Sodium borohydride as a reagent for nucleophilic aromatic substitution by hydrogen: the role of hydride Meisenheimer adducts as reaction intermediates

Abstract

The reaction of o-bromo- and p-chloro-nitrobenzenes and of 1,4-dinitrobenzene with sodium borohydride in a dilute solution in dimethyl sulphoxide solution leads to some nitrobenzene. When tritiated borohydride is employed, aromatic hydrogen exchange occurs to a comparable extent.

Under similar conditions 1-X-2,4-dinitrobenzenes mostly undergo either displacement of the 2-nitro-group by hydrogen or ring reduction. With these compounds it is possible to observe the n.m.r. spectra of the unstable cyclohexadienyl anions (hydride Meisenheimer adducts), formed by hydride addition to the 3- and/or the 5-position, which decompose into the final products.

For 2,4-dinitroanisole it is shown by an isotope-labelling experiment that the displacement of the 2-nitro-group is accompanied by intramolecular hydrogen migration from the methylene group of the Meisenheimer adduct (at the 3-position) to the 2-position. This mechanism is the nucleophilic analogue of the mechanism of ipso-attack with migration, recognised as one of the reaction paths in electrophilic aromatic substitution. The term ‘vicinal attack’ is proposed for this general class of aromatic substitution reactions in which the entering group first attacks and attaches itself to a ring position adjacent to the position that it occupies in the reaction product and from which the leaving group is displaced.