CpCo(CO)2-catalysed cyclotrimerisation of alkynes in supercritical carbon dioxide

Article abstract of DOI:10.1016/S0022-328X(01)00856-7

The reactivity of mono-substituted HCCR (R=Ph, a; CH₂OH, b; CH₂CH₂CH₂CH₃, c) and di-substituted RCCR (R=CH₂CH₃, d; CO₂CH₃, e; Ph, f) acetylenes was studied in supercritical carbon dioxide (scCO₂) using the easily available complex CpCo(CO)₂ as catalyst. The reaction of phenylacetylene produced a mixture of the isomeric cyclotrimers 1,3,5- (2a) and 1,2,4-triphenylbenzene (2a′), in a 1:5 ratio, and traces of cobaltcyclopentadienone complexes CpCo(η⁴-C₄H₂[Ph]₂CO) (6a, mixture of isomers). The possible product formed by the incorporation of CO₂ to alkynes, i.e. diphenylpyrone (7a) was not observed. The reaction of the cobaltacyclopentadiene complex CpCo(1.4-σ-C₄[Ph]₄)(PPh)₃ (8f), in scCO₂, was performed. No insertion of CO₂ into the Co-C σ-bond to form tetraphenylpyrone (7f) by reductive elimination was observed, instead the cobaltcyclobutadiene complex CpCo(η⁴-C₄[Ph]₄) (9f) was formed. In the reactions with other alkynes, lower yields were obtained in general, except in the cyclotrimerisation of the highly activated alkyne, propargyl alcohol (b). Reaction of the non-activated alkynes, 1-hexyne (c) and 3-hexyne (d), produced complex mixtures of cobalt complexes in low yield in which the alkyne was coordinated to cobalt. Finally, the highly hindered diphenylacetylene (f) gave a mixture of the known complexes CpCo(η⁴-C₄[Ph]₄) (9f) and CpCo(η⁴-C₄[Ph]₄CO) (6f) in agreement with the results observed in conventional organic solvents.

Full text of DOI:10.1016/S0022-328X(01)00856-7

Journal of Organometallic Chemistry 632 (2001) 113–118
www.elsevier.com/locate/jorganchem
CpCo(CO)2-catalysed cyclotrimerisation of alkynes in supercritical
carbon dioxide
Francisco Montilla, Teresa Avile´s *, Teresa Casimiro, Ana Aguiar Ricardo,
Manuel Nunes da Ponte
Departamento de Qu´ımica, Centro de Qu´ımica Fina e Biotecnologia, Faculdade de Ciencias e Tecnologia, Uni6ersidade No6a de Lisboa,
2829-516 Caparica, Portugal
Received 23 February 2001; accepted 7 April 2001
Abstract
The reactivity of mono-substituted HCꢀCR (R=Ph, a; CH2OH, b; CH2CH2CH2CH3, c) and di-substituted RCꢀCR (R=
CH2CH3, d; CO2CH3, e; Ph, f) acetylenes was studied in supercritical carbon dioxide (scCO2) using the easily available complex
CpCo(CO)2 as catalyst. The reaction of phenylacetylene produced a mixture of the isomeric cyclotrimers 1,3,5- (2a) and
1,2,4-triphenylbenzene (2a%), in a 1:5 ratio, and traces of cobaltcyclopentadienone complexes CpCo(h4-C4H2[Ph]2CO) (6a, mixture
of isomers). The possible product formed by the incorporation of CO2 to alkynes, i.e. diphenylpyrone (7a) was not observed. The
reaction of the cobaltacyclopentadiene complex CpCo(1.4-s-C4[Ph]4)(PPh)3 (8f), in scCO2, was performed. No insertion of CO2
into the CoꢁC s-bond to form tetraphenylpyrone (7f) by reductive elimination was observed, instead the cobaltcyclobutadiene
complex CpCo(h4-C4[Ph]4) (9f) was formed. In the reactions with other alkynes, lower yields were obtained in general, except in
the cyclotrimerisation of the highly activated alkyne, propargyl alcohol (b). Reaction of the non-activated alkynes, 1-hexyne (c)
and 3-hexyne (d), produced complex mixtures of cobalt complexes in low yield in which the alkyne was coordinated to cobalt.
