Radical anions metal-ammonia reduction

The metal-ammonia reduction proceeds by addition of an electron to the alkyne to form a radical anion, followed by protonation to give a neutral radical. Protons are provided by the ammonia solvent or by an alcohol added as a cosolvent. Addition of another electron, followed by another proton, gives the product. 

These results allow a complete description of metal-ammonia processes, as shown in Scheme IV. The upper pathway represents the classical Birch reduction (i.e., benzenes), whereas polynuclear aromatic compounds react by one of the lower routes. As indicated in Scheme IV, the protonation of dianions rather than radical anions by ammonia is favored. We previously suggested dianion protonation (8) because radical anions are not very basic (10). Recently, Mullen et al. (12) pointed out that, in some cases, dianions and radical anions of the same neutral precursor are protonated at different sites in the cases they investigated, protonation occurred with the dianions. 

An informative and amusing background to that unique material, camphor, has appeared. Its preparation by Oppenauer oxidation of the epimeric borneols occurs without epimerization. Epimerization does not occur in the presence of potassium t-butoxide in t-butyl alcohol, but it does with potassium isopropoxide in propan-2-ol." The reaction of camphor with phosphoric acid yields a complex mixture of m- and p-cymenes, 3,4-dimethylethylbenzene, 1,2,3,4-and 1,2,3,5-tetramethylbenzene, fenchone, carvenone, and carvacrol. A very detailed examination of the metal-ammonia reduction has revealed an intermediate camphor analogue of pinacol formed by association of a camphor anion radical with the metal cation. This intermediate was isolated and characterized. Other effects are discussed, such as that of adding a large excess of metal salt (LiBr, KBr, or NH Cl)." ... 

Sodium in liquid ammonia reduction of norbornadiene at — 33 °C in THF, with Bu OH present as a proton source, afforded nortricyclene and norbornene in 1 19 ratio in the absence of Bu OH a 1 200 ratio was obtained for the much slower reaction.According to the commonly accepted mechanisms of metal-ammonia reductions, the anionic species XH is the penultimate intermediate irrespective of whether the mechanism involves the preliminary formation of the dianion X (in the absence of a proton source) or of the radical HX- (presence of a proton source). The single carbanion species thus requires that the product ratio be independent of the presence or absence of a proton source more powerful than ammonia, contrary to the experimental observations. Hence, the existence of a least two discrete car-banions, namely (439) (440), is suggested. Consistent with this proposal are the results obtained for the Na-NDj reduction (CD3OD as proton source) of norbornadiene, and of the Na-NHg reduction of benzonorbornadiene. 

In a study of electron-transfer reactions. House has examined the stabilities of the anion radicals derived from a number of y-cyclopropyl-ap-unsaturated ketones. It would appear that for the conversion of substrates containing the unit depicted by (240) into (241) a rearrangement rate in excess of 10 s is necessary in order to detect the anion radical intermediate present in metal-ammonia reductions. A rate of ca. 10 s is likewise required in lithium dimethylcuprate reactions. ... 

One final example worth mentioning is the reductive alkylation/arylation with lithium and alkyl/aryl halides in liquid ammonia. This is a two-step process in which negatively charged nanotubes are formed via electron transfer from the metal. This step is relatively easy and fast due to the CNTs electron sink properties, and it enables exfoliation of the tubes through electrostatic repulsion in the second stage, the alkyl/aryl halides react with the charged tubes to form a radical anion which can dissociate into the alkyl radical and the halide anion, with the former species undergoing addition to the CNT sidewalls [42]. 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

Reactions with Protic, ionic, Poiar Reagents. The reactions of radical anions with proton donors include the reduction of arenes, the well-known Birch reduction, as well as alkynes by alkali metals in liquid ammonia. Both reactions have synthetic utility and belong to the few radical ion reactions included in elementary textbooks. 

