Catalytic intermolecular Pauson - Khand reactions in supercritical ethylene

Article abstract of DOI:10.1002/(SICI)1521-3773(20000204)39:3<636::AID-ANIE636>3.0.CO;2-S

Ethylene is not only a substrate, but also a solvent: Catalytic intermolecular Pauson - Khand reactions of terminal alkynes were carried out in supercritical ethylene to provide 2-substituted cyclopentenones in moderate to high yields. Under these conditions, even a low pressure of CO (5 atm) is sufficient for the reaction to take place.

Full text of DOI:10.1002/(SICI)1521-3773(20000204)39:3<636::AID-ANIE636>3.0.CO;2-S

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[11] The shielding constants of TMS at this level are 321.38 (for Si), 183.19
(for C) and 32.16 (for H).
[12] H.-O. Kalinowski, S. Berger, S. Braun, Carbon-13 NMR Spectroscopy,
Wiley, New York, 1986, p. 600.
[13] A. G. Brook, F. Abdesaken, G. Gutekunst, N. Plavac, Organometallics
1982, 1, 994.
[14] A. G. Brook, S. C. Nyburg, F. Abdesaken, B. Gutekunst, G. Gute-
kunst, R. K. M. R. Kallury, Y. C. Poon, Y. Chang, W. Wong-Ng, J. Am.
Chem. Soc. 1982, 104, 5668.
coupling of ethylene with the proper acetylenes under
Pauson ± Khand conditions provides an efficient and econom-
ical method for preparing synthetically useful cyclopent-
enones.[5]
Pauson ± Khand reactions of ethylene with stoichiometric
amounts of hexacarbonyldicobalt ± alkyne complexes have
been reported.[6, 7] Rautenstrauch et al. described the first
catalytic Pauson ± Khand reaction between heptyne and
ethylene in toluene in 1990.[8] This catalytic reaction showed
a high turnover number (220), but it suffered from the harsh
reaction conditions and had a low chemical yield (<50%).
This might be attributed to the low effective concentration of
ethylene in the solvent and the ill-defined, catalytic cobalt ±
carbonyl species formed under the reaction conditions
employed.
Catalytic Intermolecular Pauson ± Khand
Reactions in Supercritical Ethylene**
Encouraged by our success with CO2 as a supercritical fluid,
we first tried to apply the previously employed conditions for
the reaction with ethylene. We ran a series of reactions (48 h
at 808C) in supercritical CO2 (pfinal ˆ 200 atm) with phenyl-
acetylene (1a) and ethylene (10 ± 45 atm at 348C) with
various pressures of CO (1 ± 30 atm) and in the presence of
a catalytic amount of [Co2(CO)8] (I). The result was dis-
appointing and the chemical yield of 2-phenylcyclopentenone
(1b) never exceeded 10%. Therefore, we decided to seek the
possibility of a direct coupling reaction in supercritical
ethylene (Scheme 2).
Nakcheol Jeong* and Sung Hee Hwang
Supercritical fluids are rapidly becoming alternative reac-
tion media for conventional organic reactions.[1] The advan-
tages of reactions in supercritical fluids include the high
solubility of gaseous reactants, rapid diffusion of solutes, and
weakening of the solvation around the reacting species. In
addition, these fluids are easily recycled and allow the
separation of the dissolved compounds by selective precip-
itation, which implicates possibilities in the development of
environmentally benign processes. Sequential precipitation of
the product and catalyst is possible by a gradual release of
pressure.[2]
Recent progress has shown that supercritical fluids can
substitute conventional organic solvents in various transition
metal catalyzed transformations.[3] We demonstrated that
catalytic intra- and intermolecular Pauson ± Khand reactions
could be carried out in supercritical CO2 with high efficiency
(Scheme 1).[4] We report here a new dimension of this
O
R1
R1
cat., CO pressure
R2
R2
pressure
Scheme 2. Intermolecular Pauson ± Khand reactions between ethylene
and alkynes (see Table 1).