Finally, the highly hindered diphenylacetylene (f) gave a mixture of the known complexes CpCo(h4-C4[Ph]4) (9f) and CpCo(h4-
C4[Ph]4CO) (6f) in agreement with the results observed in conventional organic solvents. © 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Supercritical CO2; Alkyne; CpCo(CO)2; Cyclotrimerisation
1. Introduction
simultaneously as a reagent, as a C1 building block
[4,5].
Interest in supercritical carbon dioxide (scCO2) as
reaction medium has greatly expanded in the last few
years [1], mainly as an alternative to conventional or-
ganic solvents. Advantages of using scCO2 include
higher selectivity, increased reaction rates and easy
separation of reactants, catalyst and products after
reaction [2]. Moreover, scCO2 is non-toxic, non-
flammable, cheap, and presents moderate critical con-
stants (Tc=31.06 °C, Pc=73.825 bar). All these
properties demonstrate the potential of scCO2 as an
alternative substitute of the environmental unfriendly
organic solvents in a number of applications, in partic-
ular in the world of catalysis [3]. Special interest has
been focussed on the use of scCO2 as a solvent and
We are interested in this dual behaviour of scCO2
both as solvent and as reactant. For that purpose, we
have systematically studied the reactivity in scCO2 of
several unsaturated organic substrates in the presence
of transition metal complexes. Thus, we have recently
studied the metal-complex mediated activation of
arylisocyanates in scCO2 [6].
As an extension of our own work in this area, we
describe here the reactivity of alkynes in scCO2 using
the cobalt(I) complex dicarbonyl(h5-cyclopentadienyl)-
cobalt, CpCo(CO)2, as activator. As discussed below, in
all the reactions studied the observed products from
alkyne activation were either the corresponding cy-
clotrimers or cobalt compounds with the alkyne coordi-
nated. However, no coupling of CO2 and alkyne was
observed in any case.
* Corresponding author. Fax: +351-21-2948550.
E-mail address: tap@dq.fct.unl.pt (T. Avile´s).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00856-7

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F. Montilla et al. / Journal of Organometallic Chemistry 632 (2001) 113–118
CO2–alkyne coupling using Ni(cod)2/PR3 as catalyst
Cyclotrimerisation of alkynes to give benzene deriva-
was reported by Inohue et al. [18] and, recently, this
reaction has been reported in scCO2, in an interesting
example, in which CO2 is used simultaneously as sol-
vent and reactant [5].
Therefore, in each of the alkyne reactions performed
in scCO2 using CpCo(CO)2 as catalyst, the expected
products could be the cyclotrimer (2), cyclopenta-
dienone (6) and pyrone (7), either as free organic
product or coordinated to the organometallic fragment.
These products would form, respectively, by insertion
of alkyne, CO or CO2 into the metal–carbon s-bond of
the cobaltacyclopentadiene complex 1 (see Eq. (1), iso-
tives is a typical metal-catalysed reaction, proceeding
with a wide range of transition metal systems [7], the
complex CpCo(CO)2 being one of the most effective
catalysts [8]. This reaction is well known in conven-
tional solvents and has been recently carried out in
supercritical water [9], but to the best of our knowl-
edge, there are no studies performed in scCO2.
Herein, we present the reactivity of both mono-sub-
stituted HCꢀCR (R=Ph, a; CH2OH, b; CH2CH2-
CH2CH3, c) and di-substituted RCꢀCR (R=CH2CH3,
d; CO2CH3, e; Ph, f) acetylenes in scCO2, catalysed by
CpCo(CO)2. For comparison purposes, in some cases,
the reactions were performed in parallel in toluene and
in neat alkyne.
mers are omitted for clarity).
(1)
2. Results and discussion
We have studied the reactivity of several alkynes in
scCO2 using the complex CpCo(CO)2 as activator.
The cobalt complex CpCo(CO)2 is a well-known
catalyst in the intermolecular co-cyclisation of two
alkynes with an unsaturated substrate. The cobalta-
cyclopentadiene derivative 1 (Scheme 1), formed by
oxidative cyclisation of two coordinated alkynes, has
been proposed as intermediate in the catalytic cycle
[10,11]. Insertion of the unsaturated substrate into the
metal–carbon s-bond followed by reductive elimina-
tion affords the corresponding co-cyclisation product.
