Tris N-methylanilino borane

Tris(N-methylanilino)borane has previously been prepared by the reaction of boron trifluoride-ether complexes with three equivalents each of N-methylaniline and a suitable Grignard reagent,1,2 by the reaction of (N-methylanilino)potassium with boron trifluoride-ether complexes,2 and by aminolysis of boron trichloride by N-methylaniline.3 The present general procedure describes a convenient preparation of tris(N-methylanilino)-borane by the reaction of (N-methylanilino)lithium and boron trifluoride-diethyl ether in tetrahydrofuran-hexane as solvent. 

Tris(organoamino)boranes have been utilized to prepare, in reasonable yields,4,5 mono- and dihalo(organoamino)boranes which are often difficult to obtain by direct amination of the boron trihalides. Carboxylic acids, 1,3-diketones, ketones, and /3-ketoesters have been converted into carboxamides, enamino-ketones, enamines, and j -enamino-amides, respectively, by reaction with an appropriate tris(organoamino)borane under very mild conditions.6 Sulfenamides (R2NSC6H5) have also been prepared in high yield from selected tris(organoamino)boranes and sulfenic esters under relatively mild conditions.7 

A 500-mL, three-necked, round-bottomed flask, equipped with a mechanical stirrer, water-cooled condenserf topped with a calcium chloride drying 

Commercial boron trifluoride-diethyl etherj is purified by distillation at 46°/10 torr from small amounts of calcium hydride.8 (The resulting pot residue may be destroyed by the cautious addition of ice.) 

Boron trifluoride-diethyl ether (7.1 g, 0.05 mol) is added during 5 min to the previously prepared solution of (IV-methylanilino)lithium. The pale-yellow slurry is then refluxed for 3 hr. At the end of this time, the mixed solvent is concentrated to approximately one-half of its original volume by distillation from the reaction vessel. The reaction mixture is cooled to 0° and subjected to a vacuum filtration through a 9-cm Buchner funnel to afford 22.0 g of a tan precipitate. This solid is placed in 200 mL of toluene, which is then brought to a boil the mixture is subjected to a hot vacuum-filtration in order to remove the lithium fluoride co-product. A considerable reduction in yield may result unless a steam-jacketed Buchner funnel or hot sintered-glass filter is used. Upon cooling, the toluene filtrate deposits white crystals, which are collected by filtration. Yield 13.7 g (83%) of tris(N-methyl-anilino)borane, mp 211-213° (lit. mp 210° 1 214-216°3).  

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It should be noted that not all of the solid tris(organoamino)boranes are as stable to hydrolysis as is tris(N-methylanilino)borane thus, application... 

Tris(N-methylanilino)borane is a white, crystalline material having very little odor, mp 211-213°. It is relatively stable to hydrolysis by atmospheric moisture. It is not appreciably soluble in most common, nonprotonic, anhydrous, organic solvents at room temperature. Important bands in the infrared spectrum (Nujol mull) include 12 1598, 762, and 693 cm-1 (aromatic ring vibrations) 1354 cm-1 [v(nB—N)] 1285 cm 1 [aromatic v(C—N)] 1109 cm"1 [alkyl v(C-N)]. 

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