Tetrahydroxydiboron
Tetrahydroxydiboron is a reagent which has several uses in organic synthesis, notably as a precursor for boronic acids used in Suzuki-Miyaura couplings
Synthesis
The reaction of boron trichloride with alcohols was reported in 1931, and was used to prepare dimethoxyboron chloride, B2Cl. Egon Wiberg and Wilhelm Ruschmann used it to prepare tetrahydroxydiboron by first introducing the boron-boron bond by reduction with sodium and then hydrolysing the resulting tetramethoxydiboron, B24, to produce what they termed sub-boric acid. The methanol used in this process can be recycled:
Overall: 2 BCl3 + 2 Na + 4 H2O → B24 + 2 NaCl + 4 HCl
Reactions
Tetrahydrodiboron can be used in the Miyaura borylation of aryl halides and pseudo-halides to form aryl boronic acids, which are key components in Suzuki-Miyaura couplings. Tetrahydroxydiboron has also been shown to be a reducing agent for nitro groups, providing an alternative to reactions involving hydrogen gas.
Safety
Although tetrahydroxydiboron has been shown not to be shock or friction sensitive in the solid state, differential scanning calorimetry shows the compound decomposes with significant energy release starting at 90°C, which may be related to the dehydration to a polymeric boron(II) oxide. Similar thermal instability is observed in solution with organic solvents like DMF and DMSO. An additional safety consideration when using tetrahydroxydiboron is that upon exposure to water, tetrahydroxydiboron decomposes to form for two equivalents of boric acid and one equivalent of hydrogen gas.