A polymer-supported β-ketoesterate complex of palladium as an efficient, phosphane-free, air-stable, recyclable catalyst for the heck reaction

Article abstract of DOI:10.1002/1099-0682(200205)2002:5<1094::AID-EJIC1094>3.0.CO;2-C

The coupling between aryl iodides and bromides with olefins (Heck reaction) has been studied with a polymer-supported palladium catalyst in the presence of sodium or potassium acetate. The reactions can be performed in air without any activating phosphorus ligands and with non dried solvents. Under suitable conditions the catalyst, which exhibits high activity and selectivity, is recyclable several times with negligible leaching of metal. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0682(200205)2002:5<1094::AID-EJIC1094>3.0.CO;2-C

FULL PAPER
A Polymer-Supported β-Ketoesterate Complex of Palladium as an Efficient,
Phosphane-Free, Air-Stable, Recyclable Catalyst for the Heck Reaction
[
a]
[a]
[a]
[a]
Maria M. Dell’Anna, Piero Mastrorilli, Francesco Muscio, Cosimo F. Nobile,* and
Gian P. Suranna[
a]
Keywords: Heck reaction / Supported catalysts / Palladium
The coupling between aryl iodides and bromides with olefins Under suitable conditions the catalyst, which exhibits high
(
Heck reaction) has been studied with a polymer-supported
activity and selectivity, is recyclable several times with negli-
gible leaching of metal.
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
palladium catalyst in the presence of sodium or potassium
acetate. The reactions can be performed in air without any
activating phosphorus ligands and with non dried solvents. 2002)
Introduction
active in the coupling of iodobenzene with acrylic esters.
The support could be recycled, though it needed to be re-
The Heck reaction[1] remains one of the most useful and charged with Pd(dba)
in each subsequent run.
versatile CϪC bond forming reaction known. Although this
A palladium complex prepared from a triphenylphos-
2
vinylation of aryl halides has been widely studied in acade- phane amphiphilic resin and a palladium π-allyl chloride
mia, there are not many industrial examples of its exploita- dimer has been used as an active catalyst for Suzuki cross-
tion. The reasons for this are economic. Heck reactions are coupling and for the arylation of allylic acetates.[
7]
mostly catalysed by palladium complexes in homogeneous
solution in the presence of phosphorus ligands, which are
toxic and unrecoverable. In addition, these expensive cata-
lysts give problems due to their instability at high reaction
temperatures, and their separation and recovery after use.
These problems can be overcome, however, by the use of
heterogeneous systems,[ made up of supported palladium
complexes, which, moreover, are generally phosphane-free.
Palladium metal colloids, in some cases stabilised by the
All the heterogenised catalysts in which the support is
a polymer with pendant phosphanes suffer from the same
problem when used in Heck-type reactions: the irreversible
loss of metal from the matrix due both to degradation of
[2]
the phosphane moieties and to the reaction mechanism
[8a]
itself. As an example, metal leaching was observed when
2]
palladium supported on glass beads was used as catalyst for
Heck reactions, owing to the oxidation of phosphanes that
[9]
bound the metal atoms onto the matrix.
[3]
presence of the support, appear to be the active species
for phosphane-free catalytic systems.
Various examples can be found in the literature con-
cerning non-phosphinated organic polymers as supports for
Polymer-supported palladium complexes have been used
for the activation of aryl iodides by several research groups.
An early report on a palladium catalyst based on triphenyl-
phosphane-bonded polystyrene resin[ describes the aryl-
ation of olefins by iodoarenes, although no studies on the
activity of the recycled resin are reported. In a more recent
paper, polymer-supported 1,2-bis(diisopropylphosphanyl)-
benzene has been used for preparing a palladium catalyst
II
palladium-based catalysts. Among them, Pd coordinated
3
to a pincer-type κ -S,C,S ligand attached to soluble organic
polymers has been described as an active and recyclable
4]
catalyst for the olefination of iodoarenes at 95Ϫ110 °C un-
[10]
[11]
der homogeneous or liquid/liquid biphasic conditions.
