Tetraethylammonium borohydride

Selective reductions. Raber et al.1 have compared the ability of tetra-n-butylammonium borohydride, tetraethylammonium borohydride, and sodium borohydride to effect selective reduction of aldehydes in the presence of ketones, and conclude that no one of these reagents is generally effective for this purpose and that the three reagents are generally similar. 

Tetraethylammonium borohydride reduces diphenyl diselenide to phenylselenolate anion. When an acyl halide is added to the toluene solution of phenylselenolate anion, the corresponding acylation product is isolated in good yield (equation 4). Resin-supported phenyl selenide anion underwent acylation with acetyl chloride under mild reaction conditions. ... 

Tetraethylammonium borohydride (C2Hs)4NBH4,145.10 Cetyltrimethylammonium borohydride, n-C,eHn3(CH3)3NBH4,299.39 Tricaprylmethylammonium borohydride, (n-C3Hi7) ,CH ,NBH4, 383.54... 

We have investigated the solvolytic stability and reactivity of polymer-bound borohydrides and have evaluated these materials in several applications such as solvent purification, arsine generation, and metal reduction. These polymer-bound borohydrides offer several advantages over sodium or tetraethylammonium borohydride. The primary advantages are the convenience of use and the minimal introduction of ionic species or organic by-products into the treated bulk media. With the polymer-bound borohydrides, the cation is bonded covalently to the insoluble resin while the borohydride anion or its oxidation product (borate) is retained by ionic bonding. Typically, boron at levels of less than 5 ppm is the only impurity introduced into the treated medium. 

We evaluated both gel and macroreticular types of styrene-divinyl-benzene (DVB) and acrylate-DVB strong base anion-exchange resins, all having quaternary ammonium groups attached to the polymer backbone. We used commercially available resins, specifically those of Rohm and Haas Amberlyst A-26, Amberlite IRA-400, Amberlyst XE-279, and Amberlite IRA-458 (all in the chloride form). The A-26 and IRA-400 resins contain styrene-DVB skeletal structures, with IRA-400 being a gel-type resin and A-26 the macroreticular resin. Resins IRA-458 and XE-279 contain acrylate-DVB skeletal structures, where IRA-458 is a gel-type resin and XE-279 a macroreticular resin. These studies compare the properties of the borohydride form of these resins with sodium and tetraethylammonium borohydride. 

For each solvent, aldehyde and hydroperoxide reductions via the A-26, IRA-400, XE-279, and IRA-458 borohydride-form resins were investigated. Amorphous sodium borohydride and tetraethylammonium borohydride were used for comparison. Studies were run at both ambient temperature and at 45°C. Percent hydride of each resin was determined immediately prior to use. 

Aldehyde Reduction. The results of 2-ethylhexanal reduction in 95% ethanol, 2-ethylhexanol, and hexane with the various types of polymer-bound borohydrides, sodium borohydride, and tetraethylammonium borohydride are shown in Table I. 

However, in the 2-ethylhexanol, the sodium and tetraethylammonium borohydrides are, in general, more reactive than the polymer-bound boro-hydrides. Of the resins, those polymers having the styrene-DVB skeletal backbone (A-26 and IRA-400) offer some advantage over the acrylate-DVB-based resin (XE-279 and IRA-458). Of the styrene-DVB resins, the macroreticular resin (A-26) gives the best reduction, particularly at an elevated temperature. Both sodium borohydride and tetraethylammonium borohydride appeared to have limited solubility at this level in 2-ethylhexanol. However, since 2-ethylhexanol is more viscous and less polar than ethanol, reactions involving the porous ion-exchange resins would be somewhat slower than the corresponding reduction with soluble borohydrides. 

In 95% ethanol, sodium borohydride and the acrylate-DVB-based polymer-bound borohydrides (XE-279 and IRA-458) were not as reactive as tetraethylammonium borohydride or the styrene-DVB-based polymeric borohydrides (A-26 and IRA-400). In the 2-ethylhexanol and hexane evaluation, the macroreticular styrene-DVB polymer-bound borohydride (A-26) was, in general, more reactive than the other resins or sodium or tetraethylammonium borohydrides. 

Solvolytic Stability. Solvolytic stability of all four polymer-bound borohydrides was investigated in 100% ethanol as a function of temperature and in water as a function of pH. Comparative studies for sodium borohydride and tetraethylammonium borohydride were conducted. Solvolytic decomposition of the borohydride group results in the generation of hydrogen gas. Stability measurements were obtained by observing volume of hydrogen gas evolved as a function of time. Percent loss of hydride as a function of time was calculated from resin weight and initial hydride content. 

Much emphasis has been placed on the selectivity of quaternary ammonium borohydrides in their reduction of aldehydes and ketones [18-20]. Predictably, steric factors are important, as are mesomeric electronic effects in the case of 4-substituted benzaldehydes. However, comparison of the relative merits of the use of tetraethyl-ammonium, or tetra-n-butylammonium borohydride in dichloromethane, and of sodium borohydride in isopropanol, has shown that, in the competitive reduction of benzaldehyde and acetophenone, each system preferentially reduces the aldehyde and that the ratio of benzyl alcohol to 1-phenylethanol is invariably ca. 4 1 [18-20], Thus, the only advantage in the use of the ammonium salts would appear to facilitate the use of non-hydroxylic solvents. In all reductions, the use of the more lipophilic tetra-n-butylammonium salt is to be preferred and the only advantage in using the tetraethylammonium salt is its ready removal from the reaction mixture by dissolution in water. 

The reactive anionic hydridometalcarbonyl complexes can be preformed from the neutral metal carbonyls using quaternary ammonium borohydrides either under homogeneous conditions or two-phase catalytic conditions [5] and are used in a range of reductive processes. The preparation of tetraethylammonium hydridotri-iron undecylcarbonyl is used as an illustrative example. 

Bis[tetraethylammonium] Tetrakis(benzenetellurolate] ferrate(II)3 Under an atmosphere of nitrogen, 1.83 g (4.0 mmol) bis[tetraethylammonium] tetrachloroferratc(II) dissolved in 25 ml acetone are mixed with a solution of 16 mmol sodium benzenetellurolate prepared from 3.27 g (8.0 mmol) diphenyl ditellurium and 0.61 g (16 mmol) sodium borohydride, in 10 m/ ethanol. The immediately formed red precipitate is filtered and dried under a vacuum yield 100%. 

Tris[tetraethylammonium] Tetrakis [benzenetellurolate(tellurido) ferrate]3 Under an atmosphere of nitrogen, 4.54 g (4 mmol) of bis[tetraethylammonium] tetrakis[benzenetellurolato]ferrate(II) dissolved in 40 m/ acetonitrile are dropped into a solution of sodium hydrogen telluride in 30 ml acetonitrile. (The sodium hydrogen telluride was prepared from 0.51 g (4.0 mmol) tellurium powder and a three-fold excess of sodium borohydride in ethanol, evaporation of the ethanol, and dissolution of the residue in 30 ml acetonitrile.) The... 

The reaction of aluminium borohydride with tetraethylammonium chloride or bromide gives crystalline compounds (Et4N)[Al(BH4)3X] (X = Cl or Br). These are readily soluble irj ben2ene, and could be characterized by X-reLy powder diffraction and i.r. spectroscopy. These confirmed the presence of the anion shown, rather than a mixture of [A1(BH4)4] and AIX. When X = Cl, i-CAlCl) was seen at 495 cm. ... 

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