Supercritical ethylene can be obtained easily (Tc ˆ 98C,
pc ˆ 34 atm) and has been used for the preparation of the
complex [Cr(CO)5(CH2CH2)] and its polymer.[9, 10] We envi-
sioned that it can also be used for the Pauson ± Khand
reaction. Under these conditions, a high effective concen-
tration of the rather less reactive ethylene as well as of carbon
monoxide would be expected.
The catalysts examined with substrate 1a, which are all
soluble in supercritical ethyleneÐfor example, [Co2(CO)8]
(I), [Co4(CO)11{P(OPh)3}] (II), or [Co4(CO)12] (III), worked
well although the efficiency of each catalyst varied from case
to case. On the other hand, [Co2(CO)6{P(OPh)3}2] (IV), which
is a reliable catalyst in organic solvents,[5d] did not work
because of its low solubility in supercritical ethylene.
We optimized the reaction conditions with catalysts II and
III in most cases.[11] Ethylene (110 atm at 348C) and CO
(5 atm at 348C) were charged into a mixture of phenyl-
acetylene (3 mmol) and catalyst II (0.09 mmol) in a stainless-
steel reactor (80 mL). The reaction mixture was gradually
warmed to 858C over 6 h, where the pressure in the reactor
usually reached 200 ± 210 atm. The reaction mixture was
allowed to react at that temperature for 46 h. 2-Phenyl-
cyclopentenone (1b) was obtained in 80% yield (Table 1,
entry 1). Only 2-substituted cyclopentenone was obtained.
[Co2(CO)8], CO (30 atm)
sc CO2, 198 atm
EtO2C
EtO2C
EtO2C
EtO2C
90oC, 24 h
O
82 %
Scheme 1. Example for a Pauson ± Khand reaction in supercritical (sc)
CO2.
application, that is, an intermolecular Pauson ± Khand reac-
tion in supercritical ethylene. Ethylene is one of the most
abundant raw materials in the petrochemical industry. Direct
[*] Prof. Dr. N. Jeong, S. H. Hwang
Hanhyo Institutes of Technology, 461-6 Chonmin-dong
Yusong-ku, Taejon, 305-390 (Korea)
and
Department of Chemistry, Korea University
1-5ka Anamdong, Sungbukku, Seoul, 136-701 (Korea)
Fax : (‡ 82)2-3290-3121
E-mail: njeong@kuccnx.korea.ac.kr
[**] Part of this work was supported by a grant from the Center for
Molecular Design and Synthesis (CMDS) at KAIST.
Supporting information for this article is available on the WWW under
http://www.wiley-vch.de/home/angewandte/ or from the author.
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Table 1. Results of intermolecular catalytic Pauson ± Khand reactions in supercritical ethylene (Scheme 2).[a]
Entry
Substrate
R1
R2
Catalyst [mol%]
t[h]
Conversion[%]
Products
Yield[%]
1
2
3
4
5
6
7
8
9
1a
1a
2a
3a
4a
4a
5a
6a
6a
7a
8a
8a
Ph
Ph
C5H11
HO(CH2)3
Me3SiO(CH2)3
Me3SiO(CH2)3
CH3CH(OSiMe3)CH2
Me3SiOCH2
Me3SiOCH2
tBuMe2SiOCH2
MeO2C(CH2)2
MeO2C(CH2)2
H
H
H
H
H
H
H
H
H
H
H
H
II (3)
III (3)
II (3)
II (3)
II (3)
I (3)
II (5)
II (5)
III (5)
II (5)
II (5)
III (5)
46
24
42
46
64
30
44
24
48
24
30
40
100
100
100
100
100
50
100
100
100
100
100
100
1b
1b
2b
3b
3b
3b
5b
6b
6b
6b
8b
8b
80
77
70
46
82[b]
40[b]
74[b]
30[b]
23[b]
41[b]
75
10
11
12
87
[a] The optimum conditions for each substrate are given. The reactions were carried out in a bomb equipped with a sapphire window at 858C for the specified
reaction time. The initial pressure of CO was 5 atm and of ethylene was 110 atm at 348C. The liquid phase was observed at that temperature. At 858C the
reaction mixture became one red, homogeneous supercritical phase. It remained as a single phase even after completion of the reaction. [b] Yield of free
alcohol isolated after treatment with concentrated HCl.
