Aromatic rings cyclopropenyl cation

The tropylium and the cyclopropenyl cations are stabilized aromatic systems. These ions are arumatic according to Hiickel s rule, with the cyclopropeniiun ion having two n electrons and the tropyliiun ion six (see Section 9.3). Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

Structures that are also aromatic are the cyclopropenyl cation (2 jt electrons n = 0) and the cyclopentadienyl anion (6 n electrons n = 1). Although we do not wish to pursue these examples further, they are representative of systems where the number of jr electrons is not the same as the number of carbon atoms in the ring. 

Three-membered ring systems have also provided examples of aromatic and antiaromatic behavior. Despite the very substantial angle strain, Breslow and his collaborators have succeeded in preparing a number of cyclopropenyl cations (51).30 Cyclopropenone (52) has been isolated and is stable in pure form below... 

The results of this charge calculation are summarized in Fig. 4.24 the negative charge on the exocyclic carbon and the positive charges on the ring carbons are in accord with the resonance picture (Fig. 4.24), which invokes a contribution from the aromatic cyclopropenyl cation [50]. Note that the charges sum to (essentially) zero, as they must for a neutral molecule (the hydrogens, which actually also carry... 

The cyclopropenyl cation is the simplest aromatic system, and thus of some theoretical interest. In addition, the chemistry of cyclopropane derivatives is full of Interesting rearrangements to other novel structures,9 reflecting the great strain energy Of the cyclopropene ring. 

Aromatic systems that obey Hiickel s 4 + 2 rule where = 0 and so possess two 7i-electrons do exist and are indeed stable. The smallest possible ring is three membered and the derived unsaturated structure is cyclopropene. The theoretical loss of a hydride ion from this molecule leads to the cyclopropenyl cation, which contains two 7t-electrons distributed over the three carbon atoms of the planar cyclic system (Figure 1.8). 

Obviously, there can be no ring of two carbon atoms though a double bond may be regarded as a degenerate case. However, in analogy to the tropylium ion, a three-membered ring with a double bond and a positive charge on the third atom (the cyclopropenyl cation) is a 4n + 2 system and hence is expected to show aromaticity. The unsubstituted 80 has been prepared,as well as several derivatives, e.g.,... 

These molecules are cyclopropenyl cation, cyclopentadienyl anion, and pyridine respectively. Notice that in the case of pyridine, the lone pair on the nitrogen is not involved in the aromatic sextet, but rather projects from the nitrogen in the same plane as the ring, instead of being orthogonal to it, which is what would be required if it were to become involved in the aromatic ring of p sub-orbitals. 

IUPAC nomenclature is generally followed . The cyclic structures are called heterometallacycles and dimetallacycles. When the ring is composed of a metal, a nonmetal, and a carbon, the rings are numbered in that order. When two metals are present, the higher atomic number metal takes precedence. Metallacyclic three-rings which contain a double bond may possess cyclopropenyl cation-like aromaticity and such structures have been proposed <84AG(E)89>. 

The cyclopropenyl cation is aromatic because it has an unintermpted ring of p orbital-bearing atoms and the tt cloud contains one (an odd number) pair of delocalized TT electrons. The cyclopropenyl anion is not aromatic because although it has an uninterrupted ring of p orbital-bearing atoms, its tt cloud has two (an even number) pairs of tt electrons. 

It suggests that it is not the size of the ring but the number of electrons present in it determines whether a molecule would be aromatic or antiaromatic. In fact the molecules with An+ 2) n electrons are aromatic whereas with (An, 0) n electrons are antiaromatic. Thus, benzene, cyclopropenyl cation, cyclobutadiene dication (or dianion), cyclopentadie-nyl anion, tropylium ion, cyclooctatetraene dication (or dianion), etc. possess (4 + 2) ti electrons and hence aromatic whereas cyclobutadiene, cyclopentadienyl cation, cycloheptatrienyl anion, cyclooctatetraene (non-planar) etc. have An n electrons which make them antiaromatic . Systems like [10] annulene are forced to adopt a nonplanar conformation due to transannular interaction between two hydrogen atoms and hence their aromaticity gets reduced even if they have (An + 2)n electrons. On the other hand the steric constraints in systems like cyclooctatetraene force it to adopt a tube-like non-planar conformation which in turn reduces its antiaromaticity. Various derivatives of benzene like phenol, toluene, aniline, nitrobenzene etc. are also aromatic where the benzene ring and the n sextet are preserved. In homoaromatic " systems, like cyclooctatrienyl cation, delocalization does not extend over the whole molecule. 

In addition to neutral molecules, certain cation and anion intermediates meet the criteria for aromaticity. If the cyclopropenyl cation (115) and the cycloheptatrienyl cation (116) are examined, both have a continuous array of p-orbitals confined to a ring and a number of n-electrons that fit the 4n -i- 2 series (two for 115 and six for 117). Both of these carbocations are aromatic, which means that they are very stable, easy to form, and relatively long-lived intermediates. Compare these carbocations with the cyclopentadienyl cation (117), which meets the criterion of having a continuous array of p-orbitals confined to a ring, but has 4n ji-electrons (not a number in the 4n -i- 2 series) and is not aromatic. Indeed, it is considered to be antiaromatic, is very unstable, and is very difficult to form. 

Because charged ring systems can satisfy the Hiickel criteria for aromaticity, they are highly stabilized compared to other nonaromatic cations or anions. Examples include the cyclopropenyl cation, cycloheptatrienyl cation, and cyclopenta-dienyl anion. 

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