Transition metal-free hydrogenative coupling of nitroarenes mediated with dihydropyridine: chemoselective formation of aromatic azoxy, azo, hydrazine and phenazine

Due to the diversity of the compounds containing dinitrogen bonds, chemoselectivity is a key issue to be addressed for the hydrogenative coupling of nitroarenes. A system consisting of Hantzsch 1,4-dihydropyridine (HEH) and a base was developed as a transition metal-free reductant for the hydrogenative coupling of nitroarenes to provide aromatic azoxy, azo and hydrazine. Under optimized conditions, the reaction of 2-fluoronitroarene afforded phenazines. Chemoselectivity for the formation of these dinitrogen compounds was effectively regulated by the choice of the base and the amount of reductant employed. A plausible free-radical mechanism for the hydrogenative coupling of nitroarenes was proposed, wherein the combination of HEH and NaH acted as a synergistic reductant.