Effective synthesis methods of 3,4-dihydro-2h-1,3-benzoxazines with organic or organometallic catalysts

Benzoxazines are heterocyclic compounds consisting of an oxazine ring attached to the benzene ring. These compounds form polybenzoxazines, which are high performance thermoset resins with unique chemical and physical properties, through ring opening reactions at elevated temperatures or under different catalytic conditions. In general, benzoxazine monomers are synthesized from condensation reactions with primary amines (aliphatic or aromatic), phenols and formaldehyde. High temperatures, long reaction times (up to 24 hours), and rather laborious isolation steps (such as extraction, distillation and column chromatography) are often required to complete the synthesis of this monomer. On the other hand, some catalytic synthesis methods are used. However, most of the catalysts used are compounds containing expensive and toxic transition metals. These non-environmentally friendly catalysts often cause difficulties in product isolation. Moreover, they can cause structural changes in starting compounds and products containing functional groups that are sensitive to strong acid-base conditions. In this context, it is clear that more environmentally friendly and effective organocatalysts should be developed for the synthesis of benzoxazines. In this study, a series of benzoxazine monomers were successfully synthesized in a short time with high yields via the Mannich reaction using an imidazole containing zinc metal complex and carbohydrate-based fructose organocatalysts. The characterization of the synthesized benzoxazines was performed by 1H NMR, 13C NMR and FTIR spectroscopy techniques. In addition, the proposed possible mechanisms for the reactions were supported by the DFT calculations.