Microwave-mediated Biginelli reactions revisited. On the nature of rate and yield enhancements
Microwave-mediated Biginelli reactions revisited. On the nature of rate and yield enhancements
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重新审视微波介导的 Biginelli 反应。
DOI：
10.1039/b002697m
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
C. Kappe
中科院分区：
文献类型：
--
作者：
Alexander Stadler;C. Kappe
利用一个具有在线温度、压力和微波功率控制的专用商用微波反应器，对微波介导的Biginelli二氢嘧啶合成进行了重新研究。在常压微波加热下的乙醇溶液中，当温度与常规加热相同时，转化率或产率没有增加。在常压下微波辐照过热的情况下，由于热（动力学）效应，所观察到的产率和速率的增加是合理的。在密封容器条件下（20 bar, 180°C），产品的产量下降，并观察到各种副产物的形成。只有在“开放体系”条件下进行反应时，溶剂才能在微波照射下快速蒸发，才能显著提高反应速率和产率。然而，在这些实验中观察到的速率和产率的提高是无溶剂条件的结果，而不是由微波辐射引起的。通过微波和热加热下无溶剂Biginelli反应的对照实验证实了这一点。
The microwave-mediated Biginelli dihydropyrimidine synthesis was reinvestigated using a purpose-built commercial microwave reactor with on-line temperature, pressure, and microwave power control. Transformations carried out under microwave heating at atmospheric pressure in ethanol solution show no rate or yield increase when the temperature is identical to conventional thermal heating. In the case of superheating by microwave irradiation at atmospheric pressure the observed yield and rate increases are rationalized as a consequence of a thermal (kinetic) effect. Under sealed vessel conditions (20 bar, 180 °C) the yield of products is decreased and formation of various byproducts observed. The only significant rate and yield enhancements are found when the reaction is performed under “open system” conditions where the solvent is allowed to rapidly evaporate during microwave irradiation. However, the observed rate and yield enhancements in these experiments are a consequence of the solvent-free conditions rather than caused specifically by microwave irradiation. This was confirmed by control experiments of the solventless Biginelli reaction under microwave and thermal heating.