Antiaromatic 3-cyclopropenyl anion

In this structure all the hydrogens are on the same side of the ring. Delocalization is minimized because the delocalized structure is an antiaromatic cyclopropenyl anion. 

Two other systems that have been studied as possible aromatic or antiaromatic four-electron systems are the cyclopropenyl anion (59) and the cyclopentadienyl cation (60). In these cases also the evidence supports, antiaromaticity, not aromaticity. With respect to 59, HMO theory predicts that an unconjugated 61 (i.e., a single canonical form) is more stable than a conjugated 59, so that 61 would... 

Very high level ab initio [CCSD(T)//MCSF] calculations have been applied to singlet and triplet cyclopropenyl anion and cyclopropenyl radical. The anion ground state, a singlet with Cg symmetry, is destabilized relative to cyclopropyl anion as expected for an antiaromatic structure it is stabilized, with respect to its conjugate acid and the corresponding radical, by electron-withdrawing substituents such that 1,2,3-tricyanopropene has a predicted pK of 10-15. ... 

The CH2 group of cycloproparenes is relatively acidic. Extended HUckel calculations predict that the benzocyclopropenyl anion 294 should be a resonance-stabilized species, contrary to the cyclopropenyl anion 295 or cyclohepta-trienide (296), which are at least potentially antiaromatic. This prediction has been experimentally verified benzocyclopropene (1) may be deprotonated with BuLi, and the intermediate anion 294 has been trapped with trimethylsilane to afford 236. From the rate of hydrolysis of 236, the pX of 1 has been estimated to 36, i.e., some 5 units below that of toluene (pXj = 41). Theoretical calculations (STO-3G) give a pK of 33 for 1. Metallation at the CHj group of cycloproparenes is the key step for the synthesis of alkylidenecycloproparenes (see above) however, it should be noted that so far, no benzocyclopropenyl anions have... 

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. 

An ab initio study of the effect of a-substituents on the acidity of cyclopropaben-zene has shown that a-substituents stabilize the cyclopropabenzenyl anion (5) less efficiently than the cyclopropenyl anion (6).2 The attachment of induedvely/field acting substituents attached to the carbanionic site predominantly stabilize the cyclopropenyl anion by increasing the, v character of the lone pair, diminishing the antiaromatic character of the three-membered ring at the same time. 

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. 

Add electrons to your energy diagram to show the configuration of the cyclopropenyl cation and the cyclopropenyl anion. Which is aromatic and which is antiaromatic ... 

In HMO theory, the cyclopropenyl cation is aromatic (i.e., it is a closed shell system with large delocalization energy), since both electrons are in the orbital with E = a -I- 26. The Huckel cyclopropenyl anion is antiaromatic because it is an open shell system (having one electron in each of the = a - 6 orbitals) with zero delocalization energy. In contrast, the Mobius cyclopropenyl anion is aromatic, since it is a closed shell system with all four electrons in bonding orbitals, and the Mobius cyclopropenyl cation is antiaromatic. 

We end this section with a comment on how different theories produce different outlooks and create different predispositions. For example, let us consider the noncontroversial case of cyclopropenyl anion and the controversial case of cyclobutadiene, both 4k-electron "antiaromatic" annulenes. The former molecule is computed to have a triplet ground state in and the latter one is computed to have a singlet ground state in geometry. With respect to HMO theory, the CP system can be called "normal" and the CB system "abnormal". More specifically, the former is said to comply to Hund s rule but the latter is claimed to violate it. As a result, spin selection in CB becomes a topic of controversy By contrast, FC theory paints an entirely different picture in which the interesting thing is not that square CB turns out to be singlet but that CP turns out to be triplet ... 

Thus, the cyclobutadienyl 4jr-electron system is remarkably destabilized by antiaromaticity, and as noted below, the same situation holds for the 4.77-electron cyclopropenyl anion 2, cyclobutenone enolate 19a, and cyclopentadienyl cation 5. Larger and odd-electron systems suffer less destabilization, as do systems which can reduce antiaromaticity by pyra-midalization or electron delocalization into fused benzene rings. 

Calculations using the complete basis set ab initio method for the cyclopropenyl radical give an ionization energy of 6.17 eV, in good agreement with an experimental energy of 6.60 eV, and an electron affinity of 0.45 eV. The very low value of the former is indicative of the large aromatic stabilization of the cation, and the low value of the latter indicates the instability of the cyclopropenyl anion. The radical is intermediate between the two, but these results do not permit an estimate of any antiaromatic destabilization of the radical. 

Antiaromatic annulenes are cyclically conjugated hydrocarbons that contain 4nn electrons. Examples of antiaromatic annulenes containing 4n electrons are cyclopropenyl anion, cyclobutadiene, cyclopentadienyl cation, and benzene dication. At its geometry of highest symmetry an antiaromatic annulene... 

Cyclopropenyl anion adopts a geometry in the triplet state" and gives a calculated singlet—triplet gap of 13 kcal/mol. This small gap is characteristic of antiaromatic systems. The reason for the C v symmetry rather than Dzt in the triplet state is not clear. 

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. 

Figure 6.25. A depiction of the cyclopropenyl cation and the cyclopropenide anion. The former, with 4 -I- 2 electrons ( = 0) is particularly stabilized and is considered aromatic while the latter, with 4n electron (n = 1) is considered antiaromatic.

Following the instructions for drawing the tt molecular orbital energy levels of the compounds shown in Figure 8.8, draw the tt molecular orbital energy levels for the cyclo-heptatrienyl cation, the cycloheptatrienyl anion, and the cyclopropenyl cation. For each compound, show the distribution of the tt electrons. Which of the compounds are aromatic Which are antiaromatic ... 

Antiaromaticity in Open-Shell Cyclopropenyl to Cycloheptatrienyl Anions... 

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