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This article was published as part of the
Cross coupling reactions in organic
synthesis themed issue
Guest editor: Matthias Beller
All authors contributed to this issue in honour of the 2010 Nobel Prize
in Chemistry winners, Professors Richard F. Heck, Ei-ichi Negishi and
Akira Suzuki
Please take a look at the issue 10 2011 table of contents to
access other reviews in this themed issue
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Cite this: Chem. Soc. Rev., 2011, 40, 4925–4936
TUTORIAL REVIEW
www.rsc.org/csr
Microwave-assisted C–C bond forming cross-coupling reactions:
an overvieww
Vaibhav P. Mehtaa and Erik V. Van der Eycken*b
Received 7th April 2011
DOI: 10.1039/c1cs15094d
Among the fundamental transformations in the field of synthetic organic chemistry,
transition-metal-catalyzed reactions provide some of the most attractive methodologies for the
formation of C–C and C-heteroatom bonds. As a result, the application of these reactions has
increased tremendously during the past decades and cross-coupling reactions became a standard
tool for synthetic organic chemists. Furthermore, a tremendous upsurge in the development
of new catalysts and ligands, as well as an increased understanding of the mechanisms,
has contributed substantially to recent advances in the field. Traditionally, organic reactions
are carried out by conductive heating with an external heat source (for example, an oil bath).
However, the application of microwave irradiation is a steadily gaining field as an alternative
heating mode since its dawn at the end of the last century. This tutorial review focuses
on some of the recent developments in the field of cross-coupling reactions assisted
by microwave irradiation.
Introduction
a
Institut für Organische und Biomolekulare Chemie,
Georg-August-Universität, Tamannstrasse 2, Göttingen 37077,
Germany
b
Laboratory for Organic & Microwave-Assisted Chemistry
(LOMAC), Department of Chemistry, Katholieke Universiteit
Leuven, Celestijnenlaan 200F, B-3001, Leuven, Belgium.
E-mail: erik.vandereycken@chem.kuleuven.be; Tel: 0032 16327406
w Part of a themed issue on the topic of palladium-catalyzed cross
couplings in organic synthesis in honour of the 2010 Nobel Prize
winners Professors Richard F. Heck, Ei-ichi Negishi and Akira
Suzuki.
Vaibhav Mehta received his
MSc degree in Organic
Chemistry in 2005 from
Saurashtra University, Rajkot,
India, with Prof. Anamik Shah.
In the same year he joined the
group of Prof. Erik Van der
Eycken, Katholieke Universiteit Leuven, Belgium as a
doctoral student and completed
his PhD in organic chemistry
in 2010. He is currently engaged
in post-doctoral research with
Prof. Dr Lutz Ackermann at
the Georg-August-University
Vaibhav P. Mehta
in Goettingen, Germany, as
an Alexander von Humboldt fellow. His research interest
includes microwave-assisted organic synthesis, transition-metal
catalysis and synthesis of biologically important molecules.
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Transition-metal-catalyzed C–C1–3 and C–heteroatom4 bond
formation is one of the most important methods in contemporary chemistry, and is a field which has now been recognized
with several Nobel Prizes during the last decade (2001, 2005
and 2010). Transition-metal-catalyzed coupling reactions have
been used as powerful tools for the targeted or parallel
synthesis of heterocyclic compounds as well as for the synthesis
of natural products and analogues. Over the past decades,
Erik Van der Eycken received
his PhD degree (1987) in
organic chemistry from the
University of Ghent, with
Professor Maurits Vandewalle.
From 1988 to 1992 he worked
as a scientific researcher
at the R&D-laboratories of
AGFA-Gevaert, Belgium and
moved back to the University
of Ghent in 1992. In 1997 he
became Doctor-Assistant at
the University of Leuven.
After short periods of postdoctoral research work at the
Erik V. Van der Eycken
University of Graz (C. Oliver
Kappe), at The Scripps Research Institute (K. Barry Sharpless),
and at Uppsala University (Mats Larhed, Anders Hallberg), he
was appointed as Associate Professor at the University of Leuven
in 2007.
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Scheme 1
Traditional cross-coupling reactions.
these reactions have revolutionalized the art and science of
chemical synthesis. Currently the most popular methods for
such bond forming reactions employ transition-metals such
as for example [Pd], [Cu], [Fe], [Ni] and [Zn].2 There is a
plethora of literature about these transformations, which
largely involve aryl(pseudo)halides (for e.g. triflate, tosylate,
mesylate) as electrophiles and organometallic reagents as
nucleophiles (Scheme 1). Of all metals evaluated for such
cross-coupling reactions, palladium has come a long way as
a catalyst.5 The striking features of palladium compared to
other transition-metals are its reactivity, selectivity and tolerance
of a wide range of functional groups on both coupling partners.
