Palladium on pumice: New catalysts for the stereoselective semihydrogenation of alkynes to (Z)-alkenes

Article abstract of DOI:10.1016/S0040-4039(01)00065-X

High selectivities (93-99%) and excellent stereoselectivities (>99%) in the semihydrogenation of C-C triple bonds were achieved using palladium on pumice with a metal loading of 0.5, 1.5 or 3.0% wt as catalyst. The reactions were carried out in ethanol or tetrahydrofuran with only 2.5% of ethylenediamine allowing a self-terminating semihydrogenation independently on the C-C triple bond.

Full text of DOI:10.1016/S0040-4039(01)00065-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2015–2017
Palladium on pumice: new catalysts for the stereoselective
semihydrogenation of alkynes to (Z)-alkenes
a,
b
a
b,c
Michelangelo Gruttadauria, * Leonarda F. Liotta, Renato Noto and Giulio Deganello
a
Dipartimento di Chimica Organica ‘E. Patern o` ’, Viale delle Scienze, Parco d’Orleans II, 90128 Palermo, Italy
Istituto di Chimica e Tecnologia dei Prodotti Naturali (ICTPN-CNR), via Ugo La Malfa 153, 90146 Palermo, Italy
b
c
Dipartimento di Chimica Inorganica, Viale delle Scienze, Parco d’Orleans II, 90128 Palermo, Italy
Received 2 November 2000; revised 14 December 2000; accepted 10 January 2001
Abstract—High selectivities (93–99%) and excellent stereoselectivities (>99%) in the semihydrogenation of CꢀC triple bonds were
achieved using palladium on pumice with a metal loading of 0.5, 1.5 or 3.0% wt as catalyst. The reactions were carried out in
ethanol or tetrahydrofuran with only 2.5% of ethylenediamine allowing a self-terminating semihydrogenation independently on
the CꢀC triple bond. © 2001 Elsevier Science Ltd. All rights reserved.
Catalytic semihydrogenation of alkynes to (Z)-alkenes
tion of the CꢀC triple bond using other two new
Pd/pumice catalysts with a different metal loading (1.5,
0.5%).
constitutes a very important process in organic chem-
1
,2
13
istry. Indeed, the production of highly pure (Z)-alke-
nes is often a key-step during the synthesis of important
3
substances such as pheromones or other natural prod-
Preliminary investigations were carried out with com-
pound 1a, by using absolute ethanol or tetrahydrofuran
as solvent and checking how the other parameters such
as metal loading, hydrogen flow, molar ratio substrate/
Pd, stirring rate, nitrogen base (pyridine, triethylamine,
or ethylenediamine in 2.5–20% with respect to the
substrate) may influence the selectivity (Scheme 1). The
best results, without base, were obtained when we used
4
ucts. Among the catalysts reported as being efficient in
performing selective semihydrogenations of alkynes, the5
most common is the well-known Lindlar palladium
6
(
used in the presence of quinoline) and P-2 nickel (used
in the presence of ethylenediamine). In addition to
these, new catalysts have been developed such as palla-
7
dium foil, interlamellar montmorillonite–diphenyl-
8
9
14
phosphinepalladium(II) complexes, Pdc (used in the
presence of quinoline) and, as recently reported, Pd or
a hydrogen flow of 40 N mL/min, a stirring rate of
15
1300 rpm and a molar ratio substrate/Pd equal to
1
0
16
nickel boride on borohydride exchange resin and Pd
110. However, these results were not satisfactory
1
1
12
17
colloidal catalysts. In a previous communication we
have reported several examples of efficient semihydro-
genation of the CꢀC triple bond using palladium on
pumice with a metal loading of 3.0% wt as catalyst.
Here we report new examples of the semihydrogena-
because the selectivity was not excellent (ca. 80%),
whereas the stereoselectivity was found to be high
(93–96%). In EtOH good results were obtained with
10% of ethylenediamine (eda) using Pd/pumice 0.5%
(Table 1, entry 1).
Scheme 1.
Keywords: alkenes; alkynes; heterogenous catalysis; hydrogenation; palladium.
*
Corresponding author. Fax: +39 091596825; e-mail: organica@unipa.it
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00065-X

