Conformation and thermal inversion of 10,11-dihydro-5H-dibenzo[a,d]cycloheptene ring spiro-linked to homoquinones

1,3-Dipolar cycloaddition of 5-diazo-10,11-dihydro-5H-dibenzo[a,d]cyclohept ene with 2,5-dimethyl-1,4-benzoquinone gave the less stable conformer of 10 ',11'-dihydro-1,4-dimethylspiro[bicyclo[4.2.0]hept-3-ene-7,5'-(5'H-dibenzo[ a,d]cycloheptene)]-2,5-dione (alpha-conformer) via a conformationally retai ned nitrogen extrusion from the sterically congested indazole adduct. X-Ray structure analyses revealed that the cycloheptene ring moieties adopt cons iderably strained twist-boat conformations in which the dihedral angles (th eta) of the -CH2CH2- bridge are 77.1 and 27.3 degrees, respectively. At 100 degrees C the alpha-conformer underwent a one-way conformational inversion to the more stable twist-boat conformer (beta-conformer) with an almost ga uche angle of theta = 55.5 degrees. A kinetic study of the thermal inversio n exhibited a small dependency on the quinone substituents as well as negli gible solvent effects, providing the transition energy of 127 kJ mol(-1). S emiempirical (PM3) calculations were performed to determine the optimized g eometries of the alpha- and beta-conformers as well as the inversion transi tion state structure, which were compared with the X-ray data.