Sodium borohydride-metal salts

Hydrazine—borane compounds are made by the reaction of sodium borohydride and a hydrazine salt in THF (23,24). The mono-(N2H4 BH ) and di-(N2H4 2BH2) adducts are obtained, depending on the reaction conditions. These compounds have been suggested as rocket fuels (25) and for chemical deposition of nickel—boron alloys on nonmetallic surfaces (see Metallic COATINGS) (26). 

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. 

Despite the previous comments there are dangerous forms of this metal. Thus, the Ru-Zn alloy, when treated by hydrochloric acid leads to zinc dissolving into a very porous ruthenium, which detonates in air spontaneousiy. The same goes for ruthenium, which is obtained by reduction of its salts by sodium borohydride. It is recommended to reduce ruthenium salts using hydrazine, which is reputed to be not dangerous . However, with ruthenium trichioride this reaction seems to be not dangerous only when hydrazine has a very low molar ratio (0.9 mol per cent). If it is not the case, a huge volume of gas could constitute an important pressure risk. 

Toxic pollutants found in the mercury cell wastewater stream include mercury and some heavy metals like chromium and others stated in Table 22.8, some of them are corrosion products of reactions between chlorine and the plant materials of construction. Virtually, most of these pollutants are generally removed by sulfide precipitation followed by settling or filtration. Prior to treatment, sodium hydrosulfide is used to precipitate mercury sulfide, which is removed through filtration process in the wastewater stream. The tail gas scrubber water is often recycled as brine make-up water. Reduction, adsorption on activated carbon, ion exchange, and some chemical treatments are some of the processes employed in the treatment of wastewater in this cell. Sodium salts such as sodium bisulfite, sodium hydrosulfite, sodium sulfide, and sodium borohydride are also employed in the treatment of the wastewater in this cell28 (Figure 22.5). 

Colloidal metals are usually prepared by reduction of a salt with a reducing agent, such as phosphorus, acetone, tannin, or carbon monoxide. Platinum metals can also be prepared as finely divided very active blacks by reducing the metal salt in an aqueous solution of sodium or potassium borohydride. 

Hydrogen will not reduce ketones or imines using CATHy or related catalysts. Inorganic hydrogen donors that have been used include dithionite and di-hydrogenphosphite salts, metal hydrides such as sodium borohydride, and sodium cyanoborohydride. Recently, amines have been shown to function as hydrogen donors with some catalysts. The enzymic cofactor NADH can be used stoichiometrically, and the potential exists to use this catalytically [56]. 

Highly active nickel, platinum and palladium catalysts can also be prepared by reducing the metal salts with sodium borohydride. 

In another search for an alternative to Potier s modified Polonovski reaction of catharanthine A-oxide (45), it has now been found that anhy-drovinblastine (42) can be generated directly, in 77% yield, from a reaction of catharanthine and vindoline in 0.01 N acid, promoted by ionized ferric salts, followed by reduction with sodium borohydride (Scheme 30) (Wl). Remarkably, the cation radical 106 generated by Fe(III), in accord with other simple amine oxidations by Lindsay Smith and Mead (102), resulted in isoquinuclidine fragmentation and coupling to vindoline at 0°C, without the conformational inversion observed in the modified Polonovski reaction at that temperature (see Scheme 15). Other metal oxidants or ligand-bound Fe(lll) did not promote the coupling reaction. It will be of interest to see if the overwhelming competition of C-5-C-6 bond... 

It is quite difficult to reduce benzene or pyridine, because these are aromatic stmctures. However, partial reduction of the pyridine ring is possible by using complex metal hydrides on pyridinium salts. Hydride transfer from lithium aluminium hydride gives the 1,2-dihydro derivative, as predictable from the above comments. Sodium borohydride under aqueous conditions achieves a double reduction, giving the 1,2,5,6-tetrahydro derivative, because protonation through the unsaturated system is possible. The final reduction step requires catalytic hydrogenation (see Section 9.4.3). The reduction of pyridinium salts is of considerable biological importance (see Box 11.2). 

