Preparation of Sodium Cyanoborohydride

Method 1 [1]

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.

Method 2 [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.

Method 3 [2]

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 cyanoborohydride [2]

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.

[1] Inorg. Chem. 19, 3186-3189 (1980)
[2] Synthesis 135 (1975)