Syntheses with organoboranes. XIII. Synthesis of ω-(4-bromophenyl)alkanoic acids and their borylation

ω-(4-Bromophenyl)alkanoic acids 2c–e were obtained from 1-bromo-4-alkenylbenzenes 5c–e by hydroboration–thermal isomerization–oxidation. Their esters 11c–e were transformed in good yields into the corresponding boronates 12c–e by the cross-coupling reaction with 4,4,5,5,4′,4′,5′,5′-octamethyl[2,2′]bi[[1,3,2]dioxaborolanyl] (10) in an ionic liquid, [bmim][BF4]. The use of pinacolborane for the coupling reaction in the ionic liquid gave debromination products, and low yields of 12c–e. Ethyl 3-(4-bromophenyl)propanoate (7c) was transformed into ethyl 3-(4-[1,3,2]dioxaborolanyl)propanoate (9c) by the cross-coupling with [2,2′]bi[[1,3,2]dioxaborinanyl].