CONFORMATIONAL FLEXIBILITY OF 6-MEMBERED 1,2-DIHYDROCYCLES AND SUBSTITUENT ELECTRONIC EFFECTS

Conformational flexibility of six-membered 1,2-dihydrocycles has been studied using the HF/6-31G* non-empirical quantum-chemical method. It is shown that all types of such rings possess high conformational fle xibility. This property results from the presence of two groups of fac tors which determine ring geometry. One of them (conjugation effects) favours a planar conformation. The second (bending strain, non-bonded interactions, non-aromatic character of cyclic conjugated system) stab ilizes distorted sofa conformation. Substituent electronic effects and the change of exocyclic double bond polarity influence on dihydrocycl e flexibility have been studied. The evaluation of energy levels below 99% of the molecular population indicates the presence of essentially non-planar conformers with relevant torsion angle up to 30 degrees at room temperature. (C) 1998 Elsevier Science B.V. All rights reserved.