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

摘要

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. 5-(1 H -Pyrazol-1-yl)oxazoles show strong emission in acetonitrile at 360–410 nm with high quantum yields.