Nucleophilicity parameter, carbocation reactivity

Ionization rate constants in aqueous acetonitrile were obtained for trityl chlorides, bromides, and acetates covering 21 units in the pA1r+ of the trityl cation. This study observed solvolyses with and without common ion return, solvolyses where the trityl cation could be observed to form and then decay, solvolyses where water addition occurs before complete formation of the cation, and at the other extreme, solvolyses that yield persistent carbocations. Mayr and coworkers showed how electrophilicity and nucleophilicity parameters rationalize reactivity patterns and resolve mechanistic controversies in organocatalytic cyclizations. The thermodynamic affinities of a large number of Lewis bases were computed for addition to the methyl, diphenylmethyl, and triphenyl-methyl cations. ... 

Far from confirming a dependence of nucleophilic selectivity on the reactivity of the carbocations, Ritchie observed that selectivities were unchanged over a 106-fold change in reactivity.15 He enshrined this result in an equation (29) analogous to that of Swain and Scott, but with the nucleophilic parameter n modified to N+ to indicate its reference (initially) to reactions of cations, and with the selectivity parameter s taken as 1.0, that is, with no dependence of the selectivity of the cation on its reactivity (as measured by the rate constant for the reference nucleophile, kn2o for water). 

These benzhydrilium carbocations were also used to develop a comprehensive scale of hydride donor strength. Both Si-H and C-H donors were evaluated by their second-order rate constants of hydride transfers to the carbocation electrophiles. The respective nucleophilicity parameter, N and %, were then determined. The benzhydryl cation series was also used to evaluate the nucleophilic reactivities of azolium enolates (1). ... 

The roles of carbocations in commercially important hydrocarbon transformations are still not perfectly understood. The same can be said for carbocations in biological systems. Significant questions concerning reactivity still need to be explained. Why do so many reactions of carbocations show constant selectivity, in violation of the reactivity-selectivity principle Is it possible to develop a unified scale of elec-trophilicity-nucleophilicity, in particular one that incorporates these parameters into the general framework of Lewis acidity and basicity. Finally, quite sophisticated synthetic transformations are being developed that employ carbocations, based upon insights revealed by the mechanistic studies. 

The relative reactivity, solvent isotope effect (k /k -)) and activation parameters for the acid-catalysed hydration of allylic alcohols CH2=CR—CH2OH (R = H, Me) have been found to be similar to those for other alkenes. Whereas the results can be interpreted in terms of the conventional Ad-E2 mechanism, computed values for the life-time of possible carbocation intermediates suggest another feasible mechanism for CH2=CHCH20H, according to which the nucleophilic attack by the solvent is concerted with protonation55 56. 

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