2
016
M. Gruttadauria et al. / Tetrahedron Letters 42 (2001) 2015–2017
Table 1. Reaction times and selectivities for the hydrogenation of 1a with Pd/pumice
Entry
Cat. (% Pd)a
Base (%)
Solvent
t (min)
2 (%)
3 (%)
4 (%)
1
2
3
4
5
6
0.5
1.5
3.0
0.5
0.5
0.5
Eda 10
Eda 10
Eda 10
Eda 10
Eda 2.5
Py 2.5
EtOH
EtOH
EtOH
THF
THF
THF
12
200
200
80
60
15
96
96
0
99
99
62
0
0
0
0
0
1
4
4
0
1
1
37
a
(% Pd) refers to different metal loading.
Pd/pumice (3%) did not catalyze the reaction (Table 1,
entry 3). The different behavior of the three catalysts
entry 1). With tetrahydrofuran as solvent the reaction
was too slow. Hydrogenation of compound 1c was
stopped at the semihydrogenation stage when we used
Pd/pumice (1.5%) in tetrahydrofuran (Table 2, entry 2).
(
Table 1, entries 1–3) is, in our opinion, ascribed to the
different dispersion of the metal particles on the sur-
face. Palladium/pumice with the highest metal content
18
(
3.0%), shows a very low dispersion (Dx=8%) as a
The addition of a larger amount of base did not
improve the selectivity (Table 2, entry 3). Using the less
active Pd/pumice (3.0%) with 2.5% ethylenediamine we
also obtained a good selectivity (Table 2, entry 4).
Compounds 1d–g were excellently hydrogenated with
Pd/pumice (0.5%) in tetrahydrofuran with 2.5% base
(Table 2, entries 5–8). The results obtained in the
semihydrogenation of alkynes with Pd/pumice catalysts
are comparable and in some cases better than those
2
12
consequence of the low specific surface area (5 m /g)
of pumice that determines the growth of large metal
particles, producing a few neighboring catalytic sites.
Using the latter catalyst the nitrogen base will cover all
the catalytic sites blocking the adsorption of the alkyne.
Palladium/pumice (1.5 and 0.5%) have a higher disper-
1
8
sion (25 and 40%, respectively) and so nitrogen base is
not able to cover all the catalytic sites and competes
only with the re-adsorption of the alkene, blocking or
displacing it. In tetrahydrofuran excellent results were
obtained when we added 10% ethylenediamine (Table
7a,8,9,19
reported in the literature.
Moreover, they are
quite interesting since by an appropriate choice of the
metal dispersion in the catalyst, solvent and nitrogen
base added, high selectivities (93–99%) and stereoselec-
tivities (>99%) can be achieved. The best concentration
of ethylenediamine is 2.5% for any substrate. When the
substrate is less reactive, such as 1b, the most dispersed
metal catalyst, Pd/pumice (0.5%), in ethanol gives the
best results. For slightly more reactive substrates
change of the solvent is sufficient to give good selectiv-
ity (1a, 1d–g). When reactive substrate like 1c are used,
the less active Pd/pumice (1.5 or 3.0%) catalysts in tet-
rahydrofuran are preferred to achieve good selectivity.
1
, entry 4). The reaction was slower than those carried
out in ethanol, probably because in the less polar
solvent (THF) the nitrogen base competes more
efficiently for the catalytic sites. In order to achieve
high selectivity with shorter reaction times we added
only 2.5% of base (Table 1, entry 5). We were delighted
by the excellent results. It must be noted that hydro-
genation reaction with Pd/pumice (0.5%) in tetra-
hydrofuran with 2.5% pyridine was not selective and it
could not be stopped at the semihydrogenation stage
(
Table 1, entry 6). The different behavior between
5
,6,9
pyridine and ethylenediamine is probably due to the
presence of two nitrogen atoms that interact more
efficiently with the catalytic sites.
In contrast with several literature reports
where the
nitrogen base is used in large amounts, in the hydro-
genation of CꢀC triple bonds on the Pd/pumice cata-
lysts only a catalytic amount of base is necessary,
allowing a self-terminating semihydrogenation. Finally
Pd on pumice catalysts, differently from other cata-
lysts, do not need any reduction pretreatment, due to
their stability to oxidation.
On the basis of these results we then carried out the
hydrogenation reactions on substrate 1b–g (Table 2).
Diphenylacetylene (1b) was selectively and stereoselec-
tively hydrogenated in ethanol with 2.5% base (Table 2,
8,9
Table 2. Reaction times and selectivities for the hydrogenation of 1b–g with Pd/pumice
Entry
Cat. (%Pd)a
Solvent
Base (eda%)
t (min)
2 (%)
3 (%)
4 (%)
1
2
3
4
5
6
7
8
1b
1c
1c
1c
1d
1e
1f
0.5
1.5
1.5
3.0
0.5
0.5
0.5
0.5
EtOH
THF
THF
THF
THF
THF
THF
THF
2.5
2.5
10
2.5
2.5
2.5
2.5
2.5
90
30
30
50
100
50
93
94
93
93
98
99
98
98
0
0
1
0
0
0
0
0
7
6
6
7
2
1
2
2
30
30
1g
a
(% Pd) refers to different metal loading.

