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

Reactions of valencepolaromeric ketenes of mesoionic heterocyles 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 clevage 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 clevage 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).