A mechanistic analysis of the Birch Reduction.

The Birch Reduction is one of the main reactions of organic chemistry. The reaction involves the reaction of dissolving metals in ammonia with aromatic compounds to produce 1,4-cyclohexadienes. Discovered by Arthur Birch in 1944, the reaction occupies 300 pages in Organic Reactions to describe its synthetic versatility. Thus, it is remarkable that the reaction mechanism has been so very controversial and only relatively recently has been firmly established. Perhaps this is not that surprising, since the reaction also has many unusual and esoteric mechanistic facets. Here, I provide a description of how I have applied ever-evolving levels of quantum mechanics and a novel experimental test to understand details of the mechanism and the origins of the selectivities observed in the Birch reduction. The reaction involves an initial radical anion resulting from introduction of an electron from the blue liquid ammonia solution of free electrons formed by the dissolution of Li or related metals. This radical anion is protonated by an alcohol and then further reduced to a carbanion. Finally, the carbanion is protonated using a second proton to afford a nonconjugated cyclohexadiene. The regiochemistry depends on substituents present. With 18 resonance structures in the case of anisole radical anion, prediction of the initial protonation site would seem difficult. Nevertheless, computational methods from Hückel theory through modern density functional calculations do correctly predict the site of protonation. An esoteric test established this mechanism experimentally. The nature of the carbanion also is of mechanistic interest, and the preponderance of the resonance structure shown was revealed from Hückel calculations involving variable bond orders. For the trianion from benzoic acid, parallel questions about structure are apparent, and have been answered. Some mechanistic questions are answered experimentally and some by modern computations. Recently, our mechanistic understanding has led to a variety of synthetic applications. For example, the preparation of alkyl aromatics from benzoic acids makes use of the intermediates formed in these reactions. This Account provides an overview of both experimental techniques and theoretical methodology used to provide detailed mechanistic understanding of the Birch Reduction.