Volume 64 (2016) Issue 6 Pages 523-539
Our studies on tetranuclear zinc cluster-catalyzed environmentally friendly reactions are presented here. The newly developed μ-oxo-tetranuclear zinc cluster is a highly efficient catalyst for the direct formation of oxazolines from esters, carboxylic acids, lactones, and nitriles; for the transesterification of various methyl esters including α-amino esters and β-keto 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, in particular, 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. We also performed several mechanistic studies which revealed that alkoxide metal complexes are the active species in this catalytic cycle, and that the Michaelis−Menten behavior of the complexes through an ordered ternary complex mechanism is similar to that of dinuclear metalloenzymes. The deprotonation of nucleophiles was the most important step in this process, not only for achieving high catalytic activity but also for determining chemoselectivity, resulting in the chemical differentiation of alcohols and amines.