New protocol for the catalytic Pauson-Khand reaction induced by molecular sieves

Article abstract of DOI:10.1016/S0040-4039(02)01196-6

The catalytic Pauson-Khand reaction is favoured in the presence of molecular sieves. This easy protocol probably involves the adsorption of carbon monoxide by the zeolites, making it possible to effect the reaction in the absence of CO atmosphere.

Full text of DOI:10.1016/S0040-4039(02)01196-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5763–5765
New protocol for the catalytic Pauson–Khand reaction induced
by molecular sieves
Jaime Blanco-Urgoiti, Luis Casarrubios, Gema Dom´ınguez and Javier Pe´rez-Castells*
Departamento de Qu´ımica, Facultad de CC. Experimentales y de la Salud, Universidad San Pablo-CEU, Boadilla del Monte,
28668 Madrid, Spain
Received 23 April 2002; revised 14 June 2002; accepted 20 June 2002
Abstract—The catalytic Pauson–Khand reaction is favoured in the presence of molecular sieves. This easy protocol probably
involves the adsorption of carbon monoxide by the zeolites, making it possible to effect the reaction in the absence of CO
atmosphere. © 2002 Elsevier Science Ltd. All rights reserved.
There has been a considerable effort in recent years to
achieve protocols for a catalytic version of the Pauson–
Khand reaction.1 Some of the reports dealing with this
process use severe reaction conditions,2 non-commercial
metal complexes3 or other chemical additives.4
Recently, Livinghouse5 and Krafft6 have introduced
new methods that reach good results using 1 atmo-
sphere of carbon monoxide at 65°C. Still, these meth-
ods are used only with a few substrates appropriately
gem-disubstituted or bearing heteroatoms that act as
soft ligands. Also, chemical additives such as cyclohexyl-
amine or amine N-oxydes are often necessary.
sion of this process, compound 1 was submitted to the
set of reaction conditions summarised in Table 1. Con-
sidering the above mentioned studies,5,6 we fixed the
reaction temperature at 65°C for all reactions, and the
CO pressure at 1 atm. Cobalt octacarbonyl was used as
supplied, without purification.
As shown in Table 1, there are no significant differences
between toluene and DME as solvents; so we chose
toluene for the following reactions. At the beginning we
found some erratic behaviour with the results. Thus
there was an increase in yield when raising the amount
of cobalt to 10%, that seemed excessive. We realised we
had opened a new bottle of cobalt carbonyl. When
repeating the reactions with 5% of the new cobalt,
yields increased (Table 1, entries 1, 2, 5, 6). The use of
preformed cobalt–hexacarbonyl complex 3 also gives
good results but does not improve the use of commer-
cial cobalt–octacarbonyl. The conclusion is that molec-
ular sieves always increase significantly the conversion
and yield of the reaction.
The scope of the stoichiometric version of the Pauson–
Khand reaction has increased in past years as new
promoters have been found.7 In particular we have
recently introduced zeolites as new efficient promoters
for Pauson–Khand reactions.8 Herein we report a new
protocol for a catalytic version of the Pauson–Khand
reaction showing the important role of the molecular
sieves in the process. It is well known that zeolites are
able to adsorb small molecules such as CO and this
may help in the recovery of the cobalt catalyst and thus
in improving the catalytic cycle.
Molecular sieves have been used in several catalytic
organic transformations11 such as epoxidation of allylic
alcohols (Sharpless epoxidation),12 or Diels–Alder reac-
Aromatic enynes like 1 have been introduced by us as
new substrates for the intramolecular Pauson–Khand
reaction (Scheme 1).9 This substrate reacts in a totally
diastereoselective manner in the stoichiometric Pauson–
Khand reaction.10 In order to achieve a catalytic ver-
O
H
H
PKR
H
OH
OH
OH
3
1
2
Keywords: Pauson–Khand reaction; zeolites; catalytic.
* Corresponding author. Tel.: +34913724700; fax: +34913510475;
e-mail: jpercas@ceu.es
Scheme 1.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01196-6

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J. Blanco-Urgoiti et al. / Tetrahedron Letters 43 (2002) 5763–5765
Table 1. Reaction conditions for the PKR of compound 1
No
Solventa
Catalyst
Mol. sievesb
Yield (%)c
2
1
1
2
3
4
5
6
7
8
Toluene
Toluene
Toluene
Toluene
DME
DME
DME
DME
Toluene
Toluene
Toluene
Toluene
5% Co2(CO)8
5% Co2(CO)8
10% Co2(CO)8
10% Co2(CO)8
5% Co2(CO)8
5% Co2(CO)8
10% Co2(CO)8
10% Co2(CO)8
5% 3
–
Yes
–
Yes
–
Yes
–
Yes
–
Yes
–
20/35d
40/60d
40
45/25d
40/25d
30
70
10
15/20d
40/55d
45
55/50d
30/20d
30
70
40
50
50
10
45
50
35
9
10
11
12
5% 3
10% 3
10% 3
Yes
70
0
a THF was also tested observing decomposition of starting material.
b Powdered molecular sieves preheated in oven at 125°C for 4 h and cooled under argon. Two times the mass of the starting material.
c Of pure material with correct spectroscopic data (1H, 13C NMR, IR).
d Results with ‘old’/‘new’ (recently opened) cobalt hexacarbonyl.
tions.13 In the Pauson–Khand process, molecular sieves
may act by interacting with the enyne and stabilising a
pretransition state or they may protect the catalyst.
Nevertheless, we believe that adsorption of CO by the
sieves may be responsible for this favourable effect. To
support this, we submitted compound 4 to three reac-
tions in air atmosphere with 10% cobalt octacarbonyl:
blank reaction, addition of molecular sieves, and addi-
tion of molecular sieves heated to 200°C and cooled
under carbon monoxide (Scheme 2).
In conclusion, molecular sieves improve the conversion
of the catalytic Pauson–Khand reaction, probably due
to their ability to adsorb and keep carbon monoxide
which makes it possible to effect the reaction in the
absence of a CO atmosphere. We will further report the
complete study on the role of the zeolites in this
reaction.
Table 2. Compared catalytic PKR of different substrates
The results show an important increase in conversion
with sieves, reaching 65% with the pre-treated zeolite.
The latter conditions may be considered as the first
efficient catalytic conditions of the Pauson–Khand
reaction without any CO atmosphere.14 Thus we heated
a batch of molecular sieves at 200°C (3 h) and cooled
them under a CO atmosphere, and used them in PKR
of several substrates depicted in Table 2. The reactions
were carried out in toluene and in an air atmosphere
with 10% cobalt octacarbonyl (conditions A). The
results were compared with the best reaction conditions
of Table 1 (entry 4) which correspond to 10% cobalt
carbonyl in toluene with molecular sieves (conditions
B), and a blank essay without sieves and under 1 atm of
CO in toluene (conditions C).15 The results show that
yields improve remarkably when using molecular sieves,
and that conditions A also are an interesting alternative
to traditional reactions under a CO atmosphere,
although they do not reach the results of conditions B
under CO.
EtO2C
EtO2C
EtO2C
10% Co2(CO)8
O
toluene, air EtO
65 oC
2C
5
4
Without M.S.:
With M.S.:
15%
30%
With M.S. pretreated with CO: 65%
Scheme 2.

J. Blanco-Urgoiti et al. / Tetrahedron Letters 43 (2002) 5763–5765
5765
Acknowledgements
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This work was supported by DGCYT (MEC-Spain,
PB98-0053) and the Universidad San Pablo-CEU
(grant 02/01).
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,
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