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 Å 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 Å. The structural assignment of the former is supported by solid state 13C and 15N NMR studies on doubly isotopically-labelled material. Several zwitterions were converted to naphtha[1,8-bc]azepines by a mechanism involving the tertiary amino effect.