Ammonia, liquid, as solvent

Let us next consider liquid ammonia as solvent, in which the positive ion (NH4)+ corresponds to (II30)+ in water. From this ion a proton can jump to an adjacent solvent molecule. 

Proton Transfers in Various Solvents. The Autoprotolysis of Methanol. Formic Acid as Solvent. The Sulfate Ion. Autoprotolysis of Formic Add. The Urea Molecule. Sulfuric Add and Liquid Ammonia as Solvents. 

Sulfuric Acid and Liquid Ammonia as Solvents. The proton transfer that takes place in pure sulfuric acid is believed to be... 

An alternative method for the conversion of an alkyne to an alkene uses sodium or lithium metal as the reducing agent in liquid ammonia as solvent. This method is complementary to the Lindlar reduction because it produces... 

Kinetic studies of base-catalysed hydrogen exchange have been carried out by Roberts, by Shatenshtein, and by Streitweiser and their coworkers. In earlier work, potassium amide was used as base in liquid ammonia as solvent, whereas later workers used lithium and caesium cyclohexylamides in cyclohexylamine. The reaction can be represented by equilibria (239) and (240)... 

Different synthetic routes have been used to prepare these carbenes (Scheme 8.6). The most common procedure is the deprotonation of the conjugate acid. In early experiments, sodium or potassium hydride, in the presence of catalytic amounts of either f-BuOK or the DMSO anion were used. ° Then, Herrmann et al. showed that the deprotonation occurs much more quickly in liquid ammonia as solvent (homogeneous phase), and many carbenes of type IV have been prepared following this procedure. In 1993, Kuhn and Kratz" developed a new and versatile approach to the alkyl-substituted N-heterocyclic carbenes IV. This original synthetic strategy relied on the reduction of imidazol-2(3//)-thiones with potassium in boiling tetrahydrofuran (THF). Lastly, Enders et al." reported in 1995 that the 1,2,4-triazol-5-ylidene (Vila) could be obtained in quantitative yield from the corresponding 5-methoxy-l,3,4-triphenyl-4,5-dihydro-l//-l,2,4-triazole by thermal elimination (80 °C) of methanol in vacuo (0.1 mbar). 

It is a system of acids, bases and salts with liquid ammonia as solvent instead of water. Thus the water system ... 

In addition, aryl chlorides, bromides, and iodides can be converted to arene-amines ArNH2 by the conjugate bases of amines. In fact, the reaction of potassium amide with bromobenzene is extremely rapid, even at temperatures as low as —33°, with liquid ammonia as solvent ... 

H. P. Cady and R. Taft sought if electrolytic oxidations can occur in systems containing no oxygen, and found that with liquid ammonia as solvent, thallous iodide, cuprous iodide, hydrazobenzene, and methyl- and ethylamine hydrochlorides can be oxidized. The properties of soln. of salts, etc., in liquid ammonia were studied by F. W. Bergstrom, T. J. Webb, E. C. Franklin and C. A. Kraus, etc. 

The corresponding action with liquid ammonia as solvent is termed ammonolysis. E.g. ... 

A general method of synthesis of l-hetera-4-telluracyclohexa-2,5-dienes 90 is founded in the nucleophilic addition of telluride anion to type 91 diacetylene derivatives. The telluride dianion is prepared in situ from the elements in liquid ammonia. The reaction was carried out with methanol or mixtures with DMSO and liquid ammonia as solvents, with the following diacetylenes di(l-alkynyl)sulfides (73RTC1326), I-alkynylethynyl sulfides (75RTCI63), di(l-alkynyl)sulfones (78RTC244), and di(l-alkynyl)phosphi-noxides (75RTC92). 

Although this reaction appears as though it might be a simple substitution, experiments indicate that it occurs by an elimination-addition mechanism. This mechanism is outlined in Figure 17.6 for the reaction of chlorobenzene with amide anion in liquid ammonia as solvent. 

Arylphosphonates may also be synthesized from sodium or potassium dialkyl phosphite salts and aryliodides by a SrnI reaction with photostimulation and such a procedure has been described in detail using liquid ammonia as solvent.73... 

Continuous processes have been developed for the alcohols, operating under pressure with liquid ammonia as solvent. Potassium hydroxide (206) or anion exchange resins (207) are suitable catalysts. However, the relatively small manufacturing volumes militate against continuous production. For a while a continuous catalytic plant operated in Ravenna, Italy, designed to produce about 40,000 t/yr of methylbutynol for conversion to isoprene (208,209). 

Liquid ammonia as solvent sodium amide ammonium amalgam hydrazine hydroxylamine. 

Many of the SrnI reactions studied to date have employed liquid ammonia as solvent. This is an attractive solvent because it allows weak carbon acids such as acetone to be converted completely to their conjugate bases by reaction with amide... 

Where a reactive lower oxidation state results, a key concern is the necessary protection of the reduced complex from air or other potential oxidants, as they are often readily reoxidized. Usually, this requires their handling in special apparatus such as inert-atmosphere boxes or sealed glassware in the absence of oxygen. Where active metal reducing agents (such as potassium) are employed, special care with choice of solvent is also necessary. The nickel reduction reaction (6.33) can be performed in liquid ammonia as solvent, since the strongly-bound cyanide ions are not substituted by this potential ligand. 

The Birch reduction comprises a means for adding two hydrogen atoms to an aromatic ring by means of a metal, most often lithium, and an alcohol in liquid ammonia as solvent. A co-solvent, often tetrahydrofuran (THF), is often added due to the very poor solubility of steroids in ammonia. The use of the more expensive sodium was at one time precluded because traces of iron in that metal catalyzed the conversion of the metal to the strong base sodium amide. Very pure sodium, free of iron impurities, is now used for commercial-scale reductions. 

Under the usual conditions (THF or liquid ammonia as solvent), mono-substituted epoxides are attacked on the methylene carbon atom. The stereochemistry of the ring opening of vidnally di-substituted epoxides (e.g., epoxycyclohexane) is in accordance with an SN2-mechanism. 

In a sense this is a perfect example of orthogonal protection schemes in which two different chemical reactions are applied, one for the removal of the transient and the other for the cleavage of the semipermanent blocking groups. Unfortunately this combination can not be readily utilized in the synthesis of peptides with sulfur containing amino acid residues poisoning of the catalyst impedes the process. It is possible to employ countermeasures, such as addition of base or the use of liquid ammonia as solvent, when methionine residues are the source of difficulties, but cysteine residues present a less readily surmountable obstacle. 

Enolates derived from acyclic and cyclic aliphatic ketones can easily be arylated by the SrnI process. These reactions are usually conducted in dimethylsulfoxide (DMSO) or liquid ammonia as solvent. As an example, the photoinduced arylation of the enolate derived from 2-acetyl-N-methyl pyrrole with iodobenzene in... 

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