COMMUNICATIONS
[1] a) A. Koskinen, Asymmetric Synthesis of Natural Products, Wiley,
Chichester, 1993, pp. 168 ± 191; b) D. E. Cane in Comprehensive
Natural Products Chemistry, Vol. 2 (Eds.: D. Barton, K. Nakanishi),
Elsevier, Oxford, 1999, pp. 1 ± 13.
[2] Reviews for carotenoid synthesis: a) H. Pommer, Angew. Chem. 1960,
72, 911 ± 915; b) O. Isler, Pure Appl. Chem. 1979, 51, 447 ± 462; c) E.
Widmer, Pure Appl. Chem. 1985, 57, 741 ± 752; d) J. Paust, Pure Appl.
Chem. 1991, 63, 45 ± 58; e) K. Bernhard, H. Mayer, Pure Appl. Chem.
1991, 63, 35 ± 44.
[3] Lycopene synthesis: a) P. Karrer, C. H. Eugster, E. Tobler, Helv.
Chim. Acta 1950, 33, 1349 ± 1352; b) O. Isler, H. Gutmann, H. Lindlar,
M. Montavon, R. Rüegg, G. Ryser, P. Zeller, Helv. Chim. Acta 1956,
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Robson, Tetrahedron Lett. 1987, 28, 5751 ± 5754.
Dehydrogenation of [{(silox)3Nb}2(h-1,2;
h-5,6-C8H8)] (silox tBu3SiO) to
[{(silox)3Nb}2(h-1,2;h-5,6-C8H6)] and
Its Subsequent Alkene-to-Alkylidene
Rearrangement**
Adam S. Veige, Peter T. Wolczanski,* and
Emil B. Lobkovsky
The observed pyridine ring-opening of [(silox)3Nb(h-C,N-
C5H5N)] (silox tBu3SiO) to [(silox)3Nb CHCH CHCH
CHN Nb(silox)3], and related picoline chemistry[1, 2] suggest-
ed that carbon ± carbon bond scission might occur by similar
pathways. 1,3,5,7-Cyclooctatetraene (COT) was considered a
prime candidate for ring opening because of its lack of
resonance stabilization energy, but in binding to two [(silox)3-
Nb] units, COT functions as an aromatic dianion, which
directs the chemistry toward C H bond activation, dehydro-
genation, and a subsequent alkene-to-alkylidene rearange-
ment.
[4] a) M. Julia, J.-M. Paris, Tetrahedron Lett. 1973, 4833 ± 4836; b) M.
Julia, D. Arnould, Bull. Soc. Chim. France 1973, 746 ± 750; c) B. M.
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1976, 3477 ± 3478; d) M. Julia, M. Launay, J.-P. Stacino, J.-N. Verpeaux,
Tetrahedron Lett. 1982, 23, 2465 ± 2468; e) J. Bremner, M. Julia, M.
Launay, J.-P. Stacino, Tetrahedron Lett. 1982, 23, 3265 ± 3266.
[5] H. Choi, M. Ji, M. Park, I.-K. Yun, S.-S. Oh, W. Baik, S. Koo, J. Org.
Chem. 1999, 64, 8051 ± 8053.
[6] a) G. I. Zaitseva, V. M. Alꢁbitskaya, Zh. Org. Khim. 1969, 5, 612 ± 617;
b) A. Yasuda, M. Takahashi, H. Takaya, Tetrahedron Lett. 1981, 22,
2413 ± 2416; c) G. Martin, J. Sauleau, M. David, A. Sauleau, Can. J.
Chem. 1992, 70, 2190 ± 2196.
Reduction of [(silox)3NbCl2] (1)[3] with Na/Hg in THF with
three equivalents of COT present resulted in the isolation of
brown [(silox)3Nb(h-C8H8)] (2-COT, 40%) [Eq. (1)].
[7] a) T. Kar, R. N. P. Choudhary, Mater. Lett. 1997, 32, 109 ± 113; b) H.-J.
Nam, H. Kim, S. H. Chang, S.-G. Kang, S.-H. Byeon, Solid State Ionics
1999, 120, 189 ± 195.
