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NaBH3CN was made by stirring equimolar BH3·THF (~1 M) with NaCN in THF in excellent yield. Alternatively, NaBH4 and BF3·THF were reacted in THF to generate BH3·THF in situ, followed by further reaction with NaCN in the similar manner. We found that the reaction was sluggish when BF3·OEt2 was used instead. NaBH3CN, which has been prepared from NaBH4 and HCN in THF,4 often discolors after several months of storage under ambient conditions. The present synthetic route does not involve the use of HCN, and the product has much better storage stability. No discoloration was observed and the decomposition was less than 5% when a sample was stored in a capped bottle for 7 months. The properties of NaBH3CN have been well documented.2,3
Using Borane and Sodium Cyanide1
Two moles of BH3·THF (as a 1.0M solution) was added dropwise in 1.5 h to NaCN (101.2 g, 2.06 mol) in THF (300 mL) under a nitrogen atmosphere. The reaction mixture was poststirred for 4 h and then heated under reflux for 7h. When the mixture was cooled to room temperature, the small amount of unreacted NaCN was separated by filtration under a nitrogen atmosphere. Evaporation of the filtrate to dryness under reduced pressure at 60°C gave 114 g (91%) of NaBH3CN.
Using Borane, Sodium Borohydride and Sodium Cyanide 1
Alternatively, BF3·THF (2.18 mol) was added to NaBH4 (65.7g, 1.74 mol) suspended in THF (810 mL) cooled in an ice bath under a nitrogen atmosphere. After a postaddition stirring period of 1.5 h, NaCN (112.7 g, 2.30 mol) was added slowly in small batches. When addition was complete, the reaction mixture was stirred for 4 h at room temperature and then heated under reflux for 7 h. After cooling and filtration under nitrogen, the filtrate was evaporated and the residue dried in a vacuum oven to obtain 116.6 g (85%) of NaBH3CN.
Using Sodium Borohydride and Hydrogen Cyanide2
To a rapidly stirred slurry of sodium borohydride (80.2g, 2.09 mol) in THF (1000mL) in a 2-L flask is added a solution of hydrogen cyanide in THF (294g containing 58.8g of hydrogen cyanide) at 25°C. Evolution of hydrogen occurs slowly during the addition. Following the addition, the reaction mixture is stirred for 1 h at 25°C and then heated at reflux until hydrogen evolution has ceased. Filtration followed by vacuum removal of THF gives white solid sodium cyanoborohydride; yield 120g (91%).
Purification of Sodium Cyanoborohydride2
Sodium cyanoborohydride (10g) is dissolved in THF (80mL) and 1 M methanolic hydrochloric acid is added until pH reaches 9. The solution is then poured with stirring into dioxan (250mL). The precipitate is collected and stirred for 2 h in ethyl acetate (250mL). This solution is filtered, heated to reflux on a steam bath and then dioxan (150mL) is added slowly with swirling. This solution is slowly cooled to room temperature, chilled and filtered. The crystalline dioxan complex is dried in vacuo for 4 h at room temperature, then for 4 h at 80° C; yield 6.74g, purity >98% sodium cyanoborohydride by iodometric titration.