Organocatalytic aza-Michael/retro-aza-Michael reaction: pronounced chirality amplification in aza-Michael reaction and racemization via retro-aza-Michael reaction
Cai, Y.-F.; Li, L.; Luo, M.-X.; Yang, K.-F.; Lai, G.-Q.; Jiang, J.-X.; Xu, L.-W.
Chirality 23(5): 397-403
2011
ISSN/ISBN: 1520-636X
PMID: 21465570
DOI: 10.1002/chir.20940
Accession: 054790801
A detailed experimental investigation of an aza-Michael reaction of aniline and chalcone is presented. A series of Cinchona alkaloid-derived organocatalysts with different functional groups were prepared and used in the aza-Michael and retro-aza-Michael reaction. There was an interesting finding that a complete reversal of stereoselectivity when a benzoyl group was introduced to the cinchonine and cinchonidine. The chirality amplification vs. time proceeds in the quinine-derived organocatalyst containing silicon-based bulky group, QN-TBS, -catalyzed aza-Michael reaction under solvent-free conditions. In addition, we have demonstrated for the first time that racemization was occurred in suitable solvents under mild conditions due to retro-aza-Michael reaction of the Michael adduct of aniline with chalcone. These indicate the equilibrium of retro-aza-Michael reaction and aza-Michael reaction produce the happening of chirality amplification in aza-Michael reaction and racemization via retro-aza-Michael reaction under different conditions, which would be beneficial to the development of novel chiral catalysts for the aza-Michael reactions.