CATALYSIS BY FERROUS ION IN NUCLEOPHILIC AROMATIC-SUBSTITUTION REACTIONS

Efficient catalysis is provided by ferrous chloride in the nucleophili c aromatic substitution reaction of several aryl and heteroaryl halide s with a ketone enolate ion as the nucleophile in Me2SO, to give the a ryl or heteroaryl ketones in fair to good yields. The enolate ions fro m pinacolone, acetophenone, cyclohexanone and pentan-3-one behave succ essfully. A side-reaction is represented in some cases by the hydrodeh alogenation of the substrate ArX, and evidence is provided for the int ermediacy of both Ar. and Ar- species. Other significant mechanistic c lues acquired include: (i) inhibition by electron and radical scavenge rs; (ii) entrainment of poor nucleophiles by good ones; (iii) the rela tive reactivity of two nucleophiles in competition experiments with Ph l under Fe2+ catalysis having the same value as in the experiments und er both photostimulation and spontaneous initiation. All these finding s are consistent with an S(RN)1 mechanism of substitution where ferrou s ion, in combination with the nucleophile, plays an important role in the initiation step.