N.m.r. studies of the keto–enol tautomerism of acylthioacetamides

✍ Scribed by G. Klose; E. Ludwig; E. Uhlemann

• Publisher: John Wiley and Sons
• Year: 1977
• Tongue: English
• Weight: 522 KB
• Volume: 10
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1270100135

✦ Synopsis

Abstract

Acetylthioacetamides exist as different keto and enol isomers in chloroform solutions. The keto form with intramolecular hydrogen bonding between the NH and the carbonyl group is the dominant keto isomer. On the other hand the enol forms with intramolecular hydrogen bonding between the OH and the thioketo group are the dominant enol isomers in the temperature range 60°C to −60°C. The thermodynamic data of the keto‐enol equilibria were obtained by measuring the intensities of appropriate high resolution proton signals as a function of temperature. At low temperatures all lines characteristic of the enol forms are doubled in the N‐phenyl‐substituted derivatives because the rotation of the NHC~6~H~5~ group around the CN bond becomes slow and the chemical shifts characteristic of the E and Z isomers are different. We estimated approximate thermodynamic data of the E/Z equilibrium in some of the compounds. The changes of the line shape as well as the chemical shifts as a function of temperature indicate the presence of various additional exchange processes. In order to obtain further information we performed curve fittings of the chemical shifts of one acetylthioacetanilide and of a series of monothio‐β‐diketones (studied in another paper) assuming a fast two site exchange process. On the basis of the results obtained a reaction scheme for N‐substituted acylthioacetanilides in solution is proposed.