Models for incomplete nucleophilic attack on a protonated carbonyl group and electron-deficient alkenes: salts and zwitterions from 1-dimethylamino-naphthalene-8-carbaldehyde

The X-ray crystal structures of salts and zwitterionic Knoevenagel products from 1-dimethylamino-naphthalene-8-carbaldehyde show long N-C bonds between peri-groups which provide models for incomplete nucleophilic attack on a protonated carbonyl group and electron-deficient alkenes respectively. For the salts the N-C bonds lie in the range 1.625-1.638 ? with C-OH bonds intermediate in length between single and double bonds, while for the zwitterions the N-C bonds lie in the range 1.612-1.660 ?. The structural assignment of the former is supported by solid state (13)C and (15)N NMR studies on doubly isotopically-labelled material. Several zwitterions were converted to naphtha1,8-bc]azepines by a mechanism involving the tertiary amino effect.