Table 2 Cobalt-mediated PK reactions under aqueous conditions
2 (a) For the Nicholas reaction: B. J. Teobald, Tetrahedron, 2002, 58,
4133; (b) for conversion of aziridine to b-lactams: M. E. Piotti and H.
Alper, J. Am. Chem. Soc., 1996, 118, 111; (c) for conversion of
1-(1-alkynyl)cyclopropanols to 2-cyclopentenones: N. Iwasawa, T.
Matsuo, M. Iwamoto and T. Ikeno, J. Am. Chem. Soc., 1998, 120, 3903;
(d) for 1,4-reduction of unsaturated carbonyl compounds: H.-Y. Lee and
M. An, Tetrahedron Lett., 2003, 44, 2775; (e) for
a tandem
Yield (%)
Reaction
conditionsa
cycloaddition reaction: A. Oledra, C.-J. Wu, R. J. Madhushaw, S.-L.
Wang and R.-S. Liu, J. Am. Chem. Soc., 2003, 125, 9610.
3 For examples, see: (a) T. Sugihara and M. Yamaguchi, J. Am. Chem.
Soc., 1998, 120, 10782; (b) T. Sugihara, M. Yamaguchi and M.
Nishizawa, Chem. Eur. J., 2001, 7, 1589; (c) T. Sugihara, A.
Wakabayashi, Y. Nagai, H. Takao, H. Imagawa and M. Nishizawa,
Chem. Commun., 2002, 576; (d) T. Sugihara, A. Wakabayashi, H.
Takao, H. Imagawa and M. Nishizawa, Chem. Commun., 2001, 2456.
4 K. M. Brummond and J. L. Kent, Tetrahedron, 2000, 56, 3263.
5 S. E. Gibson and A. Stevenazzi, Angew. Chem., Int. Ed., 2003, 42,
1800.
PKR
Entry Substrate
adduct
I
II III
1
2
3
3
5
7
4
6
8
79 82 83
80 81 85
85 78 88
4
11
12
86 60
—
6 (a) M. E. Krafft and L. V. R. Boñaga, Angew. Chem., Int. Ed., 2000, 39,
3676 and references cited therein. Co4(CO)12 was initially used to
catalyze the PKR under harsh conditions (150 °C, 10 atm CO), and
believed to be detrimental to Co2(CO)8 catalysis in the PKR (b) J. W.
Kim and Y. K. Chung, Synthesis, 1998, 142.
5
6
13
15
14
16
78
82
—
—
78
40
7 M. E. Krafft, L. V. R. Boñaga, J. A. Wright and C. Hirosawa, J. Org.
Chem., 2002, 67, 1233.
8 Water, used as an additive, enhanced the rate of PK reactions in toluene.
T. Sugihara and M. Yamaguchi, Synlett, 1998, 1384.
9 (a) U. M. Lindström, Chem. Rev., 2002, 102, 2751; (b) Aqueous-Phase
Organometallic Catalysis, eds. B. Cornils and W. A. Herrmann, Wiley-
VCH, Weinheim, 1998.
7
8
17 X = NH n = 1
19 X = O n = 2
18
20
73
80
0
0
—
—
—
9
9
Z = NTs Y = nPr 10
84 57
10 An early attempt involved cyclizations of (enyne)Co2(CO)6 complexes
in a mixture of 1,4-dioxane and 2 M aqueous NH4OH (1 : 3, v/v) at 100
°C. T. Sugihara, M. Yamada, H. Ban, M. Yamaguchi and C. Kaneko,
Angew. Chem., Int. Ed. Engl., 1997, 36, 2801.
10
11
21 Z = O Y = Ph
22
24
70 36 74
76 73 65
23
11 2.5 mol% Co2(CO)8, PCO = 112 atm (37 °C), PCO = 30 atm, 90 °C, 24
2
h; (a) N. Jeong, S. H. Hwang, Y. W. Lee and J. S. Lim, J. Am. Chem.
Soc., 1997, 119, 10549. Co4(CO)11P(OPh)3, 5 atm CO, 110 atm
ethylene, 34 to 85 °C, 24 h (b) N. Jeong and S. H. Hwang, Angew.
Chem., Int. Ed., 2000, 39, 636.
12
25
26
68 — —
a I: water, 20 mol% Co4(CO)12, 0.5 equiv of Triton X-100, 70 °C, CO atm
in a resealable tube. II: water, 1 equiv Co4(CO)12, 0.6 equiv CTAB, 70 °C,
nitrogen atm. III: water, 1 equiv Co2(CO)8, 0.6 equiv CTAB, 70 °C,
nitrogen atm. Reactions were complete in ca. 6 h.
12 S. U. Son, S. I. Lee, Y. K. Chung, S. W. Kim and T. Hyeon, Org. Lett.,
2002, 4, 277; W. H. Suh, M. Choi, S. I. Lee and Y. K. Chung, Synthesis,
2003, 2169.