Microwave-Assisted Organic Synthesis (MAOS)6,7 is a well
accepted new concept in the present day synthetic chemistry
and has grown enormously in the last decade as can be seen
from the plethora of literature.8 This enabling technology has
now also been generally accepted in industrial research laboratories and is no longer an academic curiosity. Several organic
transformations have been successfully realized employing this
technique as for example dipolar cycloadditions,9 transitionmetal-catalyzed cross-coupling reactions,10 polymer formation11
and the synthesis of nanoparticles.12 In contrast to cross-coupling
methodologies under conventional heating, MAOS has been
proven to dramatically shorten reaction times, to deliver cleaner
reaction mixtures and hence to increase overall yields.13 The
combination of this form of non-conventional heating14 with
other techniques like ultrasound, flow chemistry and microreactors has also gained growing attention in recent years.
Microwave irradiation under controlled conditions is an
invaluable technology for medicinal chemistry and drug discovery
applications because it dramatically reduces reaction times
from days or hours to mere minutes or even seconds. Many
reaction parameters, such as reaction temperature and time,
variation of the solvents, additives and catalysts or the molar
ratios of the substrates, can be evaluated in a few hours to
optimize the desired chemistry. Conventionally, organic reactions
are carried out by conductive heating which is a rather slow
and inefficient method of transferring energy into the system
and hence results in the temperature of the reaction vessel
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Chem. Soc. Rev., 2011, 40, 4925–4936
being higher than that of the reaction mixture. In contrast,
microwave irradiation produces efficient internal heating
(in-core volumetric heating) by direct coupling of the microwaves with the molecules (solvents, reagents, catalysts) that
are present in the reaction mixture. The two general mechanisms, by which microwaves are able to heat a chemical reaction
mixture, are dipolar polarization and ionic conductance.
Hence all matter that contains charged species or dipoles can
absorb microwave energy and efficiently convert it into heat.
The choice of solvent is crucial for most of the microwaveassisted reactions,13 unless they are carried out solvent-free.
Among the many factors effecting the interaction of microwaves with a solvent, the most important parameters are the
dielectric constant (e 0 ), the dielectric loss (e00 ) and the tangent
delta (tan d). Hence under closed vessel conditions, reaction mixtures can be heated and maintained at an elevated
temperature higher than the boiling point of the solvent for
several minutes using microwave irradiation, which conventionally is difficult to achieve via oil-bath heating.
All specialized microwave reactors8a commercially available
today feature built-in magnetic stirrers, direct temperature
control of the reaction mixture with the aid of fibre-optic
probes or an infrared sensor and software that enables on-line
temperature and pressure control by regulation of microwave
power output. These reactors8e are typically available in two
different modes: (a) monomode instruments where only one
reaction can be irradiated at a time and (b) multimode
instruments where several reaction vessels can be irradiated
simultaneously in multi-vessel rotors or deep-well microtiter
plates. With the aid of integrated robotics that moves individual
reaction vessels in and out of the reaction cavity, high throughput screening can be performed at a greater speed.
Owing to the large extent of available literature about the
application of microwave irradiation for transition-metalcatalyzed cross-coupling reactions, we have restricted our
tutorial review to the developments in the last four years,
making a selection from the literature from 2007 until March
2011, with few exceptions. We opted to omit examples
dealing with C–heteroatom bond formation using microwave
irradiation.10
For the sake of clarity, we have divided the review into six
sections according to named cross-coupling reactions: (1) Suzuki–
Miyaura, (2) Mizoroki–Heck, (3) Stille–Migita, (4) Sonogashira–
Hagihara, (5) Negishi and (6) Hiyama cross-couplings. The last
section (7) deals with miscellaneous reactions. The beneficial effect
of the application of microwave irradiation as compared to
conventional heating is demonstrated.
1 Suzuki–Miyaura cross-coupling reaction
In 1979, the seminal paper of Miyaura, Yamada, and Suzuki15
laid the base for one of the most important and useful transformations for the construction of C–C bonds in the modern day
organic chemistry. Since then advances have been made
regarding the reaction scope, including the use of aryl(pseudo)halides or alkyl halides as coupling partners and the ability to
conduct couplings at very low catalyst loadings and at room
temperature. Moreover, it is now also possible to couple
hindered substrates, and even asymmetric variants have
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been reported. Of particular importance is the increased
reactivity as well as stability of the metal catalysts by carefully
designing appropriate ligand systems. The most common
ligands used today are phosphine-based, although a variety
of others, including N-heterocyclic carbenes (NHC), have been
employed in combination with [Pd], [Ni], [Fe] etc. as catalysts.
Of interest are also the procedures that utilize so-called
‘‘transition-metal-free’’ or ‘‘ligandless’’ conditions. There is
a plethora of literature available for the application of microwave irradiation for this versatile and well-investigated crosscoupling procedure.6,7,8f
Van der Eycken and co-workers16 employed a microwaveassisted Suzuki–Miyaura cross-coupling reaction for the generation of aza-analogues of the natural product ()-Steganacin. It
was demonstrated that the application of microwave irradiation
was highly beneficial for the biaryl coupling of electronically
rich aryl bromides with some substituted o-formylphenylboronic
acids. They reasoned that by speeding up the oxidative
addition, applying microwave irradiation, the protonolysis of
the sensitive boronic acid should be suppressed. The reaction
was performed at a ceiling temperature of 130 1C and a maximum
power of 150 W for 15 min delivering the newly generated biaryl
derivatives in good yields of 77–82% (Scheme 2). Under conventional heating conditions yields are far less (B42–50%).
