Polymer-supported cobalt carbonyl complexes as novel solid-phase catalysts of the Pauson-Khand reaction

Article abstract of DOI:10.1039/a909462h

Cobalt carbonyl complexes immobilised onto a 'polymer-bound triphenylphosphine' solid support are effective and practical catalysts of the Pauson-Khand reaction.

Full text of DOI:10.1039/a909462h

Polymer-supported cobalt carbonyl complexes as novel solid-phase catalysts of
the Pauson–Khand reaction
Alex C. Comely,*a Susan E. Gibson (née Thomas)a and Neil J. Halesb
a Department of Chemistry, King’s College London, Strand, London, UK WC2R 2LS. E-mail: alex.comely@kcl.ac.uk
b AstraZeneca UK Ltd., Mereside, Alderley Park, Macclesfield, UK SK10 4TG
Received (in Liverpool, UK) 29th November 1999, Accepted 10th January 2000
Cobalt carbonyl complexes immobilised onto a ‘polymer-
bound triphenylphosphine’ solid support are effective and
practical catalysts of the Pauson–Khand reaction.
The synthesis of cyclopentenone derivatives via the cobalt
carbonyl-mediated annulation of an alkyne, an alkene and
carbon monoxide, the Pauson–Khand (P–K) reaction, was first
described in 1971.1 A synthetically very important process,2 the
P–K reaction is routinely applied using a stoichiometric amount
of the transition metal complex. Although the catalytic process
was reported as early as 1973,1b it was confined to the strained
reactive alkenes norbornene and norbornadiene.
Scheme 1
Ensuing years saw little development of the catalytic P–K
reaction until a protocol developed by Rautenstrauch,3 albeit
one demanding extremely forcing conditions (310–360 bar
ethene/CO, 150 °C) to realise only moderate yields, inspired
several new approaches to catalysis. Promoters such as
1,2-dimethoxyethane or water have been investigated,4 the
addition of phosphites is shown to prevent inactive cluster
formation5 and the use of supercritical CO2 as a reaction
medium is described.6 Alternative sources of zero valent cobalt
In light of the mild and very attractive conditions reported by
Livinghouse for the catalytic P–K reaction, and in particular the
thermal window identified between 60 and 70 °C,11 it was these
conditions which were used as the basis for our initial
experiments with the cobalt–carbonyl loaded resins.¶ At 65 °C
we were very encouraged by the observation that 5 mol% of
either precursor 2 + 3 or 4 effected a moderate conversion of
enyne substrate 517 to cyclopentenone 610 after 24 h in THF
[Co(acac)2/NaBH4 and (indenyl)Co(COD)8] and cobalt car-
7
bonyl clusters9 have also been employed. Although yields and
turnovers are excellent, these systems still suffer the require-
ment of high temperatures and pressures.
The current state-of-the-art can be attributed to Livinghouse
who in 1996 reported a photochemically driven process
requiring only mild temperatures (50–55 °C) and 1 atm CO.10
A
more recent report from the same group relates that careful
control of temperature to within a narrow window (60–70 °C)
dispenses with the need for photolytic promotion.11 Rigorous
purification of Co2(CO)8 in these systems can be obviated,
reports Krafft, by prior base washing of the glassware.12
Problems associated with the very labile Co2(CO)8, however,
have also spurred the development of stable cobalt–alkyne
catalyst precursors.13
Given our current interest in polymer-supported cobalt
complexes14 and increasing awareness of the environmental and
handling advantages conferred by such solid-phase method-
ologies,15 we decided to determine the viability of the catalytic
P–K reaction using immobilised cobalt carbonyls.† Our prelim-
inary investigations, which reveal for the first time that
polymer-supported cobalt complexes do indeed catalyse the P–
K reaction, are reported herein.
under a 50 mbar overpressure of CO (Table 1, entries 1 and 2).
In preliminary steps to optimise the methodology, the first
parameter investigated was temperature. An increase to 70 °C
(Table 1, entries 3 and 4) resulted in a significant increase in
conversion for both catalyst precursors, the bisphosphine-
substituted complex 4 again proving to be the most effective
Table 1 Conversion of substrates 5 and 7 into 6 and 8 respectively via the
catalytic Pauson–Khand reaction using cobalt resins 2 + 3 and 4¶
Catalyst
precursor
(5 mol%) T/°C
Yield (%)a
(isolated)
Entry
Substrate Product
The cobalt carbonyl resins to be tested as catalyst precursors
were prepared as follows.14 Reaction of ‘polymer-bound
triphenylphosphine’ 1‡ with Co2(CO)8 in THF at room
temperature generated a resin-bound mixture of phosphine-
substituted cobalt carbonyl complexes 2 and 3 (ca. 1+1)
(Scheme 1). The purple resin, which is significantly more air-
stable than Co2(CO)8, was characterised on the basis of its IR
and 31P NMR spectra.14 Heating this resin at 75 °C in
1,4-dioxane cleanly converted it into a second form, assigned
the structure of the neutral bisphosphine 4 on the basis of its IR
and 31P NMR spectra.14 Resin 4, however, can be more
conveniently prepared directly from 1 and Co2(CO)8§ without
prior isolation of 2 + 3.
1
2
3
4
5
6
7
8
2 + 3
4
2 + 3
4
2 + 3
4
2 + 3
4
65
65
70
70
75
75
70
70
5
5
5
5
5
5
7
7
6
6
6
6
6
6
8
8
22
48
37
66 (61)
21
28
28
57 (49)
a Describes percentage conversion measured by 1H NMR spectroscopy.
Isolated yields are those obtained after flash column chromatography.
Products were characterised by comparison of their 1H, 13C and IR spectra
with the literature values (ref. 10).
Chem. Commun., 2000, 305–306
This journal is © The Royal Society of Chemistry 2000
305

