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Oxidations
Oxidation reactions are obviously quite important to synthetic organic chemistry. Traditionally, they require stoichiometric amounts of harsh, toxic oxidants and long reaction times. Microwave-induced oxidations have been extensively explored, including alcohols to carbonyl-containing compounds137,160,536-567, as well as non-oxygen compounds like aromatics568, sulfides569, and carbon–carbon double bonds570-572.
A very common reaction of alcohols is their oxidation to carbonyl compounds. Primary alcohols can yield aldehydes or carboxylic acids and secondary alcohols produce ketones. Tertiary alcohols generally will not yield any oxidized products. Numerous methods are available for oxidizing different types of alcohols. Scheme 97 shows microwave-induced oxidation of benzyl alcohols with various oxidants and methods.10,223,543-551 In all of these reactions, microwave heating increased reaction rates drastically and also increased product yields.
Scheme 97
Benzoin oxidation to benzils, or 1,2-diketones, is another widely used reaction in synthetic chemistry. 1,2-Diketones are extremely important intermediates, as they can easily be transformed into many other organic functionalities. Conventional methods require extended reaction times with highly toxic oxidants. Using microwave irradiation eliminates both of these problems, as shown in Scheme 98.564-567
Scheme 98
Aromatic tricyclic ring systems like fluorene, xanthene, diphenylmethane, and anthrone can also be oxidized. The methylene group in between the two aromatic rings can be directly oxidized to a carbonyl with potassium permanganate, but these reactions are frequently lengthy. With microwave irradiation, KMnO4 on alumina, and solvent-free conditions, successful oxidation is completed in 10-30 minutes (Scheme 99).568
Scheme 99
Sulfides can be readily oxidized to both sulfoxides and sulfones. To be selective for one or the other is the main challenge that faces organic chemists. Various methods have been employed, but most require long reaction times, the addition of extra reagent, and thus, a higher concentration of corrosive acids, peracids, or metallic compounds. Varma and co-workers, who have done extensive work with oxidants on supported mediums in solvent-free reaction environments546-551,565,566,569, have performed selective oxidation on sulfides with sodium periodate on silica gel via microwave-enhanced reaction conditions (Scheme 100).569
Scheme 100
Peracids are powerful oxidizing agents. Conventional use of m-chloroperbenzoic acid (MCPBA) on carbon– carbon double bonds forms epoxides, but these reactions are very long and usually take place at 0 °C. With microwave irradiation, epoxides are provided in only three minutes with 99% product yield (Scheme 101).570
Scheme 101
Instruments
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