Abstract
Thermodynamic stabilities of different isomers and tautomers of lithium and potassium buta-2,3-dienimidothioates (ambident N,S-centered anionic adducts derived from methoxyallenyl carbanion and methyl isothiocyanate) as real or potential precursors of 3-methoxy-N,N-dimethylthiophen-2-amine were estimated by quantum-chemical calculations in terms of the density functional theory. Gradient channels corresponding to concerted (one-step) and stepwise (two-step) mechanisms of formation of thiophene ring from the most stable conformers and tautomers were localized. The formation of thiophene structure from lithium and potassium buta-2,3-dienimidothioates in two steps via intramolecular cyclization to thiophen-2(5H)-ylidene anion and its isomerization into thienylamide was found to be more probable. The activation barriers in both stepwise and concerted cyclization mechanisms considerably decrease in going from lithium to potassium as counterion.
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