One of the most popular borylation reagents, bis(pinacolato)diboron B2Pin2, proved unstable when stored in air. According to the authors of a preprint published on ChemRxiv.org, this reagent autocatalytically oxidizes to the ester of boric acid and pinacol. Half a kilogram of B2Pin2 oxidized within a year.
Bis(pinocalotho)diboron is a colorless crystalline substance. It is commonly used to prepare organoboron compounds, for example, in the Hartwig borylation reaction. The aromatic boron derivatives obtained in this reaction can undergo cross-coupling reactions. B2Pin2 can be prepared in the laboratory from boron bromide BBr3 using a published method. It also states that the resulting reagent can be used in air and should be stored in a regular sealed jar.
But recently, chemists led by Erik A. Romero from the University of California, San Diego, discovered that B2Pin2 should be stored in an inert atmosphere. They stored it in a standard half-kilogram jar for a year and then tried a borylation reaction; the reaction failed. The chemists then recrystallized a sample of B2Pin2 and tried the reaction again, but still failed to produce the product.
To determine why the reaction wasn't working, the chemists recorded the NMR spectra of a B2Pin2 sample from the same jar. The proton and carbon spectra were clear and matched the published spectra of the reagent. However, the boron spectrum and X-ray crystallography revealed that instead of B2Pin2, the jar contained its oxidation product—boric acid ester of pinacol, PinB-OH.
The scientists then decided to determine how quickly different borylation reagents oxidize in air. It turned out that solid B2Pin2 oxidizes completely within ten days at 50 degrees Celsius, forming a mixture of BPin-OH and (BPin)2O. However, its analogs B2Nep2 and B2Cat2 proved more stable: 46 percent of B2Nep2 oxidized in the same time, while B2Cat2 did not oxidize at all.
Thus, chemists have shown that B2Pin2 and some other borylation reagents should be stored in an inert atmosphere rather than exposed to air. They suggest that 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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