Sodium/ions borohydride

The reduction of the maleic anhydride adduct (303) with lithium aluminium hydride was previously reported to occur selectively to give the lactone (304). The lower selectivity now observed with sodium aluminium hydride (none at all with sodium borohydride) is interpreted as evidence for a complex (305) of the ester and anhydride carbonyl groups with a solvated lithium ion when lithium aluminium hydride is used, leading to selective reduction of the free carbonyl group.Sodium ions are considered not to form so stable a complex. 

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... 

Cationic rings are readily reduced by complex hydrides under relatively mild conditions. Thus isoxazolium salts with sodium borohydride give the 2,5-dihydro derivatives (217) in ethanol, but yield the 2,3-dihydro compound (218) in MeCN/H20 (74CPB70). Pyrazolyl anions are reduced by borohydride to pyrazolines and pyrazolidines. Thiazolyl ions are reduced to 1,2-dihydrothiazoles by lithium aluminum hydride and to tetrahydrothiazoles by sodium borohydride. The tetrahydro compound is probably formed via (219), which results from proton addition to the dihydro derivative (220) containing an enamine function. 1,3-Dithiolylium salts easily add hydride ion from sodium borohydride (Scheme 20) (80AHC(27)151). 

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. 

Macrocycles have been prepared by formation of macrocyclic imines as well as by using variations of the Williamson ether synthesis ". Typically, a diamine or dialdehyde is treated with its counterpart to yield the Schiff s base. The saturated macrocycle may then be obtained by simple reduction, using sodium borohydride, for example. The cyclization may be metal-ion templated. In the special case of the all-nitrogen macrd-cycle, 15, the condensation of diamine with glyoxal shown in Eq. (4.14), was unsuccess-ful ... 

In analogy with the peracid attack on steroidal double bonds, the formation of the bromonium ion, e.g., (81a), occurs from the less hindered side (usually the a-side of the steroid nucleus) to give in the case of the olefin (81) the 9a-bromo-l l -ol (82). Base treatment of (82) provides the 9 5,1 l S-oxide (83). Similarly, reaction of 17/3-hydroxyestr-5(10)-en-3-one (9) with A -bromosuccinimide-perchloric acid followed by treatment with sodium hydroxide and sodium borohydride furnishes the 3, 17 5-dihydroxy-5a,l0a-oxirane. As mentioned previously, epoxidation of (9) with MPA gives the 5, 10 -oxirane. °... 

The azidohydrins obtained by azide ion opening of epoxides, except for those possessing a tertiary hydroxy group, can be readily converted to azido mesylates on treatment with pyridine/methanesulfonyl chloride. Reduction and subsequent aziridine formation results upon reaction with hydrazine/ Raney nickel, lithium aluminum hydride, or sodium borohydride/cobalt(II)... 

The reduction of the double bond of an enamine is normally carried out either by catalytic hydrogenation (MS) or by reduction with formic acid (see Section V.H) or sodium borohydride 146,147), both of which involve initial protonation to form the iminium ion followed by hydride addition. Lithium aluminum hydride reduces iminium salts (see Chapter 5), but it does not react with free enamines except when unusual enamines are involved 148). 

Two classes of aldolase enzymes are found in nature. Animal tissues produce a Class I aldolase, characterized by the formation of a covalent Schiff base intermediate between an active-site lysine and the carbonyl group of the substrate. Class I aldolases do not require a divalent metal ion (and thus are not inhibited by EDTA) but are inhibited by sodium borohydride, NaBH4, in the presence of substrate (see A Deeper Look, page 622). Class II aldolases are produced mainly in bacteria and fungi and are not inhibited by borohydride, but do contain an active-site metal (normally zinc, Zn ) and are inhibited by EDTA. Cyanobacteria and some other simple organisms possess both classes of aldolase. 

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 ... 

Bromo-6,7,8,9-tetrahydro-l//-3-benzazepin-2-amine(6) with thiocyanate ion undergoes substitution of bromide to give the thiocyanatotetrahydro-l//-3-benzazepine 7.105 Attempts to replace bromide by azide ion failed, as did diazotization of the amine group with sodium nitrite in 6 M sulfuric acid. Oddly, treatment of the aminobromo compound with sodium borohydride in methanol results not in reduction, but in methoxy-debromination to give the 2-methoxy derivative which, on the basis of HNMR spectral data, is best represented as the 2-imino tautomer 8. 

Thus, enantiomerically pure (S)35- or (R)36-acetoxysuccinimide derivatives of type 1, easily prepared from (S)- or (R)-malic acid, are diastereoselectively reduced with sodium borohydride in methanol at lower temperature to yield 85 % of an 11 1 mixture of diastereomeric hy-droxylactams of type 2, from which the enantiomerically pure chiral /V-acyliminium ions 3 are generated. 

Optically active (2R,3R)-dimethoxysuccinimide derivatives 4, prepared from (.R,./ -tartaric acid, arc reduced in excellent yield with high stereoselectivity by sodium borohydride in ethanol at 0- 5 °C to furnish a 20 1 mixture of diastereomeric hydroxylactams 543, which, on treatment with acid, give rise to the formation of the enantiomerically pure chiral /V-acylimini-um ions 6,... 

Reduction and preparation of derivatives Concentrate the aqueous mixture to 0.5 ml. Add 20 mg of sodium borohydride dissolved in 0.5 ml of ion exchange water. Let this solution stand at room temperature for 1 hour. Destroy the excess sodium borohydride by adding acetic acid until gas evolution stops. Evaporate the solution to dryness. Add 5 ml of methanol and evaporate again to dryness. 

The arenediimide anion is very likely to be involved in another type of dediazoniation, namely the reduction of arenediazonium ions by sodium borohydride (Scheme 8-54). The reaction was investigated by Musso s and Rtichardt s groups (Bloch et al., 1969 Rtichardt et al., 1970). 

As discussed earlier the whole process is a redox reaction. Selenium is reduced using sodium borohydride to give selenide ions. In the above reaction, the metal ion reacts with the polymer (PVP or PVA) solution to form the polymer-metal ion solution. Addition of the selenide ion solution to the polymer-metal ion solutions resulted in instantaneous change in the colour of the solutions from colourless to orange (PVA) and orange red (PVP). This indicates the formation of CdSe nanoparticles. The addition of the selenide solution to the polymer - metal ion solution resulted in gradual release of selenide ion (Se -) upon hydrolytic decomposition in alkaline media (equation 4). The released selenide ions then react with metal ion to form seed particles (nucleation). 

The nucleophiles that are used for synthetic purposes include water, alcohols, carboxylate ions, hydroperoxides, amines, and nitriles. After the addition step is complete, the mercury is usually reductively removed by sodium borohydride, the net result being the addition of hydrogen and the nucleophile to the alkene. The regio-selectivity is excellent and is in the same sense as is observed for proton-initiated additions.17... 

Alkylborohydrides are also used as reducing agents. These compounds have greater steric demands than the borohydride ion and therefore are more stereoselective in situations in which steric factors come into play.72 These compounds are prepared by reaction of trialkylboranes with lithium, sodium, or potassium hydride.73 Several of the compounds are available commercially under the trade name Selectrides .74... 

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