M. Gruttadauria et al. / Tetrahedron Letters 42 (2001) 2015–2017
2017
Acknowledgements
treated at −10°C with a pentane solution of [Pd(C H ) ]
3
5 2
of known concentration, under nitrogen. After 30 min the
suspension turned yellow–brown as a result of the
anchoring reaction. The mixture was then stirred at 0°C
for 1 h. Reduction to metallic Pd was obtained using
hydrogen (10%)/argon at low temperature (from −20 to
This investigation has been supported by the University
of Palermo (funds for selected research topics) and by
progetto finalizzato MSTA II-CNR (Roma).
0°C within 30 min, then from 0 to 25°C within 60 min,
then 25°C for 120 min). Compounds 1 (0.40 mmol) in
absolute ethanol or tetrahydrofuran (20 mL) were stirred
with Pd/pumice catalyst (molar ratio substrate/Pd=110)
for the time indicated in the tables. Hydrogenation reac-
tions were performed in a three-necked glass reactor with
jacket. The reactor was connected to a line operating at a
constant atmospheric pressure of H2 by using the first
two necks. The last neck, closed by a silicone septum,
allowed withdrawing of samples for GC-MS analysis.
The suspension was magnetically stirred. All the products
were identified and quantified by comparison with known
samples.
References
1
. Gutmann, H.; Lindlar, H. In Chemistry of Acetylenes;
Viehe, H. G., Ed.; Marcel Dekker: New York, 1969.
. Rylander, P. N. Hydrogenation Methods; Academic
Press: London, 1985.
. Henrick, C. A. Tetrahedron 1977, 33, 1845.
. Mori, K. In Total Synthesis of Natural Products; ApSi-
mon, J., Ed.; Wiley-Interscience: New York, 1981; Vol. 4,
pp. 9–31.
. Lindlar, H.; Dubuis, R. Org. Synth. 1966, 46, 89.
. (a) Brown, C. A.; Ahuja, V. K. J. Chem. Soc., Chem.
Commun. 1973, 553; (b) Brown, C. A.; Ahuja, V. K. J.
Org. Chem. 1973, 38, 2226.
2
3
4
5
6
1
1
1
4. We performed hydrogenations using an hydrogen flow in
the range 40–200 N mL/min.
5. At 1300 rpm the reactions were found to be under kinetic
control.
6. We also performed the hydrogenation with a molar ratio
substrate/Pd equal to 55 or 220, in the former case the
reactions cannot be stopped at the semihydrogenation
stage while in the latter case the reactions were very slow.
7. Marwell, E. N.; Li, T. Synthesis 1973, 457.
7
. (a) Ulan, J. G.; Maier, W. F.; Smith, D. A. J. Org. Chem.
1
987, 52, 3132; (b) McEwen, A. B.; Guttieri, M. J.;
Maier, W. F.; Laine, R. M.; Shvo, Y. J. Org. Chem.
983, 48, 4436.
. Sharma, G. V. M.; Choudary, B. M.; Ravichandra
Sarma, M.; Koteswara Rao, K. J. Org. Chem. 1989, 54,
1
8
9
1
1
2
998.
8. The metal dispersion (Dx%), i.e. the percentage of metal-
. Brunet, J.-J.; Caubere, P. J. Org. Chem. 1984, 49, 4058.
lic atoms exposed, was calculated by means of the mean
,
crystallite sizes D (A) of Pd estimated by using the
Scherrer equation; (Klug, H. P.; Alexander, L. E. X-ray
Diffraction Procedures for Polycrystalline and Amorphous
Materials; Wiley: New York, 1954).
1
0. (a) Yoon, N. M.; Park, K. B.; Lee, H. J.; Choi, J.
Tetrahedron Lett. 1996, 37, 8527; (b) Choi, J.; Yoon, N.
M. Tetrahedron Lett. 1996, 37, 1057.
1. Sulman, E.; Matveeva, V.; Usanov, A.; Kosivtsov, Yu.;
Demidenko, G.; Bronstein, L.; Chernyshov, D.; Valetsky,
P. J. Mol. Catal. 1999, 146, 265.
1
19. (a) Tamaru, Y.; Hojo, M.; Yoshida, Z. J. Org. Chem.
1991, 56, 1099; (b) Khai, B. T.; Arcelli, A. Chem. Ber.
1993, 126, 2265.
1
2. Gruttadauria, M.; Noto, R.; Deganello, G.; Liotta, L. F.
Tetrahedron Lett. 1999, 40, 2857.
1
3. The synthesis of pumice supported palladium catalysts
20. (a) Yermakov, Yu. I. Catal. Rev. 1986, 13, 77; (b)
Fagherazzi, G.; Benedetti, A.; Duca, D.; Martorana, A.;
Deganello, G. J. Catal. 1994, 150, 117.
20
was performed following Yermakov’s method. In a
typical experiment, a pumice suspension in pentane was
.
.