Complex aluminum and boron hydrides can contain other cations. The following compounds are prepared by metathetical reactions of lithium aluminum hydride or sodium borohydride with the appropriate salts of other metals sodium aluminum hydride [55], magnesium aluminum hydride [59], lithium borohydride [90], potassium borohydride [9i], calcium borohydride [92] and tetrabutylammonium borohydride [95]. 

In aqueous solutions, calcium chloride undergoes double decomposition reactions with a number of soluble salts of other metals to form precipitates of insoluble calcium salts. For example, mixing solutions of calcium chloride with sodium carbonate, sodium tungstate and sodium molybdate solutions precipitates the carbonates, tungstates, and molybdates of calcium, respectively. Similar precipitation reactions occur with carboxylic acids or their soluble salt solutions. CaCb forms calcium sulfide when H2S is passed through its solution. Reaction with sodium borohydride produces calcium borohydride, Ca(BH4)2. It forms several complexes with ammonia. The products may have compositions CaCl2 2NH3, CaCb dNHs, and CaCb SNHs. 

Reduction of ketones. Reduction of ketones with metals in an alcohol is one of the earliest methods for effecting reduction of ketones, and is still useful since it can proceed with stereoselectivity opposite to that obtained with metal hydrides.1 An example is the reduction of the 3a-hydroxy-7-ketocholanic acid 1 to the diols 2 and 3. The former, ursodesoxycholic acid, a rare bile acid found in bear bile, is used in medicine for dissolution of gallstones. The stereochemistry is strongly dependent on the nature of the reducing agent (equation I).2 Sodium dithionite and sodium borohydride reductions result mainly in the 7a-alcohol, whereas reductions with sodium or potassium in an alcohol favor reduction to the 7p-alcohol. More recently3 reduction of 1 to 2 and 3 in the ratio 96 4 has been achieved with K, Rb, and Cs in f-amyl alcohol. Almost the same stereoselectivity can be obtained by addition of potassium, rubidium, or cesium salts to reductions of sodium in t-amyl alcohol. This cation effect has not been observed previously. 

Highly active platinum, palladium, and nickel catalysts also can be obtained by reduction of metal salts with sodium borohydride (NaBH4). 

Methods (i) and (ii) require palladium(II) salts as reactants. Either palladium acetate, palladium chloride or lithium tetrachloropalladate(II) usually are used. These salts may also be used as catalysts in method (iii) but need to be reduced in situ to become active. The reduction usually occurs spontaneously in reactions carried out at 100 °C but may be slow or inefficient at lower temperatures. In these cases, zero valent complexes such as bis(dibenzylideneacetone)palladium(0) or tetrakis(triphenylphos-phine)palladium(O) may be used, or a reducing agent such as sodium borohydride, formic acid or hydrazine may be added to reaction mixtures containing palladium(II) salts to initiate the reactions. Triarylphosphines are usually added to the palladium catalysts in method (iii), but not in methods (i) or (ii). Normally, 2 equiv. of triphenylphosphine, or better, tri-o-tolylphosphine, are added per mol of the palladium compound. Larger amounts may be necessary in reactions where palladium metal tends to precipitate prematurely from the reaction mixtures. Large concentrations of phosphines are to be avoided, however, since they usually inhibit the reactions. 

A systematic study of the reduction of thiazolium salts by complex metal hydrides has been promised in a preliminary communication on the mechanism of the reduction with sodium borohydride. The formation of 3-benzyl-4-methylthiazolidine from the reaction of 3-benzyl-4-methylthiazolium bromide and sodium borohydride was shown to occur in a manner similar to the reduction of pyridinium ions.158... 

Satoh, T., Suzuki, S., Suzuki, Y., Miyaji, Y., and Imai, Z., Reduction of organic compound with sodium borohydride-transition metal salt systems reduction of organic nitrile, nitro and amide compounds to primary amines, Tetrahedron Lett., 10, 4555 4558, 1969. 

Treatment of potassium salts 19 and 20 with sodium borohydride, followed by deacetylation, produced the diselenides 23 and 24 which, on treatment with metallic potassium in methanol, underwent cleavage of the Se-Se bond with formation of potassium salts 25 and 26. Products 19 and 20 are excellent precursors in the synthesis of symmetrical and unsymmetrical sugar selenides 27-32, as reported by Wagner and Nuhn.25... 

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