Conformational analysis of organic carbonyl compounds. Part 5. p-Methoxybenzoyl derivatives of benzo[b]furan, benzo[b]thiophene, and naphthalene

Abstract

The conformational analysis of 2- and 3-(p-methoxybenzoyl)-benzo[b]furan and -benzo[b]thiophene and 1- and 2-(p-methoxybenzoyl)naphthalene was performed by the n.m.r. lanthanide-induced shift (LIS) method on ¹H and ¹³C chemical shifts with Yb(fod)₃ as shift reagent. In the 2-substituted benzo[b]furan a chelate structure having the lanthanide atom bound to both carbonyl and furyl oxygens is formed, so the results do not represent useful information for the conformational properties of this molecule in solution. For the 2-benzo[b]thiophene derivative the S,O-cis(Z) conformation was found to be more abundant in the equilibrium mixture of the two nearly planar conformers. In the corresponding 3-substituted heterocycles the predominant conformation is that of X,O-trans type with a similar degree of distortion from planarity in the two compounds. In all these molecules the p-methoxyphenyl ring is twisted ca. 30° from the carbonyl plane. These systems thus demonstrate the tendency of the carbonyl group to adopt the characteristic orientation of the corresponding formyl and acetyl derivatives, with deviations from planarity due to steric factors. In the 1-naphthyl derivative the naphthyl group turns out to be perpendicular to the planar methoxybenzoyl group. The situation of 2-(p-methoxybenzoyl)naphthalene resembles that of benzophenone and 4,4′-dimethoxybenzophenone: the latter compound was also analysed in the present work, with regard to its conformational properties, in order to test the degree of reliability of the experimental method employed when applied to relatively large molecular systems.