A comparative study of base-free arylcopper reagents for the transfer of aryl groups to boron halides

A comparative study on the reactivity and selectivity of arylcopper species in reactions with boron halides was performed. Mesitylcopper reacts with BX₃ (X=Cl, Br) in toluene at low temperature under highly selective formation of the monosubstituted boranes MesBX₂. The dimesitylboranes Mes₂BX are gradually formed with a twofold excess of the organocopper reagent at elevated temperature. In contrast, pentafluorophenylcopper shows a tendency for formation of B(C₆F₅) ₃ in reactions with BX₃ irrespective of the stoichiometry used, suggesting a strong impact of electronic factors on the selectivity of the aryl transfer reaction. New procedures for the synthesis of the pentafluorophenylboron halides C₆ F₅BX₂ (X=Cl: 57%; X=Br: 62%) and of tris(pentafluorophenyl)borane (80%) and related mixed-substituted triarylboranes from the base-free isolable pentafluorophenylcopper precursor have been developed.