Synthesis and Characterization of CoII Macrocycles and its Use for Promoting Hydrolysis of Esters

Abstract

The design and synthesis of hydrolytically active macrocycles mimic the substrate selectivity and rate enhancements for the hydrolysis of various organic substrates in high/low temperatures and extreme pH conditions, which is extremely challenging. In the present study, we synthesized two CoIIHMTAA-14 and CoIIHMTAA-16 macrocycles (HMTAA=hexamethyl-dibenzo-tetraaza-azulene) and used them to promote the hydrolysis of 4-nitrophenyl-2-benzamide carbonate and 4-nitrophenyl-4-benzamide carbonate esters. The effect of pH on hydrolysis of the carbonate esters was also studied at different pH 4.0, 6.5, and 8.5. The results of these studies showed that the reaction follows the first order with respect to ester concentration and is independent of the medium pH and water concentration. For the account of mechanism, hydrolysis of 4-nitrophenyl-2-benzamide carbonate and 4-nitrophenyl-4-benzamide carbonate proceeds either through oxygen or nitrogen intermolecular attack and normal H2O or OH- attacks, respectively. The plots of logkobs vs. pKa of the conjugate acid nucleophile showed leveling beyond pKa of about β = 0.3. The present macrocyclic complexes were found to provide enhanced hydrolysis of the esters.