Spectroscopic study of the keto-enol equilibrium of N-aryldiacetylthioacetamides and their reactivity

Abstract

An NMR study of the equilibrium between keto-enol tautomeric forms of N-aryl-diacetylthioacetamides enables us to estimate their population ratio in solvents of increasing polarity. An X-ray analysis confirmed the structure of the thermodynamically most stable tautomer. We presume that the course of heterocyclization processes with N-aryldiacetylthioacetamides is affected by the structure of the reacting tautomeric form. The treatment of N-aryldiacetylthioacetamides with oxalyl or bromoacetyl bromides leads to thiazolidine derivatives. The reactivity of the 5-methylene group in the obtained thiazolidin-4-one derivatives was investigated.

Spectroscopic Studies of Intramolecular Proton Transfer in 2-(4-Fluorophenylamino)-5-(2,4-Dihydroxybenzeno)-1,3,4-Thiadiazole

“…6. Clearly, the equilibrium between the enol and keto forms of FABT depends on the temperature, as observed also for other similar systems [14, 25, 26]. In polar solvents such as ethanol and methanol, the equilibrium for FABT is strongly shifted to the enol form and the temperature effect was below our detection level as a result.…”

Reaction mechanisms : Part (iii) Polar reactions

“…74 Substitution occurs at the most activated site para to ring nitrogen, despite the possible steric hindrance to substitution by adjacent chlorine atoms, reflecting the additional activating influence of ring nitrogen meta to the site of attack. When the nucleophile N,N-dimethylethylene diamine was used, a mixture of [6,6] ring fused products 135 and 136 were formed following initial substitution at the 4-position.…”

“…5 A formal oxa-ene mechanism was suggested which involves initial nucleophilic attack of Y 0 YC À of the enol at one of the nitrogens of dione 7 followed by, or coupled with, proton transfer to the other nitrogen.Keto form 10 and four enolic forms 11-14 have been detected in a study of the equilibrium between keto-enol tautomeric forms of N-aryl-diacetylthioacetamides 10 in several aprotic solvents, by use of 1 H and 13 C NMR spectroscopy (Scheme 3). 6 Enol 11 dominated in more polar solvents such as DMSO-d 6 whereas all other tautomeric forms contributed in similar proportions in less polar solvents such as chloroform and benzene. The implication of the tautomeric distribution in heterocylization reactions of ketone 10 was discussed.Rate constants of photolysis of 4-formylphenylacetic acid have been measured in a range of H 2 O (and D 2 O) solutions of different pH (pD) values.…”

“…Keto form 10 and four enolic forms 11-14 have been detected in a study of the equilibrium between keto-enol tautomeric forms of N-aryl-diacetylthioacetamides 10 in several aprotic solvents, by use of 1 H and 13 C NMR spectroscopy (Scheme 3). 6 Enol 11 dominated in more polar solvents such as DMSO-d 6 whereas all other tautomeric forms contributed in similar proportions in less polar solvents such as chloroform and benzene. The implication of the tautomeric distribution in heterocylization reactions of ketone 10 was discussed.…”