Influences of electronic effects and anions on the enantioselectivity in the oxazaborolidine-catalyzed asymmetric borane reduction of ketones

Jiaxi Xu; Tiezheng Wei; Qihan Zhang

doi:10.1021/jo048959i

Influences of electronic effects and anions on the enantioselectivity in the oxazaborolidine-catalyzed asymmetric borane reduction of ketones

J Org Chem. 2004 Oct 1;69(20):6860-6. doi: 10.1021/jo048959i.

Authors

Jiaxi Xu¹, Tiezheng Wei, Qihan Zhang

Affiliation

¹ Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Department of Chemical Biology, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People's Republic of China. jxxu@pku.edu.cn

PMID: 15387612
DOI: 10.1021/jo048959i

Abstract

The influence of electronic effects on the enantioselectivity of the oxazaborolidine-catalyzed asymmetric borane reduction of ketones has been observed and investigated with use of para-substituted acetophenones and propiophenones with a variety of functional groups and B-unsubstituted and B-methoxyoxazaborolidines derived from (S)-2-(diphenylhydroxymethyl)pyrrolidine with borane and trimethyl borate as catalysts in toluene and THF. The results indicate that Hammett linear free energy electronic effects on the enantioselectivity in the asymmetric reduction were observed and rationalized. Tuning electronic effects of the catalyst can improve the enantioselectivity in the reduction. Another interesting finding to be noted is that anions heavily affect the enantioselectivity, especially for the B-methoxy catalyst, because of their coordination with the boron atom in the catalysts.