Angewandte
Chemie
Experimental Section
Cyclobutenones 1a–d were prepared by a general two-step method
based on the [2+2] cycloaddition of alkynes with dichloroketene, and
the reductive dechlorination of the generated 4,4-dichlorocyclobute-
nones by zinc dust in the presence of tetramethylethylenediamine,
ethanol, and acetic acid.[12]
Representative procedure for the synthesis of (E)-2b from 1b
catalyzed by [{RhCl(CO)2}2]: A mixture of 2,3-dipropylcyclobut-2-en-
1-one (1b) (152 mg, 1.0 mmol), [{RhCl(CO)2}2] (19.4 mg,
0.050 mmol), and toluene (2.0 mL) was placed in a 20-mL Pyrex
flask equipped with a magnetic stirring bar under a flow of argon. The
reaction was carried out at 1108C for 12 h with stirring. After the
reaction mixture was cooled, the product, 6-((1E)-2-methyl-1-pro-
pylpent-1-enyl)-3,4-dipropylpyran-2-one ((E)-2b), was isolated by
Kugelrohr distillation as a pale yellow oil (228 mg, 0.75 mmol; 75%
yield); b.p. 170–1808C (1.0 mmHg, Kugelrohr); IR (neat): n˜ = 1562,
1635 (C C), 1712 cm (C O); 1H NMR (400 MHz, CDCl3, 258C):
d = 0.88 (t, J = 7.32 Hz, 3H), 0.94 (t, J = 7.32 Hz, 3H), 0.98 (t, J =
7.32 Hz, 3H), 0.99 (t, J = 7.32 Hz, 3H), 1.30–1.36 (m, 2H), 1.43–1.61
(m, 6H), 1.76 (s, 3H), 2.11(t, J = 7.81 Hz, 2H), 2.29 (t, J = 7.81 Hz,
2H), 2.41 (t, J = 7.81 Hz, 2H), 2.46 (t, J = 7.81 Hz, 2H), 5.83 ppm (s,
1H); 13C NMR (100 MHz, CDCl3, 258C): d = 14.0, 14.0, 14.2, 14.3 20.5
21.3 22.1 22.1, 22.5 28.6, 32.4, 34.5 36.4, 108.3, 122.3, 128.6, 139.2 153.2,
159.3, 164.3 ppm; MS (EI, 70 eV): m/z: 304 [M+]; elemental analysis
(%) calcd for C20H32O2: C 78.90, H 10.59; found: C 78.80, H 10.55.
À1
=
=
Scheme 1. Possible mechanism for the formation of 2-pyranones 2.
rhodacycloheptenone 10 is easily decarbonylated to a rhoda-
cyclohexene intermediate 11, and subsequent reductive
elimination gives the corresponding cyclopentene 4. Even
under carbon monoxide pressure, this decarbonylation pro-
cess of 10 to 11 is facile, however, it is reversible (see above).
Rapid reductive elimination from the stabilized 10 by carbon
monoxide occurs to give the corresponding cyclohexenone 5
(Scheme 2).
Received: June 18, 2004
À
Keywords: C C activation · homogeneous catalysis ·
metallacycles · rhodium · ruthenium
.
[1] For reviews, see: a) K. C. Bishop III, Chem. Rev. 1976, 76, 461 –
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Mitsudo, J. Am. Chem. Soc. 2000, 122, 6319 – 6320.
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124, 6824 – 6825.
[5] For reviews, see: a) H. W. Moore, B. R. Yerxa in Advances in
Strain in Organic Chemistry, Vol. 4 (Ed.: B. Halton), JAI, London,
1995, pp. 81–162; b) T. K. M. Shing, Methods of Organic Chemis-
try (Houben-Weyl) 4th ed. 1952-, Vol. E17 f (Ed.: A. de Meijere),
Thieme, Stuttgart, 1997, chap. 8B, pp. 898–913.
Scheme 2. Possible mechanism for the formation of cyclopentenes 4
and cyclohexenones 5.
[6] a) M. A. Huffman, L. S. Liebeskind, W. T. Pennington, Jr.,
Organometallics 1990, 9, 2194 – 2196; b) M. A. Huffman, L. S.
Liebeskind, J. Am. Chem. Soc. 1990, 112, 8617 – 8618; c) M. A.
Huffman, L. S. Liebeskind, J. Am. Chem. Soc. 1991, 113, 2771 –
2772; d) M. A. Huffman, L. S. Liebeskind, W. T. Pennington,
Organometallics 1992, 11, 255 – 266.
[7] A thermal reaction of cyclobutenones with activated alkynes to
phenols via a vinylketene intermediate has also been reported.
a) R. L. Danheiser, S. K. Gee, J. Org. Chem. 1984, 49, 1672 –
1674; b) R. L. Danheiser, A. Nishida, S. Savariar, M. P. Trova,
Tetrahedron Lett. 1988, 29, 4917 – 4920.
An alternative pathway for the formation of cyclohex-
enone 5 by a direct stereoselective Diels–Alder reaction of h4-
vinylketene rhodium intermediate 6 with 2-norbornene (3a)
is also possible, however, this mechanism cannot explain the
decarbonylative coupling of cyclobutenone with 3a under an
argon atmosphere.
In conclusion, we have developed a novel ruthenium- and
rhodium-catalyzed ring-opening dimerization of cyclobute-
nones to give 2-pyranones. The application of a rhodium
catalyst to decarbonylative and direct coupling reactions of
cyclobutenones with 2-norbornene is also successful and gives
stereoselectively cyclopentenes and cyclohexenones, respec-
tively.
[8] a) M. F. Semmelhack, R. Tamura, W. Schnatter, J. Park, M.
Steigerwald, S. Ho, Stud. Org. Chem. 1986, 25, 21 – 42; b) S. E.
Gibson, M. A. Peplow, Adv. Organomet. Chem. 1999, 44, 275 –
355, and references therein.
Angew. Chem. Int. Ed. 2004, 43, 5369 –5372
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