Polyhalogenoaromatic compounds. Part 46. Circumstantial evidence for the intermediacy of radical anions during the reaction of magnesium with ω-bromoalkoxypolychloro-arenes and -heteroarenes

Abstract

4-(ω-Bromoalkoxy)tetrachloropyridines, 4-(ω-bromoalkoxy)-3,5-dichloro-2,6-difluoropyridines, and (ω-bromoalkoxy)-pentachlorobenzenes have been prepared by reaction of the appropriate aryl and heteroaryl oxides with 1,ω-dibromoalkanes. The main products from the reactions of the bromoethoxy- or bromopropoxy-tetrachloropyridines with magnesium were the furopyridine (9a) and pyranopyridine (9b), respectively. Elimination of other possible reaction pathways leads to the hypothesis that the cyclisations proceed by electron transfer from magnesium to the tetrachloropyridyl group, followed by nucleophilic displacement of bromide from the side chain. In the absence of a leaving group in the side chain, the radical-anion intermediate leads to a Grignard reagent.