The formation of 1-aryl-substituted naphthalenes by an unusual cyclization of arylethynes catalyzed by ruthenium and rhodium porphyrins

Article abstract of DOI:10.1002/ejoc.200400746

The dimerization of arylethynes to give 1-aryl-substituted naphthalenes through catalysis by rhodium and ruthenium porphyrins has been investigated. When performed at temperatures above 130 °C, this reaction affords a mixture of triarylbenzenes and 1-aryl-substituted naphthalenes. The yields of naphthalene derivatives range from low to high, depending on the temperature and the phenyl substituents. The concentrations of the initial compounds affect the selectivity of the reaction: the dimerization/trimerization ratios in 1,2-dichlorobenzene increase as concentration decreases. The reaction mechanism is determined by the peculiar structure of the catalyst ligand and involves the formation of a vinylidene intermediate of the metalloporphyrins. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejoc.200400746

FULL PAPER
The Formation of 1-Aryl-Substituted Naphthalenes by an Unusual Cyclization
of Arylethynes Catalyzed by Ruthenium and Rhodium Porphyrins
Elfituri Elakkari,[a] Barbara Floris,[a] Pierluca Galloni,[a] and Pietro Tagliatesta*[a]
Keywords: Metalloporphyrins / Catalysis / Rhodium / Ruthenium / Cyclooligomerization
The dimerization of arylethynes to give 1-aryl-substituted
naphthalenes through catalysis by rhodium and ruthenium
porphyrins has been investigated. When performed at tem-
peratures above 130 °C, this reaction affords a mixture of tri-
arylbenzenes and 1-aryl-substituted naphthalenes. The yi-
elds of naphthalene derivatives range from low to high, de-
pending on the temperature and the phenyl substituents.
The concentrations of the initial compounds affect the selec-
tivity of the reaction: the dimerization/trimerization ratios in
1,2-dichlorobenzene increase as concentration decreases.
The reaction mechanism is determined by the peculiar struc-
ture of the catalyst ligand and involves the formation of a
vinylidene intermediate of the metalloporphyrins.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
between the yields of the triphenylbenzenes and the pheny-
lethyne conversion, ranging from 5% to 40%, depending on
the catalyst. We therefore decided to obtain deeper insight
into the process and thus found another reaction catalyzed
by rhodium porphyrins: the quite unusual cyclodimeriza-
tion of the arylethynes to give 1-aryl-substituted naphtha-
lenes (Figure 1).
Introduction
The formation of carbon–carbon bonds has long been a
central theme of research activity in organic and organome-
tallic chemistry, with several new catalytic methods having
been discovered and applied for the industrial production
of chemicals.[1] New reactions and methods are introduced
every year, and this paper reports a new application of
metalloporphyrins as catalysts in alkyne chemistry.
Synthetic metalloporphyrins have exhibited catalytic
properties in a number of organic reactions such as, for ex-
ample, the oxidation of organic substrates,[2] the cyclopro-
panation of olefins,[3] the carbonyl ylide/1,3-dipolar cyclo-
addition reactions of α-diazoketones,[4] the insertion of car-
bene into the S–H bond,[5] the amination of the double
bond,[6] and the olefination of aldehydes.[7]
Recently we demonstrated a new application of metall-
oporphyrin catalysis in the cyclotrimerization of arylethynes
by rhodium porphyrins,[8] a reaction similar to, but distinct
from, the classical Reppe reaction.[9] Such a reaction, start-
ing from monosubstituted ethynes, usually gives a 1,2,4-tri-
substituted benzene as the main product, together with dif-
ferent amounts of the 1,3,5- and 1,2,3-trisubstituted iso-
mers.[10]
Figure 1. Reaction scheme for the cyclodimerization of 1-ethynyl-
4-methoxybenzene.