polymer beads with alternate aliquots of THF and Et2O, and concentration
of the combined filtrates in vacuo afforded cyclopentenone 6 [66%
conversion by 1H NMR spectroscopy; 85 mg, 61% isolated by flash
chromatography (SiO2, EtOAc–light petroleum, 2+8 to 4+6 gradient
elution), 0.31 mmol].
producing 66% conversion and 61% isolated yield of 6. The
consequence of a further 5 °C increase in temperature, however,
was a precipitous drop in conversion for both forms of the
catalyst alike (Table 1, entries 5 and 6).
A further substrate was subjected to our preliminarily
optimised conditions. Hence, enyne 710,18 was cyclised to
cyclopentenone 610 in a respectable 49% isolated yield, catalyst
precursor 4 again resulting in the highest conversion (Table 1,
entries 7 and 8).
1 (a) I. U. Khand, G. R. Knox, P. L. Pauson and W. E. Watts, J. Chem.
Soc., Chem. Commun., 1971, 36; (b) I. U. Khand, G. R. Knox, P. L.
Pauson, W. E. Watts and M. I. Foreman, J. Chem. Soc., Perkin Trans.
1, 1973, 977.
In conclusion, we have shown for the first time that solid-
phase cobalt carbonyl complexes have significant potential as
catalysts of the P–K reaction. As with all supported catalysts, a
very simple work-up procedure is required: filtration of the
polymer-bound catalyst and concentration of the filtrate. A
further advantage conferred by the increased air-stability of the
immobilised cobalt complexes is ease of handling using readily
available laboratory equipment. Our results, together with the
environmental advantages of immobilising metal carbonyls on
a solid support, suggest that this new approach to the P–K
reaction is worthy of considerable further investigation.
The authors wish to thank AstraZeneca UK Ltd. for a
studentship (A. C. C.).
2 For recent reviews see: O. Geis and H.-G. Schmalz, Angew. Chem., Int.
Ed., 1998, 37, 911; Y. K. Chung, Coord. Chem. Rev., 1999, 188, 297.
3 V. Rautenstrauch, P. Mégard, J. Conesa and W. Küster, Angew. Chem.,
Int. Ed. Engl., 1990, 29, 1413.
4 T. Sugihara and M. Yamaguchi, Synlett, 1998, 1384.
5 N. Jeong, S. H. Hwang, Y. Lee and Y. K. Chung, J. Am. Chem. Soc.,
1994, 116, 3159.
6 N. Jeong, S. H. Hwang, Y. W. Lee and J. S. Lim, J. Am. Chem. Soc.,
1997, 119, 10549.
7 N. Y. Lee and Y. K. Chung, Tetrahedron Lett., 1996, 37, 3145.
8 B. Y. Lee, Y. K. Chung, N. Jeong, Y. Lee and S. H. Hwang, J. Am.
Chem. Soc., 1994, 116, 8793.
9 J. W. Kim and Y. K. Chung, Synthesis, 1998, 142; T. Sugihara and M.
Yamaguchi, J. Am. Chem. Soc., 1998, 120, 10782.
10 B. L. Pagenkopf and T. Livinghouse, J. Am. Chem. Soc., 1996, 118,
2285.
11 D. B. Belanger, D. J. R. O’Mahony and T. Livinghouse, Tetrahedron
