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

2-(1H-Pyrazol-1-ylcarbonyl)-2H-azirines were synthesized by in situ trapping of 2H-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 2H-azirine-2-carbonyl chloride by a pyrazole nucleophile, which is a mixture of two tautomers, is controlled by thermodynamic factors. 2-(1H-Pyrazol-1-ylcarbonyl)-2H-azirines are excellent precursors for the preparation of two other pyrazole–nitrogen heterocycle dyads: 5-(1H-pyrazol-1-yl)­oxazoles by photolysis and 1-(1H-pyrrol-2-ylcarbonyl)-1H-pyrazoles by a Ni­(II)-catalyzed reaction with 1,3-dicarbonyl compounds. 5-(1H-Pyrazol-1-yl)­oxazoles show strong emission in acetonitrile at 360–410 nm with high quantum yields.