Immobilization of molecular catalysts in supported ionic liquid phases.

Abstract

In a supported ionic liquid phase (SILP) catalyst system, an ionic liquid (IL) film is immobilized on a high-surface area porous solid and a homogeneous catalyst is dissolved in this supported IL layer, thereby combining the attractive features of homogeneous catalysts with the benefits of heterogeneous catalysts. In this review reliable strategies for the immobilization of molecular catalysts in SILPs are surveyed. In the first part, general aspects concerning the application of SILP catalysts are presented, focusing on the type of catalyst, support, ionic liquid and reaction conditions. Secondly, organic reactions in which SILP technology is applied to improve the performance of homogeneous transition-metal catalysts are presented: hydroformylation, metathesis reactions, carbonylation, hydrogenation, hydroamination, coupling reactions and asymmetric reactions.

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PERSPECTIVEwww.rsc.org/dalton | Dalton Transactions
Immobilization of molecular catalysts in supported ionic liquid phases
Charlie Van Doorslaer,aJoos Wahlen,aPascal Mertens,aKoen Binnemansband Dirk De Vos*a
Received 19th January 2010, Accepted 18th March 2010
DOI: 10.1039/c001285h
In a supported ionic liquid phase (SILP) catalyst system, an ionic liquid (IL) film is immobilized on a
high-surface area porous solid and a homogeneous catalyst is dissolved in this supported IL layer,
thereby combining the attractive features of homogeneous catalysts with the benefits of heterogeneous
catalysts. In this review reliable strategies for the immobilization of molecular catalysts in SILPs are
surveyed. In the first part, general aspects concerning the application of SILP catalysts are presented,
focusing on the type of catalyst, support, ionic liquid and reaction conditions. Secondly, organic
reactions in which SILP technology is applied to improve the performance of homogeneous
transition-metal catalysts are presented: hydroformylation, metathesis reactions, carbonylation,
hydrogenation, hydroamination, coupling reactions and asymmetric reactions.
Introduction
Ionic liquids (ILs) are organic salts with low melting points and
verylowvapourpressures.1-3Theirnon-volatilenatureisoneofthe
main motives to explore them as alternatives for volatile organic
solvents.4Depending on their composition, ILs can dissolve or
reject organic compounds. Since ILs are ionic and polar, they
are excellent media to hold charged or polar catalyst species like
transition-metal complexes.3-11In analogy to the well-established
supported aqueous-phase catalysis,12-13ILs have recently been
introduced for the immobilization of homogeneous catalysts.14-16
In a supported ionic liquid phase (SILP) catalyst system, an IL
filmisimmobilizedonahigh-surfaceareaporoussolid(e.g.,silica)
and the homogeneous catalyst is dissolved in the IL layer. The
resultingcatalystisasolid,withtheactivespeciesbeingsolubilized
intheILphaseandbehavingasahomogeneouscatalyst.Typically,
aCentre for Surface Chemistry and Catalysis, Department of Microbial and
Molecular Systems, K.U.Leuven, Kasteelpark Arenberg 23, box 2461, 3001,
Leuven, Belgium. E-mail: dirk.devos@biw.kuleuven.be
bDivision for Molecular Design and Synthesis, Department of Chemistry,
K.U.Leuven, Celestijnenlaan 200F, box 2404, 3001, Leuven, Belgium
Charlie Van Doorslaer
Charlie Van Doorslaer (born
1983) obtained his Masters de-
gree in Applied Bioscience and
Engineering at the Catholic Uni-
versity of Leuven (K.U.Leuven,
Belgium) in 2006 with a the-
sis on carbamate synthesis from
amines, alcohols and CO2at the
Centre of Surface Chemistry and
Catalysis.SinceSeptember2006,
he has been preparing a PhD
thesis on ionic liquids as media
for catalytic reactions.
Joos Wahlen
Joos Wahlen (born 1977) com-
pleted his doctoral research at
the Centre for Surface Chem-
istry and Catalysis in 2004 with
a dissertation on heterogeneous
catalysis for the generation of
singlet oxygen and performed a
post-doc in the same group. His
expertiseisinthebroaddomainof
organic catalysis with solid mate-
rials and immobilized molecular
catalysts, selective oxidation and
organocatalysis.
thereisnodirectinteractionbetweenthesupport’ssurfaceandthe
homogeneous catalyst.
