Benzo[c] Pyrlium Salts: Syntheses, Reactions, and Physical Properties

This chapter demonstrates the wide potential for using the newly developed methods to synthesize 2-benzopyrylium salts and discusses in detail the numerous and unusual transformations of this class of oxygen cations in comparison with monocyclic pyrylium salts— that is, the study of the influence of benzo[c]annelation. Two benzoannelated analogues of the pyrylium cation are known as benzo[b]pyrylium (chromylium or I-benzopyrylium) and benzo[c]pyrylium (isochromylium or 2-benzopyrylium) salts. The reactions of 2-benzopyrylium salts can be divided into five groups: (1) reactions of substituents conserving the benzo[c]pyrylium system, (2) nucleophilic addition reactions, (3) deprotonation reactions, (4) cycloaddition reactions, and (5) dimerization and further transformation of dimmers. Moreover, the chapter discusses the physical properties of benzo[b]pyrylium salts and spectroscopic analysis. The ultraviolet spectral data on 1 -arylbenzo[c]pyrylium salts is dependent on the pH values. From the pH dependence of the spectra, pKa values of the salts were computed, and although the general trend of polarographic and spectrophotometric pKa values were similar the numerical values were markedly different.