PREPARATION, REACTIVITY, NMR PROPERTIES AND SEMIEMPIRICAL MNDO/PM3 STRUCTURAL CALCULATIONS OF 2-AZIDO-SELENOPHENE AND 3-AZIDO-SELENOPHENE

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(N)A(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,dagger DELTAS(double dagger) = -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 alpha- or beta-positions of the selenophene ring, are qualitatively supported by measurement of H-1, C-13 and Se-77 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 C-13 substituent chemical shift (SCS) and charge distributions by using a semiempirical computational MNDO/PM3 method.