1,1-Dimethyldiborane is the

organoboron compound Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes. Or ...

with the formula (CH₃)₂B(μ-H)₂BH₂. 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 Diborane(6), commonly known as diborane, is the chemical compound with the formula . It is a highly toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has att ...

and

trimethylborane Trimethylborane (TMB) is a toxic, pyrophoric gas with the formula B(CH3)3 (which can also be written as Me3B, with Me representing methyl). Properties As a liquid it is colourless. The strongest line in the infrared spectrum is at 1330 cm� ...

. This reaction produces four different substitution of methyl with hydrogen on diborane. Produced are 1-

methyldiborane Methyldiborane, CH3B2H5, or monomethyldiborane is the simplest of alkyldiboranes, consisting of a methyl group substituted for a hydrogen in diborane. As with other boranes it exists in the form of a Dimer (chemistry), dimer with a twin hydrogen br ...

, 1,1-dimethyldborane, 1,1,2- trimethyldiborane, and 1,1,2,2-

tetramethyldiborane Dimethylborane, (CH3)2BH is the simplest dialkylborane, consisting of a methyl group substituted for a hydrogen in borane. As for other boranes it normally exists in the form of a dimer called tetramethyldiborane or tetramethylbisborane or TMDB (( ...

Tetramethyl lead Tetramethyllead, also called tetra methyllead and lead tetramethyl, is a chemical compound used as an antiknock additive for gasoline. It is a methyl radical synthon. Its use in gasoline is being phased out for environmental considerations. The ...

reacts with diborane in a

1,2-dimethoxyethane Dimethoxyethane, also known as glyme, monoglyme, dimethyl glycol, ethylene glycol dimethyl ether, dimethyl cellosolve, and DME, is a colorless, aprotic, and liquid ether that is used as a solvent, especially in batteries. Dimethoxyethane is misci ...

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 Borohydride refers to the anion , which is also called tetrahydroborate or more commonly tetrahydrobiopterin, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for ex ...

salts with in the presence of

hydrogen chloride The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...

aluminium chloride Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms a hexahydrate with the formula , containing six water molecules of hydration. Both the anhydrous form and the hexahydrate are col ...

, or

boron trichloride Boron trichloride is the inorganic compound with the formula BCl3. This colorless gas is a reagent in organic synthesis. It is highly reactive towards water. Production and structure Boron reacts with halogens to give the corresponding trihalides ...

. If the borohydride is

sodium borohydride Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula (sometimes written as ). It is a white crystalline solid, usually encountered as an aqueous basic solution. Sodi ...

, then

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

is a side product. If the metal is lithium then no methane is produced. dimethylchloroborane and methyldichloroborane are also produced as gaseous products. When Cp₂Zr(CH₃)₂ 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: :Ca(CH₃)₂ + 2 B₂H₆ → Ca(BH₄)₂ + B₂H₄(CH₃)₂ 1,2-dimethyldiborane slowly converts on standing to 1,1-dimethyldiborane.

Gas chromatography Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for Separation process, separating and analyzing compounds that can be vaporized without Chemical decomposition, decomposition. Typical uses of GC include t ...

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. :2 MeB₂H₅ → 1,1-Me₂B₂H₄ + B₂H₆, ''K'' = 2.8, Me = CH₃ :3 ,1-Me₂B₂H₄→ 2 Me₃B₂H₃ + B₂H₆, ''K'' = 0.00027 Trimethyldiborane partially disproportionates over a period of hours at room temperature to yield

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 (CH₃)₂BOOH. (CAS 41557-62-5) 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 (dimethyl methylboronic ester).

References

{{Reflist, 30em Alkylboranes Gases