Kremsner Jennifer M, Kappe C Oliver
Christian-Doppler-Laboratory for Microwave Chemistry and Institute of Chemistry, Heinrichstrasse 28, A-8010 Graz, Austria.
J Org Chem. 2006 Jun 9;71(12):4651-8. doi: 10.1021/jo060692v.
Microwave-assisted organic synthesis in nonpolar solvents is investigated utilizing cylinders of sintered silicon carbide (SiC)--a chemically inert and strongly microwave absorbing material--as passive heating elements (PHEs). These heating inserts absorb microwave energy and subsequently transfer the generated thermal energy via conduction phenomena to the reaction mixture. The use of passive heating elements allows otherwise microwave transparent or poorly absorbing solvents such as hexane, carbon tetrachloride, tetrahydrofuran, dioxane, or toluene to be effectively heated to temperatures far above their boiling points (200-250 degrees C) under sealed vessel microwave conditions. This opens up the possibility to perform microwave synthesis in unpolar solvent environments as demonstrated successfully for several organic transformations, such as Claisen rearrangements, Diels-Alder reactions, Michael additions, N-alkylations, and Dimroth rearrangements. This noninvasive technique is a particularly valuable tool in cases where other options to increase the microwave absorbance of the reaction medium, such as the addition of ionic liquids as heating aids, are not feasible due to an incompatibility of the ionic liquid with a particular substrate. The SiC heating elements are thermally and chemically resistant to 1500 degrees C and compatible with any solvent or reagent.
利用烧结碳化硅（SiC）圆柱体——一种化学惰性且强微波吸收材料——作为被动加热元件（PHEs），对非极性溶剂中的微波辅助有机合成进行了研究。这些加热插入物吸收微波能量，随后通过传导现象将产生的热能传递给反应混合物。使用被动加热元件可使原本微波透明或吸收性差的溶剂，如己烷、四氯化碳、四氢呋喃、二氧六环或甲苯，在密封容器微波条件下有效加热至远高于其沸点（200 - 250℃）的温度。这为在非极性溶剂环境中进行微波合成开辟了可能性，已成功证明可用于多种有机转化反应，如克莱森重排、狄尔斯 - 阿尔德反应、迈克尔加成、N - 烷基化和迪姆罗特重排。在因离子液体与特定底物不兼容而无法采用其他增加反应介质微波吸收的方法（如添加离子液体作为加热助剂）的情况下，这种非侵入性技术是一种特别有价值的工具。SiC加热元件耐热且耐化学腐蚀，温度可达1500℃，并与任何溶剂或试剂兼容。
