The highly selective formation of biaryls by the cyclization of arylethynes catalyzed by vanadyl phthalocyanine

Article abstract of DOI:10.1039/b9nj00227h

The dimerization of arylethynes catalyzed by vanadium phthalocyanine to give substituted biaryls has been investigated. The reaction yield is always high and for many examples is only slightly affected by the aryl substituents. This fact is also related to the results obtained with metalloporphyrins, which give lower selectivities due to the presence of variable amounts of triphenylbenzenes.

Full text of DOI:10.1039/b9nj00227h

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The highly selective formation of biaryls by the cyclization of arylethynes
catalyzed by vanadyl phthalocyaninew
Daniel Cicero, Angelo Lembo, Alessandro Leoni and Pietro Tagliatesta*
Received (in Victoria, Australia) 30th May 2009, Accepted 25th June 2009
First published as an Advance Article on the web 3rd August 2009
DOI: 10.1039/b9nj00227h
The dimerization of arylethynes catalyzed by vanadium phthalocyanine to give substituted biaryls
has been investigated. The reaction yield is always high and for many examples is only slightly
affected by the aryl substituents. This fact is also related to the results obtained with
metalloporphyrins, which give lower selectivities due to the presence of variable amounts
of triphenylbenzenes.
All the compounds obtained from the reactions were easily
Introduction
separated by the usual methods, i.e. low or medium pressure
The importance of the chemistry of the CRC triple bond has
been well recognized since this function was shown to be
chromatography, affording pure products, even on a gram
scale. The factors affecting the selectivity of the reaction were
one of the main building blocks of organic and material
examined in detail from the point of view of the nature of the
chemistry.1 The formation of benzene rings from alkynes,
substrates and catalysts. However, from the obtained data,5b it
named the Reppe cyclotrimerization,2 has been intensively
was clear that both parameters affected the ratio between
studied in the last fifty years because of the difficulty in
cyclodimers and cyclotrimers. In fact, examining the above
obtaining functionalized aromatic compounds.3 Recently,
cited results, the yield of cyclodimers spanned from 1%
alkyne scaffolds have been used as starting compounds for
for 4-chloro-ethynylbenzene to 78% for 4-methoxy-ethynyl-
the synthesis of nanotubes.4 In previous investigations, we
benzene. The choice of a catalyst with different electronic
found that a large number of arylethynes can undergo cyclo-
properties could offer the possibility of obtaining different
oligomerization reactions in the presence of a catalytic amount
selectivities. Phthalocyanines were taken into consideration
of rhodium or ruthenium porphyrins, giving trisubstituted
because of their higher electron-withdrawing properties
benzenes and/or biaryl derivatives, depending on the
compared to porphyrins. Although phthalocyanines are
nature of the catalyst, the reaction conditions and the solvent
well known as pigments and dyes from the beginning of
used.5
the last century, and are extensively used as colorants for
printing inks, textiles and paint,12 they have not been
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
activity. Some investigations concerning oxidation reactions
well studied in catalysis because of their low solubility and
cyclopropanation of olefins,7 the carbonyl ylide/1,3-dipolar
catalyzed by metallophthalocyanines have been reported in the
cycloaddition 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 With rhodium or
ruthenium porphyrins as catalysts, we previously obtained
1,3,5- and 1,2,4-triaryl-substituted benzenes from phenyl-
substituted ethynylbenzenes,5a but there was a discrepancy
between the yield of triphenylbenzenes and the conversion of
starting material, ranging from 1–5% to more than 70%,
depending on the catalyst and substrate. We found that
another reaction, catalyzed by the metalloporphyrins, was
operative under the same conditions, the cyclodimerization
of arylethynes to give 1-aryl-substituted naphthalenes.5b
literature,13 but the number of published papers seems low
compared to the huge number of studies about the catalytic
processes involving metalloporphyrins. During our recent
studies on the catalytic properties of metalloporphyrins,5
we found that even in the presence of an electron-poor
metalloporphyrin like Rh(TDCPP)Cl, where TDCPP is
the dianion of 5,10,15,20-tetrakis-(20,60-dichlorophenyl)-
porphyrin, we always obtained a mixture of products. Such
products were easily separated on a column, but it should be
possible to find a better catalyst giving higher selectivities.