Complications by alkyne trimerization leading to triphenyl-
benzenes (1,3,5- or 1,3,4-substituted) were not observed under
these reaction conditions. For 1a, catalyst III worked equally
well to provide 1b in 77% yield. It is noteworthy that catalyst
III was once considered as thermally inactive for the Pauson ±
Khand reaction.[12]
The reaction with aliphatic terminal alkyne 2a proceeded
uneventfully to provide 2b in 70% yield (Table 1, entry 3).
When alcohols were used as substrate the hydroxyl groups
needed to be protected for clean reaction and high chemical
yield. For example, alkynol 3a reacted without protection to
give the desired product 3b (entry 4) together with many
unidentified side products in only 46% yield after column
chromatography. Use of the trimethylsilyl-protected substrate
4a improved the reaction significantly to generate 3b in 82%
yield after treatment of the resulting Pauson ± Khand product
with acid (entry 5).
solvent. Under these conditions, even a low pressure of CO
(5 atm) is sufficient for the reaction to take place.[14]
Experimental Section
To 1a (306 mg, 3.0 mmol) in a pressure bomb was added [Co4(CO)12] (III,
54.5 mg, 0.091 mmol), and the bomb (80 mL) was flushed three times with
CO and then charged with CO (5 atm, 99.9%) at 348C. Subsequently
ethylene (99.5%) was added and then compressed to 110 atm. The reaction
mixture was warmed to 858C and allowed to react for 24 h. After that it was
cooled to room temperature, and the CO and excess ethylene were released
carefully in a well-vented hood. The reaction residue was dissolved in
acetone, concentrated in vacuo, and purified by chromatography (SiO2, n-
hexane/ethyl acetate 9/1) to give the product 2a as a colorless oil (366 mg,
2.3 mmol, 77%).[14]
Received: September 6, 1999 [Z13975]
[1] Chemical Synthesis using Supercritical Fluids (Eds.: P. G. Jessop, W.
Leitner), Wiley-VCH, Weinheim, 1999.
[2] J. F. Brennecke, Chem. Ind. (London) 1996, 831.
The trimethylsilyl-protected homopropargyl alcohol 5a
underwent the reaction very nicely to give 5b in 74% yield
(entry 7). However, propargyl alcohol derivatives 6a and 7a
were inferior to the previous substrates under these con-
ditions (entries 8 to 10). Regardless of the protecting groups
and reaction conditions employed, only modest chemical
yields were obtained (41% at best).
w-Alkynoate 8a provided the corresponding cyclopente-
none 8b in high yield (up to 87% with catalyst III; entries 11
and 12). [Co4(CO)12] (III) turned out to be the best catalyst for
this substrate. However, the limitation of the reaction in
supercritical ethylene was also evident. For example, a
disubstituted alkyne such as 2-butyne did not react under
these reaction conditions. A similar observation is also
reported in the literature.[13] [CpCo(CO)2]-catalyzed trimeri-
zation of alkyne in supercritical H2O is limited to terminal
alkynes. a,w-Bisalkyne (e.g. 1,7-octadiyne), which is a good
substrate in the intermolecular reaction with norbornadiene
in supercritical CO2, was reluctant to undergo the Pauson ±
Khand reaction.[4]
[3] a) P. G. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 1999, 99, 475, and
references therein; b) P. G. Jessop, Y. Hsiao, Y. Ikariya, R. Noyori, J.
Am. Chem. Soc. 1996, 118, 344; c) M. J. Burk, S. G. Feng, M. F. Gross,
W. J. Tumas, J. Am. Chem. Soc. 1995, 117, 8277; d) P. G. Jessop, T.
Ikariya, R. Noyori, Nature 1994, 368, 231; e) J. W. Rathke, R. J.
Klingler, T. R. Krause, Organometallics 1991, 10, 1350; f) S. Kainz, D.
