Conformational analysis. Part 7. A lanthanide induced shift (L.I.S.) nuclear magnetic resonance investigation of conformational isomerism in ortho- and meta-substituted benzaldehydes

Abstract

A lanthanide induced shift n.m.r. investigation of conformational isomerism in o- and m-methyl- and -chloro-benzaldehydes and m-methoxybenzaldehyde has been carried out. With the ¹³C diamagnetic complexation shifts removed by use of La(fod)₃ and the application of a previously described lanthanide ion complexing model, the pseudo-contact shifts have been used to obtain precise conformational preferences of the aldehydes. The m-methyl-, -chloro-, and -methoxy-benzaldehydes are 44, 42, and 32%, respectively in the O-trans conformation, whereas the o-methyl is 59% in the O-trans form and the o-chloro 100% in the O-trans form. Possible structural deformations in the ortho-substituted compounds have been considered. The technique is a sensitive probe of the methyl torsional angle for the O-cis conformer, the methyl being staggered with respect to the aldehyde group. The o-methylbenzaldehyde geometry is not appreciably distored and the aldehyde group is coplanar with the aromatic ring. In o-chlorobenzaldehyde the structural changes are small and are interpreted as arising from the attractive Cl ⋯ HCO interaction.