On the chemical, NMR and kinetic properties of 2-azido- and 3-azidothiophene: recent developments

Abstract

Different chemical reactivity and kinetic behaviour are observed for 2-azido-(1) and 3-azido-thiophene (2) in either 1,3-cycloaddition reactions with (trimethylsilyl)acetylene and trimethyl-(vinyl)silane or thermal decomposition. Compound 1 gives cyclo-A_NA_E adducts (silylated triazole and triazoline) three times faster than compound 2. Triazolines are sensitive to silica and undergo ring-contraction on chromatography, with extrusion of nitrogen, to give the corresponding 1-(thienyl)-2-(trimethylsilyl)aziridine. Kinetic measurements of the unimolecular thermal decompositions afford distinct activation parameters: E_a= 22.6 and 30.6 kcal mol^–1, ΔS^≠=–8.2 and –0.7 cal mol^–1 K^–1 for 1 and 2, respectively. The modelling of the chemical reactivity for the α- or β-azido groups in the thiophene and the consequent geometric and electronic perturbation of the azidothiophenes are qualitatively supported by measurements of either ¹H or ¹³C NMR chemical shifts.