Reduction dissolving metals also

Na or Li and ammonia, excellent yields. " A dissolving metal reduction can be effected without cleavage of a sulfur-carbon bond. Note also the unusual selectivity in the cleavage illustrated below. This was attributed to steric compression. ... 

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. 

Phosphate groups can also be removed by dissolving-metal reduction. Reductive removal of vinyl phosphate groups is one method for conversion of a carbonyl compound to an alkene.224 (See Section 5.7.2 for other methods.) The required vinyl phosphate esters are obtained by phosphorylation of the enolate with diethyl phospho-rochloridate or /V A /V -tetramethyldiamidophosphorochloridate.225... 

Reductive elimination from 1,2-dibromides generates the alkene in excellent yields. Conformationally rigid, periplanar tro 5-diaxial, also staggered trans-diequatorial, cyclohexane dibromides all afford the alkene at a mercury cathode [110]. In the bicyclo[2,2,2]octane series, the rra s-2,3-dibromide forms the alkene on dissolving metal reduction [111]. The rigid cis-periplanar 1,2 dibromobicy-clo[2,2,l]heptane, at a mercury cathode, also gives the strained alkene which can be trapped as a furan adduct [112]. 

As early as 1969, Pedersen was intrigued by the intense blue colour observed upon dissolution of small quantities of sodium or potassium metal in coordinating organic solvents in the presence of crown ethers. Indeed, the history of alkali metal (as opposed to metal cation) solution chemistry may be traced back to an 1808 entry in the notebook of Sir Humphry Davy, concerning the blue or bronze colour of potassium-liquid ammonia solutions. This blue colour is attributed to the presence of a solvated form of free electrons. It is also observed upon dissolution of sodium metal in liquid ammonia, and is a useful reagent for dissolving metal reductions , such as the selective reduction of arenes to 1,4-dienes (Birch reduction). Alkali metal solutions in the presence of crown ethers and cryptands in etheric solvents are now used extensively in this context. The full characterisation of these intriguing materials had to wait until 1983, however, when the first X-ray crystal structure of an electride salt (a cation with an electron as the counter anion) was obtained by James L. Dye and... 

This method is an inexpensive substitute for LAH reductions of esters in industrial production, and was the only alternative prior to the development of the metal hydride reducing agents. This dissolving metal reduction is also related to the Birch Reduction. 

Aromatic ketones represent a rather special case in dissolving metal reductions. Under many conditions pinacol formation is the predominent reaction path (see Volume 3, Chapter 2.6). Also, the reduction potentials of aromatic carbonyl compounds are approximately 1 V less negative than their aliphatic counterparts. The reductions of aromatic ketones by metals in ammonia are further complicated by the fact that hydrogenolysis of the carbon-oxygen bond can take place (Chapter 1.13, this volume) and Birch reduction may intervene (Chapter 3.4, this volume). 

The dissolving metal reductions are better classified as internal electrolytic reductions in which an electron is transferred from the metal surface (or from the metal in solution) to the substrate. Reduction with low-valent metal ions may also be included in this general class (vide infra). 

Benzylic amines are particularly susceptible to hydrogenolysis by catalytic hydrogenation or dissolving metal reduction. " Note that the Wolff-Kishner reduction in 19-61 involved formation of a hydrazone and deprotonation by base led to loss of nitrogen and reduction. Ceric ammonium nitrate in aqueous acetonitrile has also been shown to reductively cleave the V-benzyl group. Primary amines have been reduced to RH with hydroxylamine-O-sulfonic acid and... 

Catalytic hydrogenation of an enone would not be chemoselective if an isolated double bond were also present in the molecule. However, isolated double bonds are inert to dissolving metal reduction. On the other hand, a variety of functional groups are reduced with alkali metals in liquid ammonia. These include alkynes, conjugated dienes, allylic, or benzylic halides and ethers. 

Treatment of enolates with (R0)2P(0)C1 also results in 0-trapping to yield the corresponding enol phosphates. Dissolving metal reduction of enol phosphates is a useful procedure for the deoxygenation of ketones with concomitant, regiospecific formation of the alkene. ... 

The enolate that is required for the aldol reaction can be generated in other ways, too. For example, trimethylsilyl enol ethers react with TBAF (tetrabutylammonium fluoride, BU4ISD F ) to give the corresponding enolates. Enolates may also be prepared by the dissolving metal reduction of a.jS-unsaturated ketones (see Chapter 5). 

A radical anion can lose an anionic leaving group to give a neutral free radical, and in the reverse direction, a neutral free radical can combine with an anionic nucleophile to give a new radical anion. A radical cation can also combine with a nucleophile. Such steps occur in the SrnI mechanism (Chapter 2), in dissolving metal reductions, and in oxidative deprotections. These two-electron reactions have obvious counterparts in carbocation chemistry. 

Dissolving metal reduction of steroidal a,/ -dienones also yields products of the a-type (see Section D.2.3.2.). 

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