ORGANIC
LETTERS
2
004
Vol. 6, No. 4
93-595
Triethylborane-Induced Radical
Allylation Reaction with
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Zirconocene−Olefin Complex
Koji Hirano, Kazuya Fujita, Hiroshi Shinokubo, and Koichiro Oshima*
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Kyoto-daigaku Katsura Nishikyo-ku, Kyoto 615-8510, Japan
Received December 14, 2003
ABSTRACT
Allylzirconium reagents are effective for radical allylation of r-halo carbonyl compounds. The key steps would be homolytic cleavage of the
2
zirconium−carbon bond and halogen abstraction by the resulting Cp ZrCl(III). Zirconocene−olefin complex can be also utilized for the allylation
of r-halo compounds.
A radical allylation reaction is synthetically useful because
the introduced allyl group serves as a versatile precursor for
further functionalization. Recent advances in the radical
proved to be inferior to that of allylstannanes. Here we wish
to introduce allylzirconium as an allylating reagent via a
radical process.
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7
allylation reaction have mainly benefited from the efficiency
of allylstannanes as mediators. The use of tin-based reagents
The reagent, allylzirconium, was prepared from Cp ZrCl
2
2
2
8
and allylmagnesium chlorides [Method A]. Treatment of
Cp ZrCl (3.0 mmol) with allylmagnesium chloride (3.0
is, however, not always convenient because of the inherent
toxicity of organotin derivatives and the difficulty often
2
2
9
mmol) in THF at -78 °C provided allylzirconium 1. Benzyl
3
encountered in removing tin residues from the product. To
iodoacetate 2a (1.0 mmol) and Et B (0.2 mmol) as a radical
3
solve the problems associated with organotin reagents,
several alternative approaches have been utilized with some
success. However, the efficacy of these alternatives often
initiator were sequentially added, and the mixture was
warmed to 25 °C and stirred for 3 h. After aqueous workup,
10
4-6
(
6) Usugi, S.; Yorimitsu, H.; Oshima, K. Tetrahedron Lett. 2001, 42,
4535.
(7) Recently, we have reported the Cp2Zr(H)Cl-mediated radical reduction
reaction involving homolytic cleavage of the zirconium-hydrogen bond:
(a) Fujita, K.; Nakamura, T.; Yorimitsu, H.; Oshima, K. J. Am. Chem. Soc.
2001, 123, 3137. (b) Fujita, K.; Yorimitsu, H.; Oshima, K. Synlett 2002,
337.
(
1) (a) Radicals in Organic Synthesis; Renaud, P., Sibi, M. P., Eds.;
Wiley-VCH: Weinheim, 2001. (b) Jasperse, C. P.; Curran, D. P.; Fevig,
T. L. Chem. ReV. 1991, 91, 1237.
(
2) (a) Keck, G. E.; Enholm, E. J.; Yates, J. B.; Wiley, M. R. Tetrahedron
1
985, 41, 4079. (b) Curran, D. P. Synthesis 1998, 417 and 489.
(3) Bagley, P. A.; Walton, J. C. Angew. Chem., Int. Ed. 1998, 37, 3072.
(
4) Silicon-based radical chain carriers such as allyl tris(trimethylsilyl)-
(8) It is known that Cp2ZrCl2 reacts with 2-butenylmagnesium chloride
to provide a mixture of Cp2ZrCl(2-butenyl) and Cp2Zr(2-butenyl)2. (a)
Mashima, K.; Yasuda, H.; Asami, K.; Nakamura, A. Chem. Lett. 1983,
219. We confirmed that Cp2Zr(2-butenyl)2, generated by mixing Cp2ZrCl2
and 2-butenylmagnesium chloride in a 1:2 ratio, was also effective for the
radical allylation. The reactivity of Cp2Zr(2-butenyl)2 toward R-halo
carbonyl compounds was similar to that of Cp2ZrCl(2-butenyl).
(9) Use of 1.5 equiv of allylzirconium provided 3a in only 34% yield
along with the recovered R-iodo ester.
silane are effective alternatives to tin reagents, although these compounds
are often costly. (a) Chatgilialoglu, C.; Ballestri, M.; Vecchi, D.; Curran,
D. P. Tetrahedron Lett. 1996, 37, 6383. (b) Chatgilialoglu, C.; Ferreri, C.;
Ballestri, M.; Curran, D. P. Tetrahedron Lett. 1996, 37, 6387. (c) Guindon,
Y.; Guerin, B.; Chabot, C.; Oglivie, W. J. Am. Chem. Soc. 1996, 118, 12528.
(
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d) Porter, N. A.; Wu, J. H.; Zhang, G.; Reed, A. D. J. Org. Chem. 1997,
2, 6702.
5) (a) Quiclet-Sire, B.; Zard, S. Z. J. Am. Chem. Soc. 1996, 118, 1209.
b) Guyader, F. L.; Quiclet-Sire, B.; Seguin, S.; Zard, S. Z. J. Am. Chem.
(
(
(10) (a) Nozaki, K.; Oshima, K.; Utimoto, K. J. Am. Chem. Soc. 1987,
109, 2547. (b) Oshima, K.; Utimoto, K. J. Synth. Org. Chem. Jpn. 1989,
47, 40.
Soc. 1997, 119, 7410. (c) Sire, B.; Seguin, S.; Zard, S. Z. Angew. Chem.,
Int. Ed. 1998, 37, 2864.
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0.1021/ol036431e CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/16/2004