Polymer-supported carbene complexes of palladium have
been used as recyclable catalysts for the Heck reaction of
aryl bromides at 150 °C, although with significant loss of
[
5]
for the reaction of iodobenzene with methyl acrylate.
[12]
metal (60% ) during the first run.
II
wt
The reaction between Pd salts and a polypyrrole con-
Recently, a bis(pyrimidine)-based palladium polymer-
supported catalyst for Heck-type reactions has been syn-
thesised,[ but no data on the activity of the recovered
catalyst has been reported.
taining a phosphane-borane moiety resulted in another type
of solid-supported palladium catalyst[6] that was moderately
13]
[
a]
Centro di studi C.N.R. M.I.S.O. and Dipartimento
dЈIngegneria Civile ed Ambientale (Sezione Chimica)
Politecnico di Bari,
Another class of supports onto which catalytically active
palladium is dispersed is represented by inorganic matrices.
Pd on charcoal and Pd on different metal oxides have been
found to be active catalysts for the Heck reaction of aryl
Via Orabona, 4, 70125 Bari Italy
Fax: (internat.) ϩ 39-080/596-3611
E-mail: nobile@poliba.it
1094
 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ1948/02/0505Ϫ1094 $ 20.00ϩ.50/0 Eur. J. Inorg. Chem. 2002, 1094Ϫ1099

A Polymer-Supported β-Ketoesterate Complex of Palladium
FULL PAPER
iodides.[2] Arai et al.[14] have studied the coupling between formamide (DMF), diethylene glycol (DEG)] the best re-
iodobenzene and methyl acrylate at 160 °C in the presence sults in terms of catalyst activity were obtained using DMF
of 0.28Ϫ1.4 mol % of Pd supported on carbon or silica. (with aryl iodides) and DEG (with aryl bromides).
Unfortunately, these conventional supported palladium
Table 1. Heck coupling reactions of aryl iodides catalysed by Pd-
catalysts are unselective when the substrates are bromoar-
pol; reaction conditions: a molar ratio of 1:1.2:2.4 was used for the
enes, with dehalogenation being the dominant reaction. Pd
aryl iodide (5 mmol):alkene:base (AcOK) in 6 mL DMF; T ϭ 90
particles supported on mesoporous silica have also been
°
C; molar ratio Pd/ArI ϭ 1:1000; for all reactions the conversion
employed for the coupling of aryl bromides,[ although of the aryl iodide was quantitative
satisfactory yields were obtained only with activated sub-
strates such as p-bromonitrobenzene.
15]
Very recently, Djakovitch et al. have described Pd-modi-
fied zeolites that are very active and recyclable catalysts for
the Heck reaction of aryl bromides with olefins.[
16]
Herein we report our studies on Heck reactions using a
β-ketoesterato polymeric palladium complex (referred to as
Pd-pol) as catalyst that exhibits satisfactory activities to-
wards activated and nonactivated iodo- and bromoarene
substrates. The supported complex (1.98%w Pd) was ob-
Ϫ
tained by co-polymerisation of Pd(AAEMA) [AAEMA
2
is the deprotonated form of 2-(acetoacetoxy)ethylmethac-
rylate] with ethyl methacrylate and ethylene glycol di-
[
17]
methacrylate.
In this work we have considered the influence of solvent,
base, temperature and additives on the performance and the
recyclability of the catalyst.
Results and Discussion
Scheme 1 illustrates the scope of the investigated Heck
reactions catalysed by Pd-pol. The molar ratio between the
limiting substrate (aryl halide) and palladium was op-
timised at 1000 for reactions with aryl iodide and 500 for
reactions with aryl bromides. A general feature of all the
Pd-pol catalysed reactions is the simplicity of the set-up and
work-up. All manipulations could be performed in an air
atmosphere and with non-dried solvents because of the oxy-
gen and moisture stability of the supported catalyst.
[
a]
GLC yields. [b] Molar ratio Pd:DMG ϭ 1:20. [c] Molar ratio
Pd:PPh ϭ 1:5.
3
Table 1 summarises the results obtained in the reactions
between activated and nonactivated aryl halides with differ-
ent vinylic substrates. In all cases the conversions were
quantitative.
