Stabilities of Trityl‐Protected Substrates: The Wide Mechanistic Spectrum of Trityl Ester Hydrolyses

Abstract

Ionization rates of para-substituted triphenylmethyl (trityl) acetates, benzoates, and para-nitrobenzoates have been determined in aqueous acetonitrile and aqueous acetone at 25 degrees C. Conventional and stopped-flow techniques have been used to evaluate rate constants ranging from 1.38 x 10(-5) to 2.15 x 10(2) s(-1) by conductimetry and photospectrometry methods. The varying stabilities of the differently substituted tritylium ions account for a gradual change of reaction mechanism. Poorly stabilized carbocations are generated slowly by the ionization of their covalent precursors and trapped fast by water. Better stabilized carbocations are generated more rapidly and accumulate, so that ionization and trapping by water can be observed as separate steps in a single experiment. Finally, highly stabilized tritylium ions do not react with water, and only the rates of their formation could be measured. The ionization rate constants correlate linearly with Winstein's ionizing powers Y; the low slopes (0.17<m<0.58) indicate non-carbocation-like transition states. While the correlation between the ionization rates and Hammett-Brown's sigma(+) parameters is excellent for symmetrically substituted tritylium derivatives, deviations for unsymmetrically substituted systems are observed. The failing rate-equilibrium relationship between the rates of ionizations (log k(ion)) and the stabilities of the carbocations in aqueous solution (pK(R(+))) may be explained by the late development of resonance between a p-amino group and the carbocationic center of the tritylium ion during the ionization process.