SILPsallowamoreefficientutilizationoftheILandthecatalyst
sincetheIL’ssurfaceareaisincreasedrelativetoitsvolumeandthe
substrate can readily diffuse to the catalyst. In contrast, operation
under classical liquid/liquid biphasic conditions requires larger
amounts of ILs and the IL’s high viscosity may cause mass-
transfer limitations. The latter is particularly important for gas-
phase reactions, since ILs generally display a limited solubility for
gases such as H2, O2and CO. SILP catalysis combines the most
attractive features of homogeneous catalysis like high activity and
selectivity with benefits of heterogeneous catalysts such as large
interfacial reaction areas and ease of product separation. Indeed,
the reaction products can be recovered from the organic phase,
while the catalyst remains immobilized in the SILP. Therefore,
SILP catalysis has potential for efficient catalyst recycling and
might enable the application of fixed-bed reactor technology
to homogeneous catalysis. Some reviews about this interesting
topic have already appeared.17-20Wasserscheid et al. focused in
a microreview on the hydroformylation of propene.17In another
review they tackle the use of SILP technology in catalytic and
separation technologies.19Gua and Li deal with solid-based
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heterogeneous catalytic systems that have been developed with
the aid of ILs.18Thus, topics like the use of solid catalysts in ILs,
metal nanocatalysts in ILs and the preparation of heterogeneous
catalysts in ILs, can be found in this reference. In previous
reviews, various aspects of the immobilization of transition-metal
complexes in a SILP have only partially been covered. Clear
trends can be discerned now in the preparation procedures of the
respective SILP catalysts with regard to choice of catalyst, IL and
support. Moreover, as the body of experimental work grows, it is
now possible to make quantitative comparisons between homoge-
nous, heterogeneous and biphasic systems. Therefore the present
review focuses on recent developments in the immobilization of
molecularly defined catalysts in SILPs.
Firstly, general aspects of SILP catalysis are discussed. These
include the preparation of SILP catalysts and the choice of the
homogeneous catalyst, ionic liquid, support, and reaction condi-
tions. Next, the use of SILP catalysis is illustrated with examples
from hydroformylation, metathesis, carbonylation, hydrogena-
tion, hydroamination, C–C coupling and various enantioselective
reactions.
Pascal Mertens
of fine chemicals and pharmaceuticals.
Pascal Mertens (born 1980) ob-
tained his Masters degree in Ap-
plied Bioscience and Engineering
attheCatholicUniversityofLeu-
ven (K.U.Leuven, Belgium) with
athesisoncolloidalgoldcatalysts
inalcoholoxidationattheCentre
of Surface Chemistry and Catal-
ysis. He also completed his doc-
toral research with a dissertation
on colloidal metal catalysts in
liquid phase redox reactions. His
researchinterestsinvolveselective
transformations in the synthesis
General considerations
Catalysts,ILsandsolidsupportscanbecombinedinvariousways.
An overview is presented in Scheme 1. In this review, catalysts
are denoted SILP catalysts (SILPCs) if the support is loaded
with a separate IL phase (Type 1). The IL is deposited on the
support as a multilayer and acts as an immobilizing medium
for the catalyst, behaving as a homogeneous catalyst. The most
straightforward procedure for the preparation of a Type 1 SILPC
(Type 1a, catalyst/IL/support) is via simple impregnation of the
IL on a support’s surface. Prior to the impregnation step, the
metal and the ligand are dissolved in the IL phase. In a different
approach (Type 1b, catalyst/IL/IL-support), the surface of the
support is first modified with a monolayer of covalently anchored
ILfragments.Next,additionalILandthecatalystareimpregnated
on the modified support.
In Type 2 catalysts, the IL is directly anchored to the support
via covalent anchoring, forming a monolayer of IL fragments.
This can be achieved by grafting a functionalized IL fragment
on a preformed support or by sol–gel synthesis.21According
to our definition, the resulting solid catalysts are in fact no
“immobilized ionic liquid phase catalysts”, since there is no
separate IL phase supported on the solid. The IL in this case
should rather be considered as a covalently anchored ligand.