Polycyclic aromatic hydrocarbons (PAHs) are one of the
most widespread organic pollutants. Large numbers of such
compounds have been identified as being carcinogenic, mutagenic
or teratogenic. Sharma and co-workers17 have demonstrated
an efficient strategy for the synthesis of biaryl-containing
PAHs using microwave irradiation. PAH-bromides can be
cross-coupled with appropriately substituted o-formylphenylboronic acids using Pd(PPh3)4 or PdEnCat 30 as a catalyst
applying a ceiling temperature of 120 1C and 250 W maximum
power for 20 min (Scheme 3). The corresponding biaryl
compounds are obtained in excellent yields. It is noteworthy
that comparable or lower yields were obtained when reactions
were carried out under conventional heating conditions17
(Scheme 3).
An interesting example of the use of N-vinyl pyridinium and
ammonium tetrafluoroborate salts as new and excellent electrophilic coupling partners for the Suzuki–Miyaura cross-coupling
reaction has been described by Buszek and co-worker.18 The
authors elaborated an excellent methodology for the generation of 3-aryl-substituted enones using microwave irradiation.
The compounds were generated by reacting the salts A or B
with substituted arylboronic acids in the presence of Pd2(dba)3
(5 mol%) and PCy3 (12 mol%) at a ceiling temperature of
150 1C for 12 min delivering the substituted enone systems in
good to excellent yields (Scheme 4).
Scheme 3
Synthesis of polycyclic aromatic hydrocarbons (PAHs).
Scheme 4
Synthesis of functionalized enones.
An interesting investigation of the use of alkenyl nonaflates
(nonafluorobutane sulfonates) as excellent substrates for the
palladium-catalyzed Suzuki–Miyaura cross-coupling reaction
has been reported by Reißig and co-worker19 (Scheme 5). The
authors used nonaflates prepared from 8-oxabicyclo[3.2.1]oct6-en-3-one and its derivatives. The cross-coupling reactions
were carried out in DMF as a solvent, employing Pd(OAc)2/PPh3
as a catalyst system at a ceiling temperature of 70 1C and a
maximum power of 100–250 W for 10–40 min. It was demonstrated that the use of microwave irradiation dramatically
shortened the reaction times and gave superior yields in comparison with conventional heating conditions (Scheme 5).
Greaney and co-workers20 described an interesting protocol
for the generation of bis or tris oxazoles. The Suzuki–Miyaura
cross-coupling reaction between oxazole-4-ylboronate and
2-iodo-oxazole derivatives was carried out under microwave
irradiation at a ceiling temperature of 150 1C using 150 W
maximum power for 20 min to afford bis-oxazoles in good to
excellent yields (Scheme 6). A two fold increase in the yield and
a six fold decrease in the reaction time were observed compared
to conventional heating.
Pyridazinones are recognized as privileged scaffolds as they
are present in a wide range of commercially important drugs
and agrochemicals. Cao and co-workers21 have shown an
efficient methodology for the functionalization of 6-chloropyridazinone derivatives using the Suzuki–Miyaura cross-coupling
reaction with palladium-bis-(di-tBu-phosphino-di-hydroxy)chloride (POPd) or Pd2(dba)3 as a catalyst (Scheme 7). The
authors demonstrated a wide range of examples for efficient
decoration of the C-6 position of the pyridazinone system
under microwave irradiation. Variously substituted boronic
Scheme 2 Synthesis of hindered biaryls.
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Scheme 5 Suzuki–Miyaura cross-coupling of nonaflates.
Scheme 6 Synthesis of bis-oxazole derivatives.
Scheme 9 Tandem C–H borylation/Suzuki–Miyaura cross-coupling.
Scheme 7 C-6 functionalization of the pyridazinone scaffold.
acids were reacted with Cl–pyridazinones at a ceiling temperature of 135–140 1C for 30 min. A wide range of functionalities
are tolerated using this optimized protocol, permitting a rapid
pharmaco-modulation of the pyridazinone scaffold (Scheme 7).
Harrity and co-workers22 have described the application of
microwave irradiation to generate a range of C-4 arylated
sydnones (4-bromo-N-phenyl-NH-1,2,3-oxadiazole derivatives)
from the corresponding 4-bromosydnone via the Suzuki–
Miyaura reaction (Scheme 8). The authors carried out the
cross-coupling of this starting material with functionalized
arylboronic acids or trifluoroborate salts under focused microwave irradiation at a ceiling temperature of 130 1C for 30 min.
