Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) , Wiener Strasse 12, D-28359 Bremen, Germany.
Institute for Organic and Analytical Chemistry, University of Bremen , Leobener Strasse NW2C, D-28359 Bremen, Germany.
J Org Chem. 2017 Apr 21;82(8):4050-4055. doi: 10.1021/acs.joc.6b02727. Epub 2017 Apr 6.
The reversible ring-opening addition and fragmentation reaction of p-cresol-based N-phenylbenzoxazine with aliphatic and aromatic thiols was investigated in solvent-mediated and solvent-free reactions. Independently of the used thiol, N-phenylbenzoxazine and the thiols reacted to equilibrium with comparable amounts of reactants and products in aprotic solvent, whereas in protic solvent almost full conversions were reached. In contrast, thiol reactivity was a crucial factor in solvent-free reactions yielding fast and complete conversions for a more acidic thiol and balanced equilibrium concentrations in case of thiols with high pK values. The strong influence of thiols with low pK values emphasizes the relevance of the protonation step in the ring-opening reactions of 1,3-benzoxazines with thiols in absence of solvents where acidity predominates nucleophilicity. The reverse reactions, namely adduct dissociation and benzoxazine recovery, were successfully conducted at elevated temperatures and reduced pressure facilitated by the removal of the formed thiols yielding up to 95% recovered 1,3-benzoxazine. These results provide deeper understanding of the reversible ring-opening reaction mechanism of 1,3-benzoxazine with thiols.
研究了在溶剂介导和无溶剂条件下，以对甲酚为原料的 N-苯基苯并恶嗪与脂肪族和芳香族硫醇的可逆开环加成和碎裂反应。无论使用何种硫醇，N-苯基苯并恶嗪和硫醇在非质子溶剂中反应达到平衡时，反应物和产物的比例相当，而在质子溶剂中几乎达到完全转化。相比之下，硫醇的反应性是无溶剂反应的关键因素，对于酸性更强的硫醇，反应速度快且完全，而对于 pK 值较高的硫醇，则达到平衡时浓度平衡。具有低 pK 值的硫醇的强烈影响强调了在没有溶剂的情况下，质子化步骤在 1,3-苯并恶嗪与硫醇的开环反应中的重要性，在这种情况下，酸度占主导地位，亲核性较低。通过去除形成的硫醇，在升高的温度和减压下成功进行了逆反应，即加合物的解吸和苯并恶嗪的回收，最高可回收 95%的 1,3-苯并恶嗪。这些结果提供了对 1,3-苯并恶嗪与硫醇的可逆开环反应机制的更深入理解。