Koch, W. Baumann, W. Leitner, Angew. Chem. 1997, 109, 1699;
Angew. Chem. Int. Ed. Engl. 1997, 36, 1628; g) A. Fürstner, D. Koch,
K. Langemann, W. Leitner, C. Six, Angew. Chem. 1997, 109, 2562;
Angew. Chem. Int. Ed. Engl. 1997, 36, 2466.
[4] N. Jeong, S. H. Hwang, Y. W. Lee, J. S. Lim, J. Am. Chem. Soc. 1997,
119, 10549.
[5] a) P. L. Pauson, I. U. Khand, Ann. N. Y. Acad. Sci. 1977, 295, 2; b) N. E.
Schore in Comprehensive Organometallic Chemistry II, Vol. 12 (Eds.:
E. W. Abel, F. G. A. Stone, G. Wilkinson), Pergamon, Oxford, UK,
1995, pp. 703 ± 739; c) N. Jeong in Transition Metals for Organic
Synthesis, Vol. 1 (Eds.: M. Beller, C. Bolm), Wiley-VCH, Weinheim,
1998, pp. 560 ± 577; for catalytic reactions, see d) N. Jeong, S. H.
Hwang, Y. Lee, Y. K. Chung, J. Am. Chem. Soc. 1994, 116, 3159;
e) B. Y.Lee, Y. K. Chung, N. Jeong, Y. Lee, S. H. Hwang, J. Am. Chem.
Soc. 1994, 116, 8793; f) B. L. Pagenkopf, T. Livinghouse, J. Am. Chem.
Soc. 1996, 118, 2285; g) N. Y. Le, Y. K. Chung, Tetrahedron Lett. 1996,
37, 3145.
[6] a) C. Johnstone, W. J. Kerr, U. Lange, J. Chem. Soc. Chem. Commun.
1995, 459; b) D. C. Billington, I. M. Helps, P. L. Pauson, W. Thomson,
D. J. Willison, J. Organomet. Chem. 1988, 354, 233; c) D. C. Billington,
W. J. Kerr, P. L. Pauson, J. Organomet. Chem. 1988, 341, 181.
In summary we have demonstrated an efficient catalytic
Pauson ± Khand reaction in supercritical ethylene. Supercrit-
ical ethylene can be used not only as a substrate but also as a
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Previous work on the structure of Ca4B[3] showed that the
classification ªazo pigmentº is actually a misnomer, as in the
solid state the pigment exists largely as the keto-hydrazone
tautomer (Scheme 1). Further, dideprotonation occurs at the
[7] a) A. R. Gordon, C. Johnstone, W. R. Kerr, Synlett 1995, 1083; b) S.
Shambayati, W. E. Crowe, S. L. Schreiber, Tetrahedron Lett. 1990, 31,
5289; c) N. Jeong, Y. K. Chung, B. Y. Lee, S. H. Lee, S. E. Yoo, Synlett
1991, 204.
[8] V. Rautenstrauch, P. Megard, J. Conesa, W. Kuster, Angew. Chem.
1990, 102, 1441; Angew. Chem. Int. Ed. Engl. 1990, 29, 1413.
[9] J. A. Banister, S. M. Howdle, M. Poliakoff, J. Chem. Soc. Chem.
Commun. 1993, 1814.
[10] S. Beuermann, M. Buback, M. Busch in Chemical Synthesis using
Supercritical Fluids (Eds.: P. G. Jessop, W. Leitner), Wiley-VCH,
Weinheim, 1999, pp. 326 ± 350.
[11] Caution! Although no incident occurred during this study, we
observed that highly purified [Co2(CO)8] and [Co4(CO)12] ignited
spontaneously at times upon contact with air. Special precaution
should be paid. Therefore, we prefer to use stable [Co4(CO)11-
{P(OPh)3}] to avoid any potential danger.
[12] J. W. Kim, Y. K. Chung, Synthesis 1998, 142.
Scheme 1. Tautomerism in red azo pigments.
