Substituent effect on the planarization energy and the relative stability of Winstein and Möbius structures of the homotropylium cation

Abstract

The substituent (OH, OC 2H 5, Cl, z.sbnd;CHCHCHCH) effects on the relative stabilities of Winstein and Möbius structures of the homotropylium cation, 1, have been investigated by the MNDO model. Electron donor substituent at C 1, C 3, C 5, and C 7 favor Möbius structures in the order C 1,C 7>C 3,C 5, whereas Winstein structures are stabilized by electron donor substituents at position C 2, C 4, and C 6, and in the order C 2,C 6 > C 4. Electron withdrawing substituents at C 8 enhance stability of the homoaromatic structures, in the order OH>Cl. In any case, the substituent effect is found to be nearly additive. The reaction path for the three-carbon membered ring opening the eight-carbon membered ring inversion has been derived for all the compounds investigated. The equilibrium populations of Winstein Möbius structures were then obtained from the calculated probability distribution functions along the reaction path. The unsubstituted compound 1 was found to be a non-homoaromatic cation in the gas phase, as it adopts a Möbius conformation with a large C 1-C 7, internuclear distance.