Synthesis of chiral, enantiopure zirconocene imido complexes: Highly selective kinetic resolution and stereoinversion of allenes, and evidence for a stepwise cycloaddition/retrocycloaddition reaction mechanism

Angewandte Chemie - International Edition

Volumn 39, Issue 13, 2000, Pages 2339-2343

  Sweeney, Zachary K ^a   Salsman, Jennifer L ^a   Andersen, Richard A ^a   Bergman, Robert G ^a  

^a UNIVERSITY OF CALIFORNIA   (United States)

Author keywords

Allenes; Cycloadditions; Enantiomeric resolution; Imido complexes; Zirconium

Indexed keywords

ZIRCONOCENE;

ARTICLE; CHEMICAL REACTION; CHEMICAL STRUCTURE; CHIRALITY; COMPLEX FORMATION; MOLECULAR INTERACTION; STEREOCHEMISTRY; SYNTHESIS;

EID: 0034600906     PISSN: 14337851     EISSN: None     Source Type: Journal    
DOI: 10.1002/1521-3773(20000703)39:13<2339::AID-ANIE2339>3.0.CO;2-W     Document Type: Article

References (47)

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25. Enantioresolved complexes 1 and 5 were confirmed to be enantiopure by NMR analysis of the products formed by treating the zirconium complexes with enantiopure (R)-mandelic acid, as described by earlier workers: a) A. Schafer, L. Zsolnai, G. Huttner, H. H. Brintzinger, J. Organomet. Chem. 1987, 328, 87; b) R. B. Grossman, W. M. Davis, S. L. Buchwald, J. Am. Chem. Soc. 1991, 113, 2321; c) B. Chin, S. L. Buchwald, J. Org. Chem. 1997, 62, 2267.
26. Enantioresolved complexes 1 and 5 were confirmed to be enantiopure by NMR analysis of the products formed by treating the zirconium complexes with enantiopure (R)-mandelic acid, as described by earlier workers: a) A. Schafer, L. Zsolnai, G. Huttner, H. H. Brintzinger, J. Organomet. Chem. 1987, 328, 87; b) R. B. Grossman, W. M. Davis, S. L. Buchwald, J. Am. Chem. Soc. 1991, 113, 2321; c) B. Chin, S. L. Buchwald, J. Org. Chem. 1997, 62, 2267.
27. Enantioresolved complexes 1 and 5 were confirmed to be enantiopure by NMR analysis of the products formed by treating the zirconium complexes with enantiopure (R)-mandelic acid, as described by earlier workers: a) A. Schafer, L. Zsolnai, G. Huttner, H. H. Brintzinger, J. Organomet. Chem. 1987, 328, 87; b) R. B. Grossman, W. M. Davis, S. L. Buchwald, J. Am. Chem. Soc. 1991, 113, 2321; c) B. Chin, S. L. Buchwald, J. Org. Chem. 1997, 62, 2267.
28. The reaction was stopped 10 min after addition of the allene.
29. Enantioenrichments of the allenes were determined by chiral vapor-phase chromatography, in which the resolution is calibrated using racemic mixtures.
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32. 2. Resolution using this reagent is known to provide cyclononadiene enriched in the S enantiomer. See: a) R. D. Bach, J. W. Holubka, C. L. Willis. J. Am. Chem. Soc. 1982, 104, 3980; b) W. R. Moore, H. W. Anderson, S. D. Clark, J. Am. Chem. Soc. 1973, 95, 835.
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38. b) E. Folga, T. Ziegler, Organometallics 1993, 12, 325.
39. We postulate a diradical structure for the intermediate because the thermal dimerization of allenes is known to proceed via this type of species. For leading references, see: a) D. J. Pasto, Tetrahedron, 1984. 40, 2805; b) The Chemistry of the Allenes (Ed.: S. R. Landor), Academic Press, London, 1982. However, with our presently available data, we cannot distinguish between a diradical and the corresponding zwitterion (presumably with a positive charge on Zr and a negative charge on the allyl moiety) or a π-allyl complex in which the C3 fragment in intermediate C is coordinated to the Zr center. Evidence has been proposed for diradical intermediates in several organometallic reactions: for examples, see: c) S. L. Buchwald, E. V. Anslyn, R. H. Grubbs, J. Am. Chem. Soc. 1985, 107, 1766; d) L. Cavallo; H. Jacobsen, Angew. Chem. 2000, 112, 602; Angew. Chem. Int. Ed. 2000, 39, 589, and references therein.
