Conformational analysis of organic carbonyl compounds. Part 9. 2-Formyl-, 2-acetyl-, and 2-(p-methoxybenzoyl)-indene

Abstract

The conformational behaviour of 2-acylindenes has been examined in order to compare the results with those previously obtained for the acyl derivatives of benzo-condensed five-membered heterocycles and, from a more general point of view, with those of α,β-unsaturated carbonyl compounds. Experimentally, the problem was tackled with the LIS (lanthanide-induced shift) simulation method for ¹H and ¹³C magnetic resonances and with the NOE (nuclear Overhauser effect) difference spectra on ¹H. Ab initio MO calculations were performed on carbonyl derivatives of cyclopentadiene, chosen as model compounds. The experimental results, qualitatively from NOE measurements and more quantitatively from LIS simulation, indicate that these molecules adopt a preferential s-trans orientation for the CO bond and the internal double bond of the indene ring, planar for the formyl and acetyl derivatives. In the p-anisoyl derivative the phenyl and indene rings are twisted (the former to a greater extent) with respect to the carbonyl plane. The same situation is provided by the theoretical treatment of the corresponding cyclopentadiene derivatives.