Cinnolines. Part X. The site of protonation of cinnoline

Abstract

It is shown that 4-methylcinnoline gives the methiodide by quaternisation at N(2), and not at N(1) as previously believed, and that 3-aminocinnoline gives two methiodides, by quaternisation at N(1) and N(2), in approximately equal yields. Evidence which might indicate the site of protonation of cinnoline is discussed and it is considered that the proton magnetic resonance spectra of cinnolinium perchlorate and of 1- and 2-methylcinnolinium perchlorate do not allow any definite conclusion. The ultraviolet spectra of these perchlorates, however, indicate that protonation of cinnoline occurs predominantly at N(2). 3-Amino- and 4-methyl-cinnoline (and probably the 3- and 8-methyl isomers) behave similarly but 4-aminocinnoline is protonated mainly on N(1). The results of molecular orbital calculations on these cinnolines and on methoxycinnolines are given. The preferred site of protonation is indicated by the localisation energy index as calculated by a self-consistent version of the Hückel method.