Monocyclic -Conjugated Anions

The for conversion of phenalene to its anion is 19. The cation is estimated to have a plphenalenyl cation have been developed. Because the center carbon is part of the conjugated system, the Huckel rule, which applies only to monocyclic conjugated systems, caimot be applied to just the peripheral conjugation. The nature of the phenalenyl system is considered further in Problem 12 at the end of this chapter. 

Hiickel pointed out that, on the basis of molecular orbital theory, monocyclic conjugated polymethines have filled shells of tt-electrons when the number of TT-electrons is An + 2, where n is an integer. These systems may be expected to be stable. The rule may be illustrated by reference to Fig. 2.1. If = 0, then a system with 27r-electrons should be stable. Such a situation is found in the cyclopropenyl positive ion, which has been isolated as the hexachloroanti-monate. For n = the prediction is that the cyclopentadienyl anion, benzene and the cycloheptatrienyl (tropylium) cation are stable. This is certainly in accord with experience. The stability of benzene is well known, the cydo-pentadienyl anion is readily formed by the action of potassium metal on cyclopentadiene, and the cycloheptatrienyl cation is one of the most stable carbonium ions known. Huckel s rule also predicts that some of the larger cyclic conjugated systems are stable, e.g. those with 10,14 and 18 rr-electrons. However, the situation is complicated by steric problems (see for example Garratt, 1971) and need not be considered further here. 

In neutral 1,3-diphospholes (22) the second heteroatom is a three-coordinate pyramidal phosphorus atom which is not at all or only very weakly conjugated to the remaining Jt-system. Full cyclic conjugation occurs only in their anions, that is, in the 1,3-diphospholide anions. The same applies to the 1,3-benzodiphospholides (3) and their anions. The known monocyclic and fused 1,3-heterophospholes and 1,3-diphospholes are shown in Table 1. 

Cyclopentadienide anion is an aromatic anion. It has six ir electrons delocalized over a completely conjugated planar monocyclic array of five 5p -hybridized carbon atoms. 

This article is concerned with cationic, anionic, and free radical conjugated cyclic hydrocarbons that do not contain the Hiickel 4n -I- 2 number of -electrons. Thus, among species with no more than one charge, there are the 3, 4, 5, 7, and 8 yr-electron structures 1-10 for monocyclic rings up to seven-membered. 

Early research on boron-containing polycyclic aromatic hydrocarbons ( -PAHs) and their monocyclic archetypes was motivated in many cases by a general desire to understand the impact of an electron-deficient element such as boron on the properties of a conjugated cyclic array. A variety of different polycyclic systems have been conceived in which neutral or anionic sp -hybridized boron centers could act as surrogates for cationic or neutral carbon centers, respectively... 

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. 

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