Alkali and alkaline earth metals dissolve in liquid ammonia with the formation of solvated electrons. These solvated electrons constitute a very powerful reducing agent and permit reduction of numerous conjugated multiple-bond systems. The technique, named for Birch provides selective access to 1,4-cydohcxiidicnes from substituted aromatics.8 In the case of structures like 21 that are substituted with electron-donating groups, electron transfer produces a radical anion (here 22) such that subsequent protonation occurs se lectively in the ortho position (cf intermediate 23) A second electron-transfer step followed by another protonation leads to com pound 24... 

Extensive investigations have been made into further methods for the reduction of aromatic rings based on the use of dissolving metals in other solvents, especially the lower molecular weight amines (the Benkeser reduction), electrochemical methods (cathodic reductions), photochemical methods and the reaction of radical anions with silylating reagents rather than proton sources. The aim of much of this work has been to produce the normal Birch products more conveniently or cheaply, but very often the outcome has been quite distinct. The alternative method may then provide access to products which are not so easily obtained by the standard metal-liquid ammonia methodology. 

The dimerization products shown in Scheme 7 are generally the major ones obtained in electrochemical reductions (vide infra) or reductions at metal surfaces, - in which radical anion intermediates must diffuse to a surface before further electron transfer can occur. In metal-ammonia solutions, however, simple reduction is generally favored over dimerization. These solutions provide high concentrations of available electrons, favoring the probability of the radical ion or hydroxyallyl radical accepting a second electron. 

Although alkali metal/liquid ammonia reductions (Birch reductions) of simple alkenes is difficult, presumably as a result of the very high energy of an ethene type LUMO, the corresponding reduction of non-terminal alkynes to trawi -alkenes is an efficient and useful synthetic tool for accessing trans-alkenes [116]. The mechanism for this reaction (Scheme 69), involves the homogeneous reduction of the alkyne to the corresponding anion radical by the solvated electrons present in liquid ammonia solutions of alkali metals. 

The function of the alcohol in the metal -NH3 reduction is to provide a proton source that is more acidic than ammonia to ensure efficient quenching of the radical anion and pentadienyl anion species. Furthermore, the presence of alcohol represses the formation of the amide ion NH2 , which is more basic than RO M and is capable of isomerizing the 1,4-cyclohexadiene product to the thermodynamically more stable conjugated 1,3-cyclohexadiene. 

Aryl-substituted cyclopropyl ketones, e.g. 34, are reductively cleaved to acyclic ketones by refluxing in ethanol, or butanol, with zinc metal or a mixture of zinc metal and zinc(II) chloride. The reaction is thought to proceed via radical anion intermediates analogous to those of the lithium/ammonia process. Various substituted derivatives were studied and the aryl substituent in the cyclopropane ring was found to be essential alkyl-substituted analogs were not reductively cleaved with this reagent. 

A persistent radical-anion (4) may be prepared from dibenzofuran, the reduction of which by alkali or alkaline earth metal in liquid ammonia was reported in 1964. ° Gerdil and Lucken in 1965 also reported a preparation of 4 by reduction of dibenzofuran with potassium in DME its ESR spectrum and those of its sulfur and selenium analogs were solved. These authors also carried out a polarographic and spectroscopic study.Evans and co-workers reported ESR results for dibenzofuran closely similar to those of Gerdil and Lucken. The nature of the unimolecular decay of the anion-radical was studied and inferred to be C—O bond scission. The dependence of the rate of this reaction upon the counterion was investi-... 

The two hydrogen atoms add to the opposite faces of the alkene (i.e. anti-addition) using sodium in liquid ammonia. This dissolving metal reduction produces a solvated electron, which adds to the alkyne to produce a radical anion (bearing a negative charge and an unpaired... 

Birch reduction. Sodium or lithium metal (in liquid ammonia) can donate an electron to the benzene ring to form a radical anion. On protonation (by ethanol) and further reduction/protonation, this produces 1,4-cyclohexadiene. 

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