Carbonium ions pentadienyl

The conversion of a pentadienyl to a cyclopentenyl cation is an electrocyclic reaction of a system containing loury-electrons delocalized over five atomic orbitals, and should therefore be a csorotatory process. In the example just described, the stereochemistry of the cyclization reaction is masked by subsequent rearrangement of (alkj l groups, which ultimately affords the most stable carbonium ion. The stereochemistry of the cyclization of divinyl ketones under acidic conditions, however, is conrotatory, as predicted ... 

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. 

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