Lett., 1998, 39, 7637.
12 M. E. Krafft, L. V. R. Bonaga and C. Hirosawa, Tetrahedron Lett., 1999,
40, 9171.
13 D. B. Belanger and T. Livinghouse, Tetrahedron Lett., 1998, 39, 7641;
M. E. Krafft, C. Hirosawa and L. V. R. Bonaga, Tetrahedron Lett., 1999,
40, 9177.
14 A. C. Comely, S. E. Gibson and N. J. Hales, Chem. Commun., 1999,
2075.
Notes and references
† Polymer-bound P–K substrates have been cyclised in the presence of
stoichiometric amounts of Co2(CO)8 (ref. 16).
‡ ‘Polymer-bound triphenylphosphine’ (commercially available from
Fluka, ~ 1.6 mmol P g21) describes a diphenylphosphino polystyrene
polymer crosslinked with 1% divinylbenzene.
§ The experimental procedure for the formation of 4 is as follows:
commercial polymer-bound triphenylphosphine (1 g, ~ 1.6 mmol P) was
suspended in oxygen-free anhydrous 1,4-dioxane (15 cm3) and allowed to
swell for 30 min under N2 agitation. A solution of commercial Co2(CO)8
complex (383 mg, 1.12 mmol) in anhydrous, deoxygenated 1,4-dioxane (5
cm3) was added under nitrogen agitation, the black mixture was left at room
temperature for 30 min and subsequently heated to 75 °C for 16 h. After
cooling, the resin beads were filtered, washed with alternate aliquots of THF
and Et2O until the filtrate became colourless, and dried in vacuo to afford
deep purple beads of 4 {1.15 g, 50% P site complexation, 0.35 ± 0.05 mmol
[Co2(CO)6] g21}.
¶ A general experimental for the catalytic P–K reaction is as follows: resin
4 {63 mg, 0.025 mmol [Co2(CO)6]} and substrate 5 (125 mg, 0.5 mmol)
were combined in a 10 cm3 round-bottomed flask fitted with a condenser.
The apparatus was thoroughly purged with CO and sealed under 1.05 bar
CO. CO-saturated THF (5 cm3) was added and the mixture was heated to 70
°C for 24 h. Filtration of the pale brown mixture, thorough washing of the
15 A. R. Brown, P. H. H. Hermkens, H. C. J. Ottenheijm and D. C. Rees,
Synlett, 1998, 817; S. J. Shuttleworth, S. M. Allin and P. K. Sharma,
Synthesis, 1997, 1217.
16 For reports of the P–K reaction on polymer-bound P–K substrates: N. E.
Schore and S. D. Najdi, J. Am. Chem. Soc., 1990, 112, 441; J. L. Spitzer,
M. J. Kurth, N. E. Schore and S. D. Najdi, Tetrahedron, 1997, 53, 6791;
G. L. Bolton, Tetrahedron Lett., 1996, 37, 3433; G. L. Bolton, J. C.
Hodges and J. R. Rubin, Tetrahedron, 1997, 53, 6611.
17 F. E. Scully Jr. and K. Bowdring, J. Org. Chem., 1981, 46, 5077; W.
Oppolzer, A. Pimm, B. Stammen and W. E. Hume, Helv. Chim. Acta,
1997, 80, 623.
18 S. C. Berk, R. B. Grossman and S. L. Buchwald, J. Am. Chem. Soc.,
1994, 116, 8593.
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Chem. Commun., 2000, 305–306