The cyclooligomerization of arylethynes in ionic liquids catalysed by ruthenium porphyrins: A case of real catalyst recycling

Article abstract of DOI:10.1039/b415462b

An efficient cyclooligomerization of arylethynes, catalysed by ruthenium(II) porphyrins in environmentally friendly ionic liquids, with an effective recycling of the catalyst and easy isolation of the products is described. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b415462b

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The cyclooligomerization of arylethynes in ionic liquids catalysed by
ruthenium porphyrins: a case of real catalyst recycling{
Valeria Conte,* Elfituri Elakkari, Barbara Floris, Valentina Mirruzzo and Pietro Tagliatesta*
Received (in Cambridge, UK) 5th October 2004, Accepted 17th January 2005
First published as an Advance Article on the web 27th January 2005
DOI: 10.1039/b415462b
The commercially available substrates used for this work were
ethynylbenzene, (7), and 1-ethynyl-4-methoxybenzene, (8), (see
Chart 2) which underwent the cyclooligomerization reaction in
ionic liquids in the presence of the catalyst, under a nitrogen
atmosphere at 150 uC, for 72 hours.{ All the reaction products
were previously isolated and characterized.7,21 After cooling, the
reaction products were extracted with diethyl ether, leaving the
catalyst in the ionic liquids ready for further reaction.
An efficient cyclooligomerization of arylethynes, catalysed by
ruthenium(II) porphyrins in environmentally friendly ionic
liquids, with an effective recycling of the catalyst and easy
isolation of the products is described.
To obtain sustainable synthetic procedures is one of the most
important goals that the chemists of the 21st century should
consider when an old process needs to be improved. Most reaction
methods have been recently modified, either by introducing
environmentally friendly solvents like water or alcohols or by
using catalysts with higher selectivities and yields, with the
potential to be recycled several times.
The GC analysis of the products, showed that the porphyrin
catalyst was able to give the 1,3,5 and 1,2,4-triarylbenzenes and the
1-arylsubstituted naphthalenes with selectivities depending on the
substrates. In Table 1, the results of the cyclooligomerization
reactions in ionic liquids and, for comparison, in 1,2-dichlor-
obenzene are reported.
The chance to have, in the same process, a suitable solvent and a
catalyst that can be reused several times without any loss of
activity, should be considered highly desirable. Ionic liquids (IL)
have, in fact, recently been presented as a new and promising
family of non-conventional solvents to be used in settling sus-
tainable procedures for known reactions.1–4 Their unique physical
properties encourage the use of such solvents in homogeneous or
biphasic catalysis, so that many reactions have been studied in
such systems and there is an increasing activity in this field of
research, testified by abundant recent literature on the topic.5,6
In a previous paper, we showed that it is possible to use
rhodium porphyrins as catalysts for the cyclotrimerization of
arylethynes7 and here we report the results obtained for the same
reaction catalysed by ruthenium porphyrins in ionic liquids.
Other authors reported on the cyclotrimerization of alkynes in
supercritical CO2 catalysed by palladium complexes8 but, to our
knowledge, there are no reports on the use of metalloporphyrin
catalysts in pure ionic liquids as reaction media, although there has
been a short communication on an oxidation in a two phase
system.9 The cyclotrimerization of terminal alkynes usually affords
a mixture of trisubstituted benzenes, 2 and 3, as the main
products10–15 but sometimes the reaction also gives the 1-aryl-
naphthalene derivatives 1 by a dimerization process, as shown in
Fig. 1.16,17
In the case of substrate 7, 1-phenylnaphthalene was isolated
as the main product in both the ionic liquids and in DCB, while
Fig. 1 The cyclooligomerization of arylethynes.
The catalyst is the 5,10,15,20-tetraphenylporphyrinato
ruthenium(II) carbonyl, Ru(TPP)CO (4) whose structure is shown
in Chart 1.
The reactions were carried out in 1-butyl-3-methylimidazolium
hexafluorophosphate, {[Bmim][PF6]} (5), or in 1-octyl-3-methyli-
midazolium hexafluorophosphate, {[Omim][PF6]} (6).
These last compounds were synthesized by adaptation of
literature procedures18–20 or purchased from commercial sources.
{ Electronic supplementary information (ESI) available: experimental and
analytical data of the new products. See http://www.rsc.org/suppdata/cc/
