Fe(II)-Catalyzed Isomerization of 5-Chloroisoxazoles to 2 H-Azirine-2-carbonyl Chlorides as a Key Stage in the Synthesis of Pyrazole-Nitrogen Heterocycle Dyads.

2-(1 H-Pyrazol-1-ylcarbonyl)-2 H-azirines were synthesized by in situ trapping of 2 H-azirine-2-carbonyl chlorides, generated by Fe(II)-catalyzed isomerization of 5-chloroisoxazoles, with pyrazoles. According to DFT calculations, the selectivity of nucleophilic substitution at the carbonyl group of 2 H-azirine-2-carbonyl chloride by a pyrazole nucleophile, which is a mixture of two tautomers, is controlled by thermodynamic factors. 2-(1 H-Pyrazol-1-ylcarbonyl)-2 H-azirines are excellent precursors for the preparation of two other pyrazole-nitrogen heterocycle dyads: 5-(1 H-pyrazol-1-yl)oxazoles by photolysis and 1-(1 H-pyrrol-2-ylcarbonyl)-1 H-pyrazoles by a Ni(II)-catalyzed reaction with 1,3-dicarbonyl compounds.

Isoxazolium N-ylides and 1-oxa-5-azahexa-1,3,5-trienes on the way from isoxazoles to 2H-1,3-oxazines.

Theoretical and experimental studies of the reaction of isoxazoles with diazo compounds show that the formation of 2H-1,3-oxazines proceeds via the formation of (3Z)-1-oxa-5-azahexa-1,3,5-trienes which undergo a 6π-cyclization. The stationary points corresponding to the probable reaction intermediates, isoxazolium N-ylides, were located by DFT calculations at the B3LYP/6-31G(d) level only for derivatives without a substituent in position 3 of the isoxazole ring. These isoxazolium N-ylides are thermodynamically and kinetically very unstable.