Anion radical intermediates metal-ammonia reduction

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. ... 

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... 

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. 

Metallic bronze fh(N 11, ), can, furthermore, be prepared from calcium-ammonia solution by evaporation of the ammonia. A bronze sohd is obtained, which in TH F at -30 °C behaves as a sohd surface in its reaction with alkyl hahdes (Scheme 4.17) [39]. Its apphcation in the reduction of 61 b produces a mixture of cyclopropane derivative 62 (81%) and straight-chain diphenylbutane 64 (8%). The process involves the formation of a tight anion radical-cation radical as an intermediate. Carbon-halogen bond cleavage occurs on the surface of the metalhc cluster. 

Conjugated dienes are readily reduced to the 1,4-dihydro derivatives with metal-ammonia reagents in the absence of added proton donors. For example, isoprene is reduced to 2-methyl-2-butene by sodium in ammonia, by way of an intermediate radical anion (7.41). The protons required to complete the reduction are supplied by the ammonia. 

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... 

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. 

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