Diels-Alder reactions of oxazolidinone dienophiles catalyzed by zirconocene bis(triflate) catalysts: Mechanism for asymmetric induction

Organometallics

Volumn 17, Issue 5, 1998, Pages 914-925

  Jaquith, James B ^a   Levy, Christopher J ^a   Bondar, Georgiy V ^a   Wang, Shaotian ^a   Collins, Scott ^a  

^a UNIVERSITY OF WATERLOO   (Canada)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000744891     PISSN: 02767333     EISSN: None     Source Type: Journal    
DOI: 10.1021/om970916n     Document Type: Article

References (42)

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5. For similar approaches employing other chiral catalysts and oxazolidinone-based dienophiles, see, inter alia: (a) Evans, D. A.; Miller, S. J.; Lectka, T. J. Am. Chem. Soc. 1993, 115, 6460. (b) Corey, E. J.; Imai, N.; Zhang, H.-Y. J. Am. Chem. Soc. 1991, 113, 728. (c) Narasaka, K.; Iwasawa, N.; Inoue, M.; Yamada, T.; Nakashima, M.; Sugimori, J. J. Am. Chem. Soc. 1989, 111, 5340 and references therein. For earlier work employing chiral oxazolidinone-based dienophiles and achiral alkylaluminum catalysts, see: (d) Evans, D. A.; Chapman, K. T.; Bisaha, J. J. Am. Chem. Soc. 1988, 110, 1238.
6. For similar approaches employing other chiral catalysts and oxazolidinone-based dienophiles, see, inter alia: (a) Evans, D. A.; Miller, S. J.; Lectka, T. J. Am. Chem. Soc. 1993, 115, 6460. (b) Corey, E. J.; Imai, N.; Zhang, H.-Y. J. Am. Chem. Soc. 1991, 113, 728. (c) Narasaka, K.; Iwasawa, N.; Inoue, M.; Yamada, T.; Nakashima, M.; Sugimori, J. J. Am. Chem. Soc. 1989, 111, 5340 and references therein. For earlier work employing chiral oxazolidinone-based dienophiles and achiral alkylaluminum catalysts, see: (d) Evans, D. A.; Chapman, K. T.; Bisaha, J. J. Am. Chem. Soc. 1988, 110, 1238.
7. For similar approaches employing other chiral catalysts and oxazolidinone-based dienophiles, see, inter alia: (a) Evans, D. A.; Miller, S. J.; Lectka, T. J. Am. Chem. Soc. 1993, 115, 6460. (b) Corey, E. J.; Imai, N.; Zhang, H.-Y. J. Am. Chem. Soc. 1991, 113, 728. (c) Narasaka, K.; Iwasawa, N.; Inoue, M.; Yamada, T.; Nakashima, M.; Sugimori, J. J. Am. Chem. Soc. 1989, 111, 5340 and references therein. For earlier work employing chiral oxazolidinone-based dienophiles and achiral alkylaluminum catalysts, see: (d) Evans, D. A.; Chapman, K. T.; Bisaha, J. J. Am. Chem. Soc. 1988, 110, 1238.
8. For similar approaches employing other chiral catalysts and oxazolidinone-based dienophiles, see, inter alia: (a) Evans, D. A.; Miller, S. J.; Lectka, T. J. Am. Chem. Soc. 1993, 115, 6460. (b) Corey, E. J.; Imai, N.; Zhang, H.-Y. J. Am. Chem. Soc. 1991, 113, 728. (c) Narasaka, K.; Iwasawa, N.; Inoue, M.; Yamada, T.; Nakashima, M.; Sugimori, J. J. Am. Chem. Soc. 1989, 111, 5340 and references therein. For earlier work employing chiral oxazolidinone-based dienophiles and achiral alkylaluminum catalysts, see: (d) Evans, D. A.; Chapman, K. T.; Bisaha, J. J. Am. Chem. Soc. 1988, 110, 1238.
9. (a) Odenkirk, W.; Bosnich, B. J. Chem. Soc., Chem. Commun. 1995, 1181.
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11. (c) Hollis, T. K.; Robinson, N. P.; Bosnich, B. J. Am. Chem. Soc. 1992, 114, 5464.
12. Although we have completed analogous studies using titanocene bis(triflate) catalysts, these are less well understood from a mechanistic perspective than the zirconium systems discussed here, see: J. Guan, J. M.S. Thesis, University of Waterloo, 1994.
13. Preliminary communication: Jaquith, J. B.; Guan, J.; Wang, S.; Collins, S. Organometallics 1995, 14, 1079.
14. (a) Castellano, S.; Dwight, W. J. J. Am. Chem. Soc. 1993, 115, 2986.
