The Effect of the Benzene Ring on 1,5-Electrocyclizations: Synthesis and Thermolysis of Optically Active Benzohomofuran, Benzohomothiophene, N-Carbmethoxyhomoindole, Benzohomophosphole and Homoindene

Article abstract of DOI:10.1016/S0957-4166(00)86091-7

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(exit.) = 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(excit.) = 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(excit.) = 8.7 kcal/mol) can be attributed to the heteroatom effect on the ground state.