The first insoluble polymer-bound palladium complexes of 2-pyridyldiphenylphosphine: Highly efficient catalysts for the alkoxycarbonylation of terminal alkynes

Article abstract of DOI:10.1039/b512556a

Palladium complexes of 2-pyridyldiphenylphosphine anchored on polystyrene, polymethylmethacrylate and styrene-methylmethacrylate copolymer form highly active heterogeneous catalysts for the alkoxycarbonylation of terminal alkynes with activities approaching those obtained under homogeneous conditions. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512556a

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The first insoluble polymer-bound palladium complexes of
2-pyridyldiphenylphosphine: highly efficient catalysts for the
alkoxycarbonylation of terminal alkynes{
Simon Doherty,* Julian G. Knight* and Michael Betham
Received (in Cambridge, UK) 7th September 2005, Accepted 14th October 2005
First published as an Advance Article on the web 14th November 2005
DOI: 10.1039/b512556a
While it is relatively straightforward to immobilise triphenylpho-
sphine5 as well as bi-6 and terdentate phosphines,7 immobilisation
of 2-pyPPh2 to a solid support presents a much greater challenge.
Herein, we describe the first example of 2-pyPPh2 anchored to
insoluble polymeric supports and a preliminary investigation of
their application to the palladium-catalysed alkoxycarbonylation
of terminal alkynes.
Palladium
complexes
of
2-pyridyldiphenylphosphine
anchored on polystyrene, polymethylmethacrylate and
styrene–methylmethacrylate copolymer form highly active
heterogeneous catalysts for the alkoxycarbonylation of terminal
alkynes with activities approaching those obtained under
homogeneous conditions.
The 2-pyridylstyrylphenylphosphine monomer 3 was prepared
according to Scheme 1. Lithiation of 2-bromopyridine followed by
transmetalation with zinc chloride in pyridine gave the cyclic
2-pyridyl zinc chloride dimer 1,8 which was quenched with
dichlorophenylphosphine to afford 2-pyridylphenylchloropho-
sphine 2. The polymerisable styryl group was introduced in the
final step by reaction of 2 with styryl magnesium chloride. The
copolymerisation of 3 with styrene was carried out in benzene
using 2,29-azobis(2-methyl)propionitrile (AIBN) as radical initiator
to give polystyrene supported 2-pyridyldiphenylphosphine 4, as a
white solid after precipitation by addition of methanol. Similarly,
suspension copolymerisation of 3 with methyl methacrylate gave
the corresponding 2-pyridyldiphenylphosphine loaded copolymer
5. Both polymers have a unimodal molecular weight distribution
with polydispersities Mw/Mn of 2.1 and 2.0, respectively, and
the 2-pyridyldiphenylphosphine loading was determined to be 1.20
and 1.49 mmol g21, respectively, from elemental analysis.
The regioselective palladium catalysed inter- and intramolecular
alkoxycarbonylation of terminal alkynes has proven to be a
highly versatile process for the synthesis of acyclic and cyclic
a,b-unsaturated esters (eqn (1)).1 Some time ago researchers at
Shell discovered that catalysts formed by combining 2-pyridyldi-
phenylphosphine (2-pyPPh2), a source of palladium(II) and a
sulfonic acid were highly active and selective for the methox-
ycarbonylation of propyne, giving turn over numbers as high as
40,000 molproduct molcat21 h21 with a selectivity for methyl
methacrylate (MMA) of 99.95% under mild conditions.2
ð1Þ
The use of 2-pyPPh2 in the palladium catalysed alkoxycarbonyl-
ation of alkynes is an exceptional example of a ligand that plays a
dual role achieving high selectivity through its ability to function as
a bidentate P,N-coordinated ligand while a second P-coordinated
N-protonated 2-pyPPh2 facilitates rapid proton transfer in the rate
determining protonolysis step. The critical role of the pyridyl
group becomes clearly evident upon replacement of 2-pyPPh2 by
triphenylphosphine, which results in a dramatic decrease in
catalyst activity to ca. 10 molproduct molcat21 h21 as well as a
marked reduction in selectivity to 89%.2 Given the potential
impact of this process for the commercial production of MMA, a
large scale (2 6 106 tonnes per annum) industrial intermediate for
the production of homopolymers and copolymers,3 as well as for
the preparation of fine chemicals,4 there is likely to be interest in
the immobilisation of 2-pyPPh2 to develop a continuous process
which would combine high catalyst activity and selectivity with
facile product separation, in which catalyst leaching is limited.