The coupling between iodobenzene and styrene at 90 °C
in the presence of 0.10 mol % of supported palladium gave,
after 6 h, a 100% yield of stilbene (93% trans, 7% cis, entry
1
). Under the same reaction conditions acrylonitrile af-
forded 99% cinnamonitrile with no selectivity between the
cis and trans isomers (entry 2). The reaction between iodo-
benzene and methyl acrylate was faster, giving, at 90 °C, a
9
0% yield of trans-methyl cinnamate after 2 h (entry 3).
Reacting p-iodotoluene with styrene (entry 4), acrylonitr-
Scheme 1
The effect of the base was examined first. In preliminary ile (entry 5) or methyl acrylate (entry 6) resulted in quantit-
experiments triethylamine (the typical base for many homo- ative yields of the desired coupling products.
geneous and heterogeneous catalytic Heck reactions[
1,2]
)
The same reactions reported in entries 1, 2 and 3 of
was used, but this organic base caused significant metal Table 1 were also carried out at 140 °C, in order to study
leaching. This is probably due to the higher coordination the influence of the temperature. The reactions at 140 °C
ability of trialkylamines towards palladium[
18]
than in- were slightly faster than those carried out at 90 °C. In par-
organic bases such as alkaline acetates or carbonates. For ticular, the reaction between iodobenzene and styrene was
this reason, all subsequent tests were performed using inor- complete after 5 h, giving a 93% yield of trans-stilbene. The
ganic bases such as potassium or sodium acetate. As for the reaction between iodobenzene and methyl acrylate gave a
choice of solvent, among those tested [toluene, N-methyl- quantitative conversion of the limiting substrate after 1 h
pyrrolidine, dioxane, dimethyl sulfoxide, N,N-dimethyl- with a 98% yield of trans-methyl cinnamate. The coupling
Eur. J. Inorg. Chem. 2002, 1094Ϫ1099
1095

M. M. Dell’Anna, P. Mastrorilli, F. Muscio, C. F. Nobile, G. P. Suranna
It is possible to explain the observed metal leaching in
FULL PAPER
between iodobenzene and acrylonitrile was complete after
3
h giving a 76% yield of trans-cinnamonitrile and a 24% solutions containing PPh as additive by referring to the
3
yield of cis-cinnamonitrile. In the two latter cases the select- commonly accepted mechanism.[ Triphenylphosphane re-
1]
ivity was shifted towards the thermodynamically favoured duces palladium(II) to palladium(0) giving Ph PO and the
3
trans adduct.
coordinatively unsaturated Pd(PPh ) , which can start the
3 2
In order to study the influence of additives on the catalyst catalytic cycle through the oxidative addition of the aryl
II
performance, the reactions of entries 1 and 2 were carried halide. Hence, the Pd centre must lose its counteranion
out in the presence of N,N-dimethylglycine (DMG), which (chloride, acetate, etc.) in order to enter the reaction cycle.
Ϫ
is known to improve the activity and selectivity in the Loss of the polymer-AAEMA ligands, however, forms un-
homogeneous phosphane-free Heck coupling of aryl brom- der-ligated palladium(0), which leaches irreversibly into so-
[
19]
ides.
A slight improvement in the activity was observed. lution.
Another mechanism, recently proposed by Amatore,[21]
(entries 1, 2), whereas the TOFs for reactions carried claims the intermediacy of anionic palladium complexes. In
out in the presence of a 20:1 DMG/Pd (mol/mol) ratio were this case, the pre-activation step gives an anionic palla-
In fact, the TOFs for the reactions without DMG were 167
Ϫ1
h
Ϫ1
Ϫ1
2
00 h (styrene, entry 7) or 333 h (acrylonitrile, entry 8). dium(0) complex, which is able to start the catalytic cycle.
