Photochemical conversion of 3-azido-2-vinylthiophenes into thienopyrroles and of 2-azidostyrenes into indoles. High migratory aptitude of sulphur substituents

Abstract

On photolysis in acetonitrile 3-azido-2-vinylthiophenes (8a–h) give thieno[3,2-b]pyrroles (10), (16), (18)–(20), (23), (24), (26) and 2-azidostyrenes (9a–d) give indoles (11) and (22). A mechanism is proposed involving formation and 1,5-electrocyclisation of the corresponding nitrene, followed by sigmatropic shift of one or both of the 2-substituents (Y, Z) in the 2H-pyrrole thus formed (Scheme 3). Decomposition of the two series of azides thus provides a measure of the relative migratory aptitudes of these substituents which are found to decrease in the order RSO > RS > H > RSO₂ > RCO > EtO₂C. The sulphide, sulphoxide, and sulphone groups are thus remarkably mobile.