Conformational study of a series of 6-substituted 5,6,7,12-tetrahydrodibenzo[a,d]-cyclooctenes by nuclear magnetic resonance spectroscopy

Abstract

The conformational properties of seven 5,6,7,12-tetrahydrodibenzo[a,d]cyclooctenes bearing substituents at C-6 have been studied using variable temperature proton nmr spectroscopy. The position of the equilibrium between boat-chair (BC) and twist-boat (TB) conformers has been measured in several solvents. In contrast to the unsubstituted dibenzocyclooctene, appreciable amounts of TB conformer are present when C-6 is substituted by alkyl, hydroxy, or cyano substituents. Measurements at different temperatures showed the TB form to possess the greater entropy. Barriers to the BC → TB interconversion were determined by coalescence studies, using both approximate and complete line shape methods. Barriers ranged from 10.1 kcal/mol for the 6-keto derivative to 16.7 kcal/mol for the 6-hydroxy-6-methyl derivative.