The influence of a phosphorus additive, such as triphen- This cycle is reported in Scheme 2 for the reaction pro-
ylphosphane, was also investigated. The coupling between moted by Pd-pol in the presence of an n-fold excess PPh3.
iodobenzene and methyl acrylate carried out in the presence The anionic complex in our context should be
of triphenylphosphane (5:1 P/Pd mol/mol) gave, after 2 h, a [Pd (PPh ) (A-pol)] ([A-pol] ϭ polymer-bound β-keto-
0
Ϫ
Ϫ
3
2
quantitative yield of methyl cinnamate (95% trans and 5% esterate). Following this type of mechanism the leaching
cis, entry 9), a result similar to that obtained in the phos- should occur at the stage of the equilibrium (1) of
phane-free reaction (entry 3). However, the presence of tri- Scheme 2.
phenylphosphane caused severe metal leaching from the
supported catalyst.
In order to study the influence of equilibrium (1) on the
metal leaching, one and two equivalents of base (respect
In the case of the reaction of entry 9, the recovered cata- to the olefin) were used in two parallel phosphane-assisted
lyst became inactive after three cycles in the presence of reactions. The reaction between iodobenzene and methyl
PPh . On the contrary, the catalyst of the phosphane-free acrylate carried out at 140 °C in the presence of one equiva-
3
reaction described in entry 1 (Table 1) remained active for lent of base, 0.50 mol % of PPh and 0.10 mol % of sup-
3
at least six cycles (Figure 1).
ported Pd, gave a 90% conversion and a 94% yield of trans-
methyl cinnamate after 5 h. The same reaction carried out
in the presence of two equivalents of base gave a quantitat-
ive conversion and a 98% yield of trans-methyl cinnamate
after 1 h. In addition, the recovered resin of the former re-
action (1 equiv. of base) was inactive for a second run, while
the Pd-pol recovered after the second experiment (2 equiv.
of base) was still active even after five further catalytic runs.
However, contrary to what happens in phosphane-free reac-
tions, the leaching is not completely inhibited even in the
Figure 1. Recyclability of Pd-pol in Heck reactions between iodo-
benzene and styrene at 90 °C; in each reaction the conversion of presence of 2 equiv. of base, since the mother liquor of the
iodobenzene was quantitative; TOF ϭ [moles of coupling product
solution after each cycle revealed a high activity (80% of
fresh iodobenzene converted after 3 h reaction).
(both isomers)]/[(moles of Pd) ϫ (reaction time in hours)]
This behaviour was found to be general: the catalyst of
The lower catalyst activity shown in the reaction carried
all the phosphane-free reactions described in this work (in- out with 1 equiv. of base compared with that with 2 equiv.
Ϫ1
Ϫ1
cluding those in the presence of DMG) could be recycled of base (the relevant TOF’s are 200 h and 1000 h , re-
at least three times with no loss of activity or selectivity.
spectively) can be ascribed to the formation of the less act-
The catalyst recycling was performed by filtration, wash- ive[
21a]
cationic Pd complex [ArPd(PPh ) ] generated
ϩ
3
2
ing with acetone and diethyl ether and with water (for re- through the equilibrium (1) of Scheme 2.
moval of the excess base, its conjugate acid and the formed Although deeper mechanistic studies are required, these
alkaline halides). This latter treatment did not affect the results seem to indicate that the reaction pathway proposed
activity of the catalyst, as occurs in the case of palladium by Amatore is operative in our system.
supported on zeolites and metal oxides.[
20]
The effect of the amount of base on metal leaching and
In addition, the mother liquor of all the described phos- on catalyst activity was observed by us also in the phos-
phane-free reactions (including those in the presence of phane-free reactions. For example, the reaction between
Ϫ1
DMG) showed negligible catalytic activity. On the other iodobenzene and styrene proceeded with TOFs of 83 h
Ϫ1
hand, the mother liquor of the reaction carried out in the or 167 h when the molar ratio of olefin/base was 1 or 2,
presence of PPh (entry 9) showed a moderate activity, giv- respectively. Furthermore, in the former case the copolymer
3
ing a 50% conversion of fresh iodobenzene after 20 h reac- leached out all its palladium loading after three cycles,
tion.
