Microwave Assisted Oxidation of Benzyl Halides to Aldehydes and Ketones with 4-Hydroxypyridinium Nitrate Functionalized Silica Gel in Aqueous Media

Abstract

Aldehydes and ketones, available by numerous methods and from plentiful starting materials, are vital classes of chemicals which serve as precursors and intermediates for the preparation of such divergent products as drugs, vitamins and fragrances. One of the methods for the synthesis of aldehydes and ketones is direct oxidation of organic halides. The oxidation of benzyl halides into aldehydes is a well-known transformation in organic synthesis, although the oxidation of aliphatic halides to aldehydes is not common and is more difficult than the oxidation of benzyl halides. Up to now, several methods have been developed for direct oxidation of halides to their corresponding carbonyl compounds. Dimethyl sulfoxide, N,N-dimethyl-4-nitrosoaniline, nitronate anion and Pd(PPh3), 10 pyridine N-oxides, and other amine oxides, chromate and dichromate systems, silver nitrate, pyrazinyl sulfoxides, Mg-Al hydrotalcite, V2O5, 17 NaIO4-DMF, 18 quinolinium chlorochromate, H5IO6 in [C12mim][FeCl4], 20 and amine oxides have been used in the conversion of alkyl halides to aldehydes and ketones. Recently NaOH-modified graphitic carbon nitride (g-C3N4), 24 nano-Fe3O4@L-arginine-CD-Cu(II), 25 (NH4)3[FeMo6O18(OH)6] with O2 26 and amine-functionalized silica-coated iron-core nanoparticles have been reported for the preparation of aldehydes and ketones from alkyl halides. However these procedures have some limitations, such as long reaction times, low yields, high temperatures, being harmful to the environment and difficult separation of the product from the reaction mixture. From an environmental and green chemistry viewpoint, there is a great need to develop new procedures with a broad substrate scope that will address the above-mentioned drawbacks for the direct oxidation of organic halides to their corresponding carbonyl compounds. In this respect, leverage points for research concern the application of green oxidants, green solvents, or solvent-free processes, development of new catalytic systems, and the use of energy efficient heating methods. For example, microwave (MW) heating is one of the simplest but most effective ways to enhance energy efficiency and productivity in small-scale chemical production. Herein we wish to report the use of an eco-friendly heterogeneous oxidant based on 4-hydroxypyridinium nitrate functionalized silica gel for the oxidation of benzyl halides to the corresponding carbonyl compounds. The oxidant is a stable solid and can be