Metal hydrides sodium borohydride

In the general context of donor/acceptor formulation, the carbonyl derivatives (especially ketones) are utilized as electron acceptors in a wide variety of reactions such as additions with Grignard reagents, alkyl metals, enolates (aldol condensation), hydroxide (Cannizzaro reaction), alkoxides (Meerwein-Pondorff-Verley reduction), thiolates, phenolates, etc. reduction to alcohols with lithium aluminum hydride, sodium borohydride, trialkyltin hydrides, etc. and cyloadditions with electron-rich olefins (Paterno-Buchi reaction), acetylenes, and dienes.46... 

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

Metal hydrides Calcium hydride, lithium aluminum hydride, sodium borohydride... 

One Te-C bond in a diorgano tellurium can be cleaved by alkali metals, organic lithium compounds, sodium hydroxide, lithium aluminum hydride, sodium borohydride, Grignard reagents, tributyltin hydride, sulfuric acid, sodium sulfide, sulfuryl chloride, hydrogen bromide, bromine, or iodine. The Te-C bond can also be broken thermally or through photostimulation. 

Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. 

Hydrides commonly used in laboratories are lithium aluminum hydride, potassium hydride, sodium hydride, sodium borohydride, and calcium hydride. The following methods for their disposal demonstrate that the reactivity of metal hydrides varies considerably. Most hydrides can be decomposed safely by one of the four methods, but the properties of a given hydride must be well understood in order to select the most appropriate method. (CAUTION Most of the methods described below produce hydrogen gas, which can present an explosion hazard. The reaction shouid be carried out in a hood, behind a shield, and with proper safeguards to avoid exposure of the effluent gas to spark or flame. Any stirring device must be spark-proof.)... 

Carbonyl compounds are commonly reduced to alcohols by catalytic hydrogenation or with metal hydrides. When applied to aldehydes, reduction, represented by the symbol [H], provides a convenient route to primary alcohols (Eq. 17.15), whereas the reduction of ketones gives secondary alcohols (Eq. 17.16). Although catalytic hydrogenation of carbonyl groups is frequently the method of choice in industrial processes, lithium aluminum hydride, sodium borohydride, and their derivatives are generally used in the research laboratory. Sodium borohydride may be used in alcoholic and even aqueous solutions, because it reacts much more rapidly with the carbonyl group than with the solvent. On the other hand, lithium aluminum hydride reacts rapidly with protic solvents, so it must be used in anhi/-drous ethereal solvents such as diethyl ether or tetrahydrofuran. 

The transfonnations of gm-dihalocyclopropanes are synthetically useful because the cyclopropanes are readily prepared by the addition of dihalocarbene to olefins. In most of dehalogenation reactions to monohalocyclopropanes, the reagents are limited to metallic reductants such as organotin hydride, lithium aluminum hydride, sodium borohydride, Grignard reagent, and zinc-copper couple [1-9]. A versatile method for the reduction of gm-dibromocyclopropanes 3 with an organic reductant is achieved by use of diethyl phosphonate (commercially named diethyl phosphite) and triethylamine to give the monobromocyclopropanes 4 (Scheme 2.2) [10]. 

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... 

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

Although a few simple hydrides were known before the twentieth century, the field of hydride chemistry did not become active until around the time of World War II. Commerce in hydrides began in 1937 when Metal Hydrides Inc. used calcium hydride [7789-78-8J, CaH2, to produce transition-metal powders. After World War II, lithium aluminum hydride [16853-85-3] LiAlH, and sodium borohydride [16940-66-2] NaBH, gained rapid acceptance in organic synthesis. Commercial appHcations of hydrides have continued to grow, such that hydrides have become important industrial chemicals manufactured and used on a large scale. 

