화학공학소재연구정보센터
Journal of the American Chemical Society, Vol.132, No.31, 10634-10637, 2010
Enantioselective Synthesis of Allylboronates Bearing a Tertiary or Quaternary B-Substituted Stereogenic Carbon by NHC-Cu-Catalyzed Substitution Reactions
Allylic substitutions that afford alpha-substituted allylboronates bearing B-substituted tertiary or quaternary carbon stereogenic centers are presented. C B bond-forming reactions, catalyzed by chiral bidentate Cu-NHC complexes, are performed in the presence of commercially available bis(pinacolato)diboron. Transformations proceed in high yield (up to >98%) and site selectivity (>98% S(N)2'), and in up to >99:1 enantiomer ratio. Trans- or cis-disubstituted alkenes can be used; alkyl- (linear as well as branched) and aryl-trisubstituted allylic carbonates serve as effective substrates. Allyl-boronates that bear a quaternary carbon center are air-stable and can be easily purified by silica gel chromatography; in contrast, secondary allylboronates cannot be purified in the same manner and are significantly less stable. Oxidation of the enantiomerically enriched products fumishes secondary or tertiary allylic alcohols, valuable small molecules that cannot be easily obtained in high enantiomeric purity by altemative synthesis or kinetic resolution approaches.