Vanadyl phthalocyanine has been our choice because of its
electronic properties,14 and in this paper we report our most
recent results obtained using this catalyst under the same
reaction conditions.
`
Dipartimento di Scienze e Tecnologie Chimiche, Universita Roma Tor
Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy.
E-mail: pietro.tagliatesta@uniroma2.it; Fax: +39 06-72594754;
Tel: +39 06-72594759
Results and discussion
w Electronic supplementary information (ESI) available: 1H and
13C NMR two-dimensional experiments for the identification of the
new fluorinated compounds. Analytical data for all new compounds
and 1H NMR spectra for deuterated 1-phenyl-naphthalene. See
DOI: 10.1039/b9nj00227h
The commercially available metallophthalocyanine catalyst
used for this investigation was VOPc, 1, where Pc is the
dianion of the phthalocyanine. The structure of the catalyst
is reported in Fig. 1. The reactions were generally performed,
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Fig. 1 The structure of catalyst 1.
unless otherwise specified, without solvent at 160–180 1C
with a molar ratio substrate : catalyst = 5700 : 1, and the
final compounds were quantified by GC using an internal
standard.z
Fig. 2 The cyclooligomerization reaction catalyzed by 1.
As an example, a reaction scheme involving 4-chloro-
ethynylbenzene as starting material is reported in Fig. 2.
In Table 1, the results obtained from the cyclooligomerization
of arylethynes in the presence of catalyst 1 are reported and
compared with the data obtained using Rh(TDCPP)Cl or
Ru(OEP)CO and reported from our previously published
papers.5
Vinylidene intermediate A can react with a second molecule
of arylethyne to give adduct B, but this complex cannot
give final naphthalene derivative C, and can be only subjected
to
a retro-Diels–Alder reaction, affording again, after
decomplexation, the initial compound. In fact, from the
reaction catalyzed by 1 or Ru(OEP)CO, we were only able
to isolate the 1,3,5- and 1,2,4-triarylbenzenes, without
any trace of dimer C, as reported in Scheme 2. For
comparison, the final triarylbenzene derivatives were also
obtained by a Suzuki coupling from the 1,2,4- and 1,3,5-
tribromo-benzenes, respectively, and 2,6-dimethoxyphenyl-
boronic acid.16
From our results with catalyst 1, some new considerations
can be extrapolated. First of all, the vanadyl phtalocyanine
gives higher selectivities for the cyclodimerization vs. the
cyclotrimerization reaction, giving different results compared
to those obtained with metalloporphyrins. Looking, for
example, at the result for 4-chloro-ethynylbenzene
(entry 2), the yield of dimer increases from 1 to 50%,
useful enough for preparative synthesis. The results are in
agreement for all of the other substrates. In fact, the
cyclodimerization seems to be the preferred pathway for
reactions catalyzed by 1. Incidentally, in the case of the
4-fluoro-ethynylbenzene (entry 6), a valuable difluorinated
phenylnaphthalene was isolated. Because of the oxidation
state of the vanadium atom during the catalytic cycle,
we were not able to isolate any stable vinylidene intermediate,
and we can only suppose the presence, after oxygen cleavage,
of a V(IV)-carbene intermediate. To obtain more information
on the mechanism of the reaction, which, in the case
of porphyrins, we previously proposed to involve the
formation of a vinylidene intermediate,5b we decided to
conduct the reaction using 2,6-dimethoxy-ethynylbenzene.15
This fact, in our opinion, supports the presence of a
labile vinylidene complex during the catalytic cycle
for the other studied substrates. The presence of
a
cationic mechanism can be excluded on the basis of a
crossover experiment we performed using 4-chloro-
and 4-methoxy-ethynylbenzene in an equimolar mixture.
The expected dimers were obtained in almost the same
equimolar quantity, but also with the presence of the
scrambled examples.y Such a result is in agreement with
the lack of any polar intermediate. To further investigate
the reaction mechanism, we decided to use commercially
available 2-deuterium-labelled ethynylbenzene. Considering
the reaction mechanism proposed in previous papers,5
the [1,2]-deuterium shift should give the labile vinylidene
complex which, reacting with
a second molecule of
arylethyne, should afford the 1-phenyl-2,4-dideutero-
naphthalene through the intermediate E, as reported in
Scheme 3.
