THE EFFECT OF THE BENZENE-RING ON 1,5-ELECTROCYCLIZATIONS - SYNTHESIS AND THERMOLYSIS OF OPTICALLY-ACTIVE BENZOHOMOFURAN, BENZOHOMOTHIOPHENE, N-CARBMETHOXYHOMOINDOLE, BENZOHOMOPHOSPHOLE AND HOMOINDENE

Carbonyl ylide-like intermediates are involved in the 1,5-electrocyclization of the benzo-heterocycles 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 DELTADELTAG(not-equal) = 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 DELTADELTAG(not-equal) almost-equal-to 4 kcal/mol as expected from the difference between the bond dissociation energies of toluene and propene (DELTABDE = 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 (DELTADELTAG(not-equal) = 8.7 kcal/mol) can be attributed to the heteroatom effect on the ground state.