In this approach (Type 2, catalyst-IL-support + IL-support),
IL fragments are first covalently anchored to a support via the
organic cation, and part of the anion is then ion-exchanged
with anionic catalyst species (e.g., [WO4]2-, [RuO4]-, [PdCl4]2-,
[NiCl4]2-, [CuCl4]2-, [Al2Cl7]-, [SnCl5]-).22-25In this context, it
should be noted that earlier reported catalytic systems with
immobilized quaternary ammonium moieties and ion-exchanged
metalcatalystsarenowadaysmorepopularlycalledsupportedionic
liquidcatalysts(SILCs).AnchoringviatheanionicpartoftheILis
also possible. Alternatively, the catalyst or ligand is incorporated
in the IL fragment via covalent binding.26In a third strategy
(Type3,IL/catalyst-support),anILisimpregnatedonthesurface
of classical heterogeneous catalysts such as covalently anchored
Koen Binnemans
From 2002 until 2005, he was (part time) associate professor.
Presently, he is professor of chemistry at the Catholic University
of Leuven. His current research interests are metal-containing liquid
crystals(metallomesogens),lanthanide-mediatedorganicreactions,
lanthanide spectroscopy, luminescent molecular materials, and ionic
liquids.
Koen Binnemans (born 1970)
obtained his M.Sc. (1992) and
Ph.D.(1996)inChemistryatthe
CatholicUniversityofLeuven.In
the period 1999–2005, he was a
postdoctoral fellow of the Fund
for Scientific Research Flanders
(Belgium). He did postdoctoral
work with Prof. Jacques Lucas
(Rennes, France) and Prof. Dun-
can W. Bruce (Exeter, U.K.). In
2000, he received the first ERES
(European Rare-Earth and Ac-
tinide Society) Junior Award .
Dirk De Vos
Dirk De Vos (born 1967) is
a full professor at the Catholic
University of Leuven. After doc-
toralresearchinLeuvenandpost-
doctoral work at Purdue Univer-
sity, he focuses on selective cat-
alytic oxidation, metal–organic
frameworks, ionic liquids and in
situ spectroscopy and microscopy
of working catalysts. His awards
include the D. W. Breck Award
in Zeolite Science and Technol-
ogy (1998, together with P. Ja-
cobs and co-workers), the BASF
catalysis award (2001), the award of the Flemish scientific founda-
tion (2006), and the IPMI Student Advisor Award (2008).
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Page 3

Scheme 1
Various catalyst/ionic liquid/support combinations.
catalysts,27or supported transition-metal nanoparticles.28-30This
type of catalyst was recently named “solid catalyst with ionic liquid
layer” (SCIL).31The present review is limited to Type 1 catalysts,
for which a SILP is used to immobilize homogeneous catalyst
species. For the immobilization of transition-metal nanoparticles,
we refer to other reviews, which elaborate this topic more
exhaustively.18-19
Homogeneous catalyst
A prerequisite for the industrial application of SILP catalysis is
that the homogeneous catalyst has an inherently stable catalytic
performance, guaranteeing a long lifetime. In addition, the loss of
catalyst into the organic layer containing the products should be
negligible. Since ILs are charged and have polar properties, they
are well suited to hold ionic or highly polar catalyst species, like
ionic metal species (e.g., [PdCl4]2-, [WO4]2-). On the other hand,
the preference of the catalyst for the IL phase can be increased
by incorporation of ionic groups in the ligand of the catalyst
complex. For instance, phosphine ligands can be modified with
anionicsulfonategroupsorcationicgroupsbasedonguanidinium,
imidazoliumorpyridiniummoieties.Thisreviewfocusesonmolec-
ularly defined catalysts; therefore, the combination of ILs with
supported metal catalysts,31metal nanoparticles32or supported
enzymes33is not discussed. Regarding the field of applications, the
use of SILPs is being studied extensively for the immobilization of
rhodium and palladium complexes, since their heterogenization
via classical routes (e.g., impregnation or covalent anchoring)
has only been developed with limited success. Although less
frequently studied in this context, other metals such as Ru, Cu,
Mn, Co, Zn or Cr have also been considered for immobilization
in SILPs.
Ionic liquid
ILs are often called designer solvents, since their properties – such
as density, melting point, viscosity, or miscibility with water or
organic solvents – can be fine-tuned by an appropriate choice of
anion and cation.1-3The cations in ILs are mostly large organic
cations, like N,N¢-dialkyl-imidazolium, N-alkylpyridinium, N,N-
dialkylpyrrolidinium,tetraalkylammoniumortetraalkylphospho-
nium ions. Several cation types can be used for SILP catalysis.
