Improved procedures for the generation of diborane from sodium borohydrideand boron trifluoride

Improved procedures for the generation of diborane by the reaction of NaBH4 in triglyme or tetraglyme with the BF3 adducts of di-n-butyl ether, tert-b utyl methyl ether, monoglyme, dioxane, and tetrahydropyran were developed. In these systems, generation of diborane requires 2-4 h at 25 degrees C (fa ster reactions take place at 50 degrees C). The byproduct NaBF4 precipitate s from the reaction mixture at 25 OC as the reaction proceeds. The high-boi ling glyme can be conveniently separated from the lower boiling carrier eth ers by simple distillation of the latter. On the other hand, diborane was g enerated very slowly or not generated using the addition of each of six bor on trifluoride-etherates (di-n-butyl ether, tert-butyl methyl ether, tetrah ydrofuran, tetrahydropyran, dioxane, and monoglyme) to suspensions of sodiu m borohydride in the corresponding ether. However, diborane was generated r apidly and quantitatively by the addition of NaBH4 in triglyme (or tetragly me) to the BF3 adduct of triglyme (or tetraglyme) at room temperature. No s olid precipitation occurs during the reaction, making it convenient for lar ge-scale applications. The pure solvent triglyme (or tetraglyme) can be eas ily recovered and recycled by either crystallizing or precipitating NaBF4 f rom the generation flask. New procedures for the generation of diborane wer e also developed by the reaction of NaBF4 with NaBH4 in triglyme (or tetrag lyme) in the presence of Lewis acids such as AlCl3 and BCl3.