Calcium Birch reduction

Various other observations of Krapcho and Bothner-By are accommodated by the radical-anion reduction mechanism. Thus, the position of the initial equilibrium [Eq. (3g)] would be expected to be determined by the reduction potential of the metal and the oxidation potential of the aromatic compound. In spite of small differences in their reduction potentials, lithium, sodium, potassium and calcium afford sufficiently high concentrations of the radical-anion so that all four metals can effect Birch reductions. The few compounds for which comparative data are available are reduced in nearly identical yields by the four metals. However, lithium ion can coordinate strongly with the radical-anion, unlike sodium and potassium ions, and consequently equilibrium (3g) for lithium is shifted considerably... 

Metal-ammonia solutions reduce conjugated enones to saturated ketones and reductively cleave a-acetoxy ketones i.e. ketol acetates) to the unsubstituted ketones. In both cases the actual reduction product is the enolate salt of a saturated ketone this salt resists further reduction. If an alcohol is present in the reaction mixture, the enolate salt protonates and the resulting ketone is reduced further to a saturated alcohol. Linearly or cross-conjugated dienones are reduced to enones in the absence of a proton donor other than ammonia. The Birch reduction of unsaturated ketones to saturated alcohols was first reported by Wilds and Nelson using lithium as the reducing agent. This metal has been used almost exclusively by subsequent workers for the reduction of both unsaturated and saturated ketones. Calcium has been preferred for the reductive cleavage of ketol acetates. 

Benkeser, R. A. etal., Tetrahedron Lett., 1984, 25, 2089-2092 The use of calcium in 1,2-diaminoethane as a safer substitute for sodium or lithium in liquid ammonia for the improved Birch reduction of aromatic hydrocarbons is described in detail. 

A solution of lithium, sodium, potassium or calcium in liquid ammonia can reduce a wide variety of unsaturated groups. Thus when aromatic rings are reduced by such metals in liquid ammonia, non-conjugated cyclohexadienes are produced. The reaction is called Birch reduction. 

When tert-butyl alcohol is used with the calcium-amine system, aromatic compounds can be reduced to products identical with those obtained by Birch reduction of the same substrates [37]. Advantages of the calcium-amine-alcohol procedure are, first, that calcium is much safer to handle than sodium and lithium and is therefore more amenable to large-scale reductions, and, second, the amine solvents are relatively high-boiling and are, therefore, much easier to manipulate than liquid ammonia. 

The preeminence of lithium and sodium in metal reductions has been challenged by the use of calcium in amines, developed by Benkeser. In the initial work, naphthalene was reduced to an 80 20 mixture of A -octalin (522) and A ( )-octalin (523) with lithium in diethylamine-dimethylamine. Replacing lithium with calcium gave a 77 23 mixture of 522 and 523 in 92% yield. This method has come to be called the Benkeser reduction. This has become an important modification because Birch reductions with sodium are... 

Reduction ofarenes. Calcium in combination with amines, generally methylamine or ethylenediamine, reduces aromatic hydrocarbons to monoenes in generally high yield. Addition of t-butyl alcohol to this system effects reduction to nonconjugated dienes (Birch-type products). Ethylenediamine is essential in this case addition of a second amine is generally desirable for solubility reasons. ... 

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