Preparation, reactivity, NMR properties and semiempirical MNDO/PM3 structural calculations of 2-azido- and 3-azido-selenophene

Abstract

The preparation, by azido-transfer reaction, of 2-azido-1 and 3-azido-selenophene 2 using the appropriate heteroarylithium derivatives and tosyl azide, followed by fragmentation of the intermediate triazene lithium salts is reported here. Different chemical reactivity and kinetic behaviour were observed for azides 1 and 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 or triazoline)ca. three times faster than compound 2. Both the elusive triazoline adducts undergo rapid ring-contraction, with extrusion of nitrogen, to give rise to the corresponding 1-(selenophenyl)-2-(trimethylsilyl)aziridine (1b, 2b). Kinetic measurements of the unimolecular thermal decompositions afford distinct activation parameters: E_a= 21.5 and 30.4 kcal mol^–1, ΔS^‡=–10.7 and –0.9 cal mol^–1 K^–1 for 1 and 2. respectively. Experimental data, as a result of geometric and electronic disturbances exerted by the azido-group located at α- or β-positions of the selenophene ring, are qualitatively supported by measurement of ¹H, ¹³C and ⁷⁷Se NMR chemical shifts. The present experimental evidence and those previously obtained with related 2-azido- and 3-azido-thiophenes are corroborated with the determination of the structures and comparison between ¹³C substituent chemical shift (SCS) and charge distributions by using a semiempirical computational MNDO/PM3 method.