Methylborane

1,2-Dimethyldiborane

cis-1,2-Dimethyldiborane
trans-1,2-Dimethyldiborane
Names
IUPAC name 1,2-Dimethyldiborane
Other names Symmetrical dimethyldiborane
Identifiers
CAS Number	17156-88-6 N
3D model (JSmol)	Interactive image
InChI InChI=1S/C2H10B2/c1-3-5-4(2)6-3/h3-4H,1-2H3[1] Key: CMZSWMSJMMTTMF-UHFFFAOYSA-N[2]
SMILES C[BH]1H[BH](C)H1
Properties
Chemical formula	(CH3BH2)2
Molar mass	55.72 g mol−1
Appearance	Colorless gas
Melting point	−124.9 °C (−192.8 °F; 148.2 K)
Boiling point	+4 °C (39 °F; 277 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N (what is YN ?)
Infobox references

1,1-Dimethyldiborane

Names
IUPAC name 1,1-Dimethyldiborane
Other names Unsymmetrical dimethyldiborane
Identifiers
CAS Number	16924-32-6 7-55-1
3D model (JSmol)	Interactive image
InChI InChI=1S/C2H10B2/c1-4(2)5-3-6-4/h3H2,1-2H3[3] Key: KEWIJCBOFAPULG-UHFFFAOYSA-N[4]
SMILES C[B]1(C)H[BH2]H1
Properties
Chemical formula	(CH 3) 2B 2H 4
Molar mass	55.72 g mol−1
Appearance	Colorless gas
Melting point	−423.3 °C; −730.0 °F; −150.2 K
Boiling point	−4 °C (25 °F; 269 K)
Solubility in organic solvents	Soluble in ether, pentane, tetrahydrofuran
Thermochemistry
Std enthalpy of formation (ΔfHo298)	-25 kcal/mol
Hazards
NFPA 704	2 2
Related compounds
Related alkyl boranes	Methyldiborane Trimethyldiborane Tetramethyldiborane Trimethylborane Diethyldiborane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N (what is YN ?)
Infobox references

Dimethyldiborane is a chemical compound consisting of methyl groups attached to a diborane core. It can be considered as the dimer of methylborane, CH₃BH₂, the simplest alkylborane, consisting of a methyl group substituted for a hydrogen in borane. There are two isomers. The 1,2-dimethyldiborane form (CH₃BH₂)₂ is symmetrical with one methyl on each boron. 1,2-Dimethyldiborane can exist in a cis- and a trans arrangement. The other isomer is 1,1-dimethyldiborane, known as unsymmetrical dimethyldiborane, which has two methyl groups on one boron atom, and only hydrogen on the other. Other combinations of methylation occur on diborane, including methyldiborane, trimethyldiborane, tetramethyldiborane, and trimethylborane. Tetramethyldiborane is the dimer of dimethylborane. At room temperature the substance is at equilibrium between these forms. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.

Methylborane is formed when lithium methylborohydride LiCH₃BH₃ reacts with an acid in tetrahydrofuran.

2 LiCH₃BH₃ + 2HCl → (CH₃BH₂)₂ + 2H₂ + 2 LiCl

Instead of hydrogen chloride, methyl iodide or trimethylsilyl chloride can be used.

Lithium methylborohydride can be made by reacting a methylboronic ester with lithium aluminium hydride.

Methylboranes are also formed by the reaction of diborane and trimethylborane. This reaction produces four different substitution of methyl with hydrogen on diborane. Produced is 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane and 1,1,2,2-tetramethyldiborane. By reacting monomethyldiborane with ether, dimethyl ether borine is formed (CH₃)₂O.BH₃ leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane. The reaction is complex. At 0 °C when diborane is in excess, monomethyldiborane is initially produced, coming to a steady but low level, and 1,1-dimethyldiborane level increases over a long time, until all trimethylborane is consumed. Monomethyldiborane ends up at equilibrium with a mixture of diborane and dimethyldiborane. At 0° the equilibrium constant for 2B₂H₅Me ←→ B₂H₆ + (BH₂Me)₂ is around 0.07, so monomethyldiborane will typically be the majority of the mixture, but there will still be a significant amount of diborane and dimethyldiborane present. Monomethyldiborane yield is best with a ratio of 4 of diborane to 1 of trimethylborane. The yield of trimethyldiborane is maximised with ratio of 1 of diborane to 3 of trimethylborane.