Especially the 1-ethyl-3-methylimidazolium (emim), 1-butyl-3-
methylimidazo-lium (bmim) and 1-hexyl-3-methylimidazolium
(hmim) ions are often used as the IL’s cationic part. The
anions can vary widely, e.g., Cl-, Br-, PF6
(CF3SO2)2N-. Whereas Cl-and Br-yield hydrophilic ILs that are
miscible with water, fluorinated anions like BF4
the preparation of hydrophobic ILs immiscible with H2O. The
hydrophobicity of ILs with BF4
chain length. The most widely used IL in SILP applications is 1-
butyl-3-methylimidazoliumtetrafluoroborate,[bmim][BF4],orthe
hexafluorophosphate salt, [bmim][PF6]. These ILs combine a high
solvation power for polar catalyst complexes with a weak coordi-
nationstrength.Thisspecificcombinationmakesthemsuitablefor
reactionswithelectrophiliccatalysts.Themajorproblemisthatthe
BF4
might cause side reactions or corrosion problems, or the strongly
coordinating fluoride ions might poison the transition-metal cat-
alyst by irreversible complexation. Consequently, the application
of these ILs is restricted to anhydrous conditions. In this context,
it is not surprising that the IL’s water content is an important
issue, as the presence of small amounts of water might induce
hydrolysis of the metal catalyst or decomposition of the BF4
PF6
might bind tightly to the transition-metal complex and impede
its catalytic activity. Recently, bis(trifluoro-methylsulfonyl)imide
anion (CF3SO2)2N-has emerged as a popular anion for synthe-
sizing hydrophobic ILs, because the resulting ILs are chemically
and thermally more robust. However, disposal of such ILs, e.g., by
combustion, is more complicated due to potential HF liberation.
General concerns about environmental impact and toxicology
associated with ILs involving halogen-containing anions recently
led to the introduction of halogen-free ILs as environmentally
friendly solvents e.g., imidazolium-based ILs with weakly coordi-
nating anions like alkylsulfates.34In summary, such low-melting,
halogen-free IL systems that combine weak coordination to
electrophilic metal centres and low viscosity with high stability to
hydrolysis are highly desirable for applications in transition-metal
catalysis.
-, BF4
-, CF3SO3
-and
-or PF6
-allow
-depends on the cation’s alkyl
-and PF6
-anions are sensitive to hydrolysis. The resulting HF
-or
-anions. Moreover, impurities such as residual halide ions
Support
The majority of the supports used in SILP catalysis are porous
silica gels with a high surface area of 300–500 m2g-1. Thermal
pretreatment of the silica gel reduces the number of acidic silanol
groups. This can be important for a stable catalyst performance
since the silanol groups may react with ligands or catalysts, hereby
lowering the actual concentration of the ligand or the catalyst
in the IL phase. Mesoporous silica materials (e.g., MCM-41 and
MCM-48)andcrystallinesilica–aluminamaterialssuchaszeolites
and clays (e.g., montmorillonite) have also been used.35-39Other
inorganic materials (Al2O3, TiO2, ZrO2etc.) are less frequently
used due to their low surface areas and pore volumes. However,
under certain conditions alumina might be preferred, since this
support shows higher stability at high pH values as compared to
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SiO2.Afewstudiesfocusedontheuseoforganicpolymermaterials
including poly(diallyldimethylammonium chloride), polystyrene
based polymers and chitosan.40-42More advanced supports such
asmembranesandcarbonnanofiberssupportedonsinteredmetal
fibres have also been used.43-44
Catalyst preparation
The general method for the preparation of SILPCs is impregna-
tion. The catalyst precursor and the ligand are first dissolved in a
smallamountofavolatileorganicsolvent(e.g.,methanol,acetone,
acetonitrile, tetrahydrofuran, dichloromethane and chloroform)
that dissolves both the catalyst complex and the IL, and then the
ILisadded.Next,thissolutionismixedwiththesupportmaterial,
and upon stirring, the volatile solvent is removed by evaporation
under reduced pressure. Alternatively, the homogeneous catalyst
and the IL are impregnated on a support that already contains
a monolayer of covalently anchored IL fragments. In both cases,
the homogeneous catalyst is dissolved in the IL film. The resulting
SILPCs are characterized by the nature of the support, the
catalytic metal loading, the molar ligand/metal ratio (L/M),
the IL loading and the pore filling degree (a). The IL loading
is usually defined as the amount of IL relative to the mass of
the support (weight percentage), while the pore filling degree a
is defined as the volume of the IL divided by the pore volume
of the support. Ideally, the IL should form a thin layer on
the surface, without completely filling the pores. In general,
nitrogen adsorption measurements show that the BET surface
area, the total pore volume and the mean pore size of the
support decrease upon adsorption of the IL. Most of the SILP
catalyst systems described in the literature have IL loadings in
the range of 10–25 wt.%, while the metal loadings vary between
0.01–0.2 mmol g-1.
