Improved Procedures for the Generation of Diborane from Sodium Borohydride and Boron Trifluoride

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Abstract
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-butyl methyl ether, monoglyme, dioxane, and tetrahydropyran were developed. In these systems, generation of diborane requires 2−4 h at 25 °C (faster reactions take place at 50 °C). The byproduct NaBF4 precipitates from the reaction mixture at 25 °C as the reaction proceeds. The high-boiling glyme can be conveniently separated from the lower boiling carrier ethers by simple distillation of the latter. On the other hand, diborane was generated very slowly or not generated using the addition of each of six boron trifluoride−etherates (di-n-butyl ether, tert-butyl methyl ether, tetrahydrofuran, tetrahydropyran, dioxane, and monoglyme) to suspensions of sodium borohydride in the corresponding ether. However, diborane was generated rapidly and quantitatively by the addition of NaBH4 in triglyme (or tetraglyme) to the BF3 adduct of triglyme (or tetraglyme) at room temperature. No solid precipitation occurs during the reaction, making it convenient for large-scale applications. The pure solvent triglyme (or tetraglyme) can be easily recovered and recycled by either crystallizing or precipitating NaBF4 from the generation flask. New procedures for the generation of diborane were also developed by the reaction of NaBF4 with NaBH4 in triglyme (or tetraglyme) in the presence of Lewis acids such as AlCl3 and BCl3.