Reduction via Alkali Metals

The reduction using alkali metals is carried out in dry aprotic solvents, usually THF, and in an inert atmosphere (vacuum). The metals used are Li (wire), Na, K, Rb and Cs (mirror). The PAHs can accept several charges (up to 6 electrons). Consecutive paramagnetic and diamagnetic states can be observed by EPR and NMR spectroscopies, respectively. Many examples of reduction with alkali metals are discussed in depth here. Corannulene (2) offers an excellent example of the application of this reduction method. 2 was reduced by Li [6, 7], Na, K, Rb and Cs [8], and four reduction steps were observed, each having a different color (with the exception of sodium, which gave only two reduction steps). The four reduction steps yielded mono-, di-, tri- and tetra-anions, which were characterized by EPR or NMR [6-9]. 

Reduction of PAHs in a solution of alkali metal in liquid ammonia, known as the Birch reaction [10], is a conventional synthetic way to reduce polyaromatic rings to their dihydro derivatives [11]. Highly basic dianions are produced as intermediates, and are protonated by the ammonia to afford monoanions. In most cases, the monoanions persist in ammonia, and are quenched when a stronger proton source is added [12]. The choice of alkali metal affects the kinetics of the protonation of the monoanion intermediate. 

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