Liquid ammonia disproportionation

Reduction of Ketones and Enones. Although the method has been supplanted for synthetic purposes by hydride donors, the reduction of ketones to alcohols in ammonia or alcohols provides mechanistic insight into dissolving-metal reductions. The outcome of the reaction of ketones with metal reductants is determined by the fate of the initial ketyl radical formed by a single-electron transfer. The radical intermediate, depending on its structure and the reaction medium, may be protonated, disproportionate, or dimerize.209 In hydroxylic solvents such as liquid ammonia or in the presence of an alcohol, the protonation process dominates over dimerization. Net reduction can also occur by a disproportionation process. As is discussed in Section 5.6.3, dimerization can become the dominant process under conditions in which protonation does not occur rapidly. 

The electrochemical reduction of cycloheptatriene (CHT) in liquid ammonia takes place at about —2.5 V vs SCE and forms the radical anion of CHT. The radical anion is stable in ammonia on the voltammetric time scale but decays slowly by disproportionation and coupling reaction pathways to give respectively 1,3- and 1,4-cycloheptadienes (total yield 34-39%) and C14H18 (in yields of 55-58%) isomers which incorporate the bitropyl carbon skeleta20. 

Sulfur Sg is dissolved through a disproportionation process, which can be reversible or not reversible. The only solvent, which dissolves sulfur by a reversible disproportionation process, is liquid ammonia NH3. In this solvent, the dissolution of sulfur gives the reduced form Se in equilibrium with S3, and oxidized forms of sulfur. 

Liquid ammonia is not a classical solvent for sulfur because it dissolves sulfur in the course of a reversible and slow disproportionation process as has been described earlier. Ammonia is a very useful solvent for the investigation of polysulfides because it allows their synthesis with an easy control of their stoichiometry. After evaporation of ammonia and drying under high vacuum, a solid with the overall composition M28 is obtained. The solid is either a polysulfide or a mixture of polysulfide (s) with sulfur, depending... 

Amination of AT-alkylpyridinium salts with amide ions, which in principle should be easier than the reaction with the parent pyridine, has been little studied. The main reason for this is that solvent selection is difficult. Metal amides are only soluble in liquid ammonia (with which pyridinium salts react easily, vide infra), and pyridinium salts are soluble in solvents that are not suitable for use with metal amides. The A/ -methylacridinium cation undergoes direct imination to give (153) in 35% yield by treatment with potassium amide and iron (III) nitrate in liquid ammonia. Two other products (154) and (155) are also formed, probably by hydrolysis and subsequent disproportionation (Scheme 90). One might question whether sodamide is necessary to the above transformation in light of the fact that quin-olinium, isoquinolinium and certain pyridinium ions give cr-complexes (156), (157) and (158) in liquid ammonia alone at 0 °C (73JOC1949). 

Mono-metallation of most simple acetylenic compounds in organic solvents can be accomplished without complications. The same holds for reactions of RCsCH with alkali amides in liquid ammonia. However, HCsCSiR3 and HC=CSnR3 cannot be metallated in this way, because the bonds between C and Si and between C and Sn are cleaved. HCsCSeR and HC=CPR2 disproportionate into HCsCH and RSeCsCSeR or R2PCsCPR2, respectively, under the influence of alkali amides [40,102],... 

The product is a very pale yellow to cream-colored solid. It is hygroscopic and is soluble in liquid ammonia, with limited solubility and stability in acetonitrile, dimethyl sulfoxide, and dimethylformamide. Water or moisture from the air leads to disproportionation, and so the product must be stored under anhydrous... 

When 3-chloro- or 3-bromopyridine is heated with lithium piperidide and piperidine in boiling ether, 156 is formed, which reacts further with piperidine to give a mixture of 3- and 4-piperidinopyridine in the ratio of 48 52. No 2,3-pyridyne intermediate is apparently produced under these conditions.388 Such an intermediate is probably involved in the reaction of potassium amide in liquid ammonia with 3-bromo-4-ethoxypyridine, which gives 2-amino-4-ethoxypyridine (55-60%). The reaction is, however, complicated by the fact that 2-amino-5-bromo-4-ethoxypyridine (15-20%) and 4-ethoxypyridine (25%) are also obtained.387 The formation of these two by-products may proceed by the preliminary disproportionation of some 3-bromo-4-ethoxy-pyridine to 3,5-dibromo-4-ethoxypyridine and 4-ethoxypyridine.388 The remarkable observation that both 2-amino-6-ethoxypyridine (157) (85%) and 4-amino-2-ethoxypyridine (158) (15%) are formed during the amination of 2-bromo-6-ethoxypyridine367 still requires explanation. No such rearrangement is observed with lithium piperidide.3880... 

Lithium in liquid ammonia conditions can produce l,4-dihydroquinoline" and 3,4-dihydroisoquinoline." Conversely, lithium aluminium hydride reduces generating l,2-dihydroquinoline" and 1,2-dihydroisoquinoline. These dihydro-heterocycles can be easily oxidised back to the fully aromatic systems, or disproportionate, especially in acid solution, to give a mixture of tetrahydro and re-aromatised compounds. Stable dihydro-derivatives (see also 9.13) can be obtained by trapping following reduction, as a urethane, by reaction with a chloroformate. Quaternary salts of quinoline and isoquinoline are particularly easily reduced, either catalyticaUy or with a borohydride in protic solution, giving... 

Heptasulfur imide is formed from elementary sulfur or from the product of the reaction of sulfur with sulfur trioxide by treatment with liquid ammonia at room temperature. A disproportionation reaction takes place in which, in addition to H2S and S4N4, S7NH is produced from the elementary sulfur or S203 (1, 39). 

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