Base induced synthesis of 4H-1,4-benzothiazines and their computational studies

Abstract

A one pot synthesis of 1,4-benzothiazines from 1,3-benzothiazolium cations and α-haloketones through ring expansion under ultrasonication was carried out by employing optimal 5% sodium hydroxide and characterization was done with the help of various spectroscopic techniques viz. NMR, IR and mass spectrometry. Single crystal X-ray diffraction and computational studies of (4-bromophenyl)(4-methyl-4H-benzo[b][1,4]thiazin-2-yl)methanone (3c) were also performed. Different concentrations of different bases (NaOH, Na₂CO₃, NaHCO₃ solutions etc.) were used to obtain 1,4-benzothiazine derivatives. But, the best results were obtained with 5% NaOH solution (3.7 equivalents) for the synthesis of single product 1,4-benzothiazine derivatives while, with other concentrations, the reaction did not proceed to a single product. Quantum chemical calculations were done to find the optimized geometry and to correlate the experimental and calculated values of the bond parametres and ¹³C NMR chemical shifts of the synthesized compound. The optimized geometry was obtained using density functional theory (DFT) with the basis set 6-31G(d). The theoretical values of the ¹H NMR chemical shifts were in agreement with the experimental values. The photophysical properties of some of the synthesized compounds were studied with the help of ultraviolet-visible and fluorescence spectroscopy.