Reversible Cross-Linking and De-Cross-Linking System of Polystyrenes Bearing the Monohydrate Structure of Vicinal Tricarbonyl Group through Water–Alcohol Exchange Reactions at Ambient Conditions

We describe in this paper reversible cross-linking and de-cross-linking system based on a polystyrene derivative bearing monohydrate structure of vicinal tricarbonyl groups with 1,6-hexanediol utilizing the direct water–alcohol exchange reactions on the vicinal tricarbonyl groups. By employing diphenylpropanetrione as a unit model compound for the polymer, we have demonstrated that the water–alcohol exchange reactions could be carried out reversibly in both directions by changing solvents. Notably, the water–alcohol exchange reactions proceeded without any catalysts and under mild conditions. For example, an equimolar mixture of the hydrate of diphenylpropanetrione and benzyl alcohol in chloroform (0.5 M) reached equilibrium after standing at ambient temperature within 48 h, where the content ratio of the benzyl alcohol adduct increased up to 49%. The reaction rate and the position of the equilibrium were highly affected by the concentrations of the substrates as well as the reaction temperature. By virtue of the above characteristic features of the water–alcohol exchange reactions, the polystyrene derivative bearing monohydrate structure of vicinal tricarbonyl group (2.0 M) was cross-linked with 1,6-hexanediol (0.2 equiv of OH group to the tricarbonyl unit) in acetone at ambient temperature for 5 days to afford the networked polymer in almost quantitative yield. On the other hand, the networked polymer was treated with an excess of water at ambient temperature for 3 days to afford the original linear polymer in high yield as a result of de-cross-linking through the water–alcohol exchange reaction. The cross-linking and de-cross-linking behavior was also evidenced by SEC analysis of the reaction mixture

Intramolecular 1,3-Dipolar Cycloaddition of Nitrile <i>N</i>-Oxide Accompanied by Dearomatization

Intramolecular 1,3-dipolar cycloaddition of 2-phenoxybenzonitrile <i>N</i>-oxides to benzene rings, accompanied by dearomatization, formed the corresponding isoxazolines in high yields. The X-ray single-crystal structure analysis revealed that the reaction formed the <i>cis</i>-adduct as a single isomer. The substituents on the benzene rings markedly affected the reaction rate, yield, and structure of the final product