Reducing agents borohydride

We shall use this synthesis as a basis for discussion on chemoselectivity in reductions. In the first step, sodium borohydride leaves the black carbonyl group of the ester untouched while it reduces the ketone (in yellow) in the last step, lithium aluminium hydride reduces the ester (in black). These chemoselectivities are typical of these two most commonly used reducing agents borohydride can usually be relied upon to reduce an aldehyde or a ketone in the presence of an ester, while lithium aluminium hydride will reduce almost any carbonyl group. 

The tetrahydridoborate ion, as "sodium borohydride" NaBH is soluble in water and is similarly an excellent reducing agent in this solvent. (Lithium tetrahydridoaluminate cannot be used in water, with which it reacts violently to give hydrogen.)... 

The alkali metal tetrahydridoborates are salts those of sodium and potassium are stable in aqueous solution, but yield hydrogen in the presence of a catalyst. They are excellent reducing agents, reducing for example ion(III) to iron(II). and silver ions to the metal their reducing power is used in organic chemistry, for example to reduce aldehydes to alcohols. They can undergo metathetic reactions to produce other borohydrides, for example... 

For most laboratory scale reductions of aldehydes and ketones catalytic hydro genation has been replaced by methods based on metal hydride reducing agents The two most common reagents are sodium borohydride and lithium aluminum hydride... 

Reduction to alcohols (Section 15 2) Aide hydes are reduced to primary alcohols and ketones are reduced to secondary alcohols by a variety of reducing agents Catalytic hydrogenation over a metal catalyst and reduction with sodium borohydride or lithium aluminum hydride are general methods... 

Lithium borohydride is a more powerful reducing agent than sodium borohydride, but not as powerful as lithium aluminum hydride (Table 6). In contrast to sodium borohydride, the lithium salt, ia general, reduces esters to the corresponding primary alcohol ia refluxing ethers. An equimolar mixture of sodium or potassium borohydride and a lithium haUde can also be used for this purpose (21,22). 

Unusual reducing properties can be obtained with borohydride derivatives formed in situ. A variety of reductions have been reported, including hydrogenolysis of carbonyls and alkylation of amines with sodium borohydride in carboxyHc acids such as acetic and trifluoroacetic (38), in which the acyloxyborohydride is the reducing agent. 

In general, the aluminohydrides are more active and powerful reducing agents than the corresponding borohydrides. They decompose vigorously with water. Reaction also occurs with alcohols, although more moderately, providing a route to substituted derivatives. 

With mercuric acetate (Hg(OOCCH2)2), olefins and / fZ-butyl hydroperoxide form organomercury-containing peroxides (66,100). The organomercury compound can be treated with bromine or a mild reducing agent, such as sodium borohydride, to remove the mercury. 

The reducing agents generally used in bleaching include sulfur dioxide, sulfurous acid, bisulfites, sulfites, hydrosulfites (dithionites), sodium sulfoxylate formaldehyde, and sodium borohydride. These materials are used mainly in pulp and textile bleaching (see Sulfur compounds Boron compounds). 

Ghromium(III) Compounds. Chromium (ITT) is the most stable and most important oxidation state of the element. The E° values (Table 2) show that both the oxidation of Cr(II) to Cr(III) and the reduction of Cr(VI) to Cr(III) are favored in acidic aqueous solutions. The preparation of trivalent chromium compounds from either state presents few difficulties and does not require special conditions. In basic solutions, the oxidation of Cr(II) to Cr(III) is still favored. However, the oxidation of Cr(III) to Cr(VI) by oxidants such as peroxides and hypohaUtes occurs with ease. The preparation of Cr(III) from Cr(VI) ia basic solutions requires the use of powerful reducing agents such as hydra2ine, hydrosulfite, and borohydrides, but Fe(II), thiosulfate, and sugars can be employed in acid solution. Cr(III) compounds having identical counterions but very different chemical and physical properties can be produced by controlling the conditions of synthesis. 

Deposition reactions for some reducing agents are given in Table 1 hydrogen is a principal by-product of each reduction. Elemental phosphoms or boron is codeposited with the reduced metal from hypophosphite, borohydride, or organoborane baths (15). Other minor reactions can also occur (18). All of these reductions can be viewed as dehydrogenation reactions (16,19). 

Most classical reducing agents leave the pyrimidine nucleus unaffected, as does sodium borohydride lithium aluminium hydride usually gives a di- or tetra-hydro derivative, according to substituent(s) present (70HC(16-S1)322). 

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). 

Semicarbazones are used as protecting groups as a consequence of their stability to reducing agents such as potassium borohydride, sodium boro-hydride and lithium borohydride. Semicarbazones are cleaved by strong acids and by heating in acetic anhydride-pyridine. " ... 

At no point is H2 involved. The reducing agent is borohydride ion (BFLi ). 

Fructose bisphosphate aldolase of animal muscle is a Class I aldolase, which forms a Schiff base or imme intermediate between the substrate (fructose-1,6-bisP or dihydroxyacetone-P) and a lysine amino group at the enzyme active site. The chemical evidence for this intermediate comes from studies with the aldolase and the reducing agent sodium borohydride, NaBH4. Incubation of fructose bisphosphate aldolase with dihydroxyacetone-P and NaBH4 inactivates the enzyme. Interestingly, no inactivation is observed if NaBH4 is added to the enzyme in the absence of substrate. 

Compare atomic charges for sodium borohydride and lithium aluminum hydride. Which ion contains the most electron-rich hydride The least electron-rich hydride Based on these results alone, which hydride reagent should be the better reducing agent Explain. Obtain atomic charges for free borohydride and aluminum hydride anions. What changes, if any, does the counterion produce ... 

In an alternative process, sodium borohydride was used as the reducing agent, as follows ... 

After evaporation of the solvent, the solid residue consists of 5-(2-chlorobenzyl)-thieno[3,2-cl -pyridinium chloride which melts at 166°C (derivative n°30). This compound is taken up into a solution comprising ethanol (300 ml) and water (100 ml). Sodium borohydride (NaBH4) (20 g) is added portionwise to the solution maintained at room temperature. The reaction medium is maintained under constant stirring during 1 2 hours and is then evaporated. The residue is taken up into water and made acidic with concentrated hydrochloric acid to destroy the excess reducing agent. The mixture is then made alkaline with ammonia and extracted with ether. The ether solution is washed with water, dried and evaporated. The oily residue is dissolved in isopropanol (50 ml) and hydrochloric acid in ethanol solution is then added thereto. 

Manx- different reducing agents are effective, but the most common choice in the laboratory is sodium cyanoborohydride, NaBH3CN. Sodium cyanoboro-hydride is similar in reactivity to sodium borohydride (NaBH4) but is more stable in weak acid solution. 

Although electrothermal atomisation methods can be applied to the determination of arsenic, antimony, and selenium, the alternative approach of hydride generation is often preferred. Compounds of the above three elements may be converted to their volatile hydrides by the use of sodium borohydride as reducing agent. The hydride can then be dissociated into an atomic vapour by the relatively moderate temperatures of an argon-hydrogen flame. 

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