AletalHydrides. Metal hydrides can sometimes be used to prepare amines by reduction of various functional groups, but they are seldom the preferred method. Most metal hydrides do not reduce nitro compounds at all (64), although aUphatic nitro compounds can be reduced to amines with lithium aluminum hydride. When aromatic amines are reduced with this reagent, a2o compounds are produced. Nitriles, on the other hand, can be reduced to amines with lithium aluminum hydride or sodium borohydride under certain conditions. Other functional groups which can be reduced to amines using metal hydrides include amides, oximes, isocyanates, isothiocyanates, and a2ides (64). 

Sodium borohydride is manufactured by Morton International, Inc. Treatment of trimethyl borate with a metal hydride, eg, NaH, ia the absence of a solvent yields sodium hydrotrimethoxyborate [16940-17-3] Na[HB(OCH2)3], (eq. 50) which disproportionates ia the presence of solvents such as tetrahydrofuran at 60—70°C (eq. 51) (112). 

Sodium Tetrahydroborate, Na[BH ]. This air-stable white powder, commonly referred to as sodium borohydride, is the most widely commercialized boron hydride material. It is used in a variety of industrial processes including bleaching of paper pulp and clays, preparation and purification of organic chemicals and pharmaceuticals, textile dye reduction, recovery of valuable metals, wastewater treatment, and production of dithionite compounds. Sodium borohydride is produced in the United States by Morton International, Inc., the Alfa Division of Johnson Matthey, Inc., and Covan Limited, with Morton International supplying about 75% of market. More than six million pounds of this material suppHed as powder, pellets, and aqueous solution, were produced in 1990. 

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. 

The reduction of iminium salts can be achieved by a variety of methods. Some of the methods have been studied primarily on quaternary salts of aromatic bases, but the results can be extrapolated to simple iminium salts in most cases. The reagents available for reduction of iminium salts are sodium amalgam (52), sodium hydrosulfite (5i), potassium borohydride (54,55), sodium borohydride (56,57), lithium aluminum hydride (5 ), formic acid (59-63), H, and platinum oxide (47). The scope and mechanism of reduction of nitrogen heterocycles with complex metal hydrides has been recently reviewed (5,64), and will be presented here only briefly. 

Tertiary heterocyclic enamines are reduced with metals in acidic media 142) or electrolytically (237,238) and their salts are reduced with lithium aluminum hydride or sodium borohydride (239,240) to the corresponding saturated amines. 

Sulphoxides are reduced by the more powerful metal hydride reducing agents14, but the less reactive reagents such as sodium borohydride are ineffective. Recently, Yoon25 has... 

Although catalytic hydrogenation is the method most often used, double bonds can be reduced by other reagents, as well. Among these are sodium in ethanol, sodium and rerr-butyl alcohol in HMPA, lithium and aliphatic amines (see also 15-14), " zinc and acids, sodium hypophosphate and Pd-C, (EtO)3SiH—Pd(OAc)2, trifluoroacetic acid and triethylsilane (EtsSiH), and hydroxylamine and ethyl acetate.However, metallic hydrides, such as lithium aluminum hydride and sodium borohydride, do not in general reduce carbon-carbon double bonds, although this can be done in special cases where the double bond is polar, as in 1,1-diarylethenes and in enamines. " °... 

War research forced us to explore new synthetic routes and we discovered the alkali metal hydride route to diborane. This solved the synthetic problem. At the same time we discovered sodium borohydride and developed simple synthetic methods for its preparation and manufacture. 

The reduction of a-hydroxynitriles to yield vicinal amino alcohols is conveniently accomplished with complex metal hydrides for example, lithium aluminum hydride or sodium borohydride [69]. However, it is still worth noting that a two-step chemo-enzymatic synthesis of (R)-2-amino-l-(2-furyl)ethanol for laboratory production was developed followed by successful up-scaling to kilogram scale using NaBH4/CF3COOH as reductant [70],... 

In an irreversible metal hydride, hydrogen is generated through a chemical reaction, which is not easily reversible onboard a vehicle. Common reactant is water or an alcohol. For example, sodium borohydride can produce hydrogen as... 

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. 

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