Department of Fundamental Chemistry, Institute of Chemistry , University of São Paulo Av. Prof. Lineu Prestes, 748, Cidade Universitária , São Paulo , São Paulo 05508-000 , Brazil.
J Phys Chem A. 2019 Sep 26;123(38):8179-8187. doi: 10.1021/acs.jpca.9b05065. Epub 2019 Sep 13.
Benzoxazine formation is a fundamental step in the preparation of polybenzoxazine resins, and a detailed description of the mechanism governing the formation of benzoxazine and side products is vital for improving the properties and performance of these resins. Determination of the nature and properties of reaction intermediates is not trivial. Therefore, a Mannich-type condensation of aniline, formaldehyde, and phenol was evaluated as a potential method to form benzoxazine. Coupling positive mode electrospray ionization mass spectrometry (ESI(+)-MS) with infrared multiple photon dissociation (IRMPD) spectroscopy allowed unambiguous determination of an iminium-based mechanism and the direct observation of iminium intermediates. The benzoxazine formation mechanism was indirectly confirmed by the observation of side products that are relevant to the polymerization step, and directly confirmed by the identification of four distinct reaction intermediates that were completely characterized by IRMPD spectroscopy. The benzoxazine monomer was also shown to undergo isomerization under standard ESI-MS analysis conditions, suggesting the presence of a mixture of three isomers during their usual ESI-MS analysis.
苯并恶嗪的形成是制备聚苯并恶嗪树脂的基本步骤，详细描述控制苯并恶嗪和副产物形成的机制对于改善这些树脂的性能至关重要。确定反应中间体的性质和特性并非易事。因此，评估苯胺、甲醛和苯酚的曼尼希型缩合作为形成苯并恶嗪的潜在方法。将正模式电喷雾电离（ESI(+)-MS）与红外多光子解离（IRMPD）光谱相结合，能够明确确定基于亚胺的机制，并直接观察到亚胺中间体。通过观察与聚合步骤相关的副产物间接证实了苯并恶嗪的形成机制，并通过鉴定四个完全通过 IRMPD 光谱表征的不同反应中间体直接证实了该机制。还表明苯并恶嗪单体在标准 ESI-MS 分析条件下会发生异构化，这表明在通常的 ESI-MS 分析过程中存在三种异构体的混合物。
