Metal-carbonyl-induced reaction of isoxazoles. Ring cleavage and reduction by hexacarbonylmolybdenum, pentacarbonyliron, or nonacarbonyldi-iron

Abstract

In the presence of [Mo(CO)₆] and water the isoxazoles (1a–f) undergo thermally induced reductive cleavage of the N–O bond to give β-amino enones in good yield. Similar results were obtained by the use of [Fe(CO)₅] and water with photoirradiation, or of [Fe₂(CO)₉] and water with heating. A mechanism involving an N-complexed isoxazolepentacarbonylmolybdenum or isoxazoletetracarbonyliron, and a [Mo(CO)₅]- or [Fe(CO)₄]-complexed (β-oxo vinyl)nitrene intermediate is proposed for the reactions. The complexed nitrene moiety could be reduced by the central metal in the presence of water to give amine. Furthermore, treatment of 2-benzoyl-3-phenyl-2H-azirine (8a), which is an isomer of 3,5-diphenylisoxazole, with the metal carbonyls and water also resulted in the formation of a β-amino enone possibly via the corresponding complexed (β-oxo vinyl)nitrene. An N-complexed isoxazolepentacarbonylmolybdenum intermediate was prepared by the photoreaction of [Mo(CO)₆] with 3,5-dimethylisoxazoie. Its characterization, and chemical transformations, have been carried out to investigate the proposed mechanism.