In Birch reductions

E. Acidity of Proton Donor. Effect of Iron in Birch Reductions... 

A competing reaction in any Birch reduction is reaction of the alkali metal with the proton donor. The more acidic the proton donor, the more rapid IS the rate of this side reaction. Alcohols possess the optimum degree of acidity (pKa ca. 16-19) for use in Birch reductions and react sufficiently slowly with alkali metals in ammonia so that efficient reductions are possible with them. Eastham has studied the kinetics of reaction of ethanol with lithium and sodium in ammonia and found that the reaction is initially rapid, but it slows up markedly as the concentration of alkoxide ion in the mixture... 

The anionic intermediates formed in Birch reductions can be used in tandem alkylation reactions. 

Origin of by-products in Birch reductions / 18 Acidity of proton donor. Effect of iron in Birch reductions / 19 Requirements of Birch reduction procedures / 22 Experimental conditions for Birch reductions / 25... 

Study of the effect of small amounts of iron salts on the reaction of metals with alcohols in liquid ammonia showed that 0.5 ppm. of iron increases the rate of reaction of lithium by a factor of 2 and increases the rate of reaction of sodium by a factor of 50. High yields were then achieved in Birch reductions conducted with sodium and f-butanol in iron-free ammonia. Evidently lithium had seemed superior to sodium merely because the lithium-alcohol reaction is catalyzed by iron much less strongly than the sodium-alcohol reaction is catalyzed. Actually sodium usually is as effective as lithium and in some cases definitely superior. Only in the case of 5-methoxytetralin did lithium give a higher yield (62%) than sodium (45%). The reduction of this compound is particularly slow, and hence the greater rate of reduction by lithium becomes significant. Bothner-By found that benzene is reduced by lithium 60 times faster than by sodium. 

Addition of an alkyl group at C17 initially involves conditions fairly similar to those used in the Birch reduction of enones. A solution of the steroid in liquid ammonia and some inert co-solvent is first treated with lithium metal (Scheme 6.20). The resulting anion 20-1 is then quenched with an alkyl halide instead of the alcohol usually employed in Birch reductions. The anion at C17 then displaces the halogen, in the case at hand iodide, from the alkyl group to form a carbon-carbon bond and the 17a-methyl derivative 20-2. 

Rieke and Bales have reported further examinations of the naphthocyclobutene radical anion. Birch reduction of benzocyclopropene proceeds via the radical anion (517). No bicyclic intermediates such as those found in Birch reduction of other benzocycloalkenes are observed in this case. ... 

An innovative method of handling alkali metals is to absorb them into silica gel, thus providing a convenient source of the metals as powerful reducing agents, e.g. in Birch reductions ... 

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