REACTIONS OF VALENCE-POLAROMERIC KETENES OF MESOIONIC HETEROCYCLES WITH 3-DIMETHYLAMINO-2H-AZIRINES

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. Copyright © 1979 Verlag GmbH & Co. KGaA, Weinheim