Antiaromatic systems

Antiaromatic molecules are cyclic systems containing alternating single and double bonds, where the Jt-electron energy of antiaromatic compounds is higher than that of its open-chain counterpart Therefore, antiaromatic compounds are unstable and highly reactive often, antiaromatic compounds distort themselves out of planarity to resolve this instabihty. Antiaromatic compounds fail Hiickel s rale of aromaticity, that is, of (4 -i-2) it-electrons. Compounds that are destabilized relative to conjugated noncychc polyene models are called antiaromatic. 

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 Jt-electron energy of cydobutadiene is higher than that of its open-chain counterpart, 1,3-butadiene, and it is therefore said to be antiarornatic rather than aromatic. Recent studies on cydobutadiene show that it has a rectangular structure as opposed to a square structure and two different l,2-dideuterio-l,3-cyclobutadiene stereoisomers. This indicates that the Jt-electrons are localized and therefore not considered to be aromatic. However, it is far from stable it is highly reactive, and has a very short lifetime. Cydobutadiene dimerizes by a Diels-Alder reaction at 

The monomeric form has been studied at higher temperatures by trapping with matrix isolation in a noble gas. 

A molecule with An n electrons in the ring, with the molecular orbitals made up from An p orbitals, does not show this extra stabilisation. Molecules in this class that have been made include cyclobutadiene 1.13 (n = 1), the cyclopentadienyl cation 1.14, cyclooctatetraene 1.15 andpentalene 1.16 (n 2), [12]annulene 1.17 (n = 3) and [16]annulene 1.18 (n A). 

There is much evidence that cyclic conjugated systems of An electrons are significantly more reactive than the corresponding open-chain polyenes. There has been much speculation that they not only lack stabilisation but are actually destabilised. They have been called anti-aromatic as distinct from nonaromatic. 

A molecule with An n electrons in the ring, with the molecular orbitals made up from An p orbitals, does not show this extra stabilisation. Molecules in this class that have been studied include cyclobutadiene 1.17 

We can reach a similar conclusion from an interaction diagram, by looking at the effect of changing butadiene 1.24 into cyclobutadiene 1.25 (Fig. 1.47). This time there is one drop in n energy and one rise, and no net stabilisation from the cyclic conjugation. As with benzene, we can see that the drop is actually less (from overlap of orbitals with a small coefficient) than the rise (from overlap of orbitals with a large coefficient). Thus cyclobutadiene is less stabilised than butadiene. 

There is much evidence that cyclic conjugated systems of An electrons show no special stability. Cyclobutadiene dimerises at extraordinarily low temperatures ( 35K).28 Cyclooctatetraene is not planar, and behaves like an alkene and not at all like benzene.29 When it is forced to be planar, as in pentalene, it becomes unstable to dimerisation even at 0 °C.30 [12]Annulene and [16]annulene are unstable with respect to electrocyclic reactions, which take place below 0 °C.31 In fact, all these systems appear on the whole to be significantly higher in energy and more reactive than might be expected, and there has been much speculation that they are not only lacking in extra stabilisation, but are actually destabilised. They have been called antiaromatic 32 as distinct from nonaromatic. The problem with this concept is what to make the comparisons with. We can see from the arguments above that we can account for the destabilisation 

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Stabilizing resonances also occur in other systems. Some well-known ones are the allyl radical and square cyclobutadiene. It has been shown that in these cases, the ground-state wave function is constructed from the out-of-phase combination of the two components [24,30]. In Section HI, it is shown that this is also a necessary result of Pauli s principle and the permutational symmetry of the polyelectronic wave function When the number of electron pairs exchanged in a two-state system is even, the ground state is the out-of-phase combination [28]. Three electrons may be considered as two electron pairs, one of which is half-populated. When both electron pahs are fully populated, an antiaromatic system arises ("Section HI). 

The results of the derivation (which is reproduced in Appendix A) are summarized in Figure 7. This figure applies to both reactive and resonance stabilized (such as benzene) systems. The compounds A and B are the reactant and product in a pericyclic reaction, or the two equivalent Kekule structures in an aromatic system. The parameter t, is the reaction coordinate in a pericyclic reaction or the coordinate interchanging two Kekule structures in aromatic (and antiaromatic) systems. The avoided crossing model [26-28] predicts that the two eigenfunctions of the two-state system may be fomred by in-phase and out-of-phase combinations of the noninteracting basic states A) and B). State A) differs from B) by the spin-pairing scheme. 