The same reaction pattern was also obtained when a ru-
thenium porphyrin was used as the catalyst. The occurrence
of both cyclodimerization and cyclotrimerization products
under the same reaction conditions prompted us to examine
the factors possibly affecting the selectivity.
Reactions involving the formation of vinylidene interme-
diates from terminal alkynes to give aromatic compounds
have been reported in the literature,[11] but to the best of
our knowledge this is the first simple intermolecular cyclo-
aromatization reaction involving a dienyl system and a
monosubstituted alkyne, although there have been two re-
ports on similar reactions with disubstituted alkynes.[12]
Wilkinson-like rhodium complexes are involved as catalysts
in the first example, in the presence of HCl and azobenzene
as co-catalysts, giving moderate yields of naphthalene deriv-
atives. The second report involves a similar reaction cata-
With rhodium porphyrins as catalysts, we obtained 1,3,5-
and 1,2,4-triaryl-substituted benzenes from arylethynes,
with relative ratios depending on the porphyrin, but inde-
pendent of the temperature and solvent.[8] At the beginning
of this investigation we were focusing our attention on the
trimerization products. However, there was a discrepancy
[a] Dipartimento di Scienze e Tecnologie Chimiche, Universita’ di
Roma-Tor Vergata,
Via della Ricerca Scientifica, 00133 Roma, Italy
E-mail: pietro.tagliatesta@uniroma2.it
Eur. J. Org. Chem. 2005, 889–894
DOI: 10.1002/ejoc.200400746
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
889

E. Elakkari, B. Floris, P. Galloni, P. Tagliatesta
FULL PAPER
lyzed by a RhCl3-Aliquat® 336 ion pair, affording lower yi- ling of two terminal alkynes to give (E) and (Z) 1,4-but-3-
elds of 1-aryl-substituted naphthalenes. The mechanism of en-ynes is a well known reaction.[16]
this last reaction involves the presence of a metallacyclopen-
tadiene intermediate.
Table 1 reports the yields for the dimerization reactions
of some arylethynes with catalysis by 1 and 2, together with
Rare examples of the intramolecular cyclization of conju- the total yields of the cyclotrimerization products for all the
gated dienylalkynes via ruthenium vinylidene intermediates substrates. The data in Table 1 show that the selectivity is
and the rhodium cycloaromatization of acyclic 3-ene-1,5- affected by the nature of the substituents on the phenyl ring
diynes have also been reported in the literature.[13]
of the alkynes. There is no clear trend, although it is appar-
Our further investigations into the cyclodimerization of ent that electron-donating groups such as para-methoxy or -
arylethynes are reported here.
methyl direct the reaction toward 1-aryl-substituted naph-
thalenes as the main products, while the presence of an elec-
tron-withdrawing substituent such as chlorine results in a
mixture of 1,2,4- and 1,3,5-triaryl-substituted benzenes in
higher yield and good selectivity,[8] particularly so with the
less electron-rich rhodium porphyrin. However, the slightly
electron-attracting meta-methoxy group behaved more simi-
larly to methyl than to chlorine.
The structures of the catalysts also affected the reaction
pathway, with the rhodium porphyrin 2 more active in pro-
moting the conversion of the arylethynes into 1-aryl-substi-
tuted naphthalenes with substrates possessing para- or
meta-OCH3 and para-CH3 moieties, while catalyst 1 gave
higher yields of dimerization products with para-Cl or para-
H. As a matter of fact, with Ru(OEP)CO very similar re-
sults were obtained with almost all the substrates, in terms
of yields, conversion and selectivity.
Results and Discussion
The porphyrin catalysts used for this investigation (Fig-
ure 2) were Ru(OEP)CO (1) and Rh(TDCPP)Cl (2), where
OEP is the dianion of 2,3,7,8,12,13,17,18-octaethylporphy-
rin and TDCPP is the dianion of 5,10,15,20-tetrakis(2Ј,6Ј-
dichlorophenyl)porphyrin.
