Alcohols borohydride

PHYSICAL ORGANIC CHEMISTRY NOMENCLATURE ALDEHYDE DEHYDROGENASE ALDEHYDE HYDRATION ALDEHYDE OXIDASE ALDEHYDE OXIDOREDUCTASE ALDOSE REDUCTASE Aldehyde reduction to alcohols, BOROHYDRIDE REDUCTION ALDOLASE Aldolase reduction,... 

Ketone reduction to alcohols, BOROHYDRIDE REDUCTION KETONIZATION... 

Methanol is the most electro-reactive organic fuel, and, when it is electro-oxidized directly at the fuel anode (instead of to be transformed by steam reforming in a hydrogen-rich gas), the fuel cell is called a DMFC. More generally if the direct oxidation of a given fuel (alcohols, borohydrides, etc.)... 

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... 

Potassium and sodium borohydride show greater selectivity in action than lithium aluminium hydride thus ketones or aldehydes may be reduced to alcohols whilst the cyano, nitro, amido and carbalkoxy groups remain unaffected. Furthermore, the reagent may be used in aqueous or aqueous-alcoholic solution. One simple application of its use will be described, viz., the reduction of m-nitrobenzaldehyde to m-nitrobenzyl alcohol ... 

Direct Borohydride Reduction of Alcohols to Alkanes with Phosphonium Anhydride Activation N-Proovlbenzene. To a solution of 5.56 g (20 mmol) of triphenylphosphine oxide in 30mL of dry methylene chloride at CfC was added dropwise a solution of 1.57 mL (10 mmol) of triflic anhydride in 30mL of dry methylene chloride. After 15 min when the precipitate appeared, a solution of 1.36g (10 mmol) of 3-phenyl-1-propanol in 10 mL of dry methylene chloride was added and the precipitate vanished in 5 min. An amount of 1.5g (40 mmol) of sodium borohydride was added as a solid all at once and the slurry was stirred at room temperature for... 

Sodium borohydride is especially easy to use needing only to be added to an aque ous or alcoholic solution of an aldehyde or a ketone... 

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... 

Unlike many other borohydrides, lithium borohydride is highly soluble ia ethers including aUphatic ethers, THF, an d polyglycol ethers. It is also very soluble ia amines and ammonia. Dissolution ia water and lower aUphatic alcohols leads to extensive decomposition and hydrogen evolution. 

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). 

Sodium Borohydride. Sodium borohydride [16940-66-2] is a thermally stable, white crystalline soHd that decomposes in vacuo above 400°C. The heat of formation is —192 kJ/mol (—45.9 kcal/mol). NaBH is hygroscopic and absorbs water rapidly from moist air to form a dihydrate that decomposes slowly to sodium metaborate and hydrogen. It is soluble in many solvents including water, alcohols, Hquid ammonia and amines, glycol ethers, and dimethyl sulfoxide. 

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. 

Depending on experimental conditions, sodium borohydride reduction of anthraquinone, in a lower ahphatic alcohol, results in 9,10-dihydroxyanthracene... 

Lactitol (4-0-p -D-galactopyranosyl-D-glucitol) is obtained by sodium borohydride reduction (99,100) or catalytic hydrogenation (101) of lactose. Potentially large quantities of this sugar alcohol are available from lactose obtained from whey. 

Industrial Synthetic Improvements. One significant modification of the Stembach process is the result of work by Sumitomo chemists in 1975, in which the optical resolution—reduction sequence is replaced with a more efficient asymmetric conversion of the meso-cyc. 02Lcid (13) to the optically pure i7-lactone (17) (Fig. 3) (25). The cycloacid is reacted with the optically active dihydroxyamine [2964-48-9] (23) to quantitatively yield the chiral imide [85317-83-5] (24). Diastereoselective reduction of the pro-R-carbonyl using sodium borohydride affords the optically pure hydroxyamide [85317-84-6] (25) after recrystaUization. Acid hydrolysis of the amide then yields the desired i7-lactone (17). A similar approach uses chiral alcohols to form diastereomic half-esters stereoselectivity. These are reduced and direedy converted to i7-lactone (26). In both approaches, the desired diastereomeric half-amide or half-ester is formed in excess, thus avoiding the cosdy resolution step required in the Stembach synthesis. 

Greater selectivity in purification can often be achieved by making use of differences in chemical properties between the substance to be purified and the contaminants. Unwanted metal ions may be removed by precipitation in the presence of a collector (see p. 54). Sodium borohydride and other metal hydrides transform organic peroxides and carbonyl-containing impurities such as aldehydes and ketones in alcohols and ethers. Many classes of organic chemicals can be purified by conversion into suitable derivatives, followed by regeneration. This chapter describes relevant procedures. 

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). 

The hydride-donor class of reductants has not yet been successfully paired with enantioselective catalysts. However, a number of chiral reagents that are used in stoichiometric quantity can effect enantioselective reduction of acetophenone and other prochiral ketones. One class of reagents consists of derivatives of LiAlH4 in which some of die hydrides have been replaced by chiral ligands. Section C of Scheme 2.13 shows some examples where chiral diols or amino alcohols have been introduced. Another type of reagent represented in Scheme 2.13 is chiral trialkylborohydrides. Chiral boranes are quite readily available (see Section 4.9 in Part B) and easily converted to borohydrides. 

For example, cyclohexanone is reduced by sodium borohydride 23 times faster than cyclopentanone." The explanation for this difference lies in the relative torsional strain in the two systems. Converting an sp atom in a five-membered ring to sp increases the torsional strain because of the increase in the number of eclipsing interactions in the alcohol. A similar change in a six-membered ring leads to a completely staggered (chair) arrangement and reduces torsional strain. 

A complex of 9-BBN with MMA can be formed and compounded with sodium borohydride [92], Derivatives from the combination of 9-BBN with fatty acid or fatty alcohol give an initiator with improved stability [93], Stability appears to improve with increasing molecular weight, so oligomeric and polymeric analogs... 

In the early work on the synthesis of prostaglandins, zinc borohydride was used for the reduction of the 15-ketone function and a 1 1 mixture of epimeric 15(S)- and 15(/ )-alcohols was generally obtained. Subsequent studies led to reaction conditions for highly selective reduction to the desired 15(S)-alcohol. Some of the results are summarized in the following table. The most practical method is E which utilizes borane as the stoichiometric reductant and a chiral, enzyme-like catalyst which is shown. 

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