Semi-hydrogenation of alkynes using phosphinated polymer incarcerated (PI) palladium catalysts

Article abstract of DOI:10.1039/b517181d

Phosphinated polymer incarcerated (PI) palladium catalysts prepared from a palladium(0) complex and phosphinated styrene-based polymers were found to show good selectivity in semi-hydrogenation of alkynes without strict control of H₂ consumption. Moreover, these catalysts could be removed by simple filtration and without any leaching of Pd. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b517181d

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Semi-hydrogenation of alkynes using phosphinated polymer incarcerated
(PI) palladium catalysts
Ryo Nishio, Masaharu Sugiura and Shu¯ Kobayashi*
Received 5th December 2005, Accepted 20th January 2006
First published as an Advance Article on the web 6th February 2006
DOI: 10.1039/b517181d
Phosphinated polymer incarcerated (PI) palladium catalysts
prepared from a palladium(0) complex and phosphinated
styrene-based polymers were found to show good selectivity
in semi-hydrogenation of alkynes without strict control of H2
consumption. Moreover, these catalysts could be removed by
simple filtration and without any leaching of Pd.
Semi-hydrogenation1 of alkynes to (Z)-alkenes is an important
process not only in the laboratory but also in industry. However,
the control of the selectivity between semi-hydrogenation and
over-hydrogenation is a difficult issue. The Lindlar catalyst [Pd
on CaCO3 poisoned by Pb(OAc)2, PbSO4 or BaSO4] is prob-
ably the most widely applied heterogeneous catalyst2 for this
transformation, although other heterogeneous3 and homogeneous
catalysts4,5 have been developed. In several cases, very high semi-
Fig. 1 Structures of polymer supports.
hydrogenation selectivity has been achieved (>99%) by controlling
the H2 gas consumption.5a However, higher chemoselectivity,
reproducibility, generality and recyclability of the catalysts are
still desired.
Recently, we reported the immobilization of palladium clus-
ters into phosphinated polystyrene-based copolymers using the
polymer incarcerated (PI) method.6,7 These heterogeneous phos-
phinated PI Pd catalysts are highly active for carbon–carbon
bond-forming reactions such as the Suzuki–Miyaura coupling
even without the addition of any external phosphine ligands.6f
Moreover, these catalysts may be recovered quantitatively by
simple filtration and reused several times without loss of activity.
The phosphine moiety of the polymer supports is assumed to
Scheme 1 Preparation of phosphinated PI Pd catalysts.
have two main roles: suppression of the leaching of palladium,
and enhancement of the catalytic activity as ligands. We also
phosphine oxides on the copolymer were completely reduced to the
considered the possibility that the phosphines in the polymer
corresponding phosphines.11 Transmission electron microscopic
support might act as a poisoning agent of the metal8 realizing
(TEM) analysis showed that small Pd clusters were dispersed on
both chemoselective hydrogenation and suppression of metal
the polymer support without the formation of large Pd clusters
leaching. In this paper, we describe the chemoselective semi-
and that the size of the Pd clusters was smaller than 1.5 nm.
hydrogenation of alkynes using phosphinated PI Pd catalysts
For the purposes of comparison, non-phosphinated PI Pd 2c
and phosphine oxide-containing PI Pd 2d were prepared from
copolymers 1c and 1d without HSiCl3 reduction, respectively.
prepared from polymer supports possessing diphenylphosphino
groups as potential poisoning agents of palladium.
First, phosphinated PI Pd 2a and 2b were prepared from
We then applied PI Pd 2a–2d to the hydrogenation of dipheny-
copolymers 1a and 1b (Fig. 1) in the presence of Pd(PPh3)4
lacetylene (Table 1). In the cases of PI Pd 2c and 2d having
according to the PI method (Scheme 1). Since some phosphines
no phosphine, fast hydrogenations were observed and saturated
were oxidized to the corresponding phosphine oxides during
hydrogenated products were obtained quantitatively (entries 3
the preparation of the catalyst, the catalysts were treated with
and 4). On the other hand, phosphinated PI Pd 2a and 2b gave
semi-hydrogenated products 4 selectively under the same reaction
conditions without the need for control of H2 gas consumption
HSiCl3/Et3N9 after cross-linking to give phosphinated PI Pd 2a
and 2b. It was established by 31P SR-MAS NMR analysis10 that the
and the addition of external poisoning agents (entries 1 and 2).
