Aromatic compounds dissolving metals

Catalytic reduction of fluormated aliphatic and aromatic nitro compounds to give oximes and amines was described previously, as was the use of dissolving metals to prepare amines [Si] Refmement of these techniques has resulted in optimized yields and, as indicated in equations 69 and 70, in selective reductions [S6, 87]... 

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. 

Whatever the best explanation may be, an indication that allylic alkali metal compounds or allylic carbanions do in fact form the less stable of the two possible acids on neutralization is found in the results of the reduction of aromatic compounds by dissolving metals.376The detection of a paramagnetic intermediate in a similar system and polaro-graphic evidence indicate a one electron transfer in the rate and potential determining step.878 376 The mechanism therefore involves ions (or organometallic intermediates) like the following ... 

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. 

An unusual arene hydrogenation in ionic liquids was published by Seddon et al. in 1999 [86]. These authors reported a new means of hydrogenating aromatic compounds by dissolving electropositive metals in ionic liquids with HC1 as the... 

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... 

Thus Boehringer Sohne1 (Mannheim) patented a proc by which, when employing tin-cathodes (or cathodes of of.is indifferent metals with an addition of a small quantity of tin salt), fatty or aromatic intro-compounds, dissolved suspended in aqueous or hydro-alcoholic hydrochloric a 5 can be reduced in almost theoretical yields to the correspor ing amines. 

A number of ILs are hydrophobic, yet they readily dissolve many organic molecules—with the exception of alkanes and alkylated aromatic compounds (e.g., toluene). Among such ILs we find [bmim][PFg], which forms triphasic mixtures with alkanes and water. This multiphasic behavior has decisive implications for clean synthesis. For example, transition-metal catalysts can be exclusively dissolved in the ionic liquid, thus allowing products and by-products to be separated from the ionic liquid by solvent extraction with either water or an organic solvent. This is advantageous when using expensive metal catalysts, as it enables both the ionic liquid and the catalyst to be recycled and reused. Alternatively, some volatile products can be separated from the IL by distillation, as it has negligible vapor pressure. 

The reduction of various substrates by dissolving metals in alcoholic and aqueous media is a very old procedure in synthetic organic chemistry. In addition to aldehydes, ketones, imines and other unsaturated nitrogen compounds, many other functional groups are reduced under these conditions. Historically, the most common reduction conditions were Na in ethanol, and the reductions were carried out by adding the metal to a solution of the substrate in alcohol and the reaction mixture was heated at reflux for varying periods of time. Other reduction systems included Na-Hg amalgam in water or alcohols and, for easily reduced compounds such as aldehydes and aromatic ketones, Zn-NaOH or Fe-acetic acid have been used. ... 

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). 

In general, the reduction of aromatic carbonyl compounds to the corresponding alcohols by dissolving metals is not a particularly valuable synthetic procedure. Better yields and chemoselectivity are usually obtained using complex metal hydrides. 

One of die most popular reactions in organic chemistry is dissolving metal reductions [1-3], Two systems are frequently used - sodium dissolved in ammonia with alcohol and lithium dissolved in alkylamines [4]. Although calcium is seldom used, it has been successfully applied to the reduction of a variety of compounds and functional groups [5], including aromatic hydrocarbons, carbon-carbon double and triple bonds, benzyl ethers, allyl ethers, epoxides, esters, aliphatic nitriles, dithianes, als well as thiophenyl and sulfonyl groups. 

Processes involving the use of solid acid catalysts have also been patented. According to Chen and Yan,40 plastic and/or rubber wastes are first subjected to a size reduction step, followed by separation of any metals present and washing to remove any non-plastic material such as paper, labels, etc. Subsequently, the polymer wastes are dissolved or dispersed in a petroleum oil, with a high content of polycyclic aromatic compounds at 300 °C, and catalytically transformed in an FCC reactor at temperatures of about 500 °C. Details are given for the conversion of different wastes used whole tyres, PE bags and PS foam. 

Carbon monoxide, when passed over a heated catalyst, such as iron oxide, dissociates to form carbon dioxide and an activated carbon.54,55 Mercury and other metals react with organic halides to produce metallic halides and carbon.56 Stratton and Winkler57 describe a method in which hydrocarbons are burned in chlorine the carbon produced is activated by heating to 1,000° C for an hour. Simons and McArthur58 observed that activated carbon is formed by the action of oxygen on aromatic, alicyclic, and aliphatic compounds dissolved in liquid hydrogen fluoride. 

Birch reduction is a partial reduction of aromatic compounds by electron transfer from dissolving metals—usually Na in liquid ammonia or Li in ethylamine—in the presence of a weak proton donor—usually an alcohol. The reaction behaves as if dianion (30) were an intermediate, giving non-conjugated dienes (31). Electron-donating substituents repel the anions (32) to give products like (33). whilst electron-withdrawing substituents attract the anions (34), to give products like (35). 

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