Study of the mechanism of recyclization of furans into thiophenes and selenophenes in conditions of acid catalysis. 5. Direction of protonation of furans

Abstract

,5-Dimethyl-3-D-furan was prepared for the first time. Its recyclization into the corresponding thiophene and selenophene was investigated. The possibility of f3-C-protonation of the furan ring was confirmed by experimental and quantum chemical data.Many studies have been dedicated to answering the question of the. direction of protonation of furans in the reaction of acid hydrolysis, c~-C-Protonation of the furan ring was selected apriori by analogy with reactions of electrophilic substitution in [2]. Finnish researchers believe that hydrolysis of 2,5-dimethyl-and 2-methoxyfurans begins with/3-C-protonation of the ring [3, 4]. However, it was suggested in [5] that this hypothesis was based on incorrect determination of the structure of the products of hydrolysis of 2-methoxyfuran. The furan ions formed in the gas phase in conditions of mass spectrometry with chemical ionization (reagent gases: isobutane and methane) are described as products of t~-C-protonation in [6]. Stable furanium ions were obtained from tert-butylfurans in the liquid phase, and 2,5-dimethylfuran was completely resinified in the same conditions in [7].Acid-catalyzed isotope exchange is widely used for assessing the reactivity of heterocycles. In studying the rate of deuterium exchange of benzene, furan, thiophene, selenophene, and pyrrol, protonation of the/3-carbon atom of the furan ring was found. Only the mobility of the ~-H atoms could be measured in the deuterium exchange reaction, since hydrolytic splitting of the furan ring was simultaneously observed in the region of concentrations of H2SO 4 greater than 30 wt. %, where exchange of protons in/3 positions takes place with a sufficient rate [8].We continued the study of the mechanism of the reaction of recyclization of furans into thiophenes and selenophenes in conditions of acid catalysis. We investigated the possibility of/~-C-protonation as the ftrst stage of recyclization. 2,5-Dimethyl-3-D-furan (Ia) was investigated. Incorporation of a label in position 3 of 2,5-dimethylfuran (I) was conducted similar to the method described in [9] via mercury chloride (II) according to the following scheme:Recyclization of compound Ia into 2,5-dimethylthiophene (III) and 2,5-dimethylselenophene (IV) was conducted with hydrogen sulfide and hydrogen selenide in absolute ethanol saturated with dry hydrogen chloride, similar to the method described in [10]. The evolution of the reaction was monitored with the GLC data, and after incorporation and preservation of the label, it was monitored by PMR spectroscopy and mass spectrometry.

Self-oscillating reaction in the furan series

“…The UV spectra were recorded on an HP 8452A spectrometer in the region of 200-300 nm at steps x = 2 nm. The quantum-chemical computations were performed by the SCF LCAO MO PM3 method using MOPAC software by analogy with [5].…”

“…Kinetic investigations of the hydrolysis of 2,5-dimethylfuran (1) in 50% ethanol under the influence of HCl revealed the formation of 2,5-hexanedione (2) and an unidentified compound X. During the reaction the ratio of the concentrations of the products varied with some degree of periodicity, i.e., as it accumulated one product changed into the other, and these transformations were self-oscillating in nature [1,2].…”

“…Kinetic investigations of the hydrolysis of 2,5-dimethylfuran (1) in 50% ethanol under the influence of HCl revealed the formation of 2,5-hexanedione (2) and an unidentified compound X. During the reaction the ratio of the concentrations of the products varied with some degree of periodicity, i.e., as it accumulated one product changed into the other, and these transformations were self-oscillating in nature [1,2].Study of the behavior of furan compounds in aqueous and water-alcohol media under the conditions of acid catalysis is of great theoretical and practical interest in so far as oscillating chemical reactions take place in various types of processes that determine the vital activity of living organisms [3], while furan derivatives are physiologically active substances.In the present work in order to determine the boundary conditions for the appearance of the abovementioned transformations the kinetics of the hydrolysis of dimethylfuran 1 in 50% aqueous ethanol with various hydrochloric acid concentrations (1.5, 2.0, 3.0, 3.5 N) were investigated by GLC. The results of the experiments are presented in Fig.…”

Self-oscillating reaction In the Furan series

“…The quantum-chemical computations were performed by the SCF LCAO MO PM3 method using MOPAC software by analogy with [5]. Gas-liquid chromatography was conducted on a Tsvet-101 chromatograph with a flame-ionization detector on columns (1 m × 3 mm and 3 m × 3 mm) with 15% Apiezon L on Chromaton N-AW: Thermostat temperature 100°C, carrier gas (helium) flow rate 2 l/h.…”

“…Kinetic investigations of the hydrolysis of 2,5-dimethylfuran (1) in 50% ethanol under the influence of HCl revealed the formation of 2,5-hexanedione (2) and an unidentified compound X. Kinetic investigations of the hydrolysis of 2,5-dimethylfuran (1) in 50% ethanol under the influence of HCl revealed the formation of 2,5-hexanedione (2) and an unidentified compound X.…”