Ammonia hydride generators

Ammonia hydride generators are relatively simple systems [104]. Ammonia was fed into the lithium aluminium hydride reactor and hydrogen was generated until a pressure equal to the vapour pressure of ammonia had built up. The fuel cell connected to the system (not shown here) consumed the hydrogen, which decreased the pressure. This caused the ammonia to flow into the reactor again. An ammonia getter had to be placed downstream of the reactor to remove traces of unconverted ammonia. While the ammonia reservoirs could be exchanged, the reactor was of course exhausted when all of the lithium aluminium hydride was consumed. 

Sifer, N. and Gardner, K. (2004) An analysis of hydrogen production from ammonia hydride hydrogen generators for use in military fuel cell environments. J. Power Sources, 132, 135—138. 

A AlI lation. 1-Substitution is favored when the indole ring is deprotonated and the reaction medium promotes the nucleophilicity of the resulting indole anion. Conditions which typically result in A/-alkylation are generation of the sodium salt by sodium amide in Hquid ammonia, use of sodium hydride or a similar strong base in /V, /V- dim ethyl form am i de or dimethyl sulfoxide, or the use of phase-transfer conditions. 

Powdered sodium amide reacts with dimethyl sulfoxide to generate the sodium salt under the same conditions, with the evolution of ammonia, and is comparable to sodium hydride in its reactivity. 

Herrmann et al. reported for the first time in 1996 the use of chiral NHC complexes in asymmetric hydrosilylation [12]. An achiral version of this reaction with diaminocarbene rhodium complexes was previously reported by Lappert et al. in 1984 [40]. The Rh(I) complexes 53a-b were obtained in 71-79% yield by reaction of the free chiral carbene with 0.5 equiv of [Rh(cod)Cl]2 in THF (Scheme 30). The carbene was not isolated but generated in solution by deprotonation of the corresponding imidazolium salt by sodium hydride in liquid ammonia and THF at - 33 °C. The rhodium complexes 53 are stable in air both as a solid and in solution, and their thermal stability is also remarkable. The hydrosilylation of acetophenone in the presence of 1% mol of catalyst 53b gave almost quantitative conversions and optical inductions up to 32%. These complexes are active in hydrosilylation without an induction period even at low temperatures (- 34 °C). The optical induction is clearly temperature-dependent it decreases at higher temperatures. No significant solvent dependence could be observed. In spite of moderate ee values, this first report on asymmetric hydrosilylation demonstrated the advantage of such rhodium carbene complexes in terms of stability. No dissociation of the ligand was observed in the course of the reaction. 

Metal Hydride Process for Ammonia Purge Gas, The metal hydride process will be illustrated using the case of hydrogen recovery from an ammonia purge gas stream generated during ammonia manufacture. 

The decay involves attack by HO at phosphorus followed by hydride shift to cobalt or a direct hydride shift to cobalt from coordinated ammonia. The hydrido-cobalt intermediate so generated is believed to rapidly reduce another molecule of the hypophosphito complex in a post-rate-determining step to produce one equivalent of hypophosphite. 

In a similar way, it has been possible to form NH4 ions in the gas phase by reaction of the amide ion NH2 with formaldehyde (Kleingeld et al., 1983). In this case the proton abstraction (61a) from formaldehyde by NH2, which is a stronger base than OH-, is exothermic and results in the formation of HCO. This ion then transfers a hydride to ammonia in a subsequent ion/ molecule reaction (61b) to give NH4 and carbon monoxide. D-labelling experiments have proved that the hydride ion transferred to ammonia retains its identity so that the NH4 ion, like the H30 ion discussed above, can best be described as a hydride ion solvated by an ammonia molecule. This has also been confirmed recently both by photoelectron spectroscopy (Coe et al., 1985), where the NH4 ion was generated with a nozzle-ion source, and by theoretical calculations (Cardy et al., 1986 Cremer and Kraka, 1986 Kalcher et al., 1984 Squires, 1984). 

An elegant method to generate the free carbene is the reaction of the imidazoUum salt precursor with sodium hydride in liquid ammonia. In contrast to other solvents, liquid ammonia dissolves both, the imidazolinm salt and the base (NaH) providing a medium for smooth, efficient deprotonation and carbene formation in high yields [57,58] (see Figure 1.8). 

Secondary aromatic phosphines can be prepared from tertiary phosphines by cleavage with sodium in liquid ammonia, and the detailed preparation of diphenylphosphine by this method has been reported. Diphenylphosphine has also been prepared by the reaction of chlorodiphenylphosphine with alkali metals or with lithium tetrahydroaluminate. This phosphine has been also obtained from diphenyltrichlorophosphorane or tetraphenyldiphosphine-disulfide with lithium aluminum hydride. A faster and easier method of preparation, which gives equally high yields, consists in the cleavage of tri-phenylphosphine with lithium metal in tetrahydrofuran, followed by hydrolysis of lithium diphenylphosphide with water to generate the phosphine. ... 

Imidazole alkylations can be carried out under a variety of reaction conditions. For conventional iV-alkylations which are unlikely to be complicated in terms of regiochemistry, it is preferable to alkylate the imidazole anion (an Se2cB process). Such reactions are faster, higher yielding and less prone to azole salt formation than those in neutral conditions. The anion is generated best by the use of sodium in ethanol or liquid ammonia, with sodium or potassium hydroxide or carbonate, or by use of sodium hydride in dry DMF [3]. Addition of the alkylating agent to the deprotonated substrate completes the reaction. 

Syntheses that exploit the solubility of the alkaline-earth metals in liquid ammonia have proven practical for alkoxide work, as they generate high yields, reaction rates, and purity (Table 8, Equation (3)). In a refinement of this approach, Caulton and co-workers have used dissolved ammonia in an ethereal solvent, usually THF, to effect the production of a number of alkoxides of barium, and this method has also been examined with calcium and strontium (Table 8, Equations (4a) to (4c)). Displacement reactions using alkali metal alkoxides and alkaline-earth dihalides (Table 8, Equation (5)), and between alkaline-earth hydrides or amides and alcohols (Table 8, Equations (6) and (7)), have been examined, but alkali-metal halide impurities, incomplete reactions, and unexpected equilibria and byproducts can affect the usefulness of these approaches. 

Numerous methods for the generation of imidazole carbenes have been reported. For example, starting from an imidazolium halide, the use of systems such as sodium hydride in ammonia or dimethylsulphoxide (DMSO), sodium in ammonia, alkali metals in tetrahydrofuran (THF), metal ferf-butoxides in THF or DMSO, etc. Recently, Seddon and Earle reported a simple procedure for the generation of the imidazolium carbene 2 in 90-95% yield from an imidazolium chloride 1 which does not require solvents, filtrations, or produce noxious waste products (Scheme 4) [40],... 

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