Heterocyclic Pseudobases

This chapter discusses the extensive quantitative studies of pseudobase formation. The detailed application of spectroscopic techniques for the determination of pseudobase structure and tautomerism is also reviewed. Pseudobase formation by heteroaromatic cations is related to the covalent hydration of heteroaromatic molecules and to Meisenheimer complex formation. All such reactions involve the formation of σ-complexes by nucleophilic addition to electron-deficient aromatic species. The pH-dependent electronic spectrum for a heteroaromatic cation which contains no readily ionizable protons is usually attributable to pseudobase formation. Pseudobase formation from an unsaturated heterocyclic cation is accompanied by a significant change in the PMR spectrum. The saturation of an unsaturated carbon atom as a result of pseudobase formation results in an upfield chemical shift of approximately 4 ppm for the signal from a hydrogen atom on that carbon atom. The equilibrium constants for pseudobase formation, and the kinetics and mechanism of cation-pseudobase equilibration are also discussed.