Potential surface study of a thermally allowed double hydrogen shift in an isomer of [12] annulene

Abstract

Trans-4a,10a-dihydrobenzocyclooctene (1) rearranges thermally into 7,8-dihydro-benzocyclooctene ( 3). This rearrangement is formally a double 1,5-hydrogen-shift; it may proceed either via two consecutive hydrogen shifts, 4a,7-dihydrobenzocyclooctene ( 2) being implied as an intermediate, or the two hydrogens could migrate synchronously. In the latter case, the C 2-symmetry axes present in the starting, 1, and final compound, 3, is preserved along the reaction pathway. In order to determine which is the preferred pathway, a relevant part of the potential surface corresponding to this reaction was investigated using MO-computations. The single determinant UHF-MINDO/3 method, which takes into account electron—electron correlation, was adopted. The two consecutive 1,5-hydrogen-shifts mechanism is shown to be the preferred reaction pathway.