Mapping the Active Site in a Chemzyme: Diversity in the <i>N</i>-Substituent in the Catalytic Asymmetric Aziridination of Imines
作者:Yu Zhang、Zhenjie Lu、Aman Desai、William D. Wulff
DOI:10.1021/ol802431v
日期:2008.12.4
The active site of the aziridination catalyst derived from either the VANOL or VAPOL ligand and B(OPh)(3) is larger than expected and can accommodate not only significant substitution on the diarylmethyl unit of the imine but also that alkyl (but not perfluorylalkyl) substituents on the aryl groups lead to enhanced rates and enantioselection. The screen of diarylmethyl N-substituents on the imine revealed that the 3,5-di-tert-butyldianisylmethyl group (BUDAM) gave exceptionally high asymmetric inductions for imines of aryl aldehydes.
Evidence for a Boroxinate Based Brønsted Acid Derivative of VAPOL as the Active Catalyst in the Catalytic Asymmetric Aziridination Reaction
作者:Gang Hu、Li Huang、Rui H. Huang、William D. Wulff
DOI:10.1021/ja904589k
日期:2009.11.4
Studies are described that were designed to determine the structure of the active catalyst in the asymmetric catalytic aziridination of imines with ethyl diazoacetate (AZ reaction). Evidence suggests that the active catalyst contains a boroxine ring in which one of the three boron atoms is spiro-fused with the two phenol groups of the VAPOL tigand. B-11 and H-1 NMR evidence supports the boroxinate structure B in which the counterion to the boroxinate is the protonated form of the imine. The boroxinate structure is also supported by two solid state structures of a VAPOL boroxinate in which the gegen cation is tetramethyl ammonium and 4-dimethylaminopyridinium.