whereas in the second case the metal content passed from
1096
Eur. J. Inorg. Chem. 2002, 1094Ϫ1099

A Polymer-Supported β-Ketoesterate Complex of Palladium
FULL PAPER
Scheme 2. Catalytic cycle via anionic palladium(0) complexes
1
.98% before reaction to 1.67% after six cycles. It is possible Table 2. Suzuki and Sonogashira coupling reactions of aryl iodides
to assume that the double amount of base inhibits the pro- catalysed by Pd-pol; typical reaction conditions: a molar ratio of
:1.2:2.4 was used for ArI (5 mmol):phenyl boronic acid or phenyl-
1
tonation of the polymer-supported β-ketoesterate ligand
acetylene:base (AcOK) in 6 mL DMF; T ϭ 90 °C; molar ratio Pd/
aryl iodide ϭ 1:1000; for all reactions the conversion of the aryl
iodide was quantitative
Ϫ
(AAEMA ). Only if it remains anionic, in fact, is the poly-
meric ligand able to bind the palladium(II) and palla-
dium(0) species involved in the different stages of the reac-
tion cycle. The leaching of the metal could therefore be due
to the protonation of the polymeric ligand by the acetic
acid formed during the reaction.
In order to verify the influence of the presence of an acid
in the reaction medium on metal leaching, the Suzuki reac-
tion between aryl iodides and phenylboronic acid was car-
ried out (Table 2). Quantitative yields of the corresponding
coupling products were obtained starting from iodobenzene
Ϫ1
(
(
entry 1) or p-iodotoluene (entry 2) with TOFs of 67 h
Ϫ1
iodobenzene) and 83 h (p-iodotoluene). However, as ex-
pected, the initially yellow catalyst leached out all its palla-
dium loading into solution and was inactive in subsequent
runs.
Although in the case of the polymeric β-ketoesterato
support the presence of a double amount of acetate results
in increasing activity and recyclability of the supported
catalyst, this effect is not generalizable.
The results reported in the literature seem to indicate that
the amount of base and its nature can influence the metal
leaching, in function of the support used. In fact, it has been
[a]
GLC yields.
[
22]
recently claimed
that when Pd/alumina, Pd/C or sulfon-
ated polystyrene-supported palladium are used to promote
Heck reactions, the metal loss is enhanced by the presence of Na CO3 and 1 equiv. triethylamine were used at the
2
of acetate ions (and of the aryl halide), presumably due to same time.
the formation of soluble ArPdBr(OAc) or ArPd(OAc)n
species. Moreover, Arai et al. reported that the Pd spe- ing aryl bromides was also examined (Table 3). Pd-pol was
cies leached out into the solvent during Heck reactions pro- used as catalyst for the Heck reaction of bromobenzene
The possibility of activating economically more interest-
n
[
8]
[14]
moted by Pd/C or Pd/SiO redeposited onto the surface of with styrene at 160 °C using a 1:500 Pd/substrate molar
2
the supports at the end of the reaction only when 1 equiv. ratio, in the presence of DMG. Under these reaction condi-
Eur. J. Inorg. Chem. 2002, 1094Ϫ1099
1097

M. M. Dell’Anna, P. Mastrorilli, F. Muscio, C. F. Nobile, G. P. Suranna
FULL PAPER
tions the conversion of the substrate was quantitative after Experimental Section
2 h affording a 95% yield of trans-stilbene (entry 1). In the
1
absence of DMG the conversion was quantitative after 24 h All preparations, manipulations and reactions were carried out un-
der air. All chemicals were purchased from Aldrich and used as
and the yield of trans-stilbene was 91% (entry 2). After the
initial catalytic run, the resin used in the reaction of entry
received. The supported catalyst (Pd-pol) was prepared according
to the procedure reported in ref.[ The palladium content in the
17]
1
was recovered, washed and recycled five times with negli-
supported catalyst was determined by atomic absorption spectro-
metry using a PerkinϪElmer 3110 instrument. Chromatographic
analyses were carried out on Hewlett Packard 6890 instruments
using an HP-PLOT Molecular Sieve 5A (15.0m ϫ 0.53 mm ϫ 25.0
µm) column (injector temperature 280 °C, FID temperature 280
gible loss of activity (Figure 2).