The 2-pyridyl(p-methacryloyloxyphenyl)phenylphosphine
monomer 9 was also targeted for the preparation of a 2-pyPPh2
anchored methacrylate homopolymer and details of the synthesis
are outlined in Scheme 2. Initial attempts to reduce 2 with LiAlH4,
NaH and DIBAL all proved unsuccessful either resulting in partial
School of Natural Sciences, Chemistry, Bedson Building, University of
Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK.
E-mail: simon.doherty@ncl.ac.uk; j.g.knight@ncl.ac.uk;
Fax: +44 (0) 191 222 6929;
Tel: +44(0) 191 222 6537; +44(0) 191 2227068
{ Electronic supplementary information (ESI) available: synthesis and
characterisation of monomers and polymers and details of catalysis. See
DOI: 10.1039/b512556a
Scheme 1 Reagents and conditions: (i) BuLi, ZnCl2, py; (ii) PPhCl2,
THF–py; (iii) styrylMgCl, THF 70 uC; (iv) styrene, AIBN, C6H6 (v)
MMA, AIBN, C6H6.
88 | Chem. Commun., 2006, 88–90
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Table 1 Methoxycarbonylation of phenylacetylene and propynea
Activityc mol
product mol
Entry Ligand/support Substrateb cat21 h21
Selectivityd
1
2
3
4
5
6
7
8
9
2-pyPPh2
3
4
5
10
11
PPh3
PhCMCH
PhCMCH
PhCMCH
PhCMCH
PhCMCH
PhCMCH
PhCMCH
PhCMCH
PhCMCH
MeCMCH 1171
MeCMCH 1098
MeCMCH 284
MeCMCH 608
MeCMCH 1022
MeCMCH 55
MeCMCH 17
306
300
273
225
260
20
11.7
2.0
4.3
97.4
97.1
97.5
97.3
98.0
.99.5
81.8
80.3
.99.5
97.8
97.7
97.9
98.0
97.8
.99.5
.99.5
12
13
11
12
13
14
15
16
17
a
2-pyPPh2
3
4
5
10
11
13
Scheme 2 Reagents and conditions: (i) Mg, H2O; (ii) 4-IC6H4OH, DMA,
10 mol% Pd(OAc)2, 70 uC, 12 h; (iii) BH3?SMe2, THF, 12 h; (iv)
methacryloyl chloride, NEt3, CH2Cl2 278–0 uC (v) C6H6, AIBN, MMA,
60 uC, 12 h (vi) CH2Cl2, DABCO, RT 12 h.
Conditions: batch, intake: 30 ml methanol, 40 bar CO, 0.1 mmol
b
ligand, 0.01 mmol Pd(OAc)2, 0.3 mmol CH3SO3H, 50 uC. 10 mmol
phenylacetylene or 2.0 bar propyne. Calculated by GC analysis,
c
average of three runs. Determined by GC analysis and 1H NMR.
d
reduction or formation of phosphine oxide. Fortunately, the
desired secondary phosphine 6 was successfully prepared by
careful hydrolysis of the corresponding magnesium phosphide
which was readily generated in situ from 2-pyridylphenylchloro-
by ICP-OES, confirming that leaching was negligible. In addition,
the filtered solutions showed no activity indicating that any
palladium in solution was either very low in concentration or was
in an inactive form.
phosphine and magnesium.
