Effect of benzannelation on the conformational flexibility of the rings in oxo, imino, and methylene derivatives of cyclohexa-1,4-diene

The effect of benzannelation on the equilibrium conformation and flexibility of the dihydrocycle in cyclohexa-1,4-dienone, para-quinone, and their imino and methylene analogs was studied by the semiempirical quantum-chemical AMI method. The equilibrium conformations of the carbonyl derivatives are planar. In the imino-and methylene-substituted analogs, the dihydrocycle adopts a boat conformation due to repulsions between the hydrogen atoms at the exocyclic double bond and in the peri positions of the benzene rings. Annelation of cyclohexa-2,5-dien-1-one and para-quinone with benzene rings at the C=C double bonds causes an increase in the conformational flexibility of the partially hydrogenated ring owing to an increase in the bending strain in the first compound and a decrease in the conjugation between the carbonyl groups and the remaining part of the molecule in the second compound.