Sodium cyanoborohydride are frequently used for reductive aminations. Since the reaction rate for the reduction of iminium ions is much faster than for ketones or even aldehydes, the reductive amination can be carried out as a one-pot procedure by introducing the reducing agent into a mixture of the amine and carbonyl compound.
Contact with strong acids liberates the highly toxic gas HCN. A safer reducing agent with comparable reactivity is sodium triacetoxyborohydride. Reduction with Sodium Cyanoborohydride:
- Tin-free Giese reaction of alkyl iodides with electron-deficient alkenes and the related radical carbonylation process proceeded efficiently in the presence of sodium cyanoborohydride and tetrabutylammonium cyanoborohydride. Transfer of iodine followed by hydride reduction of the resulting carbon-iodine bond is proposed as a possible mechanism.
- Borch and co-workers showed that sodium cyanoborohydride and lithium cyanoborohydride are acid-stable reagents capable of rapidly reducing carbonyl compounds to alcohols at pH 3–4, presumably via a protonated carbonyl cation.
- With care to maintain a pH of 6–7, a mixture of a ketone or aldehyde reactant, an amine, and sodium cyanohydride provides products of reductive amination selectively, without competitive reduction of the carbonyl substrate. Though the conditions of the Borch reduction are mild, sodium cyanoborohydride is highly toxic, as are its byproducts. The pH was maintained by addition of HCl and/or KOH as needed using bromocresol green as an indicator.
It is avery hygroscopic solid, soluble in H2O (212% at 29o, 121% at 88o), tetrahydrofuran (37% at 28o, 42.2% at 62o), very soluble in MeOH, slightly soluble in EtOH but insoluble in Et2O, *C6H6 and hexane. It is stable to acid up to pH 3 but is hydrolysed in 12N HCl. The rate of hydrolysis at pH 3 is 10-8 times that of NaBH4. The fresh commercially available material is usually sufficiently pure. If very pure material is required, one of the following procedures can be used [Lane Synthesis 135 1975]: (a) The NaBH3CN is dissolved in tetrahydrofuran (20% w/v), filtered and the filtrate is treated with a fourfold volume of CH2Cl2. The solid is collected and dried in a vacuum [Wade et al. Inorg Chem. 9 2146 1970]. (b) NaBH3CN is dissolved in dry MeNO3, filtered, and the filtrate is poured into a 10-fold volume of CCl4 with vigorous stirring; the white precipitate is collected, washed several times with CCl4 and dried in a vacuum (yield 75%) [Berschied & Purcell Inorg Chem 9 624 1970]. (c) When the above procedures fail to give a clean product, then dissolve NaBH3CN (10g) in tetrahydrofuran (80mL) and add N MeOH/HCl until the pH is 9. Pour the solution with stirring into dioxane (250mL). The solution is filtered and heated to reflux. A further volume of dioxane (150mL) is added slowly with swirling. The solution is cooled slowly to room temperature, then chilled in ice and the crystalline dioxane complex is collected, dried in a vacuum for 4hours at 25o, then 4hours at 80o to yield the amorphous dioxane-free powder (6.7g) with purity >98% [Borch et al. J Am Chem Soc 93 2897 1971]. The purity can be checked by iodometric titration [Lyttle et al. Anal Chem 24 1843 1952].
Selective reducing agent for aldehydes, ketones, oximes, enamines; does not reduce amides, ethers, lactones, nitriles, nitro Compounds and epoxides. Also used for reductive amination of ketones and aldehydes, reductive alkylation of amines and hydrazines, reductive displacement of halides and tosylates, deoxygenation of aldehydes and ketones. See Lane, loc. cit.
- Sodium Cyanoborohydride is a commonly used as a reagent in the reductive amination of aldehydes and ketones and in the reductive alkylation of amines.
- Reagent for selective reductions.
- Used in the synthesis of a novel phenolate-bridged dilanthanum(III) complex of interest as a model for metalloproteins as well as for its importance in basic and applied chemistry.