Reaction conditions
SILP catalysis is ideally suited for gas-phase reactions since no
problems regarding catalyst leaching or washing out of the IL
are encountered if the reaction products are not too polar. In
addition, several ILs show high thermal stability and low vapour
pressure,andarethereforeexcellentmediaforreactionsatelevated
temperatures at which conventional solvents would evaporate or
decompose. On the other hand, liquid-phase reactions require
careful tuning of the polarities of all reaction constituents in
order to prevent leaching of the catalyst or the IL. Application
of a SILPC in liquid-phase conditions supposes extremely low
solubilityoftheILintheliquidreactant/productmixture.Physical
removal of the IL film from the support by mechanical forces
should also be prevented. The SILPC can be used in solvent-free
conditions, with the reagents and products forming a separate
organic layer. Alternatively, an additional solvent may be used.
This solvent contains the reagents and the products and should
be immiscible with the IL phase to prevent leaching of the IL
from the surface of the support. Obviously, the solvent should not
dissolve the catalyst to any extent. For this purpose, non-polar
solvents such as alkanes (e.g., n-hexane, n-heptane, n-dodecane)
are commonly used. Slightly more polar solvents are toluene
and dichloromethane. On the other hand, water, ethanol, or 2-
propanol should be used in combination with hydrophobic ILs
(e.g., containing BF4
in this case, limited extraction of the IL and the catalyst in
the solvent is difficult to avoid. Supercritical carbon dioxide
(scCO2) has recently been introduced as an organic solvent.
Although scCO2shows good solubility in some ILs, the reverse
is not the case, with no detectable IL solubilization in the
scCO2 phase. scCO2 reduces the viscosity of the IL and thus
improves the solubility and mass transport of permanent gases.
Disadvantagesarethehighinvestmentandoperatingcostsandthe
limited solvating ability of scCO2compared with classical organic
solvents.
-or (CF3SO2)2N-anions), although even
Selected examples
1.Olefin hydroformylation
Rhodium complexes are the most effective catalysts for olefin
hydroformylation under mild conditions (100◦C, 15 bar CO/H2).
Typically, Rh(CO)2(acac) (acac = acetylacetonate) is used as
the catalyst precursor and a phosphine ligand is added to
favour the formation of linear over branched aldehydes (n/iso-
aldehyde ratio). Since the heterogenization of Rh catalysts
via classical routes (e.g., impregnation or covalent anchoring)
has only been achieved with limited success, the Rh com-
plex immobilization in supported liquid phases emerged as an
alternative.12-13
Liquid-phase hydroformylation in batch reactors.
formylation of 1-hexene was one of the first reactions carried out
with a homogeneous catalyst immobilized in a SILP (Scheme 2).32
Initially, thehydroformylation
monophosphine complexes (Scheme 3, 1). As support a [1-butyl-
3-(3-triethoxysilyl-propyl)-4,5-dihydroimidazolium][BF4] or [PF6]
modified dehydratedsilica gel
(Scheme 4).45
The hydro-
wasperformedwithRh-
(474m2
g-1)wasused
Scheme 2
Hydroformylation of 1-hexene.
The resulting SILP catalyst ([Rh]/IL/IL-SiO2] activity of the
SILPC (TOF = 3900 h-1) was almost three times higher compared
to the biphasic IL system (Table 1). This improvement was
attributed to a higher concentration of the active Rh species at the
interfaceandthegenerallylargerinterfaceareaofthesolidsupport
in comparison with the biphasic system. The selectivity (n/iso
aldehyde ratio = 2.4) was comparable for both systems. Note that
thehomogeneoussystemisstillsignificantlymoreactive(Table1),
which is probably due to mass transfer limitations, caused by the
lowersolubilityofthegasesintheIL.HowevertheILintroduction
enables the recycling of the catalytic system. Although the active
Rh species are insoluble in the organic medium, at high aldehyde
concentrations [bmim][BF4] partially dissolved in the organic
phase which led to rhodium complex leaching (Rh loss ª 2 mol%).
