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

Abstract

1,3-Dipolar cycloaddition of 5-diazo-10,11-dihydro-5H-dibenzo[a,d]cycloheptene 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 (α-conformer) via a conformationally retained nitrogen extrusion from the sterically congested indazole adduct. X-Ray structure analyses revealed that the cycloheptene ring moieties adopt considerably strained twist-boat conformations in which the dihedral angles (θ) of the –CH₂CH₂– bridge are 77.1 and 27.3°, respectively. At 100 °C the α-conformer underwent a one-way conformational inversion to the more stable twist-boat conformer (β-conformer) with an almost gauche angle of θ = 55.5°. A kinetic study of the thermal inversion exhibited a small dependency on the quinone substituents as well as negligible solvent effects, providing the transition energy of 127 kJ mol^–1. Semiempirical (PM3) calculations were performed to determine the optimized geometries of the α- and β-conformers as well as the inversion transition state structure, which were compared with the X-ray data.