Solvent free permanganate oxidations

The oxidations of organic compounds by permanganate under solvent free conditions have been studied. Thiols and primary aromatic amines undergo oxidative coupling reactions to give disulfides and diazenes, respectively, sulfides are oxidized to sulfones, primary and secondary alcohols are converted to aldehydes and ketones, 1,4-diols and cyclic ethers give lactones and arenes are oxidized to the corresponding -ketones. The experimental procedure is simple and the products are easily isolated in good yields. © 2001 Elsevier Science Ltd. All rights reserved. Although solvent free reactions are of general interest because of their potential applications in combinatorial chemistry, only a limited number of useful oxidation procedures have been reported. In this paper we wish to describe a simple and general procedure that can be used for the oxidative transformation of many organic functional groups using an inexpensive and environmentally friendly oxidant, potassium permanganate. Best results are obtained when KMnO4 is first mixed with copper sulfate pentahydrate (or a 20/80 mixture of copper sulfate pentahydrate and alumina) to give a reagent that has previously been used extensively as a heterogeneous oxidant in inert solvents. The reductant is then added dropwise while the solid oxidant is tumbled using a magnetic stirrer. Most reactions can be carried out at room temperature; however, improved yields for solid reductants are obtained when the reaction temperature is near or above their melting points. Some reactions benefit from the application of additional energy in the form of microwaves. Product isolation can be achieved by washing the spent oxidant with an organic solvent, which can subsequently be recovered by simple distillation. Many of the reactions are nearly quantitative giving products of relatively high purity. The oxidant is prepared by grinding equal amounts of potassium permanganate and copper sulfate pentahydrate in a mortar until homogeneous or by adding a concentrated aqueous solution of potassium permanganate to alumina, giving a paste which is then ground with an equal amount of copper sulfate pentahydrate. Reductant (2 mmol) is added to a portion of the oxidant ( 4 g) in a 25 mL round bottomed flask and stirred magnetically until TLC analysis indicates a completed reaction. The residue is then washed with a minimum amount of solvent (methylene chloride, hexane and/or ether). Distillation of the solvent gives a product that is of acceptable purity for most purposes. If greater purity is required, the product can be distilled or recrystallized. As can be seen from the first three reactions in Table 1, thiols are converted into the corresponding disulfides in good yields under very mild conditions (Scheme 1). The times required compare favorably with those for solvent free oxidations of thiols using other oxidants. Addition of alumina to the solid support did not improve the yields of these reactions. Primary aromatic amines are also oxidatively coupled under these conditions (Scheme 2). As indicated by reactions 4–6 in Table 1, the presence of electron withdrawing substituents such as chloro decreases the reaction rate, but not the yield of diazene. The reaction times are shorter than those reported for the corresponding heterogeneous reactions, but the yields appear to be slightly lower. Cleaner products were obtained from these reactions when alumina was added to the solid support. Aryl and alkyl sulfides are converted into the corresponding sulfones in good yields (reactions 7–13, Table