Birch reduction pathway

Reduction of substituted benzenes with sodium (or lithium) in liquid ammonia in the presence of a proton source (such as methanol, ethanol, etc.) leads to a substituted, non-conjugated cyclohexadiene as a result of 1,4-addition of hydrogen (the Birch reduction).16 With benzene the product is cyclohexa-1,4-diene as a result of the following mechanistic pathway. 

These results allow a complete description of metal-ammonia processes, as shown in Scheme IV. The upper pathway represents the classical Birch reduction (i.e., benzenes), whereas polynuclear aromatic compounds react by one of the lower routes. As indicated in Scheme IV, the protonation of dianions rather than radical anions by ammonia is favored. We previously suggested dianion protonation (8) because radical anions are not very basic (10). Recently, Mullen et al. (12) pointed out that, in some cases, dianions and radical anions of the same neutral precursor are protonated at different sites in the cases they investigated, protonation occurred with the dianions. 

Scheme 6.80. A potential pathway for the reduction of benzene with sodium in Uquid ammonia in the presence of ethanol as a proton source. This is an example of the Birch reduction.

Scheme 10.66. A potential pathway for protodesilylation and enol hydrolysis of the Birch reduction product of l-methoxy-4-(trimethylsilyl)methylbenzene to generate 4-methylene-cyclohexanone (see Rabideau, P. W. Marcinow, Z. Organic Reactions, Paquette, L. A., ed. Vol. 42, Wiley, Hoboken, NJ, 1992).

---