A comprehensive view on stabilities and reactivities of triarylmethyl cations (tritylium ions)

Abstract

By combining conventional photometric and conductimetric methods with stopped‐flow techniques, we were able to investigate ionization rate constants of trityl bromides, chlorides, and carboxylates in aqueous acetonitrile in a reactivity range from 10−5 to 103 s−1. In this way, it became possible to compare ionization processes yielding tritylium ions which differ by 21 units in pKR+, corresponding to 120 kJ mol−1 in free energy. Analysis of such far‐reaching correlations provided new insights into several aspects of organic reactivity. We observed (i) the change of solvolysis mechanisms from SN1 reactions without and with common ion return, (ii) reactions where formation and consumption of the carbocations could be observed visually, (iii) solvolyses where the ionization step is almost complete before the reaction with water becomes significant, and (iv) ionizations that yield solutions of persistent carbocations. The continuous change from reactions with carbocation‐like transition states to reactions with transition states of little carbocation character caused large variations of Winstein–Grunwald m values. Values of m = 0.2 for ionization processes show that low values of m do not necessarily imply SN2 reactions. Although the 4,4′,4″‐trimethoxytritylium ion reacts more than 1000 times faster with water than the 4‐dimethylaminotritylium ion and has a pKR+ value that is more negative by three units, both carbocations are formed with almost equal rates by ionization of the corresponding acetates in aqueous acetonitrile. This comparison again illustrates that electrofugality is not simply the inverse of electrophilicity and presents a challenge for further study of the importance of intrinsic barriers. Copyright © 2012 John Wiley & Sons, Ltd.

Benzhydrylium and tritylium ions: complementary probes for examining ambident nucleophiles

“…It has been shown, however, that the same E parameters of tritylium ions that were derived from their reactions with n-nucleophiles could also be used for hydride transfer reactions [26,29,58]. Kinetic data that were obtained by the benzhydrylium method could, therefore, be supplemented by literature data for hydride transfer reactions towards tritylium ions.…”

“…(1) by a steric parameter, it was recommended not to use eq. (1) for reactions of bulky reagents, for example, reactions of tritylium ions with π-nucleophiles [20,26,29].…”

“…In addition, ambident properties as well as hydride transfer reactions complicate the kinetics of the reactions of tritylium ions with π-nucleophiles. It has, therefore, repeatedly been recommended that tritylium ions should not be used as reference electrophiles for the determination of nucleophilic reactivities [20,26].…”

“…With the goal to include these rate constants in Mayr's comprehensive nucleophilicity scale, we have compared 108 measured rate constants of hydride abstractions by tritylium ions with rate constants calculated by using eq. (1) and the E parameters of Ar 3 C + derived from their reactions with n-nucleophiles (see previous Section) [26,29]. The diagonal in Fig.…”

Carbocations as Lewis Acid Catalysts: Reactivity and Scope

“…Several triaryl cations have been previously reported in the literature and the electronic effects on stability and reactivity towards nucleophiles have been thoroughly studied. [18] The stability or Lewis acidity of relatively stable carbocations can be described by their pK R+ values. The pK R+ value was introduced more than 60 years ago as a reference system for comparing carbocation stabilities in solution and it is defined by Eq.…”