A number of [2+2+2] cycloaddition examples of
alkynes with a variety of unsaturated substrates such as
alkynes, nitriles, olefins, isocyanates and carbon
monoxide has been reported, giving, respectively, arene
(2) [12,13], pyridine (3) [14], cyclohexadiene (4) [15],
pyrimidone (5) [16] and cyclopentadienone (6) [17] (see
Scheme 1, alkyne substituent omitted for clarity).
Carbon dioxide should be a suitable unsaturated
substrate to be inserted into the cobaltacyclopentadiene
derivative 1 to form pyrones. Formation of pyrones by
In general, the reactions performed in scCO2 were
carried out in the presence of CpCo(CO)2 in a alkyne/
catalyst ratio of 30:1 (3 mol%). Both catalyst and
alkyne were placed in a high-pressure cell, charged with
CO2 to the required pressure and heated to the reaction
temperature, giving, in all cases, a fully homogenous
single-phase. Two different high-pressure cells were
used. A 30 ml cell equipped with two sapphire windows
for photochemically activated reactions and a steel
reactor (165 ml) for non-photochemically activated re-
actions requiring higher temperatures.
Activation of alkynes by the complex CpCo(CO)2, in
common organic solvents, can be achieved either pho-
tochemically or thermally. The reaction of pheny-
lacetylene, using CpCo(CO)2 as activator in scCO2, and
performed at 90°C did not take place at all; however,
when the system was photochemically activated using
visible light (200 W), a colourless solid precipitated out
inside the cell. Analysis of the products revealed a
mixture of the isomeric cyclotrimers 1,3,5- (2a) and
1,2,4-triphenylbenzene (2a%), in a 1:5 ratio yield (22%)
(see Eq. (2)) and traces of cobalt-coordinated cyclopen-
tadienone complexes (6a, mixture of isomers), but no
pyrone was observed.
(2)
Scheme 1.

F. Montilla et al. / Journal of Organometallic Chemistry 632 (2001) 113–118
115
Table 1
Reactivity of PhCCH in scCO2
a
Entry
A/C
P(CO2) (bar)
T (°C)
Benzene derivative (%)
Pyrone (%)
Cyclopentadienone derivative (%)
1
30:1
30:1
3:1
180
180
180
190
90
90
90
–
22
15
70
–
0
0
0
–
0
B5
0
2 b
3 b
4
30:1
150
a A=alkyne, C=CpCo(CO)2.
b Reaction performed using visible light (200 W).
Similar results were found when the reaction was
repeated with a stoichiometric ratio (3:1) of pheny-
lacetylene/CpCo(CO)2, although a higher yield of com-
plex 6a with respect to the cyclotrimers was obtained.
Finally, when thermal activation was used (T=
150 °C), the only observed products were a mixture of
cyclotrimers. Yield 70%. Table 1 shows the data for the
activation of phenylacetylene in scCO2.
The absence of pyrone-derivative 7a in the products
show that no insertion of CO2 into the metal–carbon
s-bond of the cyclopentadiencobalt complex 1 (Eq. (1))
took place, which could be explained due to competi-
tion between CO2 and the alkyne and CO present in the
reaction medium.
The proposed mechanism for this reaction involves a
first step consisting of the coordination of the unsatu-
rated substrate to the coordinatively unsaturated cobalt
centre in complex 1 followed by insertion into the
metal–carbon s-bond. Since CO2 is a much weaker
s-donor and a weaker p-acceptor than CO or alkyne,
in the presence of these two substrates, CO2 could not
form the corresponding adduct with complex 1. How-
ever, several MꢁCO2 adducts, formed by substitution of
CO in carbonyl transition metal complexes, in scCO2,
have been detected [19].
toluene as co-solvent, affording an orange homoge-
neous single phase (presence of toluene did not affect
the reactivity of 8f). Heating was required in order to
generate the coordinatively unsaturated intermediated
1f from 8f by removing phosphine. Complex 8f did not
react at temperatures below 100 °C, but at tempera-
tures up to 120 °C the colour of the supercritical
solution changed to yellow. After cooling at room
temperature and slow decompression, an orange crys-
talline solid was obtained, which was characterised as
(h5 - cyclopentadienyl)(h4 - tetraphenylcyclobutadiene)-
cobalt(I), CpCo(h4-C4[Ph]4) (9f) [12,22] (see Eq. (3)).