This exploration is of particular interest as the generated
functionalized sydnones are further converted into pyrazole
compounds via cycloaddition reaction with various alkynes22
(Scheme 8).
Steel, Marder and co-workers23a have recently reported a
microwave-assisted tandem Ir-catalyzed C–H borylation/
Pd-catalyzed Suzuki–Miyaura cross-coupling reaction (Scheme 9).
Of the methods employed towards the synthesis of arylboronic
acids, direct borylation23b–d of arenes and alkanes provides
Scheme 8 Arylation of sydnone derivatives.
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access to synthetically useful compounds without relying on
the accessibility of aryl or alkyl halides. The authors developed
an elegant methodology for the direct C–H borylation using a
pinacoldiborane dimer under microwave irradiation at a ceiling
temperature of 80 1C for 5–60 min. The subsequent Suzuki–
Miyaura cross-coupling of the generated boronate esters
with 4-iodomethylbenzoate under microwave irradiation at a
ceiling temperature of 80 1C for 5 min using a Pd-catalyst
affords the biaryl compounds in excellent yields (Scheme 9).
Focused microwave irradiation has been proven to be highly
beneficial during the borylation step, reducing reaction times
from hours to mere minutes without affecting the yields.
The most frequently used haloaromatic electrophilic species
in many metal-catalyzed processes are aryl iodides and bromides
due to the relative ease of the oxidative addition of the catalyst.
In contrast, aryl chlorides are used less often as coupling
partners due to the higher C–Cl bond strength rendering them
much less reactive. To overcome this difficulty, recently the
group of Sanford24 have elaborated a Pd-catalyzed C–F activation of polyfluoronitrobenzene derivatives via Suzuki–Miyaura
cross-coupling (Scheme 10). The authors investigated a range
of polyfluoroaromatics for cross-coupling using microwave
irradiation at an elevated temperature of 150 1C for 15 min.
The mono ortho-arylated products were obtained in good yields
(Scheme 10).
Scheme 10 ortho-Functionalization via C–F activation.
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2
Mizoroki–Heck cross-coupling reaction
The palladium-catalyzed arylation of alkenes, commonly known
as the Mizoroki–Heck cross-coupling reaction, was developed
in 1971.25 This reaction has special value for industrial and
academic research due to the mild conditions applied to
activate the olefin. The procedure is now broadly defined as
the Pd(0)-mediated cross-coupling of an aryl or vinyl halide or
triflate with an alkene. There are many catalytic systems that
can be used to catalyze this cross-coupling reaction. Reactive
aryl halides (Br or I) and activated alkenes are used frequently.
Aryl chlorides, due to their readily availability, are more
attractive substrates; however, due to the relatively strong
C–Cl bond, they are rather reluctant to undergo oxidative
addition.
Van der Eycken and co-worker described26 an interesting
approach for the generation of the 3-benzazepine framework
Scheme 11 Intramolecular reductive Heck cyclization for the generation of the 3-benzazepine framework.
Scheme 12 Intramolecular decarboxylative allylic Heck coupling.
Scheme 13 Intermolecular Heck coupling using arene diazonium
salts.
by intramolecular reductive Heck cyclization. As a result of
the reaction mechanism, the formation of the medium-sized
ring occurred with full regio- and stereoselectivity delivering
exclusively the seven-membered ring with the Z-configuration
of the exocyclic double bond. The reaction was carried
out under microwave irradiation using catalytic Pd(0) and
HCOONa as reducing agents at a ceiling temperature of
110–120 1C for 15 min using a maximum power of 300 W
(Scheme 11).
An interesting one-pot tandem decarboxylative allylation–
Heck cyclization for the synthesis of 1-amino indanes, a class
of atypical anti-psychotic agents, has been recently reported.
Chruma and co-workers27 elaborated a rapid and mild procedure
for the diversity oriented generation of such compounds
using microwave irradiation (Scheme 12). The decarboxylation of the starting allyl esters proceeds at room temperature.
This is followed by a microwave-assisted intramolecular Heck
coupling at a ceiling temperature of 150 1C for 5–10 min
(Scheme 12).
The use of alternative electrophiles other than halides such
as triflates, sulfamates and hydrazones, for the inter- or intramolecular Heck arylation, has attracted much attention.
Recently Correia and co-workers28 described an efficient
microwave-assisted process for the regio- and stereoselective
Heck reaction of allylic esters employing arene diazonium
salts. They demonstrated the applicability of their strategy
for the synthesis of the natural compounds Yangonin and
Methysticin (Scheme 13).
An interesting study where the intramolecular Heck cyclization was employed to generate high molecular complexity was
described by Riva and co-workers.29 It concerns a tandem
process of an SN2 0 reaction and a Heck coupling to generate
a 7-membered ring containing a skeleton. The consecutive
Pd-catalyzed steps were performed under microwave irradiation at a ceiling temperature of 60 1C and 120 1C, respectively, yielding the compounds as a diastereomeric mixture
(Scheme 14).
