sterochemistry of the addition across the alkene are best understood with a phenonium ion structure for the adduct. The nucleophile entering in beta can be varied under conditions in which the adductcation is trapped more efficiently than the starting phenyl cation. Thus, beta-methoxyalkylanilines are formed when the irradiation is carried out in methanol. beta-Iodoalkylanilines are obtained in acetonitrile
Smooth Synthesis of Aryl- and Alkylanilines by Photoheterolysis of Haloanilines in the Presence of Aromatics and Alkenes
作者:Maurizio Fagnoni、Mariella Mella、Angelo Albini
DOI:10.1021/ol990982g
日期:1999.10.1
[GRAPHICS]Irradiation of 4 chloro-N,N-dimethylaniline in acetonitrile in the presence of benzene and of various alkenes leads to heterolytic dehalogenation and trapping of the cation. 4-(Dimethylamino)biphenyl is formed in the first case, while with alkenes beta-chloroalkylanilines, stilbenes, or allylanilines are obtained depending on the alkene structure. 4-Fluoroaniline is similarly dehalogenated.
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.
Looking for a Paradigm for the Reactivity of Phenonium Ions
intermediate (phenonium character), particularly with electron-donating substituents on the aromatic ring, and the main products in acetonitrile are phenethyl chlorides. Variation of the ion-stabilizing solvent such as trifluoroethanol (TFE) leads to alkyl and hydride migration before chloride addition. In contrast, with less electron-rich aromatics and highly substituted alkenes the stabilization of the intermediate