THF; 24 h
[(silox)3NbCl2] L (excess)
[(silox)3NbL]
(1)
!
[8] a) C. Y. Meyers, A. M. Malte, W. S. Matthews, J. Am. Chem. Soc. 1969,
91, 7510 ± 7512; b) G. Büchi, R. M. Freidinger, J. Am. Chem. Soc. 1974,
96, 3332 ± 3333; c) J. S. Grossert, J. Buter, E. W. H. Asveld, R. M.
Kellogg, Tetrahedron Lett. 1974, 2805 ± 2808; d) F. Näf, R. Decorzant,
S. D. Escher, Tetrahedron Lett. 1982, 23, 5043 ± 5046; e) M. Julia, D.
Lave, M. Mulhauser, M. Ramirez-Munoz, D. Uguen, Tetrahedron
Lett. 1983, 24, 1783 ± 1786; f) T.-L. Chan, S. Fong, Y. Li, T.-O. Man, C.-
D. Poon, J. Chem. Soc. Chem. Commun. 1994, 1771 ± 1772.
[9] a) P. S. Manchand, M. Rosenberger, G. Saucy, P. A. Wehrli, H. Wong,
L. Chambers, M. P. Ferro, W. Jackson, Helv. Chim. Acta 1976, 59, 387 ±
396; b) P. Chabardes, J. P. Decor, J. Varagnat, Tetrahedron 1977, 33,
2799 ± 2805; c) G. L. Olson, H.-C. Cheung, K. D. Morgan, C. Neukom,
G. Saucy, J. Org. Chem. 1976, 41, 3287 ± 3293; d) J. Otera, H. Misawa,
T. Mandai, T. Onishi, S. Suzuki, Y. Fujita, Chem. Lett. 1985, 1883 ±
1886.
Na=Hg; 2 NaCl
2-L (L COT, cC6H10)
Abstraction of 4-picoline from [(silox)3Nb(h-C,N-4-
MeC5H4N)] (2-4-pic) by [(silox)3Ta][2] in the presence of 2-
COT afforded [(silox)3Ta(h-4-pic)] and burgundy, crystalline
[{(silox)3Nb}2(h-1,2;h-5,6-C8H8)] (22-COT, 33%, Scheme 1).
The synthesis of 22-COT must occur under mild conditions to
avoid further reaction (vide infra). An X-ray crystal structure
determination of 22-COT[4, 5] revealed the [(silox)3Nb] moi-
eties (d(Nb C) 2.20(5) (av))[6, 7] in an anti-h2,h2-config-
uration about a planar COT ligand, although disorder
problems hampered further analysis. The insolubility of 22-
COT in unreactive hydrocarbon solvents prevented spectral
characterization.
Upon thermolysis of two equivalents of [(silox)3Nb(h-C,N-
4-MeC5H4N)] (2-4-pic) and COT, 2-COT and presumably 22-
COT were generated in situ, and dehydrogenation led to the
gold-brown cyclooctatrieneyne[8] complex, [{(silox)3Nb}2(h-
1,2;h-5,6-C8H6)] (4; Scheme 1). Although 4 was isolated in
50% yield, 1H NMR spectroscopy revealed the conversion to
be >95% when the reaction was monitored in a sealed tube
(C6D6). Olefin substitution reactions of 2-4-pic, such as the
synthesis of the 1-butene complex, [(silox)3Nb(h2-C4H8)] (2-
[10] Trans-lycopene was obtained as a major product (80% or more) with
a small amount (20% or less) of cis-lycopene. This latter product was
believed to be the 9-cis isomer on the basis of characteristic peaks at
d 6.06 and 6.79 in the 1H NMR spectrum.[3c] Base-promoted
elimination of the sulfonyl groups produced mostly trans double
bonds.
[*] Prof. P. T. Wolczanski, A. S. Veige, E. B. Lobkovsky
Department of Chemistry & Chemical Biology
Baker Laboratory, Cornell University
Ithaca, New York 14853 (USA)
Fax : (1)607-255-4137
[**] We thank the US National Science Foundation for support of this
research.
Angew. Chem. Int. Ed. 2001, 40, No. 19
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