Electrocyclic Ring Opening Modes of Dewar Benzenes:  Ab Initio Predictions for Möbius Benzene and trans-Dewar Benzene as New C₆H₆ Isomers

Ab initio calculations have been performed on the two modes of electrocyclic ring opening of Dewar benzene ( 1 ), as well as related structures. Both CASSCF and MP2 optimizations predict the existence of Möbius benzene ( 3, cis, cis, trans -1,3,5-cyclohexatriene) in a very shallow minimum ca. 100 kcal/mol above benzene ( 2 ). This structure has alternating single and double bonds, with one fully trans double bond. The barrier for π bond rotation ( 3 → 2 ) is predicted to be less than 3 kcal/mol. Stationary points have been located at the TCSCF/6-31G* level for both conrotatory and disrotatory ring opening of 1 . The disrotatory path proceeds through a second-order saddle point, which appears to be a general characteristic of symmetry-forbidden reactions. At every level of calculation, conrotatory ring opening of 1 proceeds through a barrier that is 1.5−5.1 kcal/mol lower than disrotation. The predicted conrotatory enthalpic barrier of 24−28 kcal/mol agrees well with the experimental value of 25.1 ± 2. Intrinsic reaction cordinate calculations suggest that conrotation from 1 may lead to either 2 or 3 . Calculations also support the existence of trans -Dewar benzene ( 4 ), a substance originally suggested by Woodward and Hoffmann in 1971. This is predicted to be 158 kcal/mol above benzene, but still has a barrier of ca. 13 kcal/mol for ring opening to benzene.