3 - Dissolving Metal Reductions

Abstract

This chapter discusses dissolving metal reductions. Although once used extensively for the reduction of functional groups, reactions employing dissolving metals have been replaced by other convenient methods. However, certain synthetic sequences that may require stereospecific or functionally selective reductions may best be executed by means of metals in solution. The Birch reduction, or its modifications, employs a solution of an alkali metal in liquid ammonia or an aliphatic amine and is still widely used in connection with the reduction of aromatic or conjugated systems. The reduction of benzene to 1, 4-cyclohexadiene is typical of the one-electron transfer mechanism commonly understood to pertain in such reductions. The reduction of an aromatic system under controlled conditions is an important source of cycloalkanes. Hexamethylphosphoric triamide has been found to be an ideal solvent in which to conduct the reduction of α, β-unsaturated ketones by alkali metals. The reduction of α, β -unsaturated γ-diketones can conveniently be done with zinc in acetic acid.