b4/b415462b/
*pietro.tagliatesta@uniroma2.it (Pietro Tagliatesta)
valeria.conte@uniroma2.it (Valeria Conte)
Chart 1 The structure of the catalyst.
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Table 2 Conversion of 7 with subsequent use of the same catalyst–
ionic liquid system
Conversion
(%)a
Yield
(%)a
Yielda
(%) Napht.
Yielda
(%) 1,3,5
Yielda
(%) 1,2,4
Run
1
2
3
4
5
a
100
100
98
100
99
75
70
68
70
66
60
56
53
57
53
6
6
7
6
6
9
8
8
7
7
Chart 2 The structures of the ionic liquids and substrates.
Based on the substrate and determined by GC analysis
Table 1 The cyclooligomerization of arylethynes 7 and 8 catalysed by
4 in ionic liquids 5 and 6 and in 1,2-dichlorobenzene (DCB)a
The possibility of recycling the catalyst several times and the use
of environment friendly solvents, compared with the other
cyclotrimerization catalyst–solvent systems, can be considered of
high value as also shown by other authors for the olefination of
aldehydes catalysed by an iron porphyrin in the same media.25
Entry Solvent Substrate Yieldb (%) 1 Yieldb (%) 2 Yieldb (%) 3
1
2
3
4
5
6
a
5
6
5
7
7
8
8
7
8
32
17
6
30
9
60
21e
67e
59
c
c
c
c
6
DCBd
DCBd
7
4
Valeria Conte,* Elfituri Elakkari, Barbara Floris, Valentina Mirruzzo
and Pietro Tagliatesta*
Dipartimento di Scienze e Tecnologie Chimiche, Universita’ di Roma Tor
Vergata. Via della Ricerca Scientifica, 00133 Rome, Italy.
E-mail: pietro.tagliatesta@uniroma2.it; valeria.conte@uniroma2.it;
Fax: +3906-72594754; Tel: +3906-72594759
78e
c
c
Reactions carried out at 150 uC with a molar ratio Substrate/
Porphyrin 5 5700:1 and a volume ratio substrate/ionic liquids 1:3;
Yields determined by GC analysis; Trace; Reactions carried out
b
c
d
at 190 uC with a molar ratio Substrate/Porphyrin 5 5700:1 and a
volume ratio substrate/ionic liquids 1:3; Two isomers
e
Notes and references
1,3,5- and 1,2,4-triphenylbenzenes were obtained as minor
products (entries 1, 2 and 5). For compound 8, two naphthalene
derivatives were isolated, 8-methoxy-1-(3-methoxyphenyl)naphtha-
lene and 6-methoxy-1-(3-methoxyphenyl)naphthalene in 1:1 iso-
meric ratio, while the triarylbenzenes were detected in trace
amounts (entries 3, 4 and 6). The turnover number for some
experiments was over 3000 for the first run and this characteristic
is highly desired when the cost of the catalysts is an important
parameter. Table 1 also shows that the results obtained by using
ionic liquid 6 are superior, for both the substrates, in terms of yield
of naphthalene derivatives, to those obtained in 5. The results
reported are interesting considering the few reports on the one-pot
synthesis of arylnaphthalene derivatives, which are an important
class of biologically active natural compounds.22–24
{ Before any experiment, the ionic liquids were dried under high vacuum at
80 uC for at least 8 hours to avoid the presence of water. After that, dry
nitrogen was used for the reaction but even using such precautions, the
corresponding acetophenones were produced in variable yields.
1 T. Welton, Chem. Rev., 1995, 99, 2071.
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The analysis of the data also shows that the results in DCB are
comparable to those obtained in 6 and this fact could be due to the
lower polarity of 6 which makes this solvent more similar to DCB.
Reactivity and selectivity found with the title reaction are
important per se. However, we feel that even more important is
the re-use of the ionic liquid–catalyst system. In fact we tested the
recyclability in consecutive experiments performed with the
solution of metalloporphyrin in ionic liquid after the stripping of
the reaction products.
Table 2 shows the catalytic efficiency of the catalyst 4 in IL 6
after 5 runs with ethynylbenzene as substrate. As reported in the
table, it is clear that the recycling of the catalyst slightly affects the
conversion and the total yield of the cyclooligomerization
products.
19 C. M. Gordon, J. D. Holbrey, A. R. Kennedy and K. R. Seddon,
J. Mater. Chem., 1998, 8, 2627.
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M. Gra¨tzel, Inorg. Chem., 1996, 35, 1168.
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in press.
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After the first reaction, the yield is influenced by the water
content of the IL but it can be almost stabilized by high vacuum
pumping.
In conclusion, the properties of the ionic liquid–metallopor-
phyrin system makes the oligomerization of arylethynes in this
system very appealing.
1588 | Chem. Commun., 2005, 1587–1588
This journal is ß The Royal Society of Chemistry 2005