15. At a 10:1 ratio of 3a:4, two signals were present at -77.9 and -78.2 ppm in a 1:1.3 ratio, while at a 1:2 ratio of 3a:4, two signals were present at -77.5 and -77.9 ppm in a 1:1.1 ratio. The signal at -77.5 corresponds to 4 in nitromethane at this temperature, while the signal at -78.2 is close to that of free triflate ion. Due to the limited temperature range available in this solvent, we were unable to characterize these complexes to the same extent as for those formed from 3a and [S]-8 (vide infra).
16. For a discussion of bonding in five-coordinate, bent metallocene complexes, see: Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729.
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22. 5
23. Identical behavior was exhibited by 3a and optically pure [S]-8. Also, the ratio of 9a to 9b was invariant to changes in the concentration of either 3a or 8 (and their ratio, vide infra), indicating that the formation of bimetallic complexes under these conditions is not significant.
24. The astute reader will note that a prominent correlation exists between (broadened) signals at 6.7 and 6.1 ppm. These signals are due to the α and β CpH protons of residual 8 and are line broadened, we suspect, due to triflate exchange (vide infra).
25. 2. At this temperature, 2D-NOESY spectra indicated that exchange processes involving 9a and 9b were sufficiently slow that NOE difference spectra, free from exchange-related artifacts, could be obtained.
26. 2, it is apparent that slow tumbling conditions apply here. Consistent with this interpretation, the magnitude of the negative NOE decreased with increasing temperature and could not be (easily) observed at, e.g., -30°C.
27. 2, there is no, significant, dissociation of triflate ion from 8 at -30°C, suggesting nitromethane is a less effective ligand than triflate ion.
28. See: Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper & Row: New York, 1981; p 212.
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30. In addition, even under Curtin-Hammett conditions, the relative rates of interconversion of A and B compared to their reaction with CpH will be dependent on both relative and absolute concentrations of the species involved as the former is process is unimolecular and only depends on catalyst concentration while the latter depends upon both the catalyst concentration and [CpH]. For a discussion, see: Seeman, J. I. Chem. Rev. (Washington, D. C.) 1983, 83, 83 and references therein.
31. - was no longer 1:1), suggesting formation of a dicationic complex (which would be expected to have similar enantioselectivity as 9a).
32. -1 at this temperature, which is of similar magnitude to the barrier for interconversion of 9a and 9b, providing a further indication that non-Curtin-Hammett conditions apply.
33. In addition, as alluded to earlier (ref 21), the relative rates for interconversion and chemical reaction will be concentration dependent. One would expect kinetic quenching to be valid at early conversions (i.e., lower ee) where k[CpH][Zr] ≥ k′[Zr] and a tendency towards C-H conditions (higher ee) at high conversion, but detailed study of this would require that the concentration dependence of all species on the ee be determined. We thank a referee for suggesting these experiments and bringing these features to our attention.
34. 2). Predictably, the enantioselectivity of the Diels-Alder reaction with CpH was lower, ca. 56% ee (-30°C, 1 mol % [S]-8). Jaquith, J. B. M.S. Thesis, University of Waterloo, 1995. equation presented
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