Surfactant micelles as microreactors for the synthesis of quinoxalines in water: scope and limitations of surfactant catalysis
作者:Dinesh Kumar、Kapileswar Seth、Damodara N. Kommi、Srikant Bhagat、Asit K. Chakraborti
DOI:10.1039/c3ra41038b
日期:——
The scope and limitations of surfactants as catalysts for the synthesis of quinoxalines using microreactors made of the surfactants in water has been assessed. The catalytic potential followed the order: non-ionic surfactants > anionic surfactants > Brønsted acid surfactants > cationic surfactants. The non-ionic surfactant, Tween 40, is the most effective catalyst affording excellent yields within a short reaction time at room temperature and is compatible with different variations of the 1,2-diketones and 1,2-diamines. The reaction medium (spent water) containing the catalyst, as well as the catalyst itself (recovered Tween 40) can be reused for five consecutive reactions. The better catalytic efficiency of the surfactant (Tween 40) compared to the various Lewis/Brønsted acids, as well as the surfactant combined Lewis acid, suggests that surfactants, which generate microreactor assemblies at the interface, are better suited as catalytic aids to promote organic reactions in water. The inferior results obtained in organic solvents, which provide a homogeneous reaction mixture compared to those obtained in water, indicate the specific role of water. This has been depicted as a synergistic dual activation through the hydrogen bond mediated formation of supramolecular assemblies involving a water dimer and the reactants. The catalytic assistance of the surfactant could be ascribed to the ability of the surfactant molecule to undergo hydrophobic and hydrogen bond forming interactions with water and the reactants in orienting the reactants at the water interface and encapsulating inside the microreactors to facilitate the cyclocondensation.
已经评估了表面活性剂作为催化剂在水相中使用微反应器合成喹噁啉的范围和局限性。催化潜力的顺序为:非离子表面活性剂 > 阴离子表面活性剂 > 布朗斯特酸表面活性剂 > 阳离子表面活性剂。非离子表面活性剂Tween 40是最有效的催化剂,在室温下能够在短时间内提供优异的产率,并且对不同类型的1,2-二酮和1,2-二胺具有良好的兼容性。含有催化剂的反应介质(使用过的水)以及催化剂本身(回收的Tween 40)可以重复使用五次连续反应。与各种路易斯/布朗斯特酸及其复合路易斯酸相比,表面活性剂(Tween 40)展现出更好的催化效率,表明产生微反应器组装的表面活性剂更适合作为促进水中有机反应的催化辅助剂。在有机溶剂中获得的结果较差,这意味着有机溶剂提供一致的反应混合物,而在水中获得的结果表明水的特定作用。这一过程被描述为通过氢键介导的超分子组装的协同双重活化,涉及氢水二聚体和反应物。表面活性剂的催化辅助作用可以归因于表面活性剂分子与水和反应物之间进行疏水和氢键形成相互作用的能力,这有助于在水界面上定向反应物并将其封装在微反应器内,从而促进环缩合反应。