Cryptands as bases in proton-transfer reactions. The rates and hydrogen isotope effects in the ionization of ethyl nitroacetate by cryptands

Abstract

The rates and kinetic hydrogen isotope effects for the proton-transfer reaction between ethyl nitroacetate and three cryptands in the free base and monoprotonated forms have been measured. The catalytic constants for the free bases are ca. 1 order of magnitude smaller, and those of the monoprotonated cryptands ca. 2 orders of magnitude smaller, than those of ‘normal’ bases of comparable basicity. Two possible explanations for the slow rates are discussed: sterically hindered direct transfer of the proton to within the cavity of the ligand, and the reaction of a small, but kinetically active, fraction of molecules containing the free nitrogen atom in an exo-conformation. It is argued that the catalytic activity of the unprotonated cryptands is mainly due to the direct transfer of the proton to the cavity, but it is difficult to decide between the two possibilities for the monoprotonated cryptands. The observed kinetic hydrogen isotope effects were 7.2 for the unprotonated cryptand and in the range 8–8.5 for the monoprotonated cryptands.