Reductive generation of enolates using a chromium(III) ate-type reagent as a reductant and reactions of the enolates with electrophiles

Organometallics

Volumn 20, Issue 24, 2001, Pages 5014-5016

  Hojo, Makoto ^a   Sakata, Kyosuke ^a   Ushioda, Nobuo ^a   Watanabe, Takeshi ^a   Nishikori, Hisashi ^a   Hosomi, Akira ^a  

^a UNIVERSITY OF TSUKUBA   (Japan)

Author keywords

[No Author keywords available]

Indexed keywords

ELECTROPHILES; ENOLATES; REDUCTIVE GENERATION;

CHROMIUM COMPOUNDS; ESTERS; KETONES; MIXTURES; MOLECULAR STRUCTURE; REDUCTION; SUBSTITUTION REACTIONS; SYNTHESIS (CHEMICAL); THERMAL EFFECTS;

ORGANOMETALLICS;

EID: 0035956454     PISSN: 02767333     EISSN: None     Source Type: Journal    
DOI: 10.1021/om010749y     Document Type: Article

References (19)

1. Organometallic Ate Compounds in Organic Synthesis, 39. Part 38: Tateiwa, J.; Hosomi, A. Eur. J. Org. Chem. 2001, 4(8), 1445-1448.
2. Fürstner, A. Chem. Rev. 1999, 99, 991-1045.
3. Hojo, M.; Sakuragi, R.; Okabe, S.; Hosomi, A. Chem. Commun. 2001, 357-358.
4. For other recent uses of chromate reagents, Nishikawa, T.; Kakiya, T.; Shinokubo, H.; Oshima, K. J. Am. Chem. Soc. 2001, 123, 4629-4630.
5. (a) Dubois, J.-E.; Axiotis, G.; Bertounesque, E. Tetrahedron Lett. 1985, 26, 4371-4372.
6. (b) Wessjohann, L.; Gabriel, T. J. Org. Chem. 1997, 62, 3772-3774.
7. (c) Wessjohann, L.; Wild, H. Synthesis 1997, 512-514.
8. (d) Wessjohann, L.; Wild, H. Synlett 1997, 731-733.
9. (e) Gabriel, T.; Wessjohann, L. Tetrahedron Lett. 1997, 38, 4387-4388.
10. (a) Okude, Y.; Hiyama, T.; Nozaki, H. Tetrahedron Lett. 1977, 3829-3820.
11. (b) Hiyama, T.; Okude, Y.; Kimura, K.; Nozaki, H. Bull. Chem. Soc. Jpn. 1982, 55, 561-568.
12. 3" was prepared as follows: to a suspension of ground chromium(III) chloride (0.52 mmol) in THF (4.0 mL) was slowly added a solution of butyllithium in hexane (3.12 mmol/1.99 mL; 1.57 M) at -78°C under argon with stirring. The resultant mixture was further stirred for 30 min at the same temperature.
13. 4CrLi" at -78°C for 1 h, after quenching with benzaldehyde, gave 34% of 2a (syn/anti = 56/44).
14. The reaction conditions were not fully optimized, and the amount of electrophiles may be reduced.
15. For aliphatic ketones, an acetoxy group did not work for the generation of enolates.
16. In the present procedure, an excess amount of the reductant is not necessary, possibly because the reaction mixture is nearly homogeneous.
17. Aldol-type reactions of halo ketones with aldehydes under Barbier-type conditions were reported to be syn-selective, while those of esters and amides were anti-selective. Although it is premature to discuss about the difference in selectivity with those of previously reported reactions, the difference may possibly come from reaction conditions and/or the different environments around the chromiums of the chromium enolates. If the chromium has many ligands and the aldol reaction proceeds through the open TS, the reaction becomes syn-selective.
18. For comparison with eq 2, we examined the reaction of lithium enolate with enone. The reaction of lithium enolate generated from ethyl isobutyrate (with lithium diisopropylamide in THF at -20°C for 1 h) and 2-cyclohexen-1-one at -78 to -50°C for 2 h gave 82% of the 1,2-adduct and 9% of the 1,4-adduct 2k.
19. In previous reports, chemoselectivity under the Barbier-type conditions was examined by the intermolecular competitive reactions with aldehydes. In these experiments, it was reported that only ketones competed with aldehydes (to aldehyde/to ketone = (7-30)/1), and with other electrophiles such as Michael acceptors, imines, alkyl halides, and even more reactive Eschenmoser's salt, no products derived from these electrophiles were formed under the conditions. Therefore, it is uncertain whether these less reactive electrophiles react with halo ketones and h esters in the absence of competing aldehydes under Barbier conditions.