Study of the mechanism of recyclization of furans into thiophenes and selenophenes in conditions of acid catalysis. 6. Experiments with labeled atoms. Quantum chemical calculations of intermediates of recyclization and hydrolysis

Abstract

The reaction of 2-methyl-5-R-furans (R = Me, Bu, 2, Kinetic studies of recyclization of 2,5-dialkylfuram into thiopbenes and selenophenes in conditions of acid catalysis in ethanol and water-ethanol media were conducted previously [2][3][4][5]. It was found that in recyclization in the presence of a competing nucleophile --water --the processes can take place in two parallel directions: through intermediate formation of dicarbonyl compounds, and as a result of direct transformation of furans into isologs [2, 4]. The subsequent study of the kinetics of hydrolysis and recyclization of different alkyl-substituted furans showed that the rates of these reactions are differently dependent on the position and structure of the substituents and the concentration of the acid component and water [4, 5].The results of studying the mechanism of the hydrolysis and recyclization reactions with labeled atoms are reported here.The reaction of 2,5-dialkylfurans (I-liD with H2180 was investigated in the conditions of the recyclization reaction. The composition of the reaction mixture was determined by GLC. The incorporation of the "label" was monitored with the mass spectrometric data. It was shown that substitution of the 160 atom of the furan ring by the 180 isotope takes place together with the hydrolysis reaction (see Fig. 1). As a result, the products of isotope exchange of furans (IV-VI) and diketones (VII-IX) were obtained.

Acid-Catalyzed Ring Opening of Furan in Aqueous Solution

“…27 It has been shown that protonation at the ring oxygen only slightly alters the electron density distribution of the furanic ring, thereby reducing the possibility of a subsequent nucleophilic attack. 28,29 While furan is detected only in a low concentration during the decomposition of monosaccharides in acid solutions, its ring configuration provides a framework to study the furanic ring-opening reaction without the interference of formyl and hydroxymethyl groups associated with the more prevalent furanic products. Therefore, furan has been selected as the model furanic compound in molecular modeling studies.…”

“…The ring oxygen has lower proton affinity than the C α and C β positions . It has been shown that protonation at the ring oxygen only slightly alters the electron density distribution of the furanic ring, thereby reducing the possibility of a subsequent nucleophilic attack. , …”