Reductions in liquid ammonia

The apparatus used for reductions in liquid ammonia is a three-necked flask equipped with a stirrer, dropping funnel and soda lime drying tube. The flask is filled from one-third to no more than one-half its capacity, to aid in control of foaming which occurs in some instances toward the end of alcohol addition. 

Reductions in liquid ammonia run at atmospheric pressure at a temperature of —33°. If higher temperatures are necessary for the reduction, other solvents of alkali metals are used methylamine (b.p. — 6.3°), ethylamine (b.p. 16.6°), and ethylenediamine (b.p. 116-117°). 

Voltammetry studies on pyridine and pyridine derivatives have delineated some of the initial mechanistic steps of reduction.9-11 The half-wave potential was found to correlate with the n,n triplet energy.10 The mechanism of pyridine reduction in liquid ammonia was studied by Talcott among others this process was also studied on a synthetic scale.9,12 Evidence for an ECE... 

The anions PhjAs and PhjSb have been prepared in situ by alkali metal reduction in liquid ammonia and, like the corresponding PI12P , they react with haloarenes (ArX) by a photostimulated Sjj l-mechanism ° i.e. by a mechanism involving ET as an important step. The mechanism may be formulated as in equations 84-87. 

A somewhat similar cell design has been used in the equipment for electrolytic reductions in liquid ammonia for both static and circulating media in this case the center electrode is a sacrificial magnesium anode [54]. 

Here are some other compounds which are easily reduced347-351 in a comparably explicable manner, except that, with reductions in liquid ammonia without alcohol present, two electrons must be added to the LUMO of the starting material before it is basic enough to abstract a proton from ammonia. The addition of the second electron (to the radical anion) is often so slow that dimerization of the radical-anion occurs to some extent. 

Hueckel and co-workers indicate three methods by which reductions in liquid ammonia (Birch reduction) can be performed. They show that the reduction is substantially affected by the sequence in which reagents are added. 

For instance, benzene is one of the most hard-to-reduce compounds. It is not active polarographically and is not subjected to direct electrode reduction in liquid ammonia . Benzene does not react with solvated electrons obtained by dissolving alkali metals in liquid ammonia in the absence of proton donors However, in the presence of proton donors benzene is hydrogenated by solvated electrons (Table 10). 

Apparatus Reductions in liquid ammonia are carried out in a cylindrical vessel of 11 capacity and 8 cm diameter. This vessel is fitted with two ground necks at the top to take a dropping funnel and a stirrer, two glass tubes with taps for introduction and removal of ammonia, and a position for insertion of a cold-reading thermometer. Ammonia, pre-dried by soda lime and potassium hydroxide, is introduced with cooling by methanol and solid carbon dioxide and evacuation of the vessel and, whilst in the liquid state, freed from the last traces of moisture by small pieces of sodium. 

Dissolving Alkali Metal Reductions in Liquid Ammonia... 

Reductions in liquid ammonia have been useful for this purpose, as is illustrated by reaction (22). The oxidation state of nickel in this compound is zero. The... 

The authors analyze the results to exclude a mechanism involving the ketone dianion. From a practical viewpoint, the sonochemical reductions in THF occur in 0.5-1 h, making this procedure much easier than the reductions in liquid ammonia. Quinones and a-diketones are reduced by zinc and TMSCl (Fig. 17).i07... 

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