Iridium and 2-methylphenanthroline accelerate catalytic borylation reactions

2-methylphenanthroline used with iridium to accelerate catalytic borylation reactions
Profiles of the Ir-catalyzed reactions of THF and dibutyl ether with B2pin2 with variously substituted phenanthrolines as ligands. The reactions are catalyzed by the combination of 5 mol % [Ir(mesitylene)(Bpin)3] and 5 mol % 2-mphen (red), tmphen (blue), or 2,9-dmphen (green). The relative rates were estimated from the slope of the curves for the initial rates of reactions with 2-mphen (red) versus those for the reactions with the other two ligands after the induction period. n-Bu2O, n-dibutyl ether; Me, methyl. Credit: Science (2020). DOI: 10.1126/science.aba6146

A team of researchers at the University of California, Berkeley, has found that 2-methylphenanthroline can be used with iridium to accelerate catalytic borylation reactions. In their paper published in the journal Science, the group describes their new approach and the ways it might prove useful.

Catalytic borylation is a type of reaction that can be used to target stronger saturated carbon–hydrogen (C–H) bonds over weaker bonds. Unfortunately, to date, those attempting to use such reactions have found them to be slow and to use too much of the hydrocarbon. In this new effort, the researchers have addressed and overcome both issues.

As the researchers note, with C–H functionalization reactions, the reagent that is used typically attacks the weakest carbon bond (the one with the most electrons). The new reaction developed by the team at UoC overcomes such limitations and targets the primary bonds over those that are secondary. It does so by using 2-methylphenanthroline with —an approach that has been found to speed up reaction times by approximately 50 to 80 percent. The faster reaction time meant that the team could use less substrate. The researchers note that the selectivity in targeting the primary was particularly noticeable with dehydroabietic acid. They also noted that when primary C–H bonds are either blocked or absent, secondary C–H bonds are targeted.

The fact that their reaction is a borylation should make it particularly useful to chemists, the researchers believe, because boron is one of the more versatile functional groups. Their new approach will allow for taking unreactive alkyl groups through a synthesis process where they can be converted into carbon-boron bonds—which, they note, should be quite useful because afterward, the C–B can be transformed into a wide variety of functional groups.

One possible problem with the reaction could be choosing the right solvent—it will need to be one that does not itself undergo a reaction. The researchers used cyclooctane, but acknowledge that a more polar solvent needs to be developed for more general use. They also note that they next plan to look into understanding why the reaction targets the toughest bonds.

Scientists finally crack nature's most common chemical bond
A catalyst (center) based on iridium (blue ball) can snip a hydrogen atom (white balls) off a terminal methyl group (upper and lower left) to add a boron-oxygen compound (pink and red) that is easily swapped out for more complicated chemical groups. The reaction works on simple hydrocarbon chains (top reaction) or more complicated carbon compounds (bottom reaction). The exquisite selectivity of this catalytic reaction is due to the methyl group (yellow) that has been added to the iridium catalyst. The black balls are carbon atoms; red is oxygen; pink is boron. (UC Berkeley image by John Hartwig) Credit: John Hartwig, UC Berkeley

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More information: Raphael Oeschger et al. Diverse functionalization of strong alkyl C–H bonds by undirected borylation, Science (2020). DOI: 10.1126/science.aba6146

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Journal information: Science

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Citation: Iridium and 2-methylphenanthroline accelerate catalytic borylation reactions (2020, May 20) retrieved 17 April 2021 from
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