Oxepin and its derivatives have attracted attention for several reasons. Oxepin is closely related to cycloheptatriene and its aza analog azepine and it is a potential antiaromatic system with 871-elcctrons. Oxepin can undergo valence isomerization to benzene oxide, and the isomeric benzene oxide is the first step in the metabolic oxidation of aromatic compounds by the enzyme monooxygenase. 

Both peracyclene (105), which because of strain is stable only in solution, and dipleiadiene (106) are paratropic, as shown by NMR spectra. These molecules might have been expected to behave like naphthalenes with outer bridges, but instead, the outer n frameworks (12 and 16 electrons, respectively) constitute antiaromatic systems with an extra central double bond. 

Gygax R, Wirz J, Sprague JT, Allinger NL. Electronic structure and photophysical properties of planar conjugated hydrocarbons with a 4n-membered ring. Part III. Conjugative stabilization in an antiaromatic system The conformational mobility of l,5-bisdehydro[12]annulene. Helv Chim Acta 1977 60 2522-9. 

Of enormous interest to our present discussion would be XRD data on monomeric boroles in order to determine the structural consequences of 7r-electron delocalization in this four-electron, formally antiaromatic system.30 However, many attempts to grow suitable crystals of 94 have failed, and prolonged storage of solutions has produced only yellow dimers, presumably similar in structure to 100. Hence, in estimating the relative importance of structures 94a-c, spectral data must be our guide. 

The l,5-dibora-2,5-cyclohexadiene system (7) is of unusual interest because such a planar array of atoms is predicted to be a Hiickel antiaromatic system, similar to boroles themselves. Indeed, when 103 is generated at low temperatures, it readily rearrange into the nWo-2,3,4,5-tetracarba-hexaboranes6 (8)" (Eq. 33). 

Figure 7. Aromatic and antiaromatic systems in the ground state (GS) and the twin excited state (ES). The parameter E, is the coordinate that transforms A to B.

The resultant material (isolated in low yield) is an intriguing tricyclic system that could be viewed as an overall antiaromatic system or as two aromatic rings with two phosphorus sites connecting them. In any event, this tricyclic product readily undergoes addition of water in even trace amounts to generate a bis-secondary phosphine oxide (Equation 4.26). 

The H NMR spectrum showed the paratropic shift expected of protons in an antiaromatic system. Because it is difficult to evaluate the magnitude of paratropic shift in a charged species, the average shift of protons in 3 were com-... 

Once again, we may identify the cis-1,2-difluoroethylene pi dication as a 4N pi electron Hiickel antiaromatic system and the trans isomer as a 4N pi electron non-aromatic system. 

The differences between chemical shifts for the outer and inner protons are quite sizeable for the aromatic and the antiaromatic systems they are of opposite sign. 

For the antiaromatic three-membered heterocycles, experimental data are available only for thiirenes (and there is some doubt about the true antiaromaticity of thiirenes). Bond lengths have been calculated, however, for these antiaromatic -systems (80PAC1623). In comparison with the corresponding saturated heterocycles, the C—X bond lengths are increased by 0.05 to 0.17 A and the C—C bond length is decreased by 0.2 A. 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

Very strong bases can extract a proton from the 1,2- or 1,4-dihydropyridine ring giving a fully conjugated eight-7T-electron antiaromatic system, which can be trapped by electrophiles. 

An alternative theory associates electron transfer with transfer of a state of aromaticity from molecule to molecule within the stack (77AG(E)519). Efficient charge transport was identified with conversion of a neutral, antiaromatic system to a charged, aromatic radical by electron transfer. This interpretation has been eroded by the synthesis of conductors from aromatic systems such as perylene hexafluoroarsenate (81MI11301) or polypyrrole tetrafluoroborate (80CC397, 81MI11300) where an electron is transferred from a neutral, aromatic molecule to a non-aromatic charged radical. 

T809). In the bis-furan (122) the chemical shifts are close to those in unperturbed furans and give no support to the idea that a 167r-electron antiaromatic system is present (70JA973). 

Some Examples of Aromatic and Antiaromatic Systems Neutral Even-Membered Rings... 

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