Crucial Role of Elusive Isomeric h-Complexes in Gas-Phase Electrophilic Aromatic Alkylations

The kinetics and stereochem. of the protonation-induced unimol. isomerization of (R)-1-D1-3-(p-fluorophenyl)butane have been investigated in the gas phase at 40-100 C and 70-760 Torr. This process leads to the formation of the relevant meta and ortho isomers with partial racemization of the migrating sec-Bu moiety. Complete racemization is obsd., instead, when the isomerization reaction involves a 1,2-H shift in the moving alkyl group. These results, together with the relevant activation parameters, fully confirm the previous evidence of the occurrence in the alkyl cation/arene PES of noncovalent .eta.-type intermediates of defined structure and stability, lying well below the classical .pi.-complexes, as confirmed by ab initio calcns. Their crucial role in detg. the positional selectivity of gas-phase electrophilic arom. substitutions clearly emerges from the comparison of the present results with the site selectivity measured in the corresponding bimol. arene alkylation carried out at the same temps. and pressures.