Graduate School of Pharmaceutical Sciences, The University of Tokyo, The
Leaching of the palladium was examined by fluorescence X-ray
HFRE Division, ERATO, Japan Science and Technology Agency (JST),
(XRF) analysis after removal of the catalyst, and no leaching
was detected in all cases. These results indicate that phosphine
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: skobayas@mol.f.
u-tokyo.ac.jp; Fax: +81 3 5684 0634; Tel: +81 3 5841 4790
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Table 1 Hydrogenation of diphenylacetylene using phosphinated PI Pd 2a–2d
Yield (%)b
Entry
Catalyst (P/Pd)a
Reaction time/h
Recovery of 3 (%)
cis-4
trans-4
5
Pd leaching (%)c
1
2
3
4
PI Pd 2a (2.1)
PI Pd 2b (1.8)
PI Pd 2c (0)
1
1
1
1
0
14
0
59
69
0
5
6
0
0
32
17
100
100
nd
nd
nd
nd
PI Pd 2d (2.9)
0
0
a The ratio of the diphenylphosphino groups in the polymer to the Pd atoms. b Yield was determined by 1H NMR analysis using durene as an internal
standard. c Determined by XRF analysis, nd = not detected (<0.94%).
moieties can act as moderate poisoning agents of Pd playing the
same role as PbSO4 or BaSO4 in the Lindlar catalyst systems.
In general, the poisoning of palladium by phosphines is too
strong because phosphorus atoms of phosphines coordinate to
palladium atoms strongly, and adsorption of hydrogen molecules
and substrates is suppressed by the steric bulkiness and electronic
effect of phosphines.8a Therefore, hydrogenation doesn’t proceed
in the presence of phosphines in homogenous catalytic systems.
In the case of phosphinated PI Pd catalytic systems, however,
diphenylphosphino groups in the polymer support might act as
‘weak’ poisoning agents because coordination of the phosphines
to the palladium atoms is restricted by the steric bulkiness of the
polymer supports.
In order to confirm the effect of phosphines as poisoning agents,
a combination of non-phosphinated PI Pd 2c and externally
Fig. 2 H2 consumption in the hydrogenation of diphenylacetylene using
added PPh3 was tested (Scheme 2). Diphenylacetylene (3) was
hydrogenated to give cis- and trans-stilbene (4) in good yields with
high selectivity, while the hydrogenation of cis-stilbene (4) was
almost completely suppressed. These results indicate that PPh3,
which should be a very strong poisoning agent in homogeneous
Pd systems, acted as a ‘weak’ poisoning agent within the sterically
restricted polymer supports. However, 2.8% of palladium was
leached from the polymer support into the solvent by the addition
of PPh3. Therefore, it is better to introduce phosphine moieties on
polymer supports like PI Pd 2a or 2b rather than to add external
phosphines.
phosphinated PI Pd 2a vs. non-phosphinated PI Pd 2c.
was converted rapidly and that the proportion of cis-stilbene
reached the maximum around after 60 min (Fig. 3). Although
cis-stilbene was slowly over-hydrogenated to 1,2-diphenylethane,
there was little appreciable change in the product distribution
after 150 min. These results indicate that the over-hydrogenation
step is much slower than the first semi-hydrogenation step,
because the catalytic activity is suppressed by the ‘weak’ co-
ordination of phosphines to the Pd clusters in the polymer
support.
Scheme 2 Effect of PPh3 on the catalysis of PI Pd 2c.
Monitoring the H2 consumption showed that the consumption
rate with phosphinated PI Pd 2a was slower than that with non-
phosphinated PI Pd 2c, and became even slower after about 1.5
equivalents of H2 were consumed (Fig. 2). The reaction profile of
the product distribution also demonstrated that diphenylacetylene
Fig. 3 Reaction profile of the hydrogenation of diphenylacetylene using
PI Pd 2a.
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Table 2 Phosphinated PI Pd-catalyzed semi-hydrogenation of several types of alkynes
Yield (%)a
Entry
6
Catalyst
PI Pd 2a
PI Pd 2a
PI Pd 2a
Reaction time/min
Recovery of 6 (%)
cis-7
trans-7
7
8
Pd leaching (%)b
1
2
3
60
30
30
7
5
0
58
75
82
22
13
15
nd
nd
nd
Trace
4
PI Pd 2c
30
0
90
10
nd
5
6
PI Pd 2a
PI Pd 2a
20
20
20
11
74
74
Trace
Trace
Trace
9
nd
nd
a Yield was determined by 1H NMR analysis using durene as an internal standard. b Determined by XRF analysis, nd = not detected (<0.94%).