A number of questions arise when the reaction mecha-
nism is considered, such as the role of the porphyrin ligand,
if any, or whether cyclodimerization and cyclotrimerization
follow different routes (competitive reactions) or proceed
along the same pathway (consecutive reactions).
In our opinion, the reaction output must be related to
the particular catalyst used. In fact, the metalloporphyrin
complexes do not have cis coordination sites available on
the same face of the macrocycle,[17] thus ruling out the
mechanism proposed for cyclooligomerization catalyzed by
other systems: the coordination of two molecules of alkyne
and the formation of metallacyclopentadiene.[12b,17,18]
We think that, after a preliminary π-coordination of a
molecule of alkyne to the metal, the formation of a metal
vinylidene complex may occur through a [1,2]-hydrogen
shift, possibly helped by acid-base reaction with one of the
nitrogen atoms of the porphyrin ligand.[19] The η2-1-alky-
neǞη1–vinilydene rearrangement is a well known pro-
cess,[20] but we can rule out the intermediate formation of
a metal hydride because of the lack of the suitable coordi-
Figure 2. Structures of the catalysts 1 and 2.
We used different macrocycles for our experiments be- nation site. The reaction pathway with phenylethyne and
cause after a check of other metalloporphyrins, we found Ru(OEP)CO is depicted in Scheme 1.
that the compounds used in this work gave the best results.
The unstable vinylidene complex 3, activated by the me-
The reactions were generally performed without solvent tal coordination, undergoes the attack of a second molecule
at 180 °C and with a substrate/porphyrin molar ratio of of alkyne in a (formal) Diels–Alder reaction, giving the in-
5700:1, and the final products were separated by column termediate 4 in a similar way to that reported for the syn-
chromatography on silica gel columns. All the catalysts were thesis of 2-arylated quinolines.[20b] Finally, complex 4 gives
recovered by column chromatography in more than 95% the dimerization product 5 after decomplexation and rearo-
yield and could be recycled at least three times without any matization.
loss of catalytic activity.
It was not possible to isolate the intermediates 3 or 4 to
The dimerization reaction of arylethynes to give naph- support the above interpretation, but other authors have
thalene derivatives is an unusual process, whereas the coup- reported that similar cyclizations proceed through vinyli-
890
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2005, 889–894

The Formation of 1-Aryl-Substituted Naphthalenes by an Unusual Cyclization of Arylethynes
FULL PAPER
Table 1. Cyclooligomerization of substituted arylethynes, p,m-X-C6H4-CϵCH.[a]
Entry
Substrate X
Conversion (%)
Yield of cyclodimers (%)[b]
Yield of cyclotrimers (%)[b]
With Ru(OEP)CO as catalyst
1
2
3
4
5
H
p-Cl
p-OCH3
m-OCH3
p-CH3
91
88
60
61
71
23
12
68
75
30
30
39
28
31[c]
31
With Rh(TDCPP)Cl as catalyst
6
7
8
9
H
p-Cl
p-OCH3
m-OCH3
p-CH3
42
99
63
99
99
17
1
25
98
14
21
30
49
78[c]
69
10
[a] Reactions carried out at 180 °C with a substrate/porphyrin molar ratio of 5700:1. [b] Yields determined by GC analysis. [c] Two isomers
(1:1 ratio).
Scheme 1. Tentative reaction pathway for the cyclodimerization of phenylacetylene.
dene derivatives.[11,20] Furthermore, Ogoshi et al. have pro-
posed the hypothesis of the presence of a vinylidene inter-
mediate in the reaction between phenylacetylene and a rho-
dium porphyrin to give a metal-coordinated acetyl resi-
due.[21]
Experiments to verify the mechanistic hypothesis of
Scheme 1 were designed.
