文档介绍：The Birch Reduction of pounds
Peter W. Rabideau, Louisiana State University, Baton Rouge, Louisiana
Zbigniew Marcinow, Academy of Agriculture, Wroclaw, Poland
1. Introduction
The reduction of pounds by alkali metals in liquid ammonia
represents an important method for the preparation of partially unsaturated
six-membered rings. The reaction was discovered by Wooster and Godfrey, (1)
but the major development resulted from the efforts of A. J. Birch, (2, 3) and
the reaction has e to bear his name. Although a variety of metals
can be used, the mon are sodium and lithium, and, to a lesser extent,
potassium. Cosolvents such as ether or tetrahydrofuran (THF) are often used
to improve solubility, and weak acids such as alcohols may be employed
during the reaction as proton sources. The latter are necessary for the
reduction of benzene and its unactivated derivatives. Improvement in
experimental procedures by Wilds and Nelson, (4) the application to
pounds by Hückel (5) and later by Harvey, 3f and the
development of methods for the alkylation of the anions generated in this
process (i.
e., reductive alkylation) have made this reaction an important
approach to the synthesis of a wide variety pounds. (6)
2. Mechanism, Regiochemistry, and Stereochemistry
. Mechanism
Alkali metals dissolve in liquid ammonia to produce deep blue solutions that
behave as if they contain metal cations and solvated electrons, and provide an
excellent reducing medium. (7, 8) As shown in Eq. 1, the aromatic substrate
accepts an electron to produce a radical anion in equilibrium with the reactant.
In the presence of an alcohol, the radical anion may be protonated to furnish a
radical which quickly adds an electron resulting in a monoanion (Eq. 2a). In the
absence of an alcohol, the radical anion may simply persist in low
concentration,