed
reactions was developed by Liebeskind and co-workers
towards the end of the 1990s:[27] They employ various
organosulfur compounds, and thiols or thioethers act as
leaving groups. As sulfur in low oxidation states generally
coordinates well to palladium and thus blocks free coordination sites, the corresponding sulfur compounds must be
trapped or their complexation ability restricted to enable
palladium-catalyzed coupling reactions. Initially this was
accomplished by using sulfonium salts. For instance, S-aryl
and S-heteroaryl tetramethylenesulfonium hexafluorophosphates were coupled with aryl or heteroaryl boronic acids
under mild conditions [Eq. (6), 40 8C in ethanol or THF/
water].[28] [Pd(dppf)Cl2] was used as catalyst and potassium
carbonate monohydrate as base. Tetrahydrothiophene, which
is released during the coupling reaction, does not poison the
palladium catalyst, in contrast to thiols.
Further investigations have shown that the leaving group
does not have to be preformed within the substrate. Thiol
esters of 4-iodobutylthiol can also be coupled with aryl
boronic acids to yield diaryl ketones. It is assumed that
cyclization of the iodoalkyl thiolate does not occur before
insertion of the Pd catalyst into the carbonyl C S bond to
form an acyl–Pdii–thiolate complex.[29] In this way, the thiolate
is removed from the catalyst and allows the aryl group of the
boronic acid to be transferred to the palladium center.
For coupling reactions with organozinc reagents, alkyl
thiol esters do not require any masking. Zinc(ii) compounds
simultaneously act as trapping agents for thiolates because of
their high thiophilicity. Thus, ethanethiol esters of substituted
benzoic acids (as well as of aliphatic carbonic acids), for
instance, can be coupled with ethylzinc iodide to give the
corresponding propiophenone derivatives [Eq. (7)].[30] As the
reactions proceed even at room temperature, functional
groups such as aldehydes and esters remain unchanged.
Methylthioheterocycles can also be alkylated in this way.[31]
For the coupling reactions of aryl boronic acids with thiol
esters or heteroaryl thioethers, a new method was found that
avoids blocking of the Pd catalyst: The leaving thiolate is
removed from the catalyst by complexation to copper(i).[32]
Angew. Chem. Int. Ed. 2003, 42, 5394 –5399
However, an excess amount of a suitable copper salt has to be
added, which makes the reaction less attractive from both an
economical and ecological point of view. On the other hand,
the substrate scope tolerated by this “external” masking is
much larger than that of the “internal” masking through
alkylation of the leaving group. Accordingly, simple thiol
esters of aryl carbonic acids can be coupled with aryl boronic
acids in good yields in the presence of 1.6 equivalents of
copper(i) thiophene-2-carboxylate (CuTC).[33] Alkylthio-substituted heterocycles can be arylated with the same catalyst
system[34a] or by utilizing copper(i) 3-methylsalicylate (CuMeSal)[34b] [Eq. (8)]. However, this reaction is thus far limited to
various p-electron-deficient heterocycles such as pyridines,
pyrazines, and benzothiazoles.
The analogous coupling of heteroaryl thioethers with aryl
or vinyl stannanes was described very recently.[35] Again, only
p-electron-deficient heterocycles can be converted in this
reaction. Nevertheless, some progress has been reported: A
stoichiometric amount of a simple cocatalyst, copper(i)
bromide, can be used in this Stille-type coupling instead of
the copper complexes.[35b]
4. Summary and Outlook
The introduction of aryl carbonic acid derivatives for
palladium-catalyzed coupling reactions has significantly
broadened the substrate scope of these reactions. These new
substrates are interesting alternatives to aryl halides under
conditions that lead to the elimination of carbon monoxide. If
the carbonyl group is preserved in the product, this new
method for the synthesis of ketones is even superior to the
carbonylation cross-coupling sequence with haloarenes or the
coupling of highly reactive acid chlorides. However, the
accessibility of substituted aryl carbonic acids will be the
limiting factor in most cases.
In terms of atom economy, the novel coupling reactions
do not have significant general advantages over the old ones.
Stoichiometric amounts of base are often consumed in
coupling reactions with aryl halides. However, the halide
released in the coupling reaction can be reused for the
synthesis of haloarenes after oxidation (bromide, iodide) or it
2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
A. Zapf
can be disposed economically (chloride). The coupling of
carbonic acids does produce relatively small amounts of
waste, but the synthesis of the carbonic acids themselves often
generates stoichiometric amounts of salts as by-products.
The “old” reactions have at least one important advantage: They have reached the stage at which the catalyst
productivities are suitable for many industrial applications. In
most cases, the new reactions still have to be improved
significantly towards this goal. The coupling reactions of thiosubstituted (hetero)arenes is of great interest for researchers,
especially as a method to avoid deactivation of the catalyst,
but are most likely to remain laboratory methods for the
derivatization of special substrates. The functionalization of
aryl fluorides, one of the last great challenges in the area of
coupling reactions with haloarenes, is also more of academic
interest. First such successful reactions were described
The opinion that “arene chemistry is dead” has been quite
common at universities during the last decades. The work on
transition-metal-catalyzed coupling reactions shows, however, that this field of research is alive and well and still
holds much potential.
Received: July 11, 2003 [M1681]
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2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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[27] The coupling of allyl sulfides with Ni or Pd catalysts has already
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[28] J. Srogl, G. D. Allred, L. S. Liebeskind, J. Am. Chem. Soc. 1997,
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[32] A “simple” coupling of aryl thiol esters with aryl boronic acids
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of acyl thiophenyl bromides: B. Zeysing, C. Gosch, A. Terfort,
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[33] L. S. Liebeskind, J. Srogl, J. Am. Chem. Soc. 2000, 122, 11 260 –
11 261. In the same way alkynyl thioethers can be coupled to aryl
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3, 91 – 93) as well as methylthiopseudoureas to aryl amidines
Angew. Chem. Int. Ed. 2003, 42, 5394 –5399
Cross-Coupling Reactions
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[34] a) L. S. Liebeskind, J. Srogl, Org. Lett. 2002, 4, 979 – 981; b) F.-A.
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Angew. Chem. Int. Ed. 2003, 42, 5394 –5399
[36] For the Ni-catalyzed Kumada reaction of aryl fluorides, see:
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Chem. Soc. 2003, 125, 5646 – 5647; for the Pd-catalyzed coupling
of o-nitrofluorobenzenes, in which Pd presumably inserts into
the C F bond through an SNAr mechanism, see: Y. M. Kim, S.
Yu, J. Am. Chem. Soc. 2003, 125, 1696 – 1697.
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