a) In the dimerization of 1-ethynyl-3-methoxybenzene,
with both catalysts, two isomers, 6 and 7 (Scheme 2), were
isolated in a 1:1 molar ratio (Table 1, entries 4 and 9), con-
sistently with a statistical attack of the second molecule of
arylethyne at the phenyl 2- or 6-positions in the vinylidene
intermediate, as a consequence of the rotation around the
Cβ–Ph bond.
b) No reaction occurred when disubstituted alkynes (1-
phenylprop-1-yne or diphenylethyne) were treated with cat-
alysts 1 or 2 at 180 °C. However, a mixture of phenylacety-
Scheme 2. Reaction mechanism for the cyclodimerization of 1-ethy-
nyl-3-methoxybenzene.
lene and diphenylacetylene in a 1:1 molar ratio reacted in of the vinylidene derivative through the hydrogen mi-
the presence of Ru(OEP)CO, thus attesting to the necessity gration.
of the hydrogen atom on the sp carbon. In fact, only the
The vinylidene derivatives are also involved, in our opin-
presence of the terminal alkyne proton allows the formation ion, in the formation of the cyclotrimerization products,
Eur. J. Org. Chem. 2005, 889–894
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
891

E. Elakkari, B. Floris, P. Galloni, P. Tagliatesta
FULL PAPER
and this is supported by the same reaction between phenyl- ratio of 2.5:1 in 10 mL of 1,2-dichlorobenzene. Further-
acetylene and diphenylethyne in 1:1 molar ratio in the pres- more, the reaction of an alkyne different from phenylethyne,
ence of Ru(OEP)CO. At 180 °C we observed (by GC-MS 4-chloro-1-ethynylbenzene, with catalyst 2, which gave a
analysis) traces of tetraphenylbenzenes originating from the 98% yield of the cyclotrimerization products in the absence
attack of diphenylethyne on the porphyrin vinylidene deriv- of solvent,[8] afforded a 56% yield of 7-chloro-1-(4-chlo-
ative among the cyclotrimerization products, together with rophenyl)naphthalene and a 42% yield of triphenylbenzenes
the diphenylnaphthalene (Scheme 3).[22]
when performed in 3 mL of 1,2-dichlorobenzene. These re-
sults are in line with the proposed mechanism.
Conclusions
Rhodium and ruthenium porphyrins are efficient cata-
lysts for the cyclooligomerization of arylethynes, depending
on the metal, the electronic situation of the macrocycles,
and the substituents on the substrates. The reaction path-
way is also affected by temperature and initial concentra-
tion of the arylethynes: at higher temperatures and lower
concentrations we observed the formation of phenylnaph-
thalenes and triarylbenzenes in different yields, and the pro-
duct selectivity can be tuned on the basis of the parameters
cited above.
In our opinion these properties are interesting and make
the rhodium and ruthenium porphyrins a family of catalysts
capable of forming aromatic compounds. In comparison
with the other catalysts, they can be reusable and give high
turnover numbers that in many cases are over 4000 for the
first run. Furthermore, the metalloporphyrins used in this
work afford a new synthetic method for the one-pot synthe-
sis of arylnaphthalene derivatives, which are an important
class of biologically active natural compounds. Although
cyclodimers are always accompanied by cyclotrimers, they
can be very easily separated. This, together with the recov-
ery and potential recycling of catalysts, the option of avoid-
ing solvents, and the simple reaction procedure, makes the
metalloporphyrin-catalyzed cyclodimerization an appealing
reaction.
Scheme 3. Reaction of phenylacetylene with diphenylacetylene in
the presence of ruthenium catalysts 1.
Thus, we can deduce that the metal vinylidene derivative
is a common intermediate giving rise both to cyclodimers
and to cyclotrimers.
(c) Dependence on the temperature. Other conditions be-
ing equal, heating of phenylethyne in the presence of Ru-
(OEP)CO at 140 °C essentially gave only the naphthalene
derivatives, in low yields, while at 180 °C the triphenylben-
zenes were prevalent and the yields higher (see Table 2, en-
tries 1 and 2). The result can be explained in terms of kin-
etic control, driving the reaction to the dimerization pro-
ducts at lower temperature. The formation of the adduct
may be reversed (retro-Diels–Alder) at higher temperatures,
resulting in the more stable triphenylbenzenes (thermo-
dynamic control).