[13] K. S. Jerome, E. J. Parsons, Organometallics 1993, 12, 2991.
[14] See ref. [8]. Note that a much higher pressure of CO (100 bar at the
inception) was required under their conditions.
sulfonate and carboxy groups but not at the hydroxy/hydrazo
group, and there is only one type of calcium binding site.
Harris et al.[3a] suggested from solid-state NMR measure-
ments that the calcium atom is complexed by sulfonate and
carboxylate groups from different molecules and thus pro-
posed a simple polymeric structure with azo molecules
bridged by Ca2‡ ions, whereas Fryer and coworkers[3c] showed
evidence from electron microscopy that Ca4B exists as head-
to-tail dimers.
We have now succeeded in growing, by slow evaporation of
a DMF solution, small crystals of a dichloro analogue of Ca4B
(1) suitable for characterization by synchrotron radiation.[4]
Compound 1 displays a complex, supramolecular structure of
a type not previously suspected for red azo pigments but
which we believe to be consistent with the experimental
evidence in the published work on Ca4B, and which we
propose to be a suitable model structure to relate to the
structure of many other red azo pigments.
The First Red Azo Lake Pigment whose
Structure is Characterized by Single Crystal
Diffraction**
Alan R. Kennedy,* Craig McNair, W. Ewen Smith,
Greig Chisholm, and Simon J. Teat
As organic pigments are used in the form of crystals
dispersed throughout the media to be colored, their color
characteristics depend not only on their molecular structure
but also crucially upon their crystal structure. Furthermore,
the crystal structure influences the stability of the pigment to
solvent, heat, and light and also determines the crystal
morphology which is in turn related to color strength, hiding
power, stability, flow, and dispersion properties.[1] As a good
pigment is, by design, highly insoluble, it is difficult to grow
the large, high-quality crystals needed for conventional
diffraction methods and thus little detailed structural infor-
mation is available. These generalizations are true of the red
azo pigments as exemplified by Ca4B toner (the calcium salt
The structural analysis confirms that the azo dianion is
deprotonated at the sulfonate and carboxylate groups. The Ca
atoms are eight coordinate through bonding to three sites
(carboxylate, keto, and sulfonate) on one azo anion, to two
terminal water molecules, and to three oxygen atoms supplied
by bridging interactions to the carboxylate groups of neigh-
boring azo anions (Figure 1). That the azo anion acts in such a
of
4-(4-methyl-2-sulfophenyl)azo-3-hydroxy-2-naphthalic
acid; also known as CIPR57:1) which is considered the
worldwide standard process red for printing and its continuing
industrial importance is attested to by the patent literature.[2]
[*] Dr. A. R. Kennedy, Dr. C. McNair, Prof. W. E. Smith
Department of Pure and Applied Chemistry
University of Strathclyde
295 Cathedral Street, Glasgow G1 1XL (UK)
Fax: (‡45)141-552-0876
E-mail: a.r.kennedy@strath.ac.uk
Dr. G. Chisholm
Ciba Specialty Chemicals PLC
Hawkhead Road, Paisley PA2 7BG (UK)
Figure 1. Molecular structure of 1 showing the coordination geometry
about Ca1. Selected distances [Š]: Ca1-O1 2.542(2), Ca1-O4 2.394(2), Ca1-
O5 2.385(2), Ca1-O1W 2.449(2), Ca1-O2W 2.352(2), Ca1-O5* 2.440(2),
Ca1-O6* 2.881(2), Ca1-O6** 2.374(2), N1-N2 1.304(2), N1-C6 1.395(2), N2-
C7 1.333(2), C8-O4 1.257(2), C7-C8 1.458(3), C8-C9 1.451(3), C9-C10
1.349(3). Symbols: * ˆ x ‡ 1, y ‡ 0.5, z ‡ 0.5; ** ˆ x, y ‡ 1, z.
Dr. S. J. Teat
CCLRC Daresbury Laboratory
Warrington WA4 4AD (UK)
[**] We thank Prof. R. E. Mulvey for valuable discussions and EPSRC for
funding.
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