Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part IV. Mechanistic aspects of the reduction of 2,4,6-trimethyl-2′-nitrobiphenyl, 2-nitrobiphenyl, and nitrobenzene

Abstract

Reduction of 2′,4′,6′-trimethyl-2-nitrobiphenyl by triethyl phosphite gives 2-amino-2′,4′,6′-trimethylbiphenyl and triethyl N-(2′,4′,6′-trimethylbiphenyl-2-yl)phosphorimidate. The same reduction, carried out in cumene or in t-butylbenzene gives, additionally, 8,10-dimethylphenanthridine, and, in cumene, 2,3-dimethyl-2,3-diphenyl-butane, indicating the intermediacy of a triplet nitrene. Reduction of nitrobenzene by diethyl methylphosphonite in an excess of diethylamine gives 2-diethylamino-3H-azepine (83%), while the corresponding reaction with 2-nitrobiphenyl gives 2-diethylamino-3-phenyl-3H-azepine (13%) in addition to carbazole (63%), pointing to an intermediate similar to that present in decompositions of azides, i.e. a nitrene or nitrenoid species, the nature of which is discussed.

The order of reducing reactivity of a series of tervalent phosphorus reagents towards 2-nitrobiphenyl is (EtO)₂PMe (Et₂N)₃P ∼(EtO)P(NEt₂)₂ > (EtO)₃P ∼(PrⁱO)₃P ⋙ PCl₃(inactive).