Reaktionen der valenzpolaromeren Ketenform mesoionischer Heterocyclen mit 3-Dimethylamino-2H-airinen

Reactions of valencepolaromeric ketenes of mesoionic heterocycles with 3-dimethylamino-2H-azirines : Reactions of the 3-dimethylamino-2H-azirines 1a and 1b with the mesoionic oxazole 5 and the mesoionic dithiole 6 in acetonitrile at room temperature yield the 1: 1 adducts 11, 12, 19 and 20, respectively (Schemes 5 and 8). These products can be formulated as adducts of the aminoazirines and the ketenes 5a and 6a, which are valence polaromeric forms of the mesoionic heterocycles 5 and 6 (Scheme 2). The structure of the adducts has been elucidated by spectral data and their comparison with the data of (Z)-11, the structure of which has been established by X-ray [19]. Oxidation of the 1:1 adducts with KMnO4 in a two-phase system yields 4-dimethylamino-3-oxazolin-2-ones (cf. Scheme 6) by cleavage of the exocyclic C,C-double bond. A mechanism for the formation of the adducts is given in Scheme 9: Nucleophilic attack of 1 on the ketene leads to a primary adduct of type a, which undergoes cleavage of the former N(1),C(2)-azirine bond to give adducts of type 11 or 19. The N(1),C(2)-ring opening of 1a in the reaction with ketenes contrasts with the N(1),C(3)-opening of 1a in the addition with, for instance, isothiocyanates. These different ring openings are explained by the difference in nucleophilicity of the heteroatoms X and Y in a' (Scheme 10)