Ruthenium porphyrin bound to a Merrifield resin as heterogeneous catalyst for the cyclooligomerization of arylethynes

Article abstract of DOI:10.1039/c0nj00349b

Ruthenium meso-tetraphenylporphyrin was bound to a solid support, the Merrifield resin, and used in the cyclooligomerization of arylethynes, obtaining high yields and selectivities in the final products with a complete recycling of the catalyst.

Full text of DOI:10.1039/c0nj00349b

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Ruthenium porphyrin bound to a Merrifield resin as heterogeneous
catalyst for the cyclooligomerization of arylethynesw
Alina Ciammaichella, Alessandro Leoni and Pietro Tagliatesta*
Received (in Montpellier, France) 7th May 2010, Accepted 4th August 2010
DOI: 10.1039/c0nj00349b
Ruthenium meso-tetraphenylporphyrin was bound to a solid
support, the Merrifield resin, and used in the cyclooligomerization
of arylethynes, obtaining high yields and selectivities in the final
products with a complete recycling of the catalyst.
The carbon–carbon triple bond has been recently recognized
to be one of the most important organic functions used as a
building block in synthetic and material chemistry.1–3 More
recently, the alkyne scaffolds have been used as starting
compounds for the synthesis of nanotubes.4 In previous papers
we showed that many arylethynes can undergo the cyclo-
oligomerization reaction catalyzed by rhodium or ruthenium
porphyrins and vanadium phthalocyanines.5
Synthetic metalloporphyrins are well known for their catalytic
properties in many important organic reactions, such as, for
example, the oxidation of organic substrates,6 the cyclo-
propanation of olefins,7 the carbonyl ylide/1,3-dipolar cyclo-
Fig. 1 Synthesis of the catalyst 4.
addition reactions of a-diazoketones,8 the insertion of carbene
into the S–H bond,9 the amination of hydrocarbons10 and the
olefination of aldehydes.11 All the above cited reactions were
performed in homogeneous organic solutions and in many
cases the recycling of the catalyst, even if possible, must
undergo a tedious chromatographic recovery.
the synthesis of ethers.16 The synthetic steps are reported in
Fig. 1.
From the data of the elemental analysis obtained for the
catalyst 4, we estimate the content of the ruthenium porphyrin
to be around 1 mmolar per gram of solid, a value very close to
those obtained for other porphyrin functionalized solids.12
The binding of the catalyst 4 to the solid through the ethereal
function allowed to separate the functionalized resin from the
bulk of the reaction by vacuum filtration and reuse it for
several times.
The cyclooligomerization porphyrin catalysts can be reused
several times but need to be separated from the starting and
final compounds by a silica gel column but this process is
affected by the waste of time and money.5 Otherwise several
examples of solids supporting metalloporphyrins for catalysis
were reported in the literature12 but recently few results were
presented to the scientific community. In this paper we present
the synthesis of an heterogeneous porphyrin catalyst obtained
by the coupling of the macrocycle with the Merrifield resin and
its use in the cyclooligomerization reaction of some arylethynes
with slightly complete retention of the activity.
Furthermore, the robustness of the system at the high
reaction temperature (160 1C) can be verified by UV/Vis
methods. In fact after 48 hours of reaction no trace of free
metalloporphyrin was detected in the reaction media at such
temperature. In Table 1 we report the data obtained using
several arylethynes as substrates in the cyclooligomerization
reaction catalyzed by 4.z
The heterogeneous catalyst was synthesized by using the
well known Merrifield technique13 applied for the first synthesis
of peptides in the solid phase. The starting porphyrin free
base, 1, was synthesized as reported in the literature14 and
ruthenium insertion was obtained by a standard method.15
After saponification with KOH, the ruthenium porphyrin 3
was bound to the solid resin using the Williamson method for
In Fig. 2 the reaction scheme is reported.
The catalytic activity of porphyrin bound to the resin is
compared with that obtained from our previous papers
(in parentheses).5a,b,d
An inspection of the table shows that the efficiency of
the new catalytic system does not seem to be affected by
the presence of the matrix. In fact the conversion of the
substrates is always high and comparable with that previously
reported.5a,b,d,e
Dipartimento di Scienze e Tecnologie Chimiche,
Univ. Roma Tor Vergata, Via della Ricerca Scientifica, 00133 Rome,
Italy. E-mail: pietro.tagliatesta@uniroma2.it;
Fax: +39 06-72594754; Tel: +39 06-72594759
w Electronic supplementary information (ESI) available: Analytical
data for all the new porphyrin compounds. See DOI: 10.1039/
c0nj00349b
The sterically hindered substrate 2,6-dimethoxy-ethynyl-
benzene only gives poor results in terms of conversion
(Table 1, entry 7) and this fact in our opinion can be due to
