Ammonia dissolving metal reductions

When we use sodium metal dissolved in liquid ammonia (dissolving-metal reduction) as the reagent for the reduction of alkynes, we obtain trans alkenes as the products. For example, 3-heptyne is reduced to trans-3-heptene in this way. Unlike sodium amide in liquid ammonia, which functions as a strong base, elemental sodium in liquid ammonia acts as a powerful electron donor (i.e., a reducing agent). 

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. 

Dissolving-Metal Reduction of Aromatic Compounds and Alkynes. Dissolving-metal systems constitute the most general method for partial reduction of aromatic rings. The reaction is called the Birch reduction,214 and the usual reducing medium is lithium or sodium in liquid ammonia. An alcohol is usually added to serve as a proton source. The reaction occurs by two successive electron transfer/proto-nation steps. 

The stereochemistry of the photocycloadducts can be predicted from the result of a dissolving metal reduction of the same ,/)-unsaturated ketone. For example, sodium/ammonia reduction of 3,4-dimethylcydohexenone yielded trans- and cis-dimethylcyclohexanone 37 in a ratio of 84 16, which was similar to the ratio of the two photocycloadducts 38 (80 20) [33b, 42],... 

Double bonds conjugated with benzene rings are reduced electrolytically [344] (p. 23). Where applicable, stereochemistry can be influenced by using either catalytic hydrogenation or dissolving metal reduction [401] (p. 24). Indene was converted to indane by sodium in liquid ammonia in 85% yield [402] and acenaphthylene to acenaphthene in 85% yield by reduction with lithium aluminum hydride in carbitol at 100° [403], Since the benzene ring is not inert toward alkali metals, nuclear reduction may accompany reduction of the double bond. Styrene treated with lithium in methylamine afforded 25% of 1-ethylcyclohexene and 18% of ethylcyclohexane [404]. 

Various examples of the use of dissolving metal reduction on exocyclic double bonds, conjugated either with another C —C double bond53 or a carbonyl group,54 have been reported. For example, reaction of 2,6,6-trimethyltricyclo[5-4.0.0I,5]undec-7-en-9-one with lithium metal in liquid ammonia gave, after trapping of the intermediate enolate, 2,6,6-trimethyl-9-tri-fyloxy[5.4.0.015]undec-8-enc (3).53... 

It was observed in 1941 that with sodium in liquid ammonia, called dissolving-metal reduction, different dialkylacetylenes were converted to the corresponding trans alkenes in good yields and with high selectivity 195... 

Alkali metals in liquid ammonia represent the most important class of the so-called dissolving-metal reductions of aromatics. First described in 1937, it is a highly efficient and convenient process to convert aromatic hydrocarbons to partially reduced derivatives.201 The recognition and extensive development of this electron-transfer reduction came from A. J. Birch,202,203 and the reaction bears his name. 

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... 

Reduction of an alkyne to an (E)-alkene can be achieved by treating the alkyne with lithium or sodium metal in ammonia at low temperatures (Following fig.). This is called dissolving metal reduction. 

Reactions of 1,2,4-thiadiazoles with radicals and electron deficient species are virtually unknown. Catalytic and dissolving metal reductions usually result in S—N bond cleavage. For example, the 5-anilino-3-hydroxy derivative (51) gives a good yield of l-phenyl-2-thiobiuret (52) on Zn-HCl reduction (Scheme 27). Reduction of the diamino derivative (53) gives amidinothiourea (54) from which it may be prepared by oxidation (Scheme 28). Under similar conditions, cleavage of the 3,5-diphenyl derivative (55) results in loss of sulfur and formation of benzylbenzamidine (56 Scheme 29). Reduction of 5-alkylamino-or 5-arylamino-3-alkylthio derivatives (57) with H2S in pyridine-triethylamine or sodium in liquid ammonia yields 1-substituted dithiobiurets (58 Scheme 30). 

Na, or Li in liquid ammonia, for example) to reduce aromatic rings and alkynes. The dissolving metal reduction of enones by lithium metal in liquid ammonia is similar to these reactions—the C=C bond of the enone is reduced, with the C=0 bond remaining untouched. An alcohol is required as a proton source and, in total, two electrons and two protons are added in a stepwise manner giving net addition of a molecule of hydrogen to the double bond. 

The diphenylmethylene group provided an easy solution to the problem of selectively deprotecting the terminal 1,2-diol of a polyhydroxylated fragment of the antibiotic Roxaticin.183 Dissolving metal reduction of the diphenylmethylene group [Scheme 3.99] was accomplished in 96% yield with lithium in liquid ammonia without harm to three isopropyl idene groups and a rm-bu ty 1 d im ethyl si ly] ether. 

The second way to deliver H2 in a reduction is to add two protons and two electrons to a substrate—that is, H2 = 2H + 2e . Reducing agents of this sort use alkali metals as a source of electrons and liquid ammonia (NH3) as a source of protons. Reductions with Na in NH3 are called dissolving metal reductions. 

Dissolving metal reduction (Section 12.2) A reduction reaction using alkali metals as a source of electrons and liquid ammonia as a source of protons. 

Liquid ammonia (b.p. -33°C) is a solvent which is not encountered frequendy, but which does have several important general uses, in particular dissolving metal reductions ("Birch" type reductions) and most reactions involving lithium amide or sodium amide as bases. Ammonia gas from a cylinder is condensed directly into the flask (Fig. 14.5). 

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). 

DISSOLVING METAL REDUCTIONS 4.7.4. Lithium and Sodium in Liquid Ammonia... 

In order to accomplish a total dehalogenation to hydrocarbons, dissolving metal reductions are most commonly employed. Cyclopropyl chlorides and bromides are satisfactorily reduced by alkali metal in alcohoF or in liquid ammonia. However, the fluorides are usually resistant to the hydrogenolysis. gem-Difluorides have been... 

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. 

Dissolving metal reduction of a,(3-unsaturated ketones regiospecifically produces enolates that, on removal of ammonia, may be reacted with electrophiles. 

Sinclair, S., Jorgensen, W. L. Computer Assisted Mechanistic Evaluation of Organic Reactions. 23. Dissolving Metal Reductions with Lithium in Liquid Ammonia Including the Birch Reduction. J. Org. Chem. 1994, 59, 762-772. 

---