Over the years, extensive work on microwave-assisted Heck
reactions has been carried out by Larhed and co-workers.30 A
detailed study of a base-free Pd(II)-dmphen-catalyzed oxidative
Heck reaction with arylboronic acids has been performed
using air as a reoxidant. The authors have demonstrated
the applicability by generating a wide variety of arylated
compounds using terminal olefins and different arylboronic
acids (Scheme 15). The reactions were shown to be dramatically accelerated upon microwave irradiation at 100 1C ceiling
temperature for 10–20 min, giving comparable yields as when
performed at room temperature (24 h).
Scheme 14 Intramolecular Heck cyclization sequence.
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range of 7–10 kg mol1 was obtained. Upon microwave
irradiation at a ceiling temperature of 120–170 1C, the reaction
time could be dramatically reduced to 10–40 min resulting in
yields ranging from 14–34 kg mol1 (Scheme 17).
4 Sonogashira–Hagihara cross-coupling reactions
Scheme 15 Base free Pd(II)-dmphen-catalyzed oxidative Heck coupling
using boronic acids.
3
Stille–Migita cross-coupling reaction
The palladium-catalyzed Stille–Migita cross-coupling31 of aryl
or vinyl (pseudo)halides with organostannanes is another
widely used method for C–C bond formation. The reaction
has also gained much attention in natural product synthesis
owing to the application of air and moisture stable organotin
reagents and the excellent functional group tolerance. The
main drawbacks are the toxicity of the tin compounds as well
as their low polarity, which make them poorly soluble in
water. A plethora of examples of microwave-assisted Stille
reactions has been described in the literature.
Hay and co-worker described32 a microwave-assisted Stille
coupling as a convenient tool in the synthesis of hypoxiaselective 3-alkyl-1,2,4-benzotriazine 1,4-dioxide anticancer
agents such as SN29751. The introduction of the ethyl substituent is a key step in their synthesis. The authors performed
the microwave-assisted Stille reaction by using Pd(PPh3)4
in MeCN at 140 1C, furnishing the target compounds in
20–60 min with good yields ranging between 54 and 88%
(Scheme 16).
Recently, Bazan and co-workers33 have shown the importance of the Stille cross-coupling reaction for the generation of
conjugated polymers having fused aromatic heterocycles,
especially thiophenes in their backbone. These p-conjugated
polymers are useful starting materials for the preparation of
organic photovoltaic cells (OPV) or field effect transistors.
When suitable monomers of type A and B were reacted at
120 1C under conventional heating for 48 h, a copolymer in the
The coupling of copper acetylides and organic halides to
prepare internal alkynes was discovered by Castro and Stephens
in 1963 and hence known as the Stephens–Castro coupling.34
The main drawback of this reaction is the need for a high
temperature and a strong base. In 1975 Sonogashira and
Hagihara35 reported that addition of a small amount of copper
iodide greatly accelerates the palladium catalyzed crosscoupling reaction between terminal alkynes and organic electrophiles and thus permits the reaction to occur at room temperature.
This protocol has become a very widely used and practical tool for
the generation of several terminal and internal p-conjugated
acetylenic compounds. The benefits of the application of microwave irradiation for this reaction can be viewed from the
available plethora of literature.36
Aryl chlorides are known to be relatively unreactive towards
cross-coupling reactions due to the difficult oxidative addition
of the transition-metal-catalyst. Liu and co-workers37 employed
sterically hindered and electron deficient aryl chlorides
in Sonogashira cross-coupling reactions under microwave
irradiation at a ceiling temperature of 150 1C for 10 min in
combination with Pd(II) and an electron rich P(tBu)3 ligand as
a catalytic system. Furthermore, a variety of electron rich,
neutral and deficient bearing functionalities like CN, SMe,
CF3 etc. on the aryl chloride were successfully utilized giving
the desired products in good to excellent yields (Scheme 18).
An
unprecedented
microwave-assisted
desulfitative
Sonogashira-type cross-coupling protocol for the efficient
alkynylation of the C3-position of phenylsulfanylated-2(1H)pyrazinones was reported by Van der Eycken and co-workers38
(Scheme 19). It has been demonstrated that the –SPh or –SMe
group, as a surrogate for halides, undergoes facile crosscoupling to give alkynylated derivatives which can be further
utilized for diverse functionalization. Applying various acetylenes in combination with Pd(PPh3)2Cl2 and CuI as catalysts
and Cs2CO3 as a base in DMF at a ceiling temperature of
Scheme 16 Stille–Migita cross-coupling of benzotriazine derivatives.
Scheme 18 Sonogashira cross-coupling of unactivated aryl chlorides.
Scheme 17 Synthesis of p-conjugated polymers.
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Scheme 19 Desulfitative Sonogashira-type alkynylation.
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Scheme 20 Heterogeneous
cross-coupling.
Pd-EnCatTM
catalyzed
Sonogashira
95 1C and 75 W maximum irradiation power for 30–60 min,
the required products were obtained in excellent yields of
72–88% (Scheme 19).