A palladium-catalysed cross
The high activity and selectivity associated with alkoxycarbony-
lation of alkynes has been attributed to an active species containing
two molecules of 2-pyPPh2, one of which functions as a bidentate
P,N-ligand and is involved in the selectivity determining step while
the other coordinates in a monodentate manner and facilitates
proton transfer by acting as a cocatalyst in the protonolysis step of
the catalytic cycle.2,10 In this regard, the performance of these
polymer supported catalysts was rather surprising and suggests
that copolymers 4, 5 and 10 have sufficient flexibility to form 2 : 1
complexes at low palladium loadings. However, at this stage we
cannot exclude the possibility that the copolymer precludes
coordination of two ligands to a single metal centre and that the
active species is actually mononuclear and coordinated by a single
ligand. Further studies are clearly required to establish the
distribution of 2-pyPPh2 ligands in these polymers and the
composition of the active species. Accepting rapid protonolysis
by a proximate pyridyl group to be an integral part of the
palladium catalysed alkoxycarbonylation, we reasoned that the
introduction of a pyridyl substituent on to the polymer backbone
would manifest itself in an increase in catalyst activity.
Polyvinylpyridine supported 2-pyPPh2 11 was prepared by
copolymerisation of 4-vinylpyridine with 3 in the presence of
AIBN. Contrary to expectation, replacement of the polystyrene
backbone with polyvinylpyridine resulted in a dramatic decrease in
activity from 273 to 20 molproduct molPd21 h21, even in the
coupling between 4-iodophenol and 6 was used to introduce
the 4-hydroxyphenyl substituent in 7, which was protected as
its borane adduct and reacted with methacryloyl chloride to
afford monomer 9, as a white crystalline air-stable solid.
Methacrylate based homopolymer 10 was prepared by
copolymerisation of 9 with methyl methacrylate in benzene
using AIBN as radical initiator, isolated as an off-white powder
by repeated precipitation with methanol and subsequently
deprotected with DABCO in dichloromethane. The 2-pyridyl-
diphenylphosphine loading in 10 was determined to be
1.42 mmol g21 from elemental analysis and the polydispersity
of 3.8 was measured by gel permeation chromatography.
Catalysts formed by combining polymers 4, 5 and 10 with
palladium acetate were first tested in the methoxycarbonylation of
phenylacetylene, the results of which are listed in Table 1. Since the
effect on catalyst performance of the amount of acid and ligand
has been well documented9 investigations were carried out to
establish the optimal reaction conditions. The best performance in
terms of activity and selectivity was most consistently obtained
with L : Pd and acid : Pd mole ratios of 10 and 30, respectively, in
agreement with the previous studies of Drent.2 Initial studies
demonstrated that introduction of a 4-vinyl substituent into
2-pyPPh2 did not effect catalyst performance since catalysts
formed from 2-pyPPh2 and 3 were both highly active and
selective for production of the branched acryclic ester, methyl
2-phenylpropenoate, I (entries 1 & 2). The performance of
supported catalysts generated from 4, 5 and 10 and palladium
acetate were next investigated and each was found to be highly
efficient, giving activities and selectivities approaching those of
their homogeneous counterpart (entries 3–5). For each of the
polymers, analysis of the reaction mixture after filtration
revealed that the palladium content was too low to be detected
presence of
a large excess of acid. At this stage we
tentatively attribute this decrease in activty to coordination of
the polyvinylpyridine matrix to the metal rather than to diffusional
limitations or intrusion of the polymer matrix into the coordina-
tion sphere of the metal.11 Interestingly, Drent has reported a
progressive decrease in catalyst activity with increasing number of
pyridyl substituents attached to the phosphine while the number of
2-pyridyl groups had negligible effect on the regioselectivity.2 In
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this regard, the increase in selectivity to >99.5% with polyvinylpyri-
dine supported 2-pyPPh2 is remarkable (entry 6). An increase in
regioselectivity to >99.95% has previously been achieved by methyl
substitution at the 6-position of the 2-pyridyl ring. Since
regioselectivity appears to be controlled by steric factors it is
tempting to attribute the increase in selectivity associated with
polyvinylpyridine supported 11 to a change in the polymer
microstructure, possibly as a result of the different swelling
capacities in methanol.
selectivity to >99.5%, which parallels that observed in the
methoxycarbonylation of phenylacetylene.
In conclusion, the first insoluble polymer supported 2-pyPPh2
ligands have been prepared and shown to form highly active and
selective catalysts for the alkoxycarbonylation of terminal alkynes.
