Title:The Syntheses, Characterization and Crystal Structures of a Series of Heterocyclic β-Diketones and Their Isoxazole Compounds
Volume: 16
Issue: 8
Author(s): Biyun Su*, Yifan Hou, Li Wang, Xiaoteng Li, Dandan Pan, Tingyu Yan, Ao Zhang, Faida Paison and Liqing Ding
Affiliation:
- College of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an,China
Keywords:
Heterocyclic β-diketone, heterocyclic isoxazole, rheological phase reaction, crystal structure, reaction mechanism, Claisen
ketoester condensation.
Abstract: 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.
