The Birch Reduction
Another way of adding hydrogen to the benzene ring is by treatment with the electron rich solution of alkali metals, usually lithium or sodium, in liquid ammonia. This general type of reaction is known as the Birch reduction after the Australian chemist, A. J. Birch. With benzene, reduction with metals leads to 1,4-cyclohexadiene:
The initial step of the Birch reduction is an electron transfer to the lowest unoccupied molecular orbital of the benzene [latex] \pi [/latex] system (see Figure 21-5) to form a radical anion:
Subsequent steps include a sequence of proton- and electron-transfer steps as follows:
Substituent effects observed for this reaction are entirely consistent with those described for electrophilic substitution and addition – only reversed. That is, the reactivity of an arene in metal reductions is increased by electron-withdrawing groups and decreased by electron-donating groups. Substituents that can stabilize the anion-radical intermediate facilitate the reduction.