ON THE CHEMICAL, NMR AND KINETIC-PROPERTIES OF 2-AZIDO-THIOPHENE AND 3-AZIDO-THIOPHENE - RECENT DEVELOPMENTS

Different chemical reactivity and kinetic behaviour are observed for 2 -azido- (1) and 3-azido-thiophene (2) in either 1,3-cycloaddition reac tions with (trimethylsilyl)acetylene and trimethyl-(vinyl)silane or th ermal decomposition. Compound 1 gives cyclo-A(N)A(E) adducts (silylate d triazole and triazoline) three times faster than compound 2. Triazol ines are sensitive to silica and undergo ring-contraction on chromatog raphy, with extrusion of nitrogen, to give the corresponding 1-(thieny l)-2-(trimethylsilyl)aziridine. Kinetic measurements of the unimolecul ar thermal decompositions afford distinct activation parameters: E(a) = 22.6 and 30.6 kcal mol-1, DELTAS(not-equal) = -8.2 and -0.7 cal mol- 1 K-1 for 1 and 2, respectively. The modelling of the chemical reactiv ity for the alpha- or beta-azido groups in the thiophene and the conse quent geometric and electronic perturbation of the azidothiophenes are qualitatively supported by measurements of either H-1 or C-13 NMR che mical shifts.