Transfer Hydrogenation of Nitro‐, Nitroso‐ and Azoarenes by Homolytic Retrodisproportionation

Nitro‐, nitroso‐ and azobenzene are reduced almost quantitatively to aniline when heated to 230–300°C with 9,10‐dihydroanthracene (DHA), xanthene or tetralin. From the effect of polar substituents and polar solvents on the reactivity and from the isotope effect kH/kD ≈︁ 2.4 (280°C), a hydrogen‐atom transfer from the H donor to the acceptor (retrodisproportionation) is proposed as the rate‐determining step. The lower reactivity of xanthene compared with 9,10‐dihydroanthracene eliminates the possibility of a rate‐determining hydride transfer. The observation of an intense ESR signal of 9‐xanthyl radicals during the reaction in diphenyl ether and the typical products support the proposed homolytic mechanism.