Conformational equilibria of 2-substituted 1,3-dithio-5,6-benzocycloheptene and 1,3-dioxa-5,6-benzocycloheptene

Abstract

The semiempirical PM3 method was used to study the Conformational equilibrium of the various chair, boat, twist and biplanar conformations of 1,3-dithio- and 1,3-dioxa-5,6-benzocycloheptene when groups having different dimensions substitute one or both of the hydrogen atoms at C 2. The results obtained show that bond lengths and bond angles are highly affected by the steric effect originating from the substituents. The chair conformation is the prevailing one in the 1,3-dithio-5,6-benzocycloheptene derivatives and the only possible when cumbersome substituents are attached at C 2, except in the di- t-butyl derivative. In the corresponding oxygenated molecules the chair form prevails when the steric effect is not relevant, otherwise the equilibrium shifts towards the twist conformation. The calculated barriers for the chair ↔ boat interconversion are in good agreement with the literature data concerning 1,3-dioxa-5,6-benzocycloheptene and its 2,2′-dimethyl derivative, whilst they are underestimated for the dithio derivatives. Barriers to the twist ↔ boat pseudorotation pathway are found only for some dithio derivatives. Ab initio 3G–21G ∗ calculations limited to the parent compounds were also performed.