Efficient chiral poisoning of racemic titanium catalysts for the asymmetric chloral-ene reaction

Tetrahedron Letters

Volumn 37, Issue 20, 1996, Pages 3449-3452

  Faller, J W ^a   Liu, X ^a  

^a YALE UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

ALKANOL; ALKENE DERIVATIVE;

ARTICLE; CATALYSIS; DRUG SYNTHESIS; ENANTIOMER; REACTION ANALYSIS; STEREOCHEMISTRY;

EID: 0029984273     PISSN: 00404039     EISSN: None     Source Type: Journal    
DOI: 10.1016/0040-4039(96)00643-0     Document Type: Article

References (18)

1. (a) Faller, J. W.; Parr, J. J. Am. Chem. Soc. 1993, 115, 804.
2. (b) Faller, J. W.; Tokunaga, M. Tetrahedron Lett., 1993, 34, 7359.
3. (c) Faller, J. W. Pure Appl. Chem. 1994, 66, 1463.
4. (a) Maruoka, K.; Yamamoto, H. J. Am. Chem. Soc. 1989, 111, 789.
5. (b) Brown, J. M.; Maddox, P. J. Chirality 1991, 3, 345.
6. Faller, J. W.; Sams, D. W. I.; Liu, X. J. Am. Chem. Soc., 1996, 118, 1217.
7. 2\(S)-BINOL is highly effective in the asymmetric catalysis of some types of ene reaction. Examples are: (a) Mikami, K.; Terada, M.; Nakai, T. J. Am. Chem. Soc. 1990, 112, 3949.
8. (b) Mikami, K.; Yajima, T.; Terada, M.; Uchimaru, T. Tetrahedron Lett. 1993, 34, 7591. The preparation of catalyst and typical reaction procedure are described in (a).
9. An acid-activated aldehyde is sufficiently nucleophilic to add to benzene, see Olah, G. A.; Rasul, G.; York, C.; Prakash, G. K. S. J. Am. Chem. Soc. 1995, 117, 11211.
10. A prior example where partitioning within diastereomers occurs after an initial irreversible enantioselective step has been observed by Zhang, W.; Lee, N. H.; Jacobsen, E. N., J. Am. Chem. Soc. 1994, 116, 425.
11. For example, the initial enantioselectivity on C=O (entry 1, Table 1) is: 67% × 24%ee + 33% × 66%ee = 38%ee
12. 2 (0.6 M toluene solution, 0.50 mL, 0.3 mmol) was also added. The mixture was stirred for 1h before the addition of chloral (0.098mL, 1 mmol). Then the mixture was cooled to -78 °C (dry ice/acetone bath) and isobutylene (∼2 mmol) was condensed into the flask. Finally the reaction vessel was placed in a freezer (-23 °C). The work up followed the published procedures. 4a
13. If poisoning only involves deactivation of one hand of a racemic catalyst, the highest enantioselectivity achievable would be that observed for the enantiomerically pure catalyst. The change of regioselectivity also magnifies the variation in ee. There is evidence (including kinetics) in analogous cases that the intact dimers are the active species (Boyle, T. J.; Eilerts, N. W.; Heppert, J. A.; and Takusagawa, F., Organometallics, 1994, 13, 2218 and references therein). Our results suggest that the enantioselectivity of a heterodimer is different and may even be greater than that of the homodimer. There are, however, proponents of a monomeric active species (Mikami, K.; Terada, M. Tetrahedron 1992, 48, 5671.)
14. If poisoning only involves deactivation of one hand of a racemic catalyst, the highest enantioselectivity achievable would be that observed for the enantiomerically pure catalyst. The change of regioselectivity also magnifies the variation in ee. There is evidence (including kinetics) in analogous cases that the intact dimers are the active species (Boyle, T. J.; Eilerts, N. W.; Heppert, J. A.; and Takusagawa, F., Organometallics, 1994, 13, 2218 and references therein). Our results suggest that the enantioselectivity of a heterodimer is different and may even be greater than that of the homodimer. There are, however, proponents of a monomeric active species (Mikami, K.; Terada, M. Tetrahedron 1992, 48, 5671.)
15. (S)-1 can be obtained >95% ee by reaction of (+)-CpMo(NO)I(methallyl) and chloral. Nguyen, J. T.; Faller, J. W., unpublished (J.T. Nguyen, dissertation, Yale University, 1995).
16. (S)-1 can be obtained >95% ee by reaction of (+)-CpMo(NO)I(methallyl) and chloral. Nguyen, J. T.; Faller, J. W., unpublished (J.T. Nguyen, dissertation, Yale University, 1995).
17. After stirring the catalyst for 1 h, the solvent (i.e., toluene, dichloromethane and some i-PrOH) was removed in vacua for 2 h, subsequently 1 mL dichloromethane was added as solvent.
18. Secondary trichloromethyl carbinols can be converted in two steps to α-amino acids which can be crystallized to enantiomerically pure form, see Corey, E. J.; Link, J. O. J. Am. Chem. Soc., 1992, 114, 1906.