For comparison, the Lindlar catalyst (Pd–CaCO3 poisoned with
Pb)12 was tested under the same reaction conditions. Although
the Lindlar catalyst showed a higher selectivity of the semi-
hydrogenation of diphenylacetylene than the phosphinated PI Pd
catalyst (Fig. 4), over-hydrogenation to 1,2-diphenylethane was
remarkably observed after all conversion of the alkyne. Therefore,
in the case of the Lindlar catalyst, exact control of H2 consumption
and/or other additives such as quinoline is necessary to obtain
semi-hydrogenated products with high selectivity.
strict control of H2 consumption. These heterogeneous catalysts
can be prepared by simple methods and recovered by simple
filtration. It is believed that the phosphine moieties act as
appropriately weak poisoning agents due to the steric effect of
the polymer supports. Further studies will focus on the effect of
the phosphinated polymer supports on the catalytic activity of the
metals.
This work was partially supported by Grant-in-Aid for Scientific
Research from the Japan Society of the Promotion of Sciences
(JSPS).
Notes and references
1 For a review of hydrogenation, see: R. L. Burwell, Jr., Chem. Rev.,
1957, 57, 895; E. N. Marvell and T. Li, Synthesis, 1973, 457; R. A.
Sanchez-Delgado and M. Rosales, Coord. Chem. Rev., 2000, 249.
2 H. Lindlar, Helv. Chim. Acta, 1952, 35, 446; H. Lindlar and R. Dubuis,
Org. Synth., 1966, 46, 89; R. O. Hutchins and M. G. K. Hutchins, in
Reduction of Triple-bonded Groups, The Chemistry of Functional Groups,
S. Patai and Z. Rappoport, eds., John Wiley & Sons Ltd., New York,
1983, vol. 1, p. 571.
3 C. Bianchini, C. Bohanna, M. A. Esteruelas, P. Frediani, A. Meli, L. A.
Oro and M. Peruzzini, Organometallics, 1992, 11, 3837; J. Navarro, M.
Sagi, E. Sola, F. J. Lahoz, I. T. Dobrinovitch, A. Katho, F. Joo and
L. A. Oro, Adv. Synth. Catal., 2003, 345, 280; M. Sodeoka and M.
Shibasaki, J. Org. Chem., 1985, 50, 1147; M. Stephan, O. Kohlmann,
H. G. Niessen, A. Eichhorn and J. Bargon, Magn. Reson. Chem., 2002,
40, 157; C. Bianchini, A. Meli, M. Peruzzini, P. Frediani, C. Bohanna,
M. A. Esteruelas and L. A. Oro, Organometallics, 1992, 11, 138.
4 (a) N. L. Holy and S. R. Shelton, Tetrahedron, 1981, 37, 25; (b) B.
Elman and C. Moberg, J. Organomet. Chem., 1985, 294, 117; (c) C.
Moberg and L. Rakos, J. Organomet. Chem., 1987, 335, 125; (d) B. M.
Choudary, G. V. M. Sharma and P. Bharathi, Angew. Chem., 1989, 101,
506; (e) C. Ferrari, G. Predieri, A. Tiripicchio and M. Costa, Chem.
Mater., 1992, 4, 243; (f) M. Islam, A. Bose, D. Mal and C. R. Saha,
J. Chem. Res. (S), 1998, 44; (g) Z. M. Michalska, B. Ostaszewski, J.
Zientarska and J. W. Sobczak, J. Mol. Catal. A: Chem., 1998, 129, 207;
(h) G. R. Cairns, R. J. Cross and D. Stirling, J. Mol. Catal. A: Chem.,
2001, 172, 207; (i) A. M. Kluwer, T. S. Koblenz, T. Jonischkeit, K.
Woelk and C. J. Elsevier, J. Am. Chem. Soc., 2005, 127, 15 470.
5 (a) G. V. M. Sharma, B. M. Choudary, M. R. Sarma and K. K. Rao,
Fig. 4 Reaction profile of the hydrogenation of diphenylacetylene using
Pd–CaCO3 poisoned with Pb.