(d) Dependence on concentration. If the results of the
reaction carried out without solvent and with increasing
amounts of solvents (Table 2, entries 1, 4 and 5) at the same
temperature are compared, the relative 1-phenylnaphtha-
lene/triphenylbenzene ratios increased from 1:3 (neat) to
1:1.5 (3 mL 1,2-dichlorobenzene), to 1:0.4 (10 mL 1,2-
dichlorobenzene). Again, with the same concentration but
at lower temperatures (Table 2, entries 3 and 4), the reaction
was directed more towards the cyclodimerization, at 120 °C
reaching a 5:1 ratio in favor of 1-phenylnaphthalene. The
dilution effects also shift the reaction output towards cyclo-
dimerization with rhodium catalyst, giving a 15% yield of
Experimental Section
General Procedures: Chromatographic purifications were per-
formed on silica gel (35–70 mesh, Merck) columns. Thin-layer
chromatography was carried out with Merck Kieselgel 60-F254
plates. 1H NMR spectra were recorded as CDCl3 solutions on a
Bruker AM 400 instrument with tetramethylsilane (TMS) as an in-
ternal standard. FAB mass spectra were measured on a VG-Quat-
tro spectrometer with m-nitrobenzyl alcohol (NBA) as a matrix.
GC-MS spectra were obtained with a VG-Quattro spectrometer
conversion and a 1-phenylnaphthalene/triphenylbenzenes with a 30 m Supelco SPB-5 capillary column.
Table 2. Reaction of phenylethyne catalyzed by Ru(OEP)CO under different conditions.[a]
Entry
Conversion (%)[b]
Yield of cyclo-dimers (%)[b]
Yield of cyclo-trimers (%)[b]
Solvent[c]
Temperature
1
2
3
4
5
91
23
6
82
27
23
13
5
31
18
68
10
1
46
7
none[d]
none[d]
3 mL[e]
3 mL[e]
10 mL[e]
180 °C
140 °C
120 °C
180 °C
180 °C
[a] Reactions carried out with a substrate/porphyrin molar ratio of 5700:1. [b] Yields determined by GC analysis. [c] 1,2-Dichlorobenzene.
[d] 18 h. [e] 36 h.
892
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Org. Chem. 2005, 889–894

The Formation of 1-Aryl-Substituted Naphthalenes by an Unusual Cyclization of Arylethynes
GC Separation Conditions: The product yields and the isomeric ra- = 6.8 Hz, 1 H), 7.23 (s, 1 H), 6.93–7.08 (m, 4 H), 3.91 (s, 3 H), 3.82
FULL PAPER
tios for all the reactions were determined by GC analyses per-
formed on a Carlo Erba HRGC 5160 instrument with a 30 m Sup-
elco SPB-5 capillary column and a FID detector. Chemical yields
were determined by addition of a suitable internal standard (do-
decane or tetradecane) to the reaction mixture at the end of each
experiment and were reproducible within 2% for multiple experi-
ments.
(s, 3 H). EI MS: m/z (%) 264 (100).
6-Methoxy-1-(3-methoxyphenyl)naphthalene: Oil. 1H NMR
(400 MHz, CDCl3): δ = 7.78 (d, J = 8 Hz, 2 H), 7.35–7.48 (m, 3 H),
7.21–7.26 (m, 3 H), 6.84–6.90 (m, 2 H), 6.76 (d, J = 7.6 Hz, 1 H),
3.79 (s, 3 H), 3.48 (s, 3 H). EI MS: m/z (%) 264 (100).
