Large pKa shifts of α-carbon acids induced by copper(II) complexes

A series of synthetic receptors (4-6) incorporating metal ions, specifically copper(II), were examined for their ability to enhance the acidity of active methylene compounds. The copper(II) complexes were observed to reduce the pK(a) of 1,3-diketone carbon acids in acetonitrile by as much as 12 pK(a), units. The relatively large pK(a) reduction achieved by the complex is attributed to the electrostatic interaction between the anionic pi system of the enolate and the copper(ii) ions. The cage structure and hydrogen bonding sites in receptors 4 and 5 lead to a very modest further enhancement of the acidity relative to that with 6. This study provides insight into the way in which metalloenzymes stabilize an enolate intermediate.