Oxidations without introduction of oxygen

These oxidations are, in fact, dehydrogenations. Aromatizations of 6-membered rings and the formation of carbonyl groups from alcohols fall into this class of reactions, for which a variety of reagents are known in the literature. 

Aromatization of Hantzsch dihydropyridines to pyridines can be effected by the solid-supported reagent Claycop (cupric nitrate on montmorillonite). The reaction is strongly accelerated by sonication, but no change in the selectivity occurs. The method is successful only when an aryl group substitutes the 4-position.202 No mechanism is proposed, although the authors mention that nitric acid or nitronium or nitrosonium ions could be involved (Fig. 36). 

The influence of sonication on the formation of these species was not studied. Changing the oxidizer to manganese dioxide allows broadening the scope of the reaction, which becomes applicable to 4-alkyl substituted compounds.203 

202 Maquestiau, A. Mayence, A. Vanden Eynde, J.J. Tetrahedron Lett. 1991,32, 3839-3840. 

Chromanones are oxidized to chromones by HTIB (p. 155).204 it is then suggested that a first step is the functionalization of the substrate at the position a to the keto group, followed by an elimination and migration of a substituent. 

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