Table 3. Heck coupling reactions of aryl bromides catalysed by Pd-
pol; typical reaction conditions: a molar ratio of 1:1.2:2.4 was used
for ArBr (5 mmol):alkene:base (NaOAc) in 6 mL DEG; T ϭ 160
°
C, carrier: nitrogen or helium). GCMS data (EI, 70 eV) were ac-
°
C; molar ratio Pd/ArBr ϭ 1:500
quired on an HP 6890 instrument (injector temperature 280 °C,
carrier: helium) using an HP-1 crosslinked methyl siloxane (60.0
m ϫ 0.25 mm ϫ 1.0 µm) capillary column coupled with a mass
spectrometer HP 5973. The products were identified by comparison
of their GC and GC-MS features with those of authentic samples.
Conversions and yields were calculated by gas chromatography us-
ing n-dodecane as an internal standard calibrated to the corres-
ponding pure compound. In the case of entry 1 of Table 1, the
isolated yield was also assessed as described below.
General Procedure for the Heck, Suzuki and Sonogashira Reactions:
A 50 mL pressure tube was charged with Pd-pol (1.98% Pd), pot-
w
assium or sodium acetate (12 mmol), aryl halide (5.0 mmol), olefin,
alkyne or arylboronic acid (6.0 mmol), n-dodecane as internal
standard, the additive (when present) and the solvent (DMF or
DEG, 6 mL). The aryl iodide/Pd molar ratio was 1000, whereas
the aryl bromide/Pd molar ratio was 500. The mixture was heated
whilst stirring at the desired temperature until reaction completion
as monitored by GLC and GCMS analyses. After reaction the
catalyst was recovered by filtration, washed with acetone, water,
acetone and diethyl ether, and dried under vacuum.
[
a]
GLC yields. [b] Molar ratio Pd:DMG ϭ 1:20.
Isolation of trans-Stilbene: After separation of Pd-pol, the reaction
mixture was diluted with CH
3 ϫ 15 mL). After separation of the organic phase and drying
over Na SO , the solution was concentrated under reduced pres-
2 2 2
Cl (20 mL) and washed with H O
(
2
4
sure. Separation of the cis/trans isomers was carried out by silica
gel chromatography with pentane/diethyl ether (1:1) solvent mix-
tures. The product was obtained in an analytically pure form after
recrystallization from diethyl ether at Ϫ18 °C (0.808 g, isolated
yield ϭ 89.8%).
Catalytic Test for the Determination of the Activity of the Mother
Liquor: The clear filtrate obtained from the catalyst was used as
solvent for these experiments. This solution was obtained by filtra-
tion at 90 °C over celite, in order to remove the fine particles. Fresh
reactants (5.0 mmol aryl halide, 6.0 mmol olefin, 12 mmol base)
were added to the filtrate and the resulting mixture was heated at
the desired temperature whilst stirring for 20 h. The reaction course
Figure 2. Recyclability of Pd-pol in Heck reactions between bromo-
benzene and styrene at 160 °C, in the presence of DMG; in each
reaction the conversion of bromobenzene was quantitative; TOF ϭ
[moles of coupling product (both isomers)]/[(moles of Pd) ϫ (reac-
tion time in hours)]
The reactions of 4-bromonitrobenzene and 4-bromo-
acetophenone with styrene took 10 h to reach completion
giving 90% (entry 3) and 92% yield (entry 4) of the trans-
coupling adduct, respectively.
(
if any) was monitored by GLC as described above.
Acknowledgments
Mr. E. Pannacciulli is gratefully acknowledged for technical assist-
ance. The Italian MIUR and CNR are gratefully acknowledged for
financial support.
The possibility to use aryl chlorides as substrates in the
Pd-pol catalytic system has also been examined. Unfortu-
nately, the reactions between chlorobenzene and methyl ac-
rylate or between p-chloronitrobenzene and styrene at 180
[19b]
°
C in the presence of DMG and Ph PCl
were not effect-
4
[
1] [1a]
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aryl iodides with phenylacetylene at 90 °C (Sonogashira re-
action), but the selectivities were poor (Table 2).
London, 1985. [ J. Tsuji, Palladium Reagents and Catalysts,
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[1c]
S. Brase, A. de Meijere, in Metal-
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Stang), Wiley-VCH, Weinheim, 1997.
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A Polymer-Supported β-Ketoesterate Complex of Palladium
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