One of the most popular borylation reagents, bis(pinacolato)diboron B2Pin2, turned out to be unstable when stored in air. As the authors of a preprint published on the ChemRxiv.org portal write, this reagent autocatalytically oxidizes into boric acid ester and pinacol. Half a kilogram of B2Pin2 oxidized in a year.
Bis(pinocalato)diboron is a colorless crystalline substance. It is usually used to obtain organoboron compounds, for example, in the Hartwig borylation reaction. The aromatic boron derivatives obtained in this reaction can enter into cross-coupling reactions. B2Pin2 can be obtained in the laboratory from boron bromide BBr3 using a published method. It also states that the resulting reagent can be used in air, and it should be stored in a regular closed jar.
But recently, chemists led by Erik A. Romero at the University of California, San Diego, figured out that B2Pin2 should be stored in an inert atmosphere. They stored it in a regular half-kilogram jar for a year, then tried a borylation reaction; the reaction didn’t work. The chemists then recrystallized a sample of B2Pin2 and tried the reaction again, and still didn’t get the product.
To find out why the reactions weren't happening, the chemists recorded NMR spectra of a B2Pin2 sample from the same jar. The proton and carbon spectra were clean and matched the published spectra of the reagent. But the boron spectrum and X-ray structural analysis showed that instead of B2Pin2, the jar contained its oxidation product — boric acid ester of pinacol PinB-OH.
Then the scientists decided to find out how quickly different borylation reagents oxidize in air. It turned out that solid B2Pin2 completely oxidizes in ten days at 50 degrees Celsius to form a mixture of BPin-OH and (BPin)2O. At the same time, its analogs B2Nep2 and B2Cat2 turned out to be more stable: 46 percent of B2Nep2 oxidized in the same time, and B2Cat2 did not oxidize at all.
Thus, chemists have shown that B2Pin2 and some other borylation reagents should be stored in an inert atmosphere, not in the air. As they suggest, oxidation occurs faster if the sample already contains BPin-OH, which acts as a catalyst. The chemists also note that reactions with B2Pin2 should be carried out in an inert atmosphere.
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