Chemicals: All the reagents and solvents (Aldrich) were of the high-
est analytical grade and were used without further purification. The
free bases H2TDCPP and H2OEP, where TDCPP is the dianion of
5,10,15,20-tetrakis(2Ј,6Ј-dichlorophenyl)porphyrin and OEP is the
dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin, were synthe-
sized by literature methods.[14] Ru(OEP)CO was obtained by litera-
ture methods.[15] Rh(TDCPP)Cl was synthesized as described in
another paper from our laboratory.[3j] All the cyclotrimerization
products reported in this paper have been described previously.[8]
Acknowledgments
We thank Mr. Alessandro Leoni and Mr. Giuseppe D’Arcangelo
for their valuable technical assistance and MIUR for financial sup-
port.
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Typical Procedure for the Reaction Catalyzed by Metalloporphyrins
1 or 2: The catalyst (1.8 mg, 1.7 or 2.7 µmol) was dissolved in phe-
nylacetylene (1.4 mL, 13.7 mmol) and the resulting solution was
warmed between 120° and 180 °C for 18 hours, under nitrogen.
Typical Procedure for the Reaction Catalyzed by Metalloporphyrin
1 or 2 in 1,2-Dichlorobenzene: The catalyst (1.8 mg, 1.7 or 2.7 µmol)
was dissolved in 1,2-dichlorobenzene (3 mL unless otherwise
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resulting solution was warmed at 130–140 °C for 36 hours or at
180 °C for 18 hours, under nitrogen.
Reaction between Phenylacetylene and Diphenylacetylene: Catalyst
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13.7 mmol). Diphenylacetylene (2,43 g, 13.7 mmol) was added and
the resulting solution was warmed at 180 °C for 18 hours under
nitrogen.
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The crude products of all the reactions were purified by column
chromatography (SiO2, hexane/diethyl ether). The fractions con-
taining the desired compounds were evaporated under vacuum, and
the 1-arylnaphthalenes and isomeric triphenylbenzenes were sepa-
1
rated by column flash chromatography and identified by their H
NMR and mass spectra. An authentic sample of 1-phenylnaphtha-
lene, available from Aldrich, was used for the identification of the
reaction products from phenylacetylene. All the elemental analysis
gave satisfactory results.
Analytical data
7-Methoxy-1-(4-methoxyphenyl)naphthalene: M.p. 62–63 °C. 1H
NMR (400 MHz, CDCl3): δ = 7.84 (d, J = 8.8 Hz,1 H), 7.80 (t, J
= 4.8 Hz, 1 H), 7.49 (d, J = 8.8 Hz, 2 H), 7.41 (d, J = 4.4 Hz, 2 H),
7.30 (d, J = 1.6 Hz, 1 H), 7.21 (dd, J = 8.8, 1.6 Hz, 1 H), 7.08 (d,
J = 8.8 Hz, 2 H). EI MS: m/z(%) = 264 (100), 240 (12).
1
7-Methyl-1-(4-methylphenyl)naphthalene: Oil. H NMR (400 MHz,
CDCl3): δ = 7.83 (d, J = 8.4 Hz, 2 H), 7.72 (s, 1 H), 7.38–7.54 (m,
4 H), 7.32–7.37 (m, 3 H). EI MS: m/z (%) 232 (100), 217 (70), 202
(50).
7-Chloro-1-(4-chlorophenyl)naphthalene: M.p. 40–41 °C. 1H NMR
(400 MHz, CDCl3): δ = 7.86 (d, J = 8.4 Hz, 2 H), 7.82 (s, 1 H),
7.45–7.57 (m, 4 H), 7.56–7.40 (m, 3 H). EI MS: m/z (%) 272 (65),
237 (14), 202 (100).
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Eur. J. Org. Chem. 2005, 889–894
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893

E. Elakkari, B. Floris, P. Galloni, P. Tagliatesta
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The yields of these compounds were low and they were pro-
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Received: October 18, 2004
894
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Eur. J. Org. Chem. 2005, 889–894