The use of heterogeneous and reusable solid-supported Pd
catalysts for Sonogashira cross-coupling reactions has attracted
a lot of attention. An interesting study of the application of the
Pd-EncatTM TPP30 catalyst for the generation of aryl and
alkenyl substituted acetylenes under microwave irradiation was
reported by Ley and co-workers.39 Several (hetero)aromatic
bromides and vinylic chlorides were successfully cross-coupled
with various aryl or alkyl acetylenes in good to excellent yields
upon microwave irradiation at a ceiling temperature of 120 1C
for 10–30 min resulting in an important cost reduction and a
lowering of the environmental impact (Scheme 20). They
demonstrated that the encapsulated catalyst can be recycled
by a single filtration of the reaction mixture.
Capretta and co-worker40 elaborated an efficient one-pot
Sonogashira cross-coupling, carbonylation, annulation strategy
for the generation of flavones employing a low catalyst loading
and PA-Ph (1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6-phosphaadamantane) as a ligand source (Scheme 21). Suitable aryl
halides underwent Sonogashira cross-coupling with trimethylsilyl acetylene. The resulting compounds were cross-coupled
with ortho-iodo phenols after cleavage of the TMS (trimethylsilyl) group, subsequent carbonylation and annulations
resulted in the desired flavones. The use of microwave irradiation allows the application of relatively mild reaction conditions
and shorter reaction times compared to conventional heating,
delivering the compounds in good yields (Scheme 21).
A one-pot, three-step synthesis of 1,4-substituted 1,2,3-triazoles
was developed by Boons and co-worker.41 The method
comprises a Sonogashira cross-coupling reaction of a suitable
aryl halide (I or Br) with trimethylsilyl acetylene, followed by
desilylation and microwave-assisted Cu(I)-catalyzed cycloaddition of the terminal acetylene with an appropriate azide
(Scheme 22). All reactions were carried out under microwave
irradiation at a ceiling temperature of 120 1C. An array of
11 examples was generated in yields ranging from 12–97%.
5 Negishi cross-coupling reaction
The first reaction that allowed the preparation of unsymmetrical
biaryls in good yields was reported by Negishi in 1977.42 Based
on the pioneering studies of Kumada–Corriu about crosscoupling reactions using nickel- or palladium-catalysts in
combination with Grignard reagents, Negishi developed a
procedure employing zinc reagents. Although, magnesium
and lithium reagents are known to be highly nucleophilic,
the following order of reactivity for palladium-catalyzed crosscoupling reactions is observed: zinc > magnesium c lithium.
The Negishi reaction is known for its broad compatibility
with various functional groups and mild reaction conditions.
The zinc reagents are easy to prepare or readily available
and cross-couplings are described with (hetero)aryl, vinyl
or alkyl(pseudo)halides. There are several recent examples
reporting the application of microwave irradiation for Negishi
coupling reactions.
Difficultly attainable biaryl motifs were generated via
Negishi cross-coupling reaction of unactivated aryl chlorides
with sterically demanding aryl zinc chlorides by Kappe and
co-worker.43 The authors carried out a microwave-assisted
cross-coupling of the aryl zinc reagent, which was also generated
under microwave irradiation, employing a Ni- or Pd-catalyst
at an elevated temperature of 175 1C for 3–10 min
(Scheme 23). It was demonstrated that the application of
microwave irradiation dramatically reduced the reaction times
from hours to minutes, delivering the compounds in good to
excellent yields.
Lipshutz and co-workers44 demonstrated several examples
of Negishi reactions applying heterogeneous catalyst conditions.
They performed the cross-coupling of substituted aryl zinc
Scheme 21 Flavones via a multicomponent Sonogashira-carbonylation–annulation reaction.
Scheme 22 Multicomponent strategy for 1,4-disubstituted-1,2,3-triazoles.
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Scheme 23 Pd- or Ni-catalyzed Negishi cross-coupling of aryl chlorides.
Scheme 24 Ni-on-charcoal catalyzed Negishi cross-couplings.
Scheme 25 Directed ortho-zincation of (hetero)aryl ester, amide or
ketone derivatives.
halides with substituted aryl chlorides employing Ni-on-charcoal
as a cheap and highly efficient alternative catalyst under microwave irradiation at a ceiling temperature of 150 1C for 15–30 min.
The required biaryl compounds were obtained in good to
excellent yields (Scheme 24). Under conventional heating the
reactions took B24 h.
A large number of zinc reagents synthesized at an elevated
temperature under conventional heating or microwave irradiation have been shown to be highly thermally stable and functional
group tolerant. An interesting approach for the use of
(tmp)2Zn2MgCl2LiCl (tmp = 2,2,6,6-tetramethylpiperidine)
as a zincation reagent for the direct ortho-metallation of
(hetero)aromatic compounds has been reported by Knochel
and co-worker (Scheme 25).45 The authors demonstrated that
microwave irradiation at a ceiling temperature of 80–120 1C
for 2–5 h is highly beneficial for the direct zincation of these
ester, amide or ketone derivatives. It is worth mentioning that
for certain substrates no reaction was observed when conventional heating was applied while excellent yields were achieved
when the reaction was performed under microwave irradiation. The newly generated zinc reagents were further used in
cross-coupling reactions to generate biaryl compounds.