This was solved by keeping the aldehyde concentration below 50
wt.% and increasing the phosphine concentration in the IL. Note
that the Rh leaching could still be minimized.
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Scheme 3
tagged Pd complex; 3 monophosphine ligands containing ionic guanidinium or sulfonate groups; 4 [1,1,3,3-tetramethylguanidinium][lactate]
(TMGL); 5 imidazolium-tagged bis(oxazoline) ligand; 6 (S)-BINAP ligand; 7 triphenylphosphine; 8 the bisphosphine ligand sulfoxantphos; 9
5,5¢-isopropylidenebis[(4R)-4-phenyl-4,5-dihydro-1,3-oxazole]; 10 (S,S)-Mn(III) salen chloride; 11 dimeric (R,R)-Cr(III) salen complex).
Classical ligands and ionic liquid tagged ligands in SILP catalysis (1 sulfonated monophosphine ligands; 2 ionic liquid–
Scheme 4
Covalently anchored imidazolium fragments on silica gel.
Table 1
and biphasic catalytic systems in the liquid-phase hydroformylation in
batch reactors
ComparisonofthecatalyticperformanceofSILP,homogeneous,
Sa
Catalytic system L/Rhbn/icTOFd/h-1Ref.
C6
C6
C6
C6
C6
C6
SILP/[bmim][BF4]-tppti-Rh
Biphasic/[bmim][BF4]-tppti-Rh
Homog/PPh3-Rh
Biphasic/TMGL-TPPTS-Rh
SILP/TMGL-TPPTS-Rh/MCM-41 5
SILP/TMGL-TPPTS-Rh/SiO2
10
10
10
5
2.4
2.2
2.6
3.8
3.0
2.1
3900
1380
24000
82
389
87
41
41
41
42
42
425
aS = substrate, C6 = 1-hexene.bL/Rh = ligand to Rh molar ratio.cn/i =
normal/iso aldehyde ratio.
(Rh) per hour.
dTOF is defined as mol (aldehyde) per mol
ArelatedRh-monophosphine basedsystemwas pre-
pared using a mesoporous MCM-41 silica (1041 m2g-1)
as the support and [bmim][BF4], [bmim][PF6] or [1,1,3,3-
tetramethylguanidinium][lactate] (TMGL) as ILs (Scheme 3, 4).46
In the hydroformylation of 1-hexene, the MCM-41 catalyst
(tppts/Rh=5)wasmoreactivethanasimilarlypreparedsilicagel
catalyst (Table 1). The highest activity (TOF = 389 h-1, n/iso = 3)
was obtained with a 15 wt.% TMGL loading. The n/iso aldehyde
ratio increased with increasing TMGL loading and tppts/Rh
molar ratio. This system is nearly 5 times more active than the
corresponding biphasic system (Table 1). The catalyst was reused
5timeswithoutadropinactivityorselectivity,andtheRhcontent
in the organic phase was below the AAS detection limit. In the
hydroformylationofhigherolefins,theactivityoftheSILPcatalyst
decreased with increasing olefin chain length.
Gas- and liquid-phase hydroformylation in fixed-bed reactors.
In a further development, the hydroformylation was studied in
continuous-flow fixed-bed reactors. Rh complexes with mono- or
bisphosphine ligands were used as the catalysts. The gas-phase
propene and liquid-phase 1-octene hydroformylation have first
been carried out using a SILPC comprised of silica (298 m2g-1),
[bmim][PF6] and ionic Rh-monophosphine complexes (0.2 wt.%
Rhloading)(Scheme3,3).47Inthecontinuous-flowhydroformyla-
tionofpropeneat120◦C,itwasobservedthatcatalystscontaining
the NORBOS-Cs3ligand were significantly more active than the
guanidinium-based analogues (Table 2, entries 3 and 4). However,
the SILPc were less active compared to the heterogeneous catalyst
(Table 2, entry 1). With respect to the ligand/Rh ratio, the
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