The thermal sigmatropic isomerization of pseudosaccharyl crotyl ether

The thermally induced sigmatropic isomerization of the pseudosaccharyl crotyl ether, 3-(E)-but-2-enoxy)-1,2-benzisothiazole 1,1-dioxide (CBID), has been investigated by using temperature dependent infrared spectroscopy, differential scanning calorimetry (DSC), and polarized light thermomicroscopy. The reaction can take place in both melted and crystalline phases, affording the product resulting from the [3,3′] migration of the allylic system from O to N, 2-(E)-1-methylprop-2-en-1,2-benzisothiazol-3(2H)-one 1,1-dioxide (CBIOD). In the melt, the activation energy of the process was determined as being 49.1±5.3 kJ mol−1, with k=(22.2±0.6)×104 s−1 at 140 °C. In the solid state, at 110 °C, the rate constant drops by one order of magnitude [k=(1.46±0.07)×104 s−1]. The enthalpy of reaction, determined by DSC, is ΔrxH=−27.0±0.8 kJ mol−1. Assignments were proposed for the infrared spectra of the observed neat condensed phases of the two compounds.