Dissolving metal reductions Birch reduction

In contrast, kinetic studies of the dissolving metal reduction (Birch reduction) of... 

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). 

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. 

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. 

The reaction differs from other dissolving-metal reductions, such as the Birch reduction (see Section 11.3.3), in that products containing unconjugated double bonds are not observed. 

Dissolving metal reduction (see Birch reduction, Reduction of specific functional groups)... 

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). 

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. 

You will appreciate that C02R groups may be removed from these various products by hydrolysis and decarboxylation. The sulfones can be removed by reduction, for example with sodium amalgam in ethanol - this is dissolving metal reduction and works by electron transfer in the style of a Birch reduction. 

Isolated carbon-carbon double bonds are not normally reduced by dissolving metal reducing agents. Reduction is possible when the double bond is conjugated, because the intermediate anion can be stabilized by electron delocalization. The best reagent is a solution of an alkali metal in liquid ammonia, with or without addition of an alcohol - the so-called Birch reduction conditions. Under these conditions conjugated alkenes, a,p-unsaturated ketones and even aromatic rings can be reduced to dihydro derivatives. 

Reduction of aromatic compounds to dihydro derivatives by dissolved metals in liquid ammonia (Birch reduction) is one of the fundamental reactions in organic chemistry308. When benzene derivatives are subjected to this reduction, cyclohexa-1,4-dienes are formed. The 1,4-dienes obtained from the reduction isomerize to more useful 1,3-dienes under protic conditions. A number of syntheses of natural products have been devised where the Birch reduction of a benzenoid compound to a cyclohex-1,3-diene and converting this intermediate in Diels-Alder fasion to polycyclic products is involved (equation 186)308f h. 

The Birch reduction is the 1,4-reduction of aromatics to their corresponding cyclohexadie-nes by alkali metals (Li, K, Na) dissolved in liquid ammonia in the presence of an alcohol. 

The deep blue solutions formed by dissolving alkali metals in ammonia do not rapidly generate the amide unless a catalyst is added.9 However, a hydrogen acceptor will also initiate the reaction and this forms the basis of the important Birch reduction of aromatic compounds (equation 2).10... 

Two reductions occur concurrently in this dissolving-metal reaction (Scheme 12.10). The first is a Birch reduction8 of the benzyl ether to give a... 

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