Radicals, anions sigma

The cyclic peralkylsilane oligomers, (R2Si) with n = 4-6, manifested especially strong electron delocalization.5 These rings are structurally analogous to those of the cycloalkanes, since the silicon atoms form four sigma bonds. However, the electronic properties of the cyclosilanes more nearly resemble those of aromatic hydrocarbons such as benzene. One example of such behavior is their reduction to anion radicals. Aromatic hydrocarbons such as naphthalene can be reduced, electrolytically or with alkali metals, to deeply colored anion radicals in which an unpaired electron occupies the lowest unoccupied molecular orbital (LUMO) of the hydrocarbon (equation (2)). 

Recall that the energy required to break the sigma bond homolytically is called the bond dissociation energy (BDE, p. 337). Note that the bond breaks to give two neutral species, two radicals, and not to give two polar species, a cation and an anion (Fig. 11.3). 

Polyolefins.—The synthesis of 1,2,3,5-hexatetraene, one of the two remaining of the 12 linear CgHf, isomers, was reported as outlined in Scheme 56. The anion derived from (107) rearranges by competing [1,2], [1,4], homo [2,5], and homo [4,5] sigma-tropic rearrangements to give allylic alcohols (108) and (109) in addition to aldehydes (110) and (111). The hydrocarbon (112) is also produced from a radical dissociation-recombination process. 

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