Conformational analysis. Part 6. A lanthanide-induced shift nuclear magnetic resonance investigation of steric effects in mesitaldehyde and 2,4,6-trimethylacetophenone

Abstract

A lanthanide-induced shift (L.I.S.) n.m.r. Investigation of the structures in solution of mesitaldehyde (1) and 2,4,6-trimethylacetophenone (2) has been performed. The use of the crystal structure geometry of the mesitaldehyde–perchloric acid complex was unsatisfactory and no reasonable fit to the L.I.S. Data was achieved. In contrast, use of a model geometry in which steric relaxation (of the methyl groups) was allowed gave good agreement. The formyl group is co-planar with the benzene ring, but the CCC angles to the adjacent methyl groups are increased (to 126°) to relieve steric interactions. In contrast, in (2) the acetyl group has a dihedral of 60–90° with the aromatic ring whilst the methyl CCC angles are unaffected. The lanthanum(III) diamagnetic complexation shifts further illustrate the lack of conjugation of the carbonyl group with the aromatic ring in (2) compared with (1).