No traces of tetraphenylpyrone were detected. Similar
results were obtained when the reaction was performed
at higher temperatures (150 °C) and in the absence of
toluene as co-solvent. Although complex 8f presented
low solubility in scCO2, the cyclobutadienecobalt com-
plex 9f exhibited a reasonable solubility (solubility of 9f
in scCO2 was not calculated).
(3)
In order to be able to confirm this hypotheses, the
complex
(h5-cyclopentadienyl)(triphenylphosphine)-
2,3,4,5-tetraphenylcobaltacyclopentadiene, CpCo(1.4-
s-C4[Ph]4)(PPh)3 (8f) was prepared according to the
previously described procedure, namely by reaction of
CpCo(PPh3)2 and diphenylacetylene (f) [20] (complex
8a was not prepared in order to avoid a complicated
mixtures of isomers).
Complex 8 has been widely employed by Yamazaki
and Wakatsuki as a versatile precursor for stoichiomet-
ric and catalytic production of a number of organic
cyclic compounds [21] and it has been used in mecha-
nistic studies of the CpCo(CO)2-catalysed cyclisation of
alkynes, as precursor for the cobaltacyclopentadiene
complex 1 [10]. Therefore, 8f provides an excellent
precursor to study the decomposition of complex 1f in
scCO2 in the absence of alkyne and CO.
These results lead us to the conclusion that scCO2 is
too weak a s-donor and p-acceptor to form the inter-
mediate adduct with the cobaltcyclopentadiene complex
1 and, consequently, cannot insert into the cobalt–car-
bon s-bond, which will afford the corresponding py-
rone, by reductive elimination.
The steric hindrance as the factor responsible for the
non-formation of pyrone can also be discarded, since
2-oxo-1,2-dihydrotetraphenylpyridines can form by re-
action of the cobaltacyclopentadiene 8f and isocyan-
tates, isoelectronic but sterically more demanding than
CO2 [23].
In Table 2 the cyclotrimerisation of phenylacetylene
in different solvents, using photochemical activation, is
compared. It can be observed that, when the reaction
was performed in scCO2, conversions to the benzene-
derivative are worse than those obtained when per-
Solubility studies of complex 8f in scCO2 were per-
formed, the solubility proved to be quite small, but
could be increased by addition of small quantities of

116
F. Montilla et al. / Journal of Organometallic Chemistry 632 (2001) 113–118
Table 2
sapphire windows (limitations on the design of the
high-pressure visual cell prevent to heat at temperatures
up to 120 °C). In general, low yields were obtained in
all the cases studied, except in the cyclotrimerisation of
the highly activated propargyl alcohol (b). Complex
mixtures of the alkyne incorporated in cobalt com-
pounds were obtained in low yield with the non-acti-
vated alkynes 1- and 3-hexyne. Finally, the highly
hindered alkyne diphenylacetylene (f) produced a mix-
ture of the previously known complexes CpCo(h4-
C4[Ph]4) (9f) and CpCo(h4-C4[Ph]4CO) (6f) in
agreement with that observed in conventional organic
solvents [12,17].
Data for the cyclotrimerisation of PhCCH in different solvents
a
Entry Solvent
A/C
T (°C) b Arene total (%)
1
2
scCO2
Neat
phenylacetylene
Toluene
30:1
30:1
90
90
22
51
3
30:1
90
84
a A=alkyne, C=CpCo(CO)2.
b Reaction performed using visible light (200 W).
formed in toluene or in neat alkyne. These differences
could be explained by limitations on the design of the
high-pressure cell, which prevent high efficiency in the
photochemical process in scCO2. When the cy-
clotrimerisation was thermally activated, results were
similar to those obtained from analogous reactions
carried out in conventional solvents. The main advan-
tage of carrying out the reaction in scCO2 with respect
to toluene or neat alkyne includes a better separation of
the resulting benzene derivative (the benzene derivative
precipitate out of the supercritical solution, while the
non-reacted alkyne can be easily removed in the decom-
pression process).
