6533b7cefe1ef96bd1256e71

RESEARCH PRODUCT

ISomerization of xylene: MINDO/3 study of the cyclization of benzenium cation into bicyclo[3.1.0]hexenyl cation

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Abstract A MINDO/3 study of thermal electrocyclic closing of the benzenium cation to form a bicyclo[3.1.0]hexenyl cation is presented. The calculated mechanism discusses the possibility of cyclization of xylenium cations on a silica catalyst or in superacid media. The structure and energy of the transition state, the enthalpies of isomerization and barriers of activation were calculated for the following processes: isomerization of 2,6-dimethylbenzenium cation into 2,6-dimethylbicyclo[3.1.0]hexenyl cation, isomerization of 2,4-dimethylbenzenium cation into 2,4-dimethylbicyclo[3.1.0]-hexenyl cation, and isomerization of benzenium cation into bicyclo[3.1.0]-hexenyl cation. Equilibrium and kinetic constants were calculated for the last process. The activation barriers seem to indicate that this usually accepted reaction step is too high in energy to make the reaction feasible. The results show that the mechanism does not follow the Woodward—Hoffmann rule, since the thermal cyclization of a 4-π electron cationic system should be forbidden.