Our studies on tetranuclear zinc cluster-catalyzed environmentally friendly transesterification are presented here. The newly developed l-oxo-tetranuclear zinc cluster is a highly efficient catalyst for the transesterification of various methyl esters including a-amino esters, b-keto esters, and even highly unstable a, b-unsaturated esters; for the acetylation of alcohols in EtOAc; and for the deacylation of esters in MeOH. A unique hydroxy group-selective acylation in the presence of inherently much more nucleophilic amino groups was also achieved by this zinc cluster. Zinc cluster-catalyzed transesterification was drastically accelerated by the addition of alkyl amine and N-heteroaromatic ligands, which coordinate with the metals, stabilize the clusters with lower nuclearities, and enhance catalytic activity for the transesterification. Based on our mechanistic studies, the deprotonation of nucleophiles was the most important step in this process, not only for achieving high catalytic activity but also for determining chemoselectivity. In addition, we also developed the second generation zinc catalyst: bis(imidazole)/zinc complexes and the third generation zinc catalyst: heterogeneous zinc/imidazole catalyst, enabling the recovery of the catalyst through simple filtration with the same or higher catalytic activity.
!!!All Science Journal Classification (ASJC) codes
- 化学 (全般)