Friedrich Scheidt

The effect of the benzene ring on 1,5-electrocyclizations: Synthesis and thermolysis of optically active benzohomofuran, benzohomothiophene, N-carbmethoxyhomoindole, benzohomophosphole and homoindene.

Abstract Carbonyl ylide-like intermediates are involved in the 1,5-electrocyclization of the benzoheterocycles 3a-d mentioned in the title. The activation barriers analyzed by the time- and temperature-dependence of the racemisation of the optically active derivatives turned out to be higher by ΔΔG ≠ = 13 ± 2 kcal/mol than those determined for the parent systems 1a-d whereas for the corresponding carbocycles 3d and 1d the difference is only ΔΔG ≠ ≈ 4 kcal/mol as expected from the difference between the bond dissociation energies of toluene and propene (ΔBDE = 3.2 kcal/mol). This result can be considered as an evidence for the electrocyclic character of the ring-opening in the heterocycles 1a-d and 3a-d . The difference between the Gibbs activation enthalpies found for the ring-opening of homofuran 1a and homothiophene 1b (ΔΔG ≠ = 8.7 kcal/mol) can be attributed to the heteroatom effect on the ground state.

Substituent effects in the thermal rearrangement of bicyclo[2,1,1]hexenes

anti-5-Methoxy-(2a) and 5-azidobicyclo[2,1,1]hexene (2b) rearrange cleanly at 30–50° to give exo-6-methoxy-(1c) and 6-azidobicyclo[3,1,0]hexene (1d) respectively; reduction of (2b) below 0° proceeds with rearrangement to 5-aminobicyclo[3,1,0]hexene.