Such
a substrate, bearing two substituents on both
ortho positions of the phenyl ring, could not undergo
the dimerization reaction because of the impossibility of
facilitating the final rearomatization due to the presence of
the methoxy group at the naphthalene ring junction, as
reported in Scheme 1.
1
2
The H- and H-NMR spectra of the final dimer show that
we obtained the final 1-phenyl-naphthalene with 58% of the
label on C-2, while C-3 and C-4 retained almost 100% of
the label. This fact can be explained only if we imagine a
reaction pathway with
a rapid intramolecular shift of
the deuterium from C-2 to C-3 for intermediate E, and a
consequent decomplexation and rearomatization, with
scrambling of H and D at position C-2 by abstraction from
the initial compound.z
z Typical procedure for the reaction catalyzed by vanadyl phthalo-
cyanine 1: 1.0 mg of catalyst (1.7 mmol) was dissolved in 1 mL of
phenylacetylene (9.77 mmol). The resulting solution was warmed at
160–180 1C for 48 h under nitrogen. At the end of the reaction,
dodecane or tetradecane was added as an internal standard and the
mixture analyzed by GC.
Typical procedure for the reaction catalyzed by vanadyl phthalo-
cyanine 1 in 1,2-dichlorobenzene: 1.0 mg of catalyst (1.7 mmol) was
dissolved in 3 mL of 1,2-dichlorobenzene and 1.38 g of 1-ethynyl-
naphthalene (9.1 mmol) was added. The resulting solution was
warmed at 160 1C for 48 h under nitrogen. At the end of the reaction,
dodecane or tetradecane was added as an internal standard and the
mixture analyzed by GC.
y It was not possible to separate and fully characterized the scrambled
dimers, which were detected using GC and GC-MS.
z A kinetic isotopic effect was also determined for 2-D-ethynylbenzene
compared with the non-deuterated compound. A modest but signifi-
cant effect (kH/kD = 2.0 ꢁ 0.08) was observed, suggesting the presence
of a secondary effect due to some rehybridization of the alkyne in the
transition state of the reaction.
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Table 1 The cyclooligomerization of substituted arylethynes p,m-X–C6H4–CRCH and 1-ethynyl- or 2-ethynyl-naphtalene using 1 as the
catalyst.a Data from previous papers are reported in parentheses
Entry
Substrate X
Conversion (%)
Yield of cyclodimers (%)b
Yield of cyclotrimers (%)b
1
2
3
4
5
6
7
8
H
p-Cl
p-OCH3
m-OCH3
p-CH3
p-F
93
98
99
99
94
99
99
98
72 (23)d
50 (1)e
66 (49)e
93 (78)c,e
82 (69)e
40
21 (68)d
48 (98)e
33 (14)e
6 (21)e
12 (30)e
59
1-Ethynyl-naphthalenef
2-Ethynyl-naphthalenef
89 (86)g
97 (89)g
— (4)g
— (7)g
a
b
c
Reactions carried out at 160–180 1C with a molar ratio of substrate : catalyst = 5700 : 1. Yields determined by GC analysis. Two isomers
(1 : 1 ratio). From ref. 5b with Ru(OEP)CO as the catalyst. From ref. 5a with Rh(TDCPP)Cl as the catalyst. In 1,2-dichlorobenzene at
d
e
f
g
160 1C. From ref. 5d with Ru(OEP)CO as the catalyst.
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Scheme 2 Reaction products and yields for the cyclooligomerization
of 2,6-dimethoxy-ethynylbenzene.
Scheme 3 The reaction mechanism and the deuterium fate of the
2-deutero-ethynylbenzene.
Conclusions
In this paper, we have reported an improvement in the dimer
yield for the cyclooligomerization of arylethynes by the use of
a vanadium phthalocyanine. Furthermore, we made observations
that support our hypothesis about the presence of a vinylidene
intermediate during the cyclodimerization reaction.
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´
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