Using phenyl cations as probes for establishing electrophilicity-nucleophilicity relations

N,N-Dimethyl-4-aminophenyl cation is used as an electrophilic probe for determining the relative reactivity of nucleophiles. The singlet state ((3)1) of this cation is completely unselective (reaction rates with benzene, MeCN, and trifluoroethanol within a factor of 5). The corresponding triplet (11) does not react with alcohols and MeCN. The rates of reaction of the latter intermediate with 23.7 pi,sigma and n nucleophiles are measured by competition experiments and found to vary over only 2 orders of magnitude over a range of 22 units of the nucleophilicity parameter N introduced by Mayr. As far as one can judge with the considerable scatter of the data, fitting the data of both amines and pi nucleophiles is possible only by using a modified Mayr's equation: log k = s(E + eN) with e = 0.33. The reduced dependence on N is explained by the fact that in the case of diradicalic triplet (3)1 interaction with the nucleophile involves a half-filled (sigma) orbital, which is empty in singlet (1)1. It is suggested that Mayr's equation can be extended to quite diverse reactions, but a scaling factor of e < I may have to be introduced in some cases, according to the electronic structure of the electrophilic probe used.