Novel synthetic approach in microwave-assisted solid-supported oxidations using ‘in situ’ generated molecular oxygen

Three different types of oxidation reactions were carried out under microwave (MW) conditions in dry media, with nearly quantitative yield, using ‘in situ’, yet separately generated molecular oxygen as the reactive gas. The latter is formed by a controlled decomposition of potassium chlorate (220-306 °C) adsorbed on zeolite support, and is used as a reactive oxidizing agent for the solid-supported oxidations. The MW-assisted oxidations include an oxidative decomplexation of (η⁶-arene)Cr(CO)₃ complexes to the corresponding arenes using silica as solid support (100 °C), an oxidation of fluorene to fluorenone induced by KF-alumina support (150 °C), and oxidation of benzyl alcohol to benzaldehyde using a supported ruthenium catalyst (150 °C). This synthetic approach allows to carry out in synchronized manner two different solid-supported reactions (oxygen generation and oxidation) at different temperatures and on different solid supports together in the same sealed system. It was made possible by tuning the absorption efficiency of MWs through accurate selection of the solid supports employed in the reactions. The high feasibility of this novel synthetic approach resulted from a preliminary study of the interaction between MWs and mineral oxides such as alumina, silica, clay, and zeolite particularly when mixed with additives such as water, ionic liquids or graphite (5% w/w). The use of these MW absorber additives allows the MW transparent or poorly absorbing mineral oxides to be efficiently heated to very high temperatures in few minutes.