the difficulty for the starting alkyne to approach the catalyst
bound to a solid support. The yield of the dimerization
c
2122 New J. Chem., 2010, 34, 2122–2124 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Table 1 Cyclooligomerization of substituted arylethynes, p,m-X–C6H4–CRCH and 1-ethynyl or 2-ethynylnaphthalene, using 4 as catalyst.a
In parentheses the data from previous papers using metalloporphyrins are reported
Yield of
cyclodimersb (%)
Yield of
cyclotrimersb (%)
Entry
Substrate X
Conversion (%)
1
2
3
4
5
7
8
9
H
p-Cl
p-OCH3
m-OCH3
p-CH3
2,6-Di (OCH3)
1-Ethynyl naphthalene
2-Ethynyl naphthalene
80(91)
90(99)
61(60)
67(99)
86(99)
12(70)
87(95)
64(95)
20(23)d
48(1)e
26(28)e
44(78)c,e
58(69)e
—
60(68)d
31(98)e
33(30)e
20(21)e
25(30)e
12(70)g
44(19)
7(4)
38(73)f
57(36)f
a
b
c
d
Reactions carried out at 160–180 1C. Yields determined by GC analysis. Two isomers (1 : 1 ratio). From ref. 5b with RuOEPCO as
catalyst. From ref. 5d with Rh(TDCPP)Cl as catalyst. From ref. 5d with RuOEPCO as catalyst. From ref. 5d with RuOEPCO as catalyst.
e
f
g
Table 3 Cyclooligomerization of phenylacetylene using 2, 3 and 4a
Yield of Yield of
Catalyst Conversion (%) cyclodimers (%) cyclotrimers (%)
b
b
2
3
4
74
91
80
36
51
20
38
40
60
a
Reactions carried out at 160–180 1C with 10 mg of catalyst 4 in 1 mL
b
of neat phenylacetylene (9.77 mmol). Yields determined by GC
analysis.
phenylacetylene as the substrate. Such experiments could
clarify which role the organic support played in directing the
selectivities of the reaction. The data from the reactions with 2
and 3 as catalyst are reported in Table 3 and compared with
those obtained using catalyst 4.
Fig. 2 The reaction scheme.
products of the substituted ethynylbenzenes slightly decreases
while the trimers are produced in higher percentage.
The ratio between the cyclodimers and trimers seems to be
affected by the solid matrix in terms of an increase of the trimers
yield. This effect in our opinion should be due to the different
steric hindrance which the porphyrin intermediates enclosed in
the resin experience along the two reaction pathways.
The resistance of the resin bound metalloporphyrin to
the reaction conditions was tested using phenylacetylene as
substrate in three consecutive experiments of recycling of the
same batch of catalyst (Table 2). The total yield of the reaction
was 80% for the first experiment, 84% for the second and 77%
for the third one while the trimers yield increases and that of
the dimers decreases. Such an effect can be due to the differences
in the conformations of the polymer induced during the first
batch by the high reaction temperature. After this treatment, the
resin can reach a more stable conformation which seems to
influence the results of the other batches in terms of a different
steric hindrance around the porphyrin catalyst.
In a previous paper5b we proposed a mechanism for the
cyclodimerization of phenylacetylene to give the 1-phenyl-
naphthalene, catalyzed by ruthenium porphyrins, in terms of
the formation of a vinylidene intermediate of the metal com-
plex by a Z2-1-alkyne - Z1-vinilydene rearrangement. Such
an intermediate could then undergo the concerted attack of a
second molecule of alkyne in a (formal) Diels–Alder reaction
(see Fig. 3) to give the final dimeric product while triphenyl-
benzenes probably derive from an open intermediate.
In conclusion we found that a ruthenium porphyrin bound
to a solid matrix, the Merrifield resin, by a covalent bond can
catalyze the oligomerization of the arylalkynes with good yield
and selectivities. Such a system can be recovered and reused
more times without any loss of catalytic activity.
The difference in the activity between catalyst 4 and the
porphyrin intermediates 2 and 3 was also investigated using
Table 2 Cyclooligomerization of phenylacetylene using 4 as catalyst
in three consecutive experimentsa
Yield of
cyclodimersb (%)
Yield of
cyclotrimersb (%)
Run
Conversion (%)
1
2
3
80
84
77
20
52
49
60
32
28
a
Reactions carried out at 160–180 1C with 10 mg of catalyst 4 in 1 mL
of neat phenylacetylene (9.77 mmol). Yields determined by GC
analysis.
b
Fig. 3 Proposed mechanism for the cyclodimerization.
c
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Notes and references
z Typical procedure for the reaction catalyzed by 4: 10 mg of catalyst 4
were placed in 1 mL of phenylacetylene (9.77 mmol). The resulting
mixture was warmed at 160–180 1C for 48 hours under nitrogen. At
the end of the reaction, dodecane or tetradecane was added as an
internal standard and the mixture analyzed by GC. Separation of the
catalyst: after cooling at 40 1C, the reaction was diluted with chloro-
form and centrifuged for 5 minutes, after that the organic solution was
separated from the catalyst using a pipette. The catalyst was washed
several times with chloroform until the initial and final products were
not present in the organic solution (GC analysis). The recovered
catalyst was dried under vacuum at 60 1C for 2 hours and reused.
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