13 F. Koji, T. Morimoto, K. Tsutsumi and K. Kakiuchi, Angew. Chem., Int.
Ed., 2003, 42, 2409.
14 (a) T. Morimoto, K. Fuji, K. Tsutsumi and K. Kakiuchi, J. Am. Chem.
Soc., 2002, 124, 3806; (b) T. Shibata, N. Toshida and K. Kagai, J. Org.
Chem., 2002, 67, 7446; (c) T. Shibata, N. Toshida and K. Takagi, Org.
Lett., 2002, 4, 1619; (d) K. H. Park, S. U. Son and Y. K. Chung, Chem.
Commun., 2003, 1898.
15 M. E. Krafft, J. A. Wright and L. V. R. Boñaga, Tetrahedron Lett., 2003,
44, 3417.
16 M. E. Krafft, R. H. Romero and I. L. Scott, J. Org. Chem., 1992, 57,
5277.
Lastly, the issue of a CO-free catalytic PKR procedure was
briefly addressed.12–14 We observed a slight decrease in the yield of
cycloadducts from reactions conducted under an argon atmosphere.
For example, PK cyclization of enynes 5 and 7 using 20 mol% of
Co4(CO)12 furnished 70% and 59% yields of cyclopentenones 6
and 8, respectively (cf. Table 2, entries 2 and 3, condition I).
Nevertheless, these results show the potential of the aqueous
protocol in the absence of CO, and warrant further studies.
In summary, we have illustrated the reactivity of Co4(CO)12
towards enynes in a water–CTAB medium, and more importantly,
its catalytic activity in the PKR in a water–Triton® X-100 medium.
These conditions have led to the first examples of a mild and simple
cobalt-catalyzed aqueous protocol for the thermal PK reaction of
enynes bearing internal alkynes. The water–Triton® X-100 medium
using Co4(CO)12 has rendered the catalytic, aqueous thermal PKR
one which proceeds with high efficiency, exceeding that of the
aqueous reactions using stoichiometric amounts of Co4(CO)12 and
Co2(CO)8. Enyne substrates that possess groups prone to undergo
hydrolysis were shown to be stable under these conditions.19
This work was generously supported by the National Science
Foundation and the MDS Research Foundation.
17 J. Cassayre and A. Z. Zard, J. Organomet. Chem., 2001, 64, 316(a) J.
Cassayre and A. Z. Zard, J. Am. Chem. Soc., 1999, 121, 6072; (b) L.
Pérez-Serrano, J. Blanco-Urgoiti, L. Casarrubios, G. Dominguez and J.
Pérez-Castells, J. Org. Chem., 2000, 65, 3513; (c) T. Sugihara, M.
Yamada, M. Yamaguchi and M. Nishizawa, Synlett, 1999, 771.
18 H.-Y. Lee, M. An and J.-H. Sohn, Bull. Korean Chem. Soc., 2003, 24,
539.
19 General procedure: To a resealable tube containing enyne 5 (80 mg, 0.29
mmol), Co4(CO)12 (32 mg, 0.057 mmol, 20 mol%) and Triton® X-100
(91 mg, 0.14 mmol, 0.5 equiv) was added water (1.5 mL). Carbon
monoxide was then bubbled through the mixture (1–2 min), and the
reaction mixture was heated at 70 °C for 18 hours. Upon completion of
the reaction (ca. 6 h), the mixture was filtered through a filter paper on
a Büchner funnel, extracted with ethyl acetate, dried over MgSO4, and
filtered through a short pad of Celite. Subsequent concentration
followed by purification by flash column chromatography provided 70
mg of enone 7 (80% yield). Co4(CO)12 was purchased from Strem
Chemical Co., and used without further purification. Compounds 1, 2, 9,
10, 25, 26: (a) M. E. Krafft, L. V. R. Boñaga and C. Hirosawa, J. Org.
Chem., 2001, 66, 3004. Compounds 5, 6: T. Kondo, N. Suzuki, T. Okada
and T. Mitsudo, J. Am. Chem. Soc., 1997, 119, 6187. Compound 15: N.
Vicart and R. Whitby, Chem. Commun., 1999, 1241; Compounds 3, 4,
7, 8, 21, 22: ref. 7. Compounds 11–14, 16, 23, 24: ref. 15. Compounds
17–20: ref. 16.
Notes and references
1 General review on tetranuclear clusters contains many references to
Co4(CO)12: (a) P. Chini and B. T. Heaton, Top. Curr. Chem., 1977, 71,
3; (b) see also: R. D. W. Kemmitt and D. R. Russell, in Comprehensive
Organometallic Chemistry, Vol 5, eds. E. W. Abel, F. A. Stone and G.
Wilkinson, Pergamon, Oxford, 1982, p. 7. Co4(CO)12 is less prone to air
oxidation than Co2(CO)8.
C h e m . C o m m u n . , 2 0 0 4 , 1 7 4 6 – 1 7 4 7
1747