In the case of polyvinylpyridine supported 2-pyPPh2, very high
selectivities were obtained but at the expense of activity. Although
the performance of the polystyrene and polymethacrylate based
catalysts did not exceed that of the corresponding homogeneous
system, in several cases it was very similar and there was no
evidence of metal leaching, which renders these immobilized
catalysts ideal candidates for multiple recycling in a range of
platinum group metal catalysed transformations. Ultimately, we
aim to identify robust processible catalysts that can either be
extruded or coated onto high surface area substrates for use in a
continuous process. Currently, studies are underway to (i)
investigate the effect of pyridine ring substitution and basicity on
catalyst performance, (ii) expand the range of substrates and
reaction types available to these supported 2-pyPPh2 ligands, (iii)
determine the nature and composition of the active species and (iv)
systematically modify the co-monomer and the tether to delineate
the factors that influence catalyst activity and selectivity.
In an attempt to glean further information about the polymer
composition–catalyst activity/selectivity relationship the perfor-
mance of polystyrene 12 and polyvinylpyridine 13 supported
triphenylphosphine based catalysts was investigated and both were
markedly less active than their polystyrene 4 and polyvinylpyridine
11 supported 2-pyPPh2 counterparts (entries 8 and 9). Moreover,
while the selectivity of 80.3% achieved with polystyrene supported
triphenylphosphine is similar to that of 81.8% obtained under
homogeneous conditions (entry 7), its polyvinylpyridine supported
counterpart 13 gave a selectivity >99.5%. This level of selectivity is
particularly exceptional for a triphenylphosphine based catalyst
and is similar to that obtained with polyvinylpyridine based
2-pyPPh2 11, described above. Thus, while the combination of
PPh3 and a pyridine backbone in 13 does not give high activity it
gives a selectivity comparable to that obtained with 2-pyPPh2
based catalyst systems.
We thank the Institute of applied catalysis (iAc) for a bursary to
MB, the University of Newcastle upon Tyne for financial support
and Johnson Matthey for generous loans of palladium salts.
Notes and references
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2 E. Drent, P. Arnold and P. H. M. Budzelaar, J. Organomet. Chem.,
1994, 475, 57; E. Drent, P. Arnold and P. H. M. Budzelaar,
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with organometallic compounds, ed. B. Cornils and W. A. Hermann,
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3 Kirk Othmer, Encyclopedia of Chemical Technology, Wiley, New York,
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Interest in the selective production of methyl methacrylate
prompted us to examine the efficiency of the same catalyst systems
in the methoxycarbonylation of propyne, the results of which are
listed in Table 1 (entries 11–17). Under standard conditions, the
performance of
a catalyst mixture generated with 3 was
qualitatively similar to that obtained with 2-pyPPh2. In stark
contrast to phenylacetylene, the performance of catalysts generated
from polymers 4, 5 and 10 in the carbonylation of propyne showed
a marked dependence on the co-monomer and the tether. For
instance, the activty of 1022 molproduct molPd21 h21 obtained
with methacrylate-based homopolymer 10 is comparable to that of
the homogeneous 2-pyPPh2 system while the activities obtained
with methacrylate-styrene copolymer 5 and the styrene homo-
polymer 4 are significantly lower, which is likely to be due to better
diffusion of the reagents in the more polar polymeric network of 10.
The performance of polyvinylpyridine supported 2-pyPPh2 13 and
triphenylphosphine 11 were also examined under the same con-
9 A. Scrivanti, V. Beghetto, M. Zanato and U. Matteoli, J. Mol. Catal. A:
Chem., 2000, 160, 331; A. Scrivanti, V. Beghetto, E. Campagna and
U. Matteoli, J. Mol. Catal. A: Chem., 2001, 168, 75.
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,
10 A. Scrivanti, V. Beghetto, E. Campagna, M. Zanato and U. Matteoli,
Organometallics, 1998, 17, 630; A. Dervisi, P. G. Edwards,
P. D. Newman, R. P. Tooze, S. J. Coles and M. B. Hursthouse,
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respectively, correspond to a 20–50 fold decrease in activity
compared with that of 1171 molproduct molPd21 h21 for
2-pyPPh2 under homogeneous conditions. However, catalyst
systems based on 11 and 13 both gave a marked increase in
11 R. A. Taylor, B. P. Santora and M. R. Gagne´, Org. Lett., 2000, 2, 1781.
90 | Chem. Commun., 2006, 88–90
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