Electrophilic aromatic trans-t-alkylation.

A review is presented of the acid-catalysed electrophilic aromatic displacement reactions. Acidity dependence, solvent isotope effects, and substituent effects are assessed as criteria for the assignation of the detailed mechanisms to these reactions. The mechanisms of reactions, including hydrogen exchenge, between tertiary-alkyl derivatives and nucleophiles are discussed in terms of carbonium-ion mechanisms. The ortho-, meta-, and para-isomers of t-outyl-toluene, labelled with tritium in the t-butyl group, have been prepared by a route involving the reduction of the 2-methyl-2-tolylpropionic acids by tritium-labelled diborane. Relative specific activities showed that sodium diborohydride completely exchanged borane with diborane in diglyme. Attempts to prepare an optically active (asymmetric at the quaternary caroon atom) tertiary-alkylaromatic, labelled with tritium at a position beta to the ring, were unsuccessful. A preliminary investigation, using vapour phase chromatography for analysis, has been made of the trans-t-butylation of t-butylaromatics in trifluoro-acetic acid, and in trifluoroacetic acid/perchloric acid. The mixed acid medium removed polyalkylaromatics, presumaoly by oxidation, at a rate comparable to the trans-t-butylation reaction. In trifluoroacetic acid an equilibrium is set up between t-butyl trifluoro-acetate and the possible t-butylaromatics.