Tautomerism of beta-Dicarbonyl and beta,beta'-Tricarbonyl Compounds in Erotogenic Media: The Prototropy of Hydroxonium Derivatives of Keto-Enols

摘要

The tautomerism of beta-dicarbonyl and beta,beta'-tricarbonyl compounds (beta-diCCs and beta,beta'-triCCs, respectively) has been known for over a century. Nonetheless, many aspects of this phenomenon remain to be studied and discussed at present. The discovery of keto-enol tautomerism of beta-diCC and beta,beta'-triCC anions made it possible to gain insight into the properties of this important and large class of organic compounds widespread in nature and often used in the laboratory and industry, but did not solve problems arising during studies in this area. These problems are exemplified by the annelation of 3,4-dihydroiso-quinolines with 2-chloroacetylcyclohexane-1,3-diones or 3-acetyltetronic, -tetramic, and 3-acylthiotetronic acids, which takes place only under strong acid catalysis conditions, or the H/D exchange of the protons of methyl, methylene, and methyne groups adjacent to beta-dicarbonyl and beta,beta'-tricarbonyl fragments in the presence of strong organic (CF3COOD) and mineral (D2SO4, HCl) acids. It should be noted that the tautomerism of beta-diCCs and beta,beta'-triCCs in protogenic media, in contrast to their tautomerism in amphiprotic and polar aprotic media, was not considered previously because it was believed to be identical to the acid-catalyzed tautomerism of carbonyl compounds and, therefore, was explained on the same basis. This is due to the erroneous notion that acid-catalyzed pro-totropic reactions of carbonyl compounds, beta-diCCs, and beta,beta'-triCCs in a protogenic medium are also identical.