Antiaromatic cyclopropene

It is clear that simple cyclobutadienes, which could easily adopt a square planar shape if that would result in aromatic stabilization, do not in fact do so and are not aromatic. The high reactivity of these compounds is not caused merely by steric strain, since the strain should be no greater than that of simple cyclopropenes, which are known compounds. It is probably caused by antiaromaticity. ... 

Another group of unstable carbanions are those with antiaromatic character (Scheme 5.71). Thus, cyclopropenyl anions or oxycyclobutadienes, generated by deprotonation of cyclopropenes or cyclobutenones, respectively, will be highly reactive and will tend to undergo unexpected side reactions. Similarly, cyclopentenediones are difficult to deprotonate and alkylate, because the intermediate enolates are electronically related to cyclopentadienone and thus to the antiaromatic cyclopenta-dienyl cation. 

Removal of one electron should make no difference to the relative stabilities of polyene molecule ions or even electron polyene fragments as compared to their neutral counterparts, e.g. butadiene and the allyl radical should have the same relative stabihties as the butadiene molecule ion, and the allyl cation. Removal of one electron will, however, alter the stabihties, and thus the reactivities of cychc polyenes. The molecule ions of aromatic hydrocarbons will be substantially less aromatic then their neutral counterparts. Correspondingly the molecule ions of antiaromatic hydrocarbons will not be as antiaromatic as their neutral analogs, e.g. cyclobutadiene + should be relatively more stable than cyclobutadiene. The largest charge effects in hydrocarbons will be observed in nonaltemant ) monocychc hydrocarbons. The cyclopropenium ion 7 and the tropillium ion 2 are both strongly aromatic as compared to their neutral analogs. Consequently CsHs is a very common ion in the mass spectra of hydrocarbons while cyclopropene is not a common product of hydrocarbon pyrolysis or photo-... 

Knowledge of the energetics of (C-C3H2 )-CH 2 (15) gives information relevant to those of other cyclopropene derivatives. If one accepts the above experimentally determined heat of formation of C4H4, the theoretical energy difference of vinylacetylene and methyl-enecyclopropene, and an ill-defined experimentally derived heat of formation of vinylacetylene S the ionization potential of methylenecyclopropene is indirectly deduced to be 8.2 eV. This value is meaningfully compared to the 9.5 eV directly measured as the ionization potential of cyclopropenone. The derived 1.3 eV difference is comparable to the 1.1 eV difference for the antiaromatic cyclopentadienone (with a vertical value of 9.5 eV) and non-aromatic methylenecyclopentadiene (i.e. fulvene, with an IP of 8.4 eV). 

Oxirene is formally a 4 71 antiaromatic system, and furthermore contains considerable ring strain. The ring strain in cyclopropene is ca. 53 kcal/mol Oxirenes are therefore expected to be short-lived high-energy intermediates. There has been much discussion among the theoreticians whether oxirene is more or less stable than the isomeric oxocarbene. The conflicting results of calculations are indicated in Fig. 1. 

Very recently, it has been shown that on the basis of the energetic criterion of antiaromaticity and the proton affinity of 3-cyclopropenyl anion (13) this ion does not merit being differentiated from other aUylic anions and is therefore best thought of as non-aromatic. Cyclopropene is the smallest cycloafkene, and its conjugate base at C3 is considered to be a special anion that is destabihzed due to the presence of 4jt electrons in this fuUy conjugated monocycHc species. Its acidity, however, follows the same correlation as for cyclobutene, cyclopentene, cyclohexene, and propene. No additional parameter beyond the central C—C—C bond angle is needed to explain or account for the weak acidity of cyclopropene. 

The 3-cyclopropenyl anion is more basic than the allyl anion and cyclopropyl anion, its acyclic and saturated counterparts. This can be accounted for by the small central C-C—C bond angle and the resulting electrostatic repulsion in the constrained anion. No additional parameter is needed to account for the weak acidity of cyclopropene at the aUyhc position. Consequently, on the basis of the thermodynamic definition of antiaromaticity, this concept is not needed to describe the 3-cyclopropenyl anion. Magnetic criteria such as nuclear independent chemical shifts (NICSs) lead to a different conclusion, but in this instance there is no energetic basis for this view. Consequently, the 3-cyclopropenyl anion is best described as non-aromatic despite 50 years of thought to the contrary. 

The focus of this study is to find out to what extent different exocyclic (C=X) substituents induce aromaticity and antiaromaticity in cyclopropene derivatives (HC)2C=X [32]. Although the consensus opinion suggests that parent methylenecy-clopropene, (HC)2C=CH2, the simplest cross-conjugated cyclic hydrocarbon, is nonaromatic, it is admitted that cyclopropenone is at least modesdy aromatic [33-35]. However, quantitative assessments of the aromaticity of other (HC)2C=X derivatives have not led to satisfactory agreement. Thus, this family of compounds represents a paramount opportunity to use the EDA-aromaticity method to give a definitive answer to a question that has been controversially discussed in the literature for several decades [33-35]. 

The metal ion may be partially covalently bonded to the anion [199], Ab-initio calculations on the substitution of the hydrogen atoms of cyclopropene by lithium indicate that substitution of the vinylic hydrogen is energetically favoured compared to substitution of a methylene hydrogen because of the development of so-called antiaromatic character in the latter process [201]. 

When one deprotonates propene, it is the methyl hydrogens that are the most acidic. Deprotonation creates the resonance stabilized allylic anion. When the analogous reaction is attempted with cyclopropene, a vinylic hydrogen is the one removed. Deprotonation of the CH2 group in cyclopropene (Eq. 2.19) would create an antiaromatic anion, an undesirable effect, and this reversal in acidities provided early support for the notion of destabilization due to antiaromaticity. 

In Chapter 2 we showed that aromaticity strongly stabilizes organic structures. Accordingly, large effects are prevalent in acid-base chemistry. Tables 5.6E and I highlight some examples. The most well known is the acidity of cyclopentadiene (pK 16.0), which is similar to that of water. The resulting anion is aromatic. However, when the resulting anion is antiaromatic the compounds are dramatically less acidic, as expected the cyclopropene pKj, is 61 and the pK of cycloheptatriene is 38.8. 

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