Alkali metal borohydrides

At this point (1943) we were visited by representatives of the Signal Corps. They had learned through our reports that we had a new chemical that might be used for the field generation of hydrogen. They felt that their present method, based on the reaction of alkali with ferrosilicon, was cumbersome and they were seeking a more convenient hydrogen source. 

We pointed out that sodium borohydride on a weight basis should be far more efficient than sodium hydroxide and ferrosilicon. Although we had never used it for hydrogen generation, we had no doubt that it would be similar to diborane in reacting readily with water to liberate hydrogen. They asked for a demonstration. 

I placed a weighed quantity of sodium borohydride in a flask and attached it to a dropping funnel which contained water. The outlet from the flask was connected to a gas meter. The entire assembly was placed behind an explosion screen, since I did not know how violent the reaction might be. 

With Professor Schlesinger and all of the representatives watching me from a safe distance, I cautiously reached behind a screen and turned the stopcock to allow water to flow onto the borohydride. 

The borohydride dissolved and the solution sat there merely looking at me  

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Pyrazoles, and some indazoles, substituted on the nitrogen by B, Al, Ga, In, Si, Ge, Sn, P and Hg are known. Poly(pyrazol-1 -yl)borates have been studied by Trofimenko (72CRV497) who found that they were excellent ligands (Section 4.04.2.1.3(vi)). The parent ligands (282), (283) and (284) are available by the reaction of an alkali metal borohydride with pyrazole, the extent of substitution depending on the reaction temperature (Scheme 22). 

Numerous organic syntheses are based on stoichiometric oxidations of hydrocarbons with sodium dichromate and potassium permanganate, or on hydrogenations with alkali metals, borohydrides or metallic zinc. In addition, there are reactions... 

H. C. Brown, and E. R. Hyde The preparation of other Borohydrides by Metathetical Reactions Utilizing the Alkali Metal Borohydrides. 

During the time of the Olin reports, the first examples of oligomeric boron-bridged (l-pyrazolyl)borate systems appeared from the laboratory of Trofimenko at DuPont Chemicals 24 He reported the synthesis of poly(l-pyrazolyl)borates (6) (Fig. 5) from the reactions of alkali metal borohydrides with the pyrazole ligand. The (l-pyrazolyl)borate ligand was obtained from two pyrazole units when bridged by a BR2 unit on one side and by a metal or onium ion on the other. Even though reports... 

Alkali metal borohydrides are frequently used for the reduction of rc-electron-deficient heteroaromatic systems, but reduction of jt-electron-excessive arenes is generally possible only after protonation of the systems [e.g. 35-37]. The use of tetra-n-butylammonium borohydride under neutral conditions for the conversion of alkylindoles into indolines [38] is therefore somewhat unusual. Reduction of indoles by diborane under strongly alkaline conditions involves the initial interaction of the indolyl anion with the diborane to form an amino-borane which, under the basic conditions, reacts with a second molecule of diborane to produce the indoline [39]. The reaction of tetra-n-butylammonium borohydride with indoles could also proceed via the intermediate formation of diborane. 

The alkali metal borohydrides reduce aminochromes in solution very rapidly 70,109,120,147,148,1B1,1B5,157 the expected 5,6-dihydroxy-indole derivative was usually obtained in high yield.161,166 Reduction of the 7-iodoaminochromes was not usually accompanied by deio-dination to any appreciable extent.109,166 A number of relatively minor, unidentified, often fluorescent, water-soluble by-products were usually also detected in the reduction mixtures.109,148,155... 

Members of this ring system (194) were synthesized by reduction of the 2-acyloxyquinazolines 192 with alkali metal borohydrides followed by dehydrative cyclization of the resultant hydroxyalkyl derivatives (193) [74JIC453 85IJC(B) 1035]. 

The only new compound produced in these studies was LiAlF4 however, this work is significant because control was achieved even with the highly exothermic reactions of lithium aluminum hydride, alkali metal borohydrides, and alkali metal hexahydroaluminated with elemental fluorine. The reactions of alkali metal amides with fluorine have also been successfully controlled. These reactions provide extremely clean routes to the fluoride analogues. 

The Zr and Hf tetrahydroborates M(BH4)4 are the most volatile compounds of these elements, with boiling points of 123 and 118°C they are also very sensitive to oxidation and hydrolysis. They can be obtained by the reaction of the chlorides with alkali metal borohydrides or of the complex fluorides NaMFs with aluminum borohydride (the fluorides MF4 do not react), followed by distillation from the reaction mixture. According to low-temperature X-ray data, all the borohydride groups are tridentate in both M(BH4)4 complexes, which have Td symmetry. The same is trae for their substituted M(BH3Me)4 derivatives, that is, the metal atom is 12-coordinate. Proton NMR indicates fast exchange of bridging and terminal protons on the NMR timescale in these fluxional molecules see Fluxional Molecule). 

