Organic heterocyclothiazenes. Part 9. The chemistry of a mesoionic bicyclic imine and its oxides

Abstract

The mesoionic bicyclic Imine (1) is stable to heat, light, and acids, and is resistant to cycloaddition. However it is readily oxidised, exclusively in the thiadiazole ring, to the mono-(4), di-(5), and tri(6) oxides, the last being an unusual hetero fused oxaziridine. In the sequence of structures (1)→(4)→(5)→(6) the interannular N ⋯ S separation gets progressively smaller and the relative lengths of the bonds from the bridging N atom to the rings invert (Scheme 1). This is attributed to enhanced dipolar attraction between the rings as the dithiazolium contribution (8b) becomes increasingly important. Chemical consequences of this interaction are the unprecedented formation of a thiadiazole trioxide (6) and the very ready hydrolysis of the monoxide (4) to the dibenzoylthiadiazole (7)(Scheme 2) which involves cleavage of the original rings and complete formation of the incipient central ‘ring’.