2.2.1 Introduction
The solution of alkali metal in ammonia (at -33°C) can generate solvated electrons and metal cations.
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These solvated electrons can reduce conjugated double bonds, triple bonds and aromatic compounds. Most of the organic compounds are not soluble in liquid ammonia and, therefore, the compounds are dissolved in THF or Et2O and are added to the dissolved metal solution.
2.2.2 Reduction of Conjugated Dienes
The conjugated dienes are readily reduced to the corresponding 1,4-dihydro compounds with dissolved metal ammonia reagents (Scheme 1).
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Scheme 1 |
2.2.3 Reduction of α,β-Unsaturated Ketones
The α,β-unsaturated ketones could be reduced to the corresponding saturated carbonyl compounds or alcohols depending on the reaction conditions with dissolved metal in ammonia (Scheme 2). For examples, the α,β-unsaturated ketone, cyperone, when treated with lithium in ammonia, it gives the corresponding saturated ketone but when treated with lithium in ammonia and ethanol, a good proton source, it gives the corresponding alcohol.
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Scheme 2 |
Mechanism
The solvated electron is transferred to the conjugated system to give the radical anion which is protonated by added alcohol or ammonia and then the second electron transfer generates the enolate anion which on protonation during work up gives the desired carbonyl compound (Scheme 3).
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Scheme 3 |