Antiaromatic compounds cyclobutadiene

Thus cyclobutadiene like cyclooctatetraene is not aromatic More than this cyclo butadiene is even less stable than its Lewis structure would suggest It belongs to a class of compounds called antiaromatic An antiaromatic compound is one that is destabi lized by cyclic conjugation... 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

The scheme for treating these subjects is as follows. We compare typical aromatic and antiaromatic compounds. Similar to the comparison between benzene and cyclobutadiene, we concern ourselves here with pyridine (34) and azete (57). Such an approach provides, apart from other advantages,... 

The most obvious compound in which to look for a closed loop of four electrons is cyclobutadiene (44).135 Hiickel s rule predicts no aromatic character here, since 4 is not a number of the form 4n + 2. There is a long history of attempts to prepare this compound and its simple derivatives, and, as we shall see, the evidence fully bears out Hiickel s prediction— cyclobutadienes display none of the characteristics that would lead us to call them aromatic. More surprisingly, there is evidence that a closed loop of four electrons is actually ami-aromatic.1 If such compounds simply lacked aromaticity, we would expect them to be about as stable as similar nonaromatic compounds, but both theory and experiment show that they are much less stable.137 An antiaromatic compound may be defined as a compound that is destabilized by a closed loop of electrons. 

An antiaromatic compound is less stable than an acyclic compound having the same number of jt electrons. Cyclobutadiene is less stable than 1,3-butadiene. 

Why does the number of n electrons determine whether a compound is aromatic Cyclobutadiene is cyclic, planar, and completely conjugated, just like benzene, but why is benzene aromatic and cyclobutadiene antiaromatic ... 

The procedure followed in Sample Problem 17.1 also illustrates why cyclobutadiene is antiaromatic. Having the two unpaired electrons in nonbonding MOs suggests that cyclobutadiene should be a highly unstable diradical. In fact, antiaromatic compounds resemble cyclobutadiene because their HOMOs contain two unpaired electrons, making them especially unstable. 

These various approaches for comparing the thermodynamic stability of aromatic compounds with reference compounds all indicate that there is a large stabilization of benzene and an even greater destabilization of cyclobutadiene. These compounds are the best examples of aromaticity and antiaromaticity, and in subsequent discussions of other systems we compare their stabilization or destabilization to that of benzene and cyclobutadiene. 

Antiaromatic compounds are especially unstable compared with their acyclic analogs. Cyclobutadiene is isolable only in an inert matrix at very low temperatures. Cyclopentadienone is extremely unstable because the C—O resonance structure is antiaromatic. Cycloctatetraene avoids being antiaromatic by bending into a tub shape so that its p orbitals don t overlap continuously. 

Whereas aromatic systems are defined by a positive resonance energy, anti-aromatic systems are characterized by a negative resonance energy. As a rule, antiaromatic compounds are unstable and contain 4n 7i-electrons in a cyclic planar, completely conjugated arrangement. Cyclobutadiene belongs to this category and is stable only in a solid matrix at very low temperatures (20 K). 

It is most significant that in this procedure negative resonance energies appear for antiaromatic compounds. This was not possible with the simple Hiickel expression of eq 1, where any localized structure including cyclobutadiene would have a vanishing delocalization energy. 

Members of another set of cyclic polyenes have (4n) ii-electrons, such as cyclobutadiene (113 4 7i-electrons) and cyclooctatetraene (114 8 ir-electrons), so they do not adhere to the Hiickel rule. Compound 113 is a cyclic compound and every carbon is sp hybridized, but it has only 4 7t-electrons and does not satisfy the 4n -i- 2 rule (4 is not part of this series, so 113 does not have 2, 6, 10, 14, etc. 7t-electrons). Because 113 does not satisfy the Hiickel rule, it is not aromatic. Likewise, 114 is cyclic and has a continuous array of sp carbons, but 8 7t-electrons do not fit the 4n -H 2 series and thus do not satisfy the Hiickel rule, and 114 is not aromatic. These compounds are not aromatic and are also very unstable and difficult to prepare. Because they are so difficult to prepare and the ring system is so unstable, cyclic compounds such as this with 4n 7i-electrons are called antiaromatic compounds. For practical purposes, assume that such compounds cannot be prepared (although they can be if extremely low temperatures and specialized conditions are used). 

The self-dimerization of ethyne (acetylene, HOCH) would give rise to the 4n antiaromatic compound cyclobutadiene and, although the metal-catalyzed trimer-ization and tetramerization (vide supra) occur, the reaction (Equation 6.72) is not observed. Interestingly, cyclobutadiene and substituted cyclobutadienes have been prepared by several more circuitous routes and one of the ways of producing cyclobutadiene (albeit temporarily) is shown in Scheme 6.77. 

It is the prototype antiaromatic compound, it has been the subject of innumerable experimental and theoretical studies, and it is discussed in several locations in this text. Cyclobutadiene did eventually succumb to experimental characterization, primarily involving spectroscopic studies in cryogenic matrices at very low temperatures that we will discuss later in this book. However, using supramolecular chemistry, cyclobutadiene can be characterized by NMR and IR spectroscopies at room temperature ... 

Antiaromatic compounds have an even number of pairs of rr electrons. Therefore, either they are unable to fill their bonding orbitals (cylopentadienyl cation) or they have a pair of TT electrons left over after the bonding orbitals are filled (cyclobutadiene). Hund s rule requires that these two electrons go into two different degenerate orbitals (Section 1.2). 

In contrast to the compounds discussed above are those that fulfill the first three requirements for aromaticity, but have only four electrons in the n-system (actually An systems with 4, 8, 12... electrons). Such compounds do not experience any stabilization (energy lowering),but are actually destabilized. These are known as antiaromatic compounds. An example of an antiaromatic compound is cyclobutadiene, which can only be isolated and studied at extremely low temperatures because of its instability. An explanation for this phenomenon requires an understanding of orbital symmetry considerations, which is beyond the scope of this chapter (Figure 1.32). 

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. ... 

It would be useful if triple bonds could be similarly epoxidized to give oxirenes. However, oxirenes are not stable compounds.Two of them have been trapped in solid argon matrices at very low temperatures, but they decayed on warming to 35 Oxirenes probably form in the reaction, but react further before they can be isolated. Note that oxirenes bear the same relationship to cyclobutadiene that furan does to benzene and may therefore be expected to be antiaromatic (see p. 58). 

In contrast to aromatic molecules which have An + 2 n electrons, cyclobutadiene and cyclooctatetraene do not have An + 2 7r electrons and are not aromatic. In fact, diese molecules, which contain An jt electrons (n is an integer), are less stable than die planar model compounds and are termed antiaromatic. Bodi of these molecules adopt shapes that minimize interactions of die n orbitals. 

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