Polycyclic reduction with alkali metals

In this chapter we concentrate on reduction processes of carbon-rich systems. The formation of anions and radical anions of -conjugated monocyclic systems, cyclophanes, bowls and fullerenes is described. Carbon-rich compounds can be reduced directly by contact with alkali metals Li, Na, K, Rb and Cs, which have a low reduction potential. Proton, carbon and lithium NMR and EPR spectroscopies are the main methods used to gain a better understanding of the mono- and polycyclic systems in solution. Special attention will be given to modes of electron delocalization, aromaticity, anti-aromaticity, as well as aggregation, bond formation and bond cleavage processes of diamagnetic electron transfer products. Electrochemical reductions will be briefly discussed. 

The polycyclic anions were first prepared by metal reduction in 1914 by Schlenk et al.5 a) and studied later by Schlenk and Bergmann 5 b). This class of conjugated anions opened a new era in carbanion chemistry by pointing out the electron transfer process as a source for charged species. The mechanism of the metal reduction of polycyclic hydrocarbons has been investigated and is well established 1,215 18-68>. The addition of two electrons to the fully conjugated (4n + 2)n-molecules yields 4mt paratropic systems 2°. 137 139>. The chemistry of this reaction is simple, with electrons initially on the alkali metal going to 7t-molecular orbitals associated with the aromatic hydrocarbon molecule (Eq. 13). 

Studies of metal reduction of polycyclic aromatic hydrocarbons (PAHs) [1] were reported as early as 1867 by Berthelot [2], where he described the fusion of metallic potassium with naphthalene 1. Studies in solution of electron transfer from Na and K to PAHs started as early as 1913 with Schlenk et al. [3] who reported that when anthracene reacted with the alkali metal in ether, 1 1 and 2 1 metal anthracene adducts were obtained. Not understanding what the 1 1 adduct really was, it was termed radical and later on it was suggested by Hvickel that the radical is a singly charged species [4],... 

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