Sodium borohydride as a reducing agent

The oxymercuration reaction of various 2-substituted 1,4-benzodioxin derivatives 35 in the presence of a suspension of mercuric acetate in water/THF followed by treatment in situ with sodium chloride and then with sodium borohydride as a reducing agent provided in excellent yields the expected hemiketals 36 (Scheme 2) <1997X2061 >. 

The basic principle of hydride generation is the conversion into volatile hydrides by use of the sodium borohydride as a reducing agent. The hydride can then be readily dissociated into atomic vapour by ICP-OES. 

Generation of Volatile Hydrides (11). The use of commercial sodium borohydride as a reducing agent for the generation of volatile arsine (AsH3) in trace arsenic analysis is often complicated by trace (ppb) arsenic impurities in the borohydride. The following procedure using polymer-bound borohydride has eliminated these problems ... 

Sodium Borohydride as a Reducing Agent for Sugar Lactones, M. L. Wolfrom and Hany B. Wood, J. Amer. Chem. Soc., 73, 2933-2934 (1951). 

Wolfrom ML, Anno K. Sodium borohydride as a reducing agent in the sugar series. J Am Chem Soc 1952 74 5583-5584. 

With silica as a support and sodium borohydride as a reducing agent, cyclohexanol has been prepared from cyclohexanone in less than 3 min using a scientific microwave unit (Scheme 6.9). The reduction of carbonyl compounds by sodium borohydride is traditionally time consuming. It can take 2-4 h to reach completion, this equating to a whole laboratory period, and usually another lab period is required for product isolation and characterization. Using the microwave procedure, the entire experiment can be completed in one 2.5 h session. The reaction was performed in closed tubes... 

Cross-linked quatemized CS membranes, fabricated by using GA as a cross-linker and sodium borohydride as a reducing agent, were the promising candidates for alkaline polyelectrolyte fuel cells [103]. The cross-linked CS/PVP membranes are prepared by Smitha et al. [18] for fuel cell applications. The manbrane showed good thermal stability... 

Fig. 9.27 SEM images of gold deposited onto Ni-B substrate using sodium borohydride as a reducing agent at 75 °C (Reproduced from Ref. [20] with permission from The Electrochemical Society)... 

Metal particles are most conveniently grown on the clay mineral surface by displacing the exchangeable cation by precursor transition-metal cations and by subsequent reduction. Au and Ag clusters were obtained in laponite clay mineral of synthetic origin by Aihara et al. [9] using this method with sodium borohydride as a reducing agent. Silver nanoparticles are widely used as photosensitive components [10], as catalysts [11], in photocatalysis [12], and in surface-enhanced Raman... 

S. Takahashi. Sodium borohydride as a reducing agent for preparing ninhydrin rea nt for amino acid analysis. /. Biochem. Tokyo), 1978, 83, 57. 

Only a few chiral catalysts based on metals other than rhodium and ruthenium have been reported. The titanocene complexes used by Buchwald et al. [109] for the highly enantioselective hydrogenation of enamines have aheady been mentioned in Section 3.4 (cf. Fig. 32). Cobalt semicorrin complexes have proven to be efficient catalysts for the enantioselective reduction of a,P-unsaturated carboxylic esters and amides using sodium borohydride as the reducing agent [ 156, 157]. Other chiral cobalt complexes have also been studied but with less success... 

The reaction is performed in two operations, the first of which is oxymercuration. In this stage the alkene is treated with mercury(II) acetate [Hg(02CCH3)2, abbreviated as Hg(OAc)2]. Mercury(II) acetate is a source of the electrophile HgOAc, which bonds to C-1 of the alkene. The oxygen of water, one of the components in the THF-H2O solvent mixture, bonds to C-2. The demercuration operation uses sodium borohydride (NaBH4, a reducing agent) to convert C—Hg to C—H. 

A rather similar method based on reductive alkylation of urea involves the use of trimethylsilyl chloride (TMSCl) as the catalyst in the condensation step and again utilizes sodium borohydride as the reducing agent [792]. Under these conditions, the parent urea can be converted into monosubstituted as well as disubstituted ureas in yields of 60-92%. 

Potassium borohydride is similar in properties and reactions to sodium borohydride, and can similarly be used as a reducing agent for removing aldehydes, ketones and organic peroxides. It is non-hygroscopic and can be used in water, ethanol, methanol or water-alcohol mixtures, provided some alkali is added to minimise decomposition, but it is somewhat less soluble than sodium borohydride in most solvents. For example, the solubility of potassium borohydride in water at 25° is 19g per lOOmL of water (as compared to sodium borohydride, 55g). 

Devaky and Rajasree have reported the production of a polymer-bound ethylenediamine-borane reagent (63) (Fig. 41) for use as a reducing agent for the reduction of aldehydes.87 The polymeric reagent was derived from a Merrifield resin and a 1,6-hexanediol diacrylate-cross-linked polystyrene resin (HDODA-PS). The borane reagent was incorporated in the polymer support by complexation with sodium borohydride. When this reducing agent was used in the competitive reduction of a 1 1 molar mixture of benzaldehyde and acetophenone, benzaldehyde was found to be selectively reduced to benzyl alcohol. 

Braman et al. [713] suggested the use of sodium borohydride (NaBH4) as a reducing agent to replace the metallic zinc used in the classical Marsh test, which is awkward to handle and often contains large blanks of the elements of interest. Sodium borohydride is now used almost exclusively in the various modifications of the hydride method. 

The preferential chemical reaction of a reagent with one of two or more different functional groups. For example, sodium borohydride exhibits greater chemoselectivity as a reducing agent than does lithium aluminum hydride, because the latter reacts with a wider spectrum of substances. 

Lithium borohydride is intermediate in activity as a reducing agent between lithium aluminium hydride and sodium borohydride. In addition to the reduction of aldehydes and ketones it will readily reduce esters to alcohols. It can be prepared in situ by the addition of an equivalent quantity of lithium chloride to a 1m solution of sodium borohydride in diglyme. Lithium borohydride should be handled with as much caution as lithium aluminum hydride. It may react rapidly and violently with water contact with skin and clothing should be avoided. 

The relatively inexpensive and safe sodium borohydride (NaBH4) has been extensively used as a reducing agent because of its compatibility with protic solvents. Varma and coworkers reported a method for the expeditious reduction of aldehydes and ketones that used alumina-supported NaBH4 and proceeded in the solid state accelerated by microwave irradiation (Scheme 7) [50]. The chemoselectivity was apparent from the reduction of frarcs-cinnamaldehyde to afford cinnamyl alcohol. 

Commonly employed reducing agents are sodium borohydride, sodium sulfide nonahy-drate, potassium sulfide, sodium or potassium hydrogen sulfite (Vol. IX, p. 1068), sodium disulfite, hydrazine hydrate, and thiourea dioxide. Lithium diethylamide1 has been found to reduce diaryl tellurium dichlorides. Methyl magnesium iodide is recommended in the older literature as a reducing agent (Vol. IX, p. 1069). 

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