Artificial Esterases Zinc II Complexes

24- Zn2, the extent of cooperation of the two metal centres is quite high for esters 22 and 19, significantly lower for ester 20 and even lower for ester 21. Clearly, catalyst 24-Zn2 experiences an increasing difficulty to fit the altered substrate in the transition state when the ester size exceeds that of 19. A similar trend of reactivity is seen in the reactions carried out in the presence of the less effective catalyst 25-Zn2. In this case, however, there is no sign of cooperation between metal centres in the reactions of 20 and 21. 

The marked superiority of the 1,2-vicinal complex 24-Zn2 over its distal regioisomer 

25- Zn2 closely parallels that observed in the basic ethanolysis catalyzed by dinuclear complexes 14-Ba2 and 15-Ba2 (Table 8.1). The results obtained in the basic methanolysis of esters catalyzed by the zinc(II) complexes of calix[4]arenes 28-31 decorated with 1,5,9-triazacyclododecane ([12]aneN3) ligands fully confirm the superiority of 1,2-vicinal bimetallic catalysts over their distal regioisomers. Thus, the order of catalytic efficiency 1,2-vicinal 1,3-distal appears to be a substrate-independent feature of upper rim calix[4]arene-based bimetallic catalysts for the cleavage of carboxylate-functional-ized esters, which is unaffected by the nature of the metal ion and of the corresponding ligating unit. 

Kinetic data obtained in the reactions catalyzed by the BAMP ligated zinc complexes permit reliable estimates of the ki u3 and kintr quantities needed for the calculation of EMs. 

---