Report

Catalyst-controlled selectivity in the C–H borylation of methane and ethane

Science  25 Mar 2016:
Vol. 351, Issue 6280, pp. 1421-1424
DOI: 10.1126/science.aad9289

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Methane borylation in a cyclohexane sea

Although methane combusts readily at high temperatures, it is generally the hardest hydrocarbon to transform under gentler conditions, owing to its particularly strong C-H bonds. Cook et al. now show that soluble rhodium, iridium, and ruthenium catalysts can slice through these C-H bonds to add boron substituents to methane at 150°C. Smith et al. report the iridium-catalyzed reaction using phosphine ligands to enhance activity. Both studies were performed in cyclohexane solvent, revealing a remarkable selective preference for the methane reaction over functionalization of the cyclic hydrocarbon.

Science, this issue pp. 1421 and 1424

Abstract

The C–H bonds of methane are generally more kinetically inert than those of other hydrocarbons, reaction solvents, and methane functionalization products. Thus, developing strategies to achieve selective functionalization of CH4 remains a major challenge. Here, we report transition metal–catalyzed C–H borylation of methane with bis-pinacolborane (B2pin2) in cyclohexane solvent at 150°C under 2800 to 3500 kilopascals of methane pressure. Iridium, rhodium, and ruthenium complexes all catalyze the reaction. Formation of mono- versus diborylated methane is tunable as a function of catalyst, with the ruthenium complex providing the highest ratio of CH3Bpin to CH2(Bpin)2. Despite the high relative concentration of cyclohexane, minimal quantities of borylated cyclohexane products are observed. Furthermore, all three metal complexes catalyze borylation of methane with >3.5:1 selectivity over ethane.

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