MOLECULAR RECOGNITION OF ENOLATES OF ACTIVE METHYLENE-COMPOUNDS IN ACETONITRILE - THE INTERPLAY BETWEEN COMPLEMENTARITY AND BASICITY, AND THE USE OF HYDROGEN-BONDING TO LOWER GUEST PK(A)S

A model system for enolase and racemase enzymes was used to explore the extent to which traditional hydrogen bonds can increase carbon acidity. Polyazaclefts 1 and 2 were investigated as receptors for active methylene enolates 10 to 19 in acetonitrile. Receptor 1 was designed to form four hydrogen bonds to the heteroatoms of the enolate guests. The synthesis of the receptors began with the central pyridine ring followed by formation of the peripheral pyrrole rings. Binding constants for 1 with the enolates 10 to 17 in acetonitrile vary from 1.75 x 10(2) to 2.72 x 10(4) M(-1). The nature of both the enolate functionality and the counterion were found to affect the strength of complexation. Molecular mechanics and X-ray analysis of host 1 were used to predict the geometries of the guest-host complexes. Nonaqueous titrations with picric acid were performed on enolates 10 to 17 in order to assess the degree to which complexation was changed by the basicity of the enolate, Complementarity of the guest to host 1 was found to be the dominant factor in enolate binding and not guest basicity. However, a correlation was found to exist between basicity and binding for enolates of the same shape and functionality. Binding by host 1 was found to increase the acidity of 1,3-cyclohexanedione by 1.0 pK(a) unit in acetonitrile. Therefore, traditional hydrogen bonds exert only enough anion stabilization to account for a small fraction of the large pK(a) shifts found for enolase and racemase enzymes. Nevertheless, this acidity increase can be exploited as a means to induce deprotonation of 1,3-cyclohexanedione in the presence of 1.