Lithium complexes reactions, borane reagents

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. 

Sodium bis-(2-methoxyethoxy)aluminium hydride (ref. 84) and lithium tri-tert-butoxyaluminium hydride (ref. 85) showed excellent stereoselectivity for podophyllotoxin compared with other reagents. However, these reagents have the disadvantage that low temperature (-75 C) is required to obtain satisfactory yield. Although the stereoselectivity was inferior to that of the above-mentioned aluminium hydride complexes, borane-terf-butylamine complex (ref. 86) and borane ammonia complex (ref. 87) gave the products in more than 90% yield. The reactions proceeded at room temperature, and borane-tert-butylamine complex did not require the use of a nitrogen stream. 

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