| Abstract: | Intramolecular general base catalysis by the imidazolyl group was found in the hydrolyses of endo-5-[4;(5')-imidazolyl]-bicyclo[2.2.1]hept-endo-2-yl trans-cinnamate and endo-5-[4'(5')-imidazolyl]bicyclo[2.2.2]oct-endo-2-yl trans-cinnamate in which the imidazolyl and trans-cinnamoyl groups are bound in close proximity to each other by rigid bicyclic rings. The rate constants for the intramolecular general base-catalyzed hydrolyses at 60 degrees are 6.4 X 10(-7) sec-1 for the former and 1.8 X 10(-7) sec-1 for the latter and the deuterium oxide solvent isotope effects are 3.0 for both. On the other hand, no intramolecular catalytic participation of the imidazolyl group was observed in the hydrolyses of the endo-exo isomers, exo- 5-[4'(5')-imidazolyl]bicyclo[2.2.1]hept-endo-2-yl trans-cinnamate and endo-5[4'(5')-imidazolyl]bicyclo[2.2.2]oct-exo-2-yl trans-cinnamate, in which the imidazolyl groups are located far from the trans-cinnamoyl groups. Intramolecular general base-catalyzed hydrolyses by the imidazolyl groups in endo-5[4'(5')-imidazolyl]bicyclo[2.2.1]hept-endo-2-yl trans-cinnamate and endo-5-[4'(5')-imidazolyl]bicyclo[2.2.2]oct-endo-2-yl trans-cinnamate can serve as models of serine esterase-catalyzed hydrolyses. |