Considering the decomposition of alkali metal borohydrides to form metal hydrides, the reaction can be described as follows (Eq. (5.2)) ... 

The first report of pure alkali metal borohydride appeared in 1940 by Schlesinger and Brown ° who synthesized LiBH4 by the reaction of ethyllithium with B2H6. The direct reaction of the corresponding metal/metal hydrides with diborane in etheral solvents under suitable conditions produces high yields of the borohydrides " " ... 

Suitable reducing agents are alkali metal borohydrides and alumino-hydrides, hydrazine (44), sodium dithionite, sodium hypophosphite (76a,... 

The alkali metal borohydrides contain more hydrogen than the alanates, e.g., 18.5 and 10.6 wt.% for the respective lithium (LiBH4) and sodium (NaBH4) analogues, but are more stable and therefore less useful as accessible hydrogen stores. For example, LiBFl4 starts to decompose only above 300 °C, whereas NaBFl4 does not decompose until 350-400 °C. For borohydrides, the main issues to be addressed are as follows. 

The first one consisted of a four-step reaction sequence [35] formation of the Schiff base (84) by condensation of the 7a-amino derivative (83) with chloral 1,4-dehydrochlorination with Hunig base affording the imine (85) borohydride reduction giving (86) and subsequent acid hydrolysis to 7p-amino-l-oxacephem (87). In this epimerization, the three-step conversion from (84) to (87) was performed as a one-pot reaction in an overall yield of 93%. Also, it should be emphasized that the alkali metal borohydride reduction of (85) proceeded in a highly stereoselective manner, with no trace of the 7a-amino isomer in the products. In contrast, the unsubstituted imino analog (88), prepared in situ by either A-chlorination of (83) or treatment of 7p-amino-7a-methoxy-l-oxacephem (41) with methanolic hydrogen chloride gave a 1 1 mixture of 7a-... 

Gupta, M.K. Trans-forming shortenings. Backing Snacks 61-66 (November 2003). Helebra, S.F., Mikulski, R.A. and Cook, M.M. Purification of natural oils with alkali metal borohydrides. European Patent No. 116,408 (1984). 

Secondary Amines.—The reduction of imines to the corresponding secondary amines can be effected by various methodologies. New additions are the sodium triacyloxyborohydrides (easily obtainable from sodium borohydride and AT-acyl derivatives of optically active amino-acids), which are used for the asymmetric reduction of cyclic imines. Also now available is a highly stereoselective reduction of N-benzylimines derived from substituted cyclohexanones, with alkali-metal borohydrides, in particular L-selectride. A fiuther addition is the first report of the reduction of aldimines by hydrogen transfer from propan-2-ol,... 

It is also possible to produce gases for blowing processes by reacting together substances such as alkali metal borohydrides and hydroxy organic compounds [56] or dried hydrofluoric acid and vinyl chloride [57]. 

Effluents from the manufacture of Pb(CH3)4 can be treated with an alkali metal borohydride, e.g., NaBH4 at pH 8 to 11 to substantially reduce the level of dissolved lead compounds, like [Pb(CH3)3][145, 146]. Zn can also be used [147]. Liquid NH3 and toluene are used to remove solid NH4Clfrom the apparatus for producing Pb(CH3)4 by the NH3- or amine-catalyzed reaction of CH3CI with a PbNa alloy [148]. Stabilization of Pb(CH3)4, and of antiknock fluids containing Pb(CH3)4, is accomplished by addition of compounds, like toluene [149], xylene [141, 150], styrenes [151], naphthalenes [149, 151, 152], anthracenes [152], substituted phenols [141, 152 to 155], olefinic hydrocarbons [152], alcohols [141, 152], amines [155], hydroquinones [156], ethers [141], saturated or unsaturated carboxylic acids [141, 152], esters of phosphoric acid [152], or of sulfuric acid [157], or of sulfonic acids [141], imidazoles [158], alkyl halides and alkyl thiocyanates [141], or tall oil [159] see also Organolead Compounds , Vol. 2, Section 1.1.1.2, to be published. 

The reaction of complexes containing anionic ligands with alkali metal borohydrides can result in the formation of complexes with coordinated borohydride. 2,43 There are three main classes of metal-borohydride complexes. Figure 10.2.-43/44... 

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