We applied phosphinated PI Pd 2a and 2b to the semi-
hydrogenation of several types of alkynes (Table 2). Alkynes hav-
ing aromatic, aliphatic, hydroxy, and/or alkoxycarbonyl groups
provided the corresponding semi-hydrogenated products in good
yields without any leaching of palladium. In these hydrogenations,
small amounts of trans-alkene products were obtained.4i,13 We
confirmed that these trans-products were formed by the Pd-
catalyzed isomerization of cis-products.
In summary, we have demonstrated that phosphinated PI Pd
served as an effective catalyst for the selective semi-hydrogenation
of alkynes without the addition of poisoning agents and without
´
J. Org. Chem., 1989, 54, 2998; (b) A. Mastalir, Z. Kira´ly and F. Berger,
994 | Org. Biomol. Chem., 2006, 4, 992–995
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
´
Kobayashi, Angew. Chem., Int. Ed., 2001, 40, 3469; R. Akiyama and S.
Kobayashi, Angew. Chem., Int. Ed., 2002, 41, 2602; S. Kobayashi and
R. Akiyama, Chem. Commun., 2003, 449.
Appl. Catal., A, 2004, 269, 161; (c) A. Mastalir, Z. Kira´ly, G. Szo¨llsi
and M. Barto´k, J. Catal., 2000, 194, 146; (d) B. M. Choudary, K. R.
Kumar and M. L. Kantam, J. Catal., 1991, 130, 41.
8 (a) C. A. Tolman, Chem. Rev., 1977, 77, 313; (b) R. Mason and D. W.
Meek, Angew. Chem., Int. Ed. Engl., 1978, 17, 183; (c) G. Carturan, R.
Dalla Pozza and G. Facchin, J. Mol. Catal., 1983, 22, 257; (d) S. Siegel
and J. A. Hawkins, J. Org. Chem., 1986, 51, 1638; (e) S. Sgorlon, F.
Pinna and G. Strukul, J. Mol. Catal., 1987, 40, 211.
9 W. Chem and J. Xiao, Tetrahedron Lett., 2000, 41, 3697; D. E.
Bergbreiter and C. Li, Org. Lett., 2003, 5, 2445.
6 For polymer incarcerated catalysts, see: (a) R. Akiyama and S.
Kobayashi, J. Am. Chem. Soc., 2003, 125, 3412; (b) K. Okamoto,
R. Akiyama and S. Kobayashi, J. Org. Chem., 2004, 69, 2871;
(c) K. Okamoto, R. Akiyama and S. Kobayashi, Org. Lett., 2004,
6, 1987; (d) K. Okamoto, R. Akiyama, H. Yoshida, T. Yoshida and
S. Kobayashi, J. Am. Chem. Soc., 2005, 127, 2125; (e) S. Kobayashi,
H. Miyamura, R. Akiyama and T. Ishida, J. Am. Chem. Soc.,
2005, 127, 9251; (f) R. Nishio, M. Sugiura and S. Kobayashi, Org.
Lett., 2005, 7, 4831; (g) H. Miyamura, R. Akiyama, T. Ishida, R.
Matsubara, M. Takeuchi and S. Kobayashi, Tetrahedron, 2005, 61,
12 177.
7 For microencapsulated catalysts, see: S. Kobayashi and S. Nagayama,
J. Am. Chem. Soc., 1998, 120, 2985; S. Nagayama, M. Endo and S.
Kobayashi, J. Org. Chem., 1998, 63, 6094; S. Kobayashi, M. Endo and
S. Nagayama, J. Am. Chem. Soc., 1999, 121, 11 229; S. Kobayashi, T.
Ishida and R. Akiyama, Org. Lett., 2001, 3, 2649; R. Akiyama and S.
10 S. Kobayashi, R. Akiyama, T. Furuta and M. Moriwaki, Molecules
Online, 1998, 2, 35.
11 Note: 31P SR-MAS NMR analysis of PI Pd 2a: d 29.6 ppm (before
reduction by HSiCl3), d −5.0 ppm (after the treatment of HSiCl3).
12 Note: Pd–CaCO3/Pb (Lindlar catalyst) was purchased from Aldrich
(catalog no. 20,573-7).
13 D. H. Goldsworthy, F. R. Hartley and S. G. Murray, J. Mol. Catal.,
1983, 19, 257; J. Yu and J. B. Spencer, Chem. Commun., 1998, 1103;
J. C. A. A. Roelofs and P. H. Berben, Chem. Commun., 2004, 970.
This journal is
The Royal Society of Chemistry 2006
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