Asymmetric aldol reactions between acetone and benzaldehydes catalyzed by chiral Zn2+ complexes of aminoacyl 1,4,7,10-tetraazacyclododecane: fine-tuning of the amino-acid side chains and a revised reaction mechanism.Chem Asian J. 2013 Sep; 8(9):2125-35.CA
We previously reported that chiral Zn(2+) complexes that were designed to mimic the actions of class-I and class-II aldolases catalyzed the enantioselective aldol reactions of acetone and its analogues thereof with benzaldehyde derivatives. Herein, we report the synthesis of new chiral Zn(2+) complexes that contain Zn(2+)-tetraazacyclododecane (Zn(2+)-aneN4) moieties and amino acids that contain aliphatic, aromatic, anionic, cationic, and dipeptide side chains. The chemical and optical yields of the aldol reaction were improved (up to 96 % ee) by using ZnL complexes of L-decanylglycyl-pendant aneN4 (L-ZnL(7)), L-naphthylalanyl-pendant aneN4 (L-ZnL(10)), L-biphenylalanyl-pendant aneN4 (L-ZnL(11)), and L-phenylethylglycyl-pendant aneN4 ligands (L-ZnL(12)). UV/Vis and circular dichroism (CD) titrations of acetylacetone (acac) with ZnL complexes confirmed that a ZnL-(acac)(-) complex was exclusively formed and not the enaminone of ZnL and acac, as we had previously proposed. Moreover, the results of stopped-flow experiments indicated that the complexation of (acac)(-) with ZnL was complete within milliseconds, whereas the formation of an enaminone required several hours. X-ray crystal-structure analysis of L-ZnL(10) and the ZnL complex of L-diphenylalanyl-pendant aneN4 (L-ZnL(13)) shows that the NH2 groups of the amino-acid side chains of these ligands are coordinated to the Zn(2+) center as the fourth coordination site, in addition to three nitrogen atoms of the aneN4 rings. The reaction mechanism of these aldol reactions is discussed and some corrections are made to our previous mechanistic hypothesis.