6
ranks as a powerful and reliable C–C bond-forming procedure.46
The generality of this approach was enhanced by the introduction of new types of stable and easy-to-handle silicon-based
coupling reagents by Nakao, Hiyama and co-workers47 as
well as by Denmark and co-workers.48 As do boronic acid
derivatives, they exhibit low toxicity when being compared
with the corresponding tin counterparts. Many of them are
relatively inexpensive and commercially available. While certain
boronic acids sometimes exhibit limited stability or are difficult
to prepare, silicon-based reagents can be prepared by a variety
of methods. Furthermore, unlike boronic acids, silicon-based
reagents are stable to most reaction conditions employed in
the synthetic organic chemistry.
The first application of the use of microwave irradiation for
a Hiyama cross-coupling reaction was reported by Matthew
Clarke.49 It was demonstrated that aryl chlorides could be
efficiently cross-coupled employing phenyltrimethoxysilane and
an appropriate Pd-catalyst under microwave irradiation at a
ceiling temperature of 115 1C for 18 min (Scheme 26). Complete
conversion was reported and the generated biaryl compounds
were obtained with a yield of over 90% (GC-analysis). The
applicability of the procedure to generate styrene derivatives
when using vinyltrimethoxysilane is also described.
Recently, Clarke and co-workers50 have shown that when
changing the promoter for the Hiyama coupling reaction,
employing vinyltrimethoxysiloxane, from TBAF to NaOH,
the reactivity pattern is changed resulting in the formation of
ether derivatives. The authors demonstrated that the methoxy
group can be installed via cross-coupling of vinyltrimethoxysilane with aryl halides (Scheme 27) using microwave irradiation
Scheme 26 Hiyama cross-coupling reaction using phenyltrimethoxysilane.
Hiyama cross-coupling reactions
Organosilicon compounds are useful and ubiquitous reagents
in the modern organic chemistry. Among their versatile transformations, transition-metal-catalyzed cross-coupling with organic
(pseudo)halides, commonly known as the Hiyama reaction,
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Scheme 27 Synthesis of ether derivatives using vinyltrimethoxysilane.
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7 Miscellaneous reactions
Scheme 28 Styrene derivatives using Hiyama cross-coupling.
at a ceiling temperature of 120 1C for 20 min resulting in the
formation of ether derivatives in good yields.
An extension of the Pd(0)-catalyzed vinylation to include
user friendly vinyltrimethoxysilane as a coupling partner was
realized under microwave irradiation. Najera and co-worker51
elaborated a procedure where TBAF was replaced for aqueous
NaOH as a promoter for Hiyama cross-coupling. The vinylation
cross-coupling reaction was performed using vinyltrimethoxysilane with activated aryl bromides or chlorides using palladacycle
under microwave irradiation at a ceiling temperature of 120 1C
for 10–20 min. The generated substituted styrene derivatives
were obtained in 41–90% yield. A large number of functional
groups were well tolerated upon applying this fluoride-free
condition (Scheme 28).
Scheme 29 Pd(0)-catalyzed cross-coupling of tosylhydrazones.
Scheme 30 Substituted cyclic homoallylic alcohols via arylative
cyclization.
The use of so-called tandem or cascade reactions in combination with enabling technologies like microwave irradiation,
microreactors and flow-chemistry has recently attracted a lot
of attention. In a related approach Barluenga and co-workers
described the cross-coupling of tosylhydrazides with orthodihaloarenes.52 The reaction follows a cascade sequence of
cross-coupling (arylation) of the aryl halide with the tosylhydrazone, followed by intramolecular Buchwald–Hartwig
type amination53 using Pd2(dba)3 as a catalyst under microwave
irradiation at a ceiling temperature of 150 1C for 30–120 min
(Scheme 29). The authors have shown that under microwave
irradiation the total reaction time could be dramatically reduced.
Allenes are interesting substrates for transition-metal-catalyzed
reactions. Tsukamoto and co-workers54 have shown that
diversely substituted allenyl aldehydes or ketones can be cyclized
using a Pd-catalyst and substituted boronic acids under microwave irradiation (Scheme 30). They have carried out different
kinds of arylation or alkenylation at a ceiling temperature
of 80 1C for 10–90 min affording the corresponding six or five
membered carbo- or heterocyclic compounds in good yields
(Scheme 30).
