The Syntheses, Characterization and Crystal Structures of a Series of Heterocyclic β-Diketones and Their Isoxazole Compounds

Background: In the field of coordination chemistry, the introduction of heterocyclic substituents into the structure of β-diketone enables ligand to produce multiple coordination sites. The adoption of small steric oxime group into the structure of heterocyclic β-diketone by Schiff-base condensation will further increase coordination sites and facilitate the generation of polynuclear structures. Objective: A series of β-diketones (2a-2c) containing different heterocycles such as pyridine, thiophene and furan and their corresponding isoxazole compounds (3a-3c) were synthesized. Materials and Methods: The Claisen condensations were investigated in a solvent-free rheological phase system at room temperature to obtain heterocyclic β-diketones 2a-2c, which further reacted with hydroxylamine hydrochloride to obtain heterocyclic isoxazoles 3a-3c. All these compounds were well characterized by EA, IR, 1H NMR and X-ray crystal diffraction to confirm the structures. Synthetic mechanisms of compounds and the effects of different heterocycles on reactivity were discussed deeply. Results: 1H NMR indicated that these β-diketones do not exist as a total diketonic form but an equilibration between diketone and enol forms in CDCl3 solvent, in which the enol form accounts for 98.0% in 2a, 94.3% in 2b, 95.5% in 2c. While the crystal structures of 2a-2c showed that the reaction allows to isolate diketones in solid state. Crystal structures of 3a-3c showed that the neutral β-ketone oximes resonate and cyclize to form the target heterocyclic isoxazoles. Conclusion: SN1 nucleophilic substitution mechanism of Claisen ketoester condensation was proposed for the syntheses of 2a-2c, and SN1 single molecule nucleophilic substitution reaction mechanism was put forward for 3a-3c.