Borylation of arenes catalyzed by transition metal complexes

Studies on the identification of transition metal complexes, that are able to catalyze the borylation reaction of unactivated arenes with a diboron reagent were performed. Iron carbonyl monophosphine complexes were found to induce the borylation of benzene under photochemical conditions with a single turnover, Iron carbonyl bis(boryl) complexes were shown to be likely intermediates in the reaction. Ruthenium carbonyl monophosphine complexes were shown to catalyze the borylation of benzene under thermal conditions at high temperatures, with up to eight catalytic turnovers. Ruthenium carbonyl bis(boryl) complexes were not reaction intermediates. No ruthenium boryl that was competent to be an intermediate to the borylation reaction was identified. Ir(I) precursors, in conjunction with bipyridine ligands, catalyze the borylation of arenes in high yields at temperatures that are significantly lower than those required for previous borylation reactions. The combination of {lcub}Ir(COE)2Cl{rcub} 2 and (4,4-di-t-butyl)bipyridine (dtbpy) allows for the reaction to take place at room temperature. The same catalyst system at 100°C provides remarkably high turnover numbers for a hydrocarbon functionalization process. Mechanistic studies show that the reaction proceeds through unusual Ir(III) tris-boryl complexes. An example of this type of complex ligated by (4,4'-di-t-butyl)bipyridine was isolated and structurally characterized. (dtbpy)Ir(COE) (Bpin)3 (Bpin = pinacolboryl) reacted fast with benzene at room temperature to produce aryl boronate esters in high yield. Kinetic studies on the catalytic reaction in the presence of {lcub}Ir(COE)2Cl{rcub}2 and dtbpy revealed that the reaction proceeds through a mechanism where the rate determining step is oxidative addition of the carbon-hydrogen bond of the substrate.