40. We postulate a diradical structure for the intermediate because the thermal dimerization of allenes is known to proceed via this type of species. For leading references, see: a) D. J. Pasto, Tetrahedron, 1984. 40, 2805; b) The Chemistry of the Allenes (Ed.: S. R. Landor), Academic Press, London, 1982. However, with our presently available data, we cannot distinguish between a diradical and the corresponding zwitterion (presumably with a positive charge on Zr and a negative charge on the allyl moiety) or a π-allyl complex in which the C3 fragment in intermediate C is coordinated to the Zr center. Evidence has been proposed for diradical intermediates in several organometallic reactions: for examples, see: c) S. L. Buchwald, E. V. Anslyn, R. H. Grubbs, J. Am. Chem. Soc. 1985, 107, 1766; d) L. Cavallo; H. Jacobsen, Angew. Chem. 2000, 112, 602; Angew. Chem. Int. Ed. 2000, 39, 589, and references therein.
41. We postulate a diradical structure for the intermediate because the thermal dimerization of allenes is known to proceed via this type of species. For leading references, see: a) D. J. Pasto, Tetrahedron, 1984. 40, 2805; b) The Chemistry of the Allenes (Ed.: S. R. Landor), Academic Press, London, 1982. However, with our presently available data, we cannot distinguish between a diradical and the corresponding zwitterion (presumably with a positive charge on Zr and a negative charge on the allyl moiety) or a π-allyl complex in which the C3 fragment in intermediate C is coordinated to the Zr center. Evidence has been proposed for diradical intermediates in several organometallic reactions: for examples, see: c) S. L. Buchwald, E. V. Anslyn, R. H. Grubbs, J. Am. Chem. Soc. 1985, 107, 1766; d) L. Cavallo; H. Jacobsen, Angew. Chem. 2000, 112, 602; Angew. Chem. Int. Ed. 2000, 39, 589, and references therein.
42. We postulate a diradical structure for the intermediate because the thermal dimerization of allenes is known to proceed via this type of species. For leading references, see: a) D. J. Pasto, Tetrahedron, 1984. 40, 2805; b) The Chemistry of the Allenes (Ed.: S. R. Landor), Academic Press, London, 1982. However, with our presently available data, we cannot distinguish between a diradical and the corresponding zwitterion (presumably with a positive charge on Zr and a negative charge on the allyl moiety) or a π-allyl complex in which the C3 fragment in intermediate C is coordinated to the Zr center. Evidence has been proposed for diradical intermediates in several organometallic reactions: for examples, see: c) S. L. Buchwald, E. V. Anslyn, R. H. Grubbs, J. Am. Chem. Soc. 1985, 107, 1766; d) L. Cavallo; H. Jacobsen, Angew. Chem. 2000, 112, 602; Angew. Chem. Int. Ed. 2000, 39, 589, and references therein.
43. We postulate a diradical structure for the intermediate because the thermal dimerization of allenes is known to proceed via this type of species. For leading references, see: a) D. J. Pasto, Tetrahedron, 1984. 40, 2805; b) The Chemistry of the Allenes (Ed.: S. R. Landor), Academic Press, London, 1982. However, with our presently available data, we cannot distinguish between a diradical and the corresponding zwitterion (presumably with a positive charge on Zr and a negative charge on the allyl moiety) or a π-allyl complex in which the C3 fragment in intermediate C is coordinated to the Zr center. Evidence has been proposed for diradical intermediates in several organometallic reactions: for examples, see: c) S. L. Buchwald, E. V. Anslyn, R. H. Grubbs, J. Am. Chem. Soc. 1985, 107, 1766; d) L. Cavallo; H. Jacobsen, Angew. Chem. 2000, 112, 602; Angew. Chem. Int. Ed. 2000, 39, 589, and references therein.
44. 1.
45. There is, of course, a higher energy path leading from intermediate C to the more sterically congested isomer of the metallacycle. Because we do not observe detectable amounts of this product, we have omitted this pathway and product from the diagram for the sake of clarity.
46. One can always postulate more complicated mechanisms that will also account for a particular set of experimental results. Therefore, as noted by two referees, we cannot rule out alternative scenarios in which the faster reacting allene enantiomer reacts by a concerted mechanism and the slower reacting allene reacts by a stepwise process proceeding through intermediate C or through a less stable diastereoisomer of (S,S,R)-10. Experiments are under way aimed at distinguishing these possibilities.
47. H. B. Kagan, J. C. Fiaud, Top. Stereochem. 1988, 18, 249.