Along with Pd-catalyzed C–N and C–O bond forming
reactions, C–P bond forming reactions are attractive methodologies to generate phosphines, phophonates or phosphorus
acid intermediates for the synthesis of ligands, natural products
and biologically active compounds. The Pd(0)-catalyzed reaction applying microwave irradiation has been described by
Stawinski and co-workers.55 Aromatic (pseudo)halides and
vinylic bromides can be efficiently converted to phosphonate
diesters using a Pd-catalyst under microwave irradiation at a
ceiling temperature of 120 1C for 10 min affording the
compounds in excellent yields (Scheme 31).
The use of diazo-compounds as efficient coupling partners
for Pd-catalyzed C–C bond forming reactions has been described
by Wang and co-workers.56 They demonstrated that substituted diazirines can be efficiently cross-coupled with aromatic
halides (Br or Cl) under microwave irradiation. The reactions
Scheme 32 Pd(0)-catalyzed cross-coupling of diazirines.
Scheme 31 Pd(0)-catalyzed generation of phosphonate diester systems.
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Chem. Soc. Rev., 2011, 40, 4925–4936
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Scheme 33 Generation of biaryl-substituted oxazabicyclo[3.3.1]nonanes.
Scheme 34 Domino approach for the synthesis of styrene potassium
sulfonate derivatives.
were carried out employing Pd2(dba)3 as a catalyst at a ceiling
temperature of 110 1C for 10 min, affording substituted alkene
derivatives in good yields (Scheme 32).
An interesting sequence employing a fluorous linker has
been described by Zhang and co-workers for the efficient
generation of complex oxazabicyclo[3.3.1]nonane derivatives,
which are useful motifs for molecular recognition.57 They applied
a microwave-assisted 3-component reaction of an aldehyde
bearing a fluorous linker, an amine and iso-butyraldehyde at a
ceiling temperature of 50 1C for 20 min. The generated
tetrahydroquinoline derivatives underwent cycloaddition with
4-hydroxycoumarin or 4-hydroxy-N-methylquinoline under
microwave irradiation at a ceiling temperature of 85 1C for
30 min. Finally, the fluorous linker was cleaved from the oxazabicyclo[3.3.1]nonane system via Suzuki–Miyaura cross-coupling
using PdCl2(dppf) as a catalyst under microwave irradiation at
a ceiling temperature of 100 1C for 30 min, affording highly
functionalized oxabicyclo[3.3.1]nonanes in good to excellent
yields (Scheme 33).
An unprecedented microwave-assisted domino process has
recently been described by Surya Prakash, Olah and co-workers
comprising subsequent hydrolysis, dehydrohalogenation and
Heck-coupling reaction starting from 2-chloroethanesulfonyl
chloride, resulting in the formation of potassium vinylsulfonates (Scheme 34).58 The 2-chloroethanesulfonyl chloride
was hydrolyzed and the obtained salt underwent dehydrohalogenation. The Heck-coupling reaction of aryl iodide with
in situ generated potassium vinylsulfonate occurred upon
irradiation at a ceiling temperature of 180 1C for 10 min using
water as a solvent and Pd(OAc)2 (2 mol%) as a catalyst.
To overcome the difficulty of catalyst deactivation and low
conversion, a fresh batch of Pd(OAc)2 (1 mol%) was added
and the mixture was again irradiated at a ceiling temperature
of 180 1C for 10 min, delivering the resulting substituted
potassium styrene sulfonate derivatives in good to excellent
yields. These are interesting substrates for further functionalization via desulfitative Heck-coupling reaction with alkenes to
generate conjugated alkenes.59
Scheme 35 One-pot domino sequence for persubstituted pyrazoles.
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Mueller and co-worker recently described the synthesis of
persubstituted pyrazoles via a four-step one-pot reaction
sequence (Scheme 35).60 The synthesis of functionalized
pyrazoles, starting from the generated ynones, was carried
out under microwave irradiation at a ceiling temperature of
150 1C for 10 min utilizing t-BuOH as a solvent. After
bromination of the resulting compounds at their 4-position,
a microwave-assisted Suzuki–Miyaura cross-coupling reaction
was performed at a ceiling temperature of 160 1C for 20 min,
delivering the corresponding persubstituted pyrazoles in good
yields over the four-step process.
9
10
11
Conclusions
12
In conclusion, there has been extensive progress in the field of
microwave-assisted transition-metal-catalyzed reactions ranging
from traditional cross-coupling reactions to recently developed
C–H bond functionalizations. In this tutorial review, we have
been specifically highlighting the generation of C–C bonds.
This is demonstrated with some examples of traditionally onestep processes as well as with some examples of one-pot domino
reactions. In each case the application of microwave irradiation is
commented. We hope that this review will be of interest to the
synthetic community and will result in further focused research
regarding the application of microwave irradiation for transitionmetal-catalyzed cross-coupling reactions.
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20
Acknowledgements
VPM is thankful to Alexander von Humboldt foundation for
obtaining a post-doctoral research fellowship. EVdE wishes to
thank the F.W.O. (Fund for Scientific Research – Flanders
(Belgium)) and the Research Fund of the Katholieke Universiteit Leuven for financial support to the laboratory.
21
22
23
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