Antiaromatic carbocations

Although five equivalent resonance structures can be drawn for all three species, Huckel s rule predicts that only the six-ir-electron anion should be aromatic. The four-77-electron cyciopentadienyl carbocation and the five-7r-electron cyciopentadienyl radical are predicted to be unstable and antiaromatic. 

The predicted antiaromaticity in fluoranthene-PAH carbocations (NICS) could well be the origin of the observed paratropicity and proton shielding in these nonalternant-PAH carbocations. The observed broadening in the proton spectra in several cases, the appearance of upfield-shifted broad humps, and the formation of insoluble precipitates (which upon quenching returned the intact PAH) were taken as evidence for the concomitant presence of the RC which could additionally contribute to proton shielding. 

Rings containing an odd number of carbon atoms can be aromatic or antiaromatic, if they are planar and have a conjugated p orbital on each ring atom. To have an even number of electrons in their odd number of p orbitals, these species must be ionic. They must be carbocations or carbanions. 

In contrast, the cyclopentadienyl carbocation, which has four pi electrons and is antiaromatic, is quite unstable. Thus, 5-iodo-l,3-cyclopentadiene is unreactive under conditions in which iodocyclopentane reacts rapidly by an SN1 mechanism. 

Carbocations in which the empty C(p) orbital is part of an aromatic system (Chapter 1) are considerably stabilized over what one would expect just from resonance stabilization alone. The most important aromatic carbocation is the tropylium (cycloheptatrienylium) ion, which is so stable that one can buy its salts from commercial suppliers. Conversely, those carbocations in which the empty C(p) orbital is part of an antiaromatic system (e.g., cyclopentadienylium) are considerably destabilized. 

Alkynes undergo the same set of reactions as alkenes but are slightly less reactive because the intermediates involved are less stable. For the Ade2 process, the vinyl cation intermediate formed is less stable than the alkyl carbocations formed when electrophiles attack alkenes. Addition to the vinyl cation produces a mixture of syn and anti addition. Stabilizing the vinyl cation by bridging is less favorable since the bridged ion is more strained and may have some antiaromatic character. 

The cycloheptatrienyl carbocation (20) is also a six n electron system, and all six electrons can go into bonding orbitals. Its delocalization energy is calculated to be 2.99)8 (Figure 4.26), which is close to the value of 50 kcal/mol of delocalization stabilization calculated by other methods. Therefore, the cycloheptatrienyl cation is an especially stable carbocation, although it is still a cation and is certainly not as stable as benzene. On the other hand, the cycloheptatrienyl anion is a 4n Ji system and thus is predicted to be antiaromatic by HMO theory. More advanced calculations suggest that any energy consequences of electron delocalization in the anion must be very small. 

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. 

Cation (13) results when the alcohol is dehydrated with acid." Aldehydes and methyl ketones readily add at the carbocation centre, and (13) has a pA(r+ value of —0.81." Species such as (14 Y = Cl, Me, Ph, OH) maintain as much aromaticity as possible they are not antiaromatic, but dication (15) is." Unfortunately it was found that the degree of antiaromaticity found depends on the calculational level." Cations (16 Y = OMe, Me, Cl, F), obtained when the appropriate fulvene precursor is treated with SbFs-SOaClF, can also be described as antiaromatic, to an extent depending on the electronic effect of the Y substituents." ... 

The solvolytic reactivity of the 9-CFs substituted fluorenyl tosylate 46 was strongly depressed compared to R = H, and a rate factor of 10 due to antiaromatic destabilization was estimated. Solvolytic studies with formation of 9-fluorenyl carbocations with COzR, - CONMez, and CR=NOCH3 substituents have also been reported. On the basis of the calculated geometry it was suggested by Creaiy et al. that such cations avoid antiaromatic structures with cyclopentadienyl cation moieties and resemble bis(dienyl) cations 28a. 

In summary, antiaromatic character of the fluorenyl carbocation is expected to be strongly attenuated. However, in a number of examples cited there is evidence for the effects of residual antiaromaticity, either in the molecule as whole or just for the cyclopentadienyl fragment. 

When 1,3,5-cycloheptatriene is heated with bromine, a stable salt is formed, cycloheptatrienyl bromide. In this molecule, the organic cation contains six delocalized tt electrons, and the positive charge is equally distributed over seven carbons (as shown in the electrostatic potential map in the margin). Even though it is a carbocation, the system is remarkably unreactive, as is expected for an aromatic system. In contrast, the cycloheptatrienyl anion is antiaromatic, as indicated by the much lower acidity of cycloheptatriene (pA"a = 39) compared with that of cyclopentadiene. 

Cations that are constrained not to be planar are not generally stable. The energy required to deform a carbocation toward a pyramidal structure with an empty sp orbital is some 100 kj mol". So in 4.69, the cation is constrained not to be planar, and this cation cannot be prepared. And as we have seen in the previous chapter, cations with aromatic character, such as the tropylium ion, 4.70, are especially stable, while those with antiaromatic character, such as the cyclopentadienyl cation, 4.71, are especially unstable. 

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