In order to check the influence of substituents of
different nature on the alkyne, we have extended our
studies to other alkynes with different electronic and
steric properties than phenylacetylene. Thus, we have
chosen more activated alkynes such as propargyl alco-
hol (b), dimethylacetylendicarboxilate (e) and dipheny-
lacetylene (f), or less activated alkynes such as 1-hexyne
(c) and 3-hexyne (d). Similar to the reaction with
phenylacetylene, no pyrone was formed in any of the
studied reactions. The corresponding cyclotrimers or
complex mixtures of cobalt compounds were obtained
instead.
3. Experimental
3.1. General
All reactions and manipulations of solutions were
performed under an Ar atmosphere. Solvents were
reagent grade and were dried according to literature
methods; alkynes were purchased from Aldrich and
were used as supplied. CpCo(CO)2 [12] and CpCo(1.4-
s-C4[Ph]4)(PPh)3 [20] were prepared as previously
reported.
A simple high-pressure apparatus, described else-
where [6], was used to study the reactivity in scCO2.
Two different cells were used. A 30-ml high-pressure
view cell, equipped with sapphire windows to allow
visual inspection, was employed in all reactions carried
out at temperatures below 120 °C. For photochemi-
cally activated reactions, a visible light lamp (200 W)
was coupled outside the cell (see Fig. 1).
When the required temperature was higher than
120 °C, a simple high-pressure stainless steel reactor
(165 ml) was used. All reagents and catalysts were
charged into the cell under Ar. The cell was placed in
the high-pressure apparatus and CO2 (purity 99.9999%
from Carburos Meta´licos) was transferred into the cell
using a liquid pump.
In Table 3 the data for the cyclotrimerisation of these
mono- and di-substituted acetylenes in scCO2 are pre-
sented. Although better conversions can be reached by
thermal activation (heating at 150 °C), all reactions
were photochemically performed in order to follow the
course of the reaction by visual inspection through
Table 3
Data for the cyclotrimerisation of alkynes in scCO2
Alkyne RCꢀCR%
A/C a
P(CO2) (bar)
T (°C) b
Arene total (%)
Isomer ratio
1, 3, 5
1, 2, 4
R=C6H5, R%=H
R=CH2OH, R%=H
R=(CH2)3CH3, R%=H
R=R%=CH2CH3
R=R%=CO2Me
30:1
30:1
30:1
22:1
30:1
180
180
185
166
220
90
90
90
90
90
22
60
0
0
17
1
1
–
–
–
5
3
–
–
–
a A=alkyne, C=CpCo(CO)2.
b Reaction performed using visible light (200 W).

F. Montilla et al. / Journal of Organometallic Chemistry 632 (2001) 113–118
117
Fig. 1. 1, CO2 cylinder; 2, pneumatic liquid pump; 3, visible light; 4, pressure transducer; 5, temperature controller; 6, heating tape; 7, stir plate;
8, Schlenck tube; 9, vent line; 10, argon line.
After reaction, the cell was depressurised using a
Schlenck tube as a trap. The contents of both, the cell
the reaction mixture. After 24 h, CO2 was vented and
the residue obtained was extracted with CH2Cl2,
filtered, and the solvent was removed under reduced
pressure. The solid residue was analysed and shown to
be the mixture of cyclotrimers 1,3,5- (2a) and 1,2,4-
triphenylbenzene (2a%) (1:5 ratio) by comparison with
an authenticated sample. Yield: 0.095 g (22%).
1
and Schlenck tube, were analysed by H-, 13C-NMR,
IR and MS and compared with authenticated samples.
Cyclotrimer ratio was determined by analysis of the
1
integrals in the H-NMR spectrum.
Traces of CpCo(h4-C4H2[Ph]2CO) (6a, mixture of
isomers) were also observed by NMR and by MS
(M+H)+ m/z: 357. Better yield of 6a can be obtained
by the reaction of CpCo(CO)2 and phenylacetylene in a
1:3 ratio. The spectroscopic data agree with those re-
ported in the literature [9b].
This reaction was repeated, heating at 150 °C, af-
fording the mixture of isomers 2a and 2a% in a 1:5 ratio
(70% yield).
