1,1-Dimethyldiborane

1,1-Dimethyldiborane is the organoboron compound with the formula ₂B₂BH₂. A pair of related 1,2-dimethyldiboranes are also known. It is a colorless gas that ignites in air.

Formation

The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s. Methylboranes are formed by the reaction of diborane and trimethylborane. This reaction produces four different substitution of methyl with hydrogen on diborane. Produced are 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane, and 1,1,2,2-tetramethyldiborane.
Tetramethyl lead reacts with diborane in a 1,2-dimethoxyethane solvent at room temperature to make a range of methyl substituted diboranes, ending up at trimethylborane, but including 1,1-dimethyldiborane, and trimethyldiborane. The other outputs of the reaction are hydrogen gas and lead metal.
Other methods to form methyldiboranes include heating trimethylborane with hydrogen. Alternatively trimethylborane reacts with borohydride salts with in the presence of hydrogen chloride, aluminium chloride, or boron trichloride. If the borohydride is sodium borohydride, then methane is a side product. If the metal is lithium then no methane is produced. dimethylchloroborane and methyldichloroborane are also produced as gaseous products.
When Cp₂Zr₂ reacts with borane dissolved in tetrahydrofuran, a borohydro group inserts into the zirconium carbon bond, and methyl diboranes are produced.
In ether dimethylcalcium reacts with diborane to produce dimethyldiborane and calcium borohydride:
1,2-dimethyldiborane slowly converts on standing to 1,1-dimethyldiborane.
Gas chromatography can be used to determine the amounts of the methyl boranes in a mixture. The order they elute are diborane, monomethyldiborane, trimethylborane, 1,1-dimethyldiborane, 1,2-dimethyldiborane, trimethyldiborane, and finally tetramethyldiborane.

Selected properties

1,1-Dimethyldiborane has a dipole moment of 0.87 d. The predicted heat of formation for the liquid is ΔH⁰_f=-31 kcal/mol, and for the gas -25 kcal/mol. Heat of vapourisation was measured at 5.5 kcal/mol.

Reactions

At −78.5 °C, methyldiborane disproportionates slowly, first to diborane and 1,1-dimethyldiborane. In solution methylborane is more stable against disproportionation than dimethylborane.
Trimethyldiborane partially disproportionates over a period of hours at room temperature to yield tetramethyldiborane and 1,2-dimethyldiborane. Over a period of weeks 1,1-dimethyldiborane appears as well.
Gentler oxidation of 1,1-dimethyldiborane at 80 °C yields 2,5-dimethyl-1,3,4-trioxadiboralane, a volatile liquid that contains a ring of two boron and three oxygen atoms. An intermediate in this reaction is two molecules of dimethylborylhydroperoxide ₂BOOH. When methyldiborane is oxidised around 150 °C a similar substance methyltrioxadiboralane is produced. At the same time dimethyltrioxadiboralane and trimethylboroxine are also formed, and also hydrocarbons, diborane, hydrogen, and dimethoxyborane.