Catalysis of decarboxylation by an adjacent negative charge: a theoretical approach.

We have calculated the rate acceleration in decarboxylation reactions that can be accomplished by charge-charge repulsion between the substrate carboxylate and an adjacent negative charge in media of various dielectric constants. It has been shown that a full negative charge or surrounding partial negative charges will have the same effect. It is concluded that the rate of decarboxylation could be greatly accelerated by the presence of a negative charge nearby. For example, in media with dielectric constants from 5.62 to 20.7, a 10(8)-fold rate acceleration could be achieved by a negative charge placed 3.77 A away from the substrate carboxylate group. However, pK(a) perturbation on two carboxylate groups at close proximity may limit the extent of catalysis. It should also be noted that the extent of catalysis does not change much when the dielectric constant varies from 5.62 to 20.7.