Reduction of quinones by hydride Meisenheimer adducts

Abstract

Hydride Meisenheimer adducts, prepared by the reaction of 1,3,5-trinitrobenzene or 2,4-dinitroaniline with tetrahydridoborate ions, can transfer hydride to a range of quinones in dimethyl sulphoxide solution. Rate constants are reported for the 1,3,5-trinitrobenzene adduct and tetrachloro-1,4-benzoquinone, tetrabromo-1,4-benzoquinone, and 2,6-dichloro-1,4-benzoquinone, and for the 2,4-dinitroaniline adduct with 1,4-benzoquinone. The rates increase with increasing reduction potential of the quinone. In all cases the hydroquinones formed are slowly oxidised back to quinones.There are indications that the formation of the products of the hydride transfer is preceded by formation of a species having a low-intensity absorption maximum at > 700 nm, which is tentatively ascribed to a charge-transfer complex.4-Nitrobenzaldehyde reacts with the hydride adduct of 1,3,5-trinitrobenzene at a much lower rate.All the reactions observed are characterised by low energies of activation (in the range 36–51 kJ mol–1) and large negative entropies of activation.