N-bromosuccinimide (NBS)-promoted, three-component synthesis of α,β-unsaturated isoxazol-5(4H)-ones, and spectroscopic investigation and computational study of 3-methyl-4-(thiophen-2-ylmethylene)isoxazol-5(4H)-one

Abstract

A three-component, NBS-promoted synthesis of α,β-unsaturated isoxazol-5(4H)-ones by reaction of aromatic aryl or hetero-aryl aldehydes, hydroxylamine hydrochloride, and 1,3-dicarbonyl compounds (ethyl acetoacetate or ethyl 4-chloroacetoacetate), under mild reaction conditions at room temperature is described. This simple, efficient, and clean reaction is an expeditious means of obtaining the corresponding isoxazol-5(4H)-one derivatives in good to high yields. Geometrical properties and vibrational wavenumbers of 3-methyl-4-(thiophen-2-ylmethylene)isoxazol-5(4H)-one (MTISO) were predicted by use of density functional theory (DFT) by use of the B3LYP level with the 6-311++G(d,p) and 6-311++G(2d,p) basis sets. Results indicate that the B3LYP method enables satisfactory prediction of vibrational frequencies and structural data. The absorption spectra of MTISO in solvents of different polarity were studied at room temperature. The UV–visible spectrum of the compound was recorded and such electronic properties as the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were determined by the time-dependent DFT (TD-DFT) approach. The stability of the molecule arising from hyper-conjugative interaction and charge delocalization was studied by NBO analysis. A molecular electrostatic potential map (MEP) of the compound was also studied to predict reactive sites. Reactivity descriptors, Fukui functions, and electrophilic sites were found and are discussed. The thermal stability of MTISO was studied by thermogravimetric analysis (TGA).