Reactions with others alkynes were performed using
the same procedure as described above. Alkyne and
CpCo(CO)2 were loaded into the high-pressure view cell
and heated at 90 °C. CO2 was then added until com-
plete dissolution of the catalyst and alkyne in the
reaction mixture. All reactions were photochemically
activated using an external visible light (200 W). After
reaction, products were analysed by NMR and MS. No
formation of pyrone was observed in any of the studied
reactions. Propargyl alcohol (b) gave a mixture of
isomers 1,3,5- (2b) and 1,2,4-tri(hydroximethil)benzene
3.2. Physical methods
Infrared spectra were recorded as mulls on NaCl
plates using an ATI Mattson Genesis FTIR
spectrometer.
Elemental analyses were performed at the Microana-
lytical Laboratory of Universidade Te´cnica de Lisboa,
Portugal.
Mass spectra were recorded on a VG Autospec using
the LSIMS technique using 3-nitrobenzylalcohol as a
matrix and a cesium gun at the Instituto de Ciencias de
Materiales de Aragon, Zaragoza, Spain. H-, and 13C-
1
NMR spectra were recorded on a Bruker ARX 400
1
spectrophotometer. H-NMR spectra were recorded us-
ing TMS as internal reference and 13C-NMR spectra
were referenced using the 13C resonance of the solvent
as internal standard.
3.3. Reacti6ity of alkynes catalysed by CpCo(CO)2 in
scCO2
(2b%) in
a 1:3 ratio (yield: 60%) and CH3CO2-
CCCO2CH3 (e) afforded hexamethylbenzenehexacar-
boxilate (2e) in a 17% yield. The remaining alkyne was
recovered unreacted.
The alkynes 1-hexyne (c) and 3-hexyne (d) were
recovered unreacted or were incorporated into cobalt
compounds giving complex mixtures, but no cy-
clotrimers were obtained.
Reaction of CpCo(CO)2 and diphenylacetylene (f)
produced traces of the known cyclopentadienylcobalt
complexes CpCo(h4-C4[Ph]4) (9f), MS (M+H)+ m/z:
480 [12,22] and CpCo(h4-C4[Ph]4CO) (6f), MS (M+
CpCo(CO)2 is highly soluble in scCO2. Its solubility
has been published recently by us [6].
A typical reaction: a mixture of CpCo(CO)2 (0.026 g,
0.14 mmol) and phenylacetylene (0.44 g, 4.4 mmol)
(30:1 ratio) was charged in the high-pressure view cell.
CO2 was transferred into the cell, immediately the cell
was heated to 90 °C and a visible light lamp (200 W)
was coupled. When the final temperature was reached,
additional CO2 was added up to a final pressure of 180
bar. The presence of a single fluid phase was confirmed
visually, with the catalyst being completely dissolved in

118
F. Montilla et al. / Journal of Organometallic Chemistry 632 (2001) 113–118
H)+ m/z: 509 [17]. The spectroscopic data of these
complexes are in agreement with those reported in the
literature.
(b) G.P. Jessop, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 118
(1996) 344.
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[6] F. Montilla, E. Clara, T. Avile´s, T. Casimiro, A.A. Ricardo, M.
Nunes da Ponte, J. Organomet. Chem. 626 (2001) 227.
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3.4. Reacti6ity of CpCo(1.4-|-C4[Ph]4)(PPh)3 (8f) in
scCO2
Complex 8f (0.077 g, 0.10 mmol) was loaded into the
high-pressure view cell under Ar. The reactor was then
sealed, heated to 120°C and, finally, CO2 was charged,
bringing the pressure to 230 bar. Compound 8f is red;
however, no colour was observed in the supercritical
phase, due to the insolubility of 8f in scCO2, although,
as the reaction proceeded a progressive colour change
to orange was observed. After 12 h, the reaction was
stopped and the CO2 was slowly vented. An orange
solid (0.03 g) precipitated inside the cell, which was
characterised by NMR and elemental analyses as
CpCo(h4-C4[Ph]4) (9f) [12,22]. Yield: 62%.
[10] D.R. McAlister, J.E. Bercaw, R.G. Bergman, J. Am. Chem. Soc.
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Similar results were obtained when the reaction was
repeated adding toluene (1 ml) as co-solvent or heating
at higher temperature (150 °C).
Acknowledgements
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The authors acknowledge the financial support from
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