THIOL ANIONS IN NUCLEOPHILIC AROMATIC-SUBSTITUTION REACTIONS WITH ACTIVATED ARYL HALIDES - ATTACK ON CARBON VS ATTACK ON HALOGEN

The reaction of o-iodonitrobenzene with thiol anions (RSNa, R - Me, 2- Pr, t-Bu) in 2-propanol solution produces the o-nitrophenyl thioether and nitrobenzene in relative proportions and with rates which depend o n R. The results of kinetic analysis and of D-incorporation experiment s are inconsistent with a radical chain mechanism as proposed earlier for the alkoxide-promoted hydrode-halogenation of aryl halides. The da ta clearly indicate the involvement of a carbanionic intermediate (the o-nitrophenyl anion) and are consistent with a stepwise mechanism in which the first step is nucleophilic attack on iodine to form a sulphe nyl halide and the o-nitrophenyl anion (a halogenophilic reaction). In contrast, the reaction of the other o-halonitrobenzenes (F, Cl, Br) p roduces exclusively the product of alkylthio-de-halogenation according to the addition/elimination (S(N)Ar) mechanism. The fact that in thes e systems the reactivity of thiol anions results from their ability as bielectronic nucleophiles rather than as reducing agents is discussed .