Abstract

Ketones are amongst the most important and versatile groups of chemicals known, with applications in industry as solvents and starting materials, as well as playing an important role in biological and medicinal applications. With their importance for synthetic chemists, the search for means of protecting ketones is ongoing. Tosylhydrazones have been suggested as such a means because they are easily formed through a simple condensation reaction between the ketone and p-tolysulfonylhydrazine, and they are resistant to many chemical conditions. Despite this, tosylhydrazones have seen little use as a protecting group, largely due to the difficulty of removing them. This study investigated the deprotection of ketones from tosylhydrazones by irradiating a solution of the tosylhydrazone with visible light, in the presence of chloranil. In addition to ketone, a precipitate byproduct and nitrogen gas was produced. Depending on the experimental procedure, ketone was regenerated quantitatively. The effect of several factors on the rate of ketone regeneration were investigated, among them were light intensity, solvent, initial concentrations of tosylhydrazone and chloranil and secondary reaction products. Several factors elucidating to the reaction mechanism were probed. The first was the source of oxygen in the regenerated ketone, with three sources probed (air, water and the tosyl group). The second factor was the participation of a tosylhydrazone radical cation, with investigations into its independent generation and trapping, and computational studies on the tosylhydrazone and its radical cation. The final factor investigated was the nature of the secondary reaction products. A reaction mechanism was proposed based on experimental and computational data.