|
|Section2= |Section3= |Section4= |Section5= }} |Section2= |Section3= |Section4= |Section5= |Section8= }} 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.〔 〕 ==Formation== 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.〔 Tetramethyl lead can react 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 reacting hydrogen with trimethylborane between 80 and 200°C under pressure, or reacting a metal borohydride with trimethylborane 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 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 + 2B2H6 → Ca(BH4)2 + B2H4(CH3)2. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dimethyldiborane」の詳細全文を読む スポンサード リンク
|