Cyclobutadiene compounds, antiaromatic destabilization

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

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. 

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. 

Both thermochemical and molecular orbital approaches agree that benzene is an especially stable molecule and are reasonably consistent with one another in the stabilization energy that is assigned. It is very significant that MO calculations also show a destabilization of certain conjugated cyclic polyenes, cyclobutadiene in particular. The instability of cyclobutadiene has precluded any thermochemical evaluation of the extent of destabilization. Compounds that are destabilized relative to conjugated but noncyclic polyene models are called antiaromatic. ... 

Cyclobutadiene (11) is the epitome of an antiaromatic neutral hydrocarbon and has been the target of chemical investigations for well over a century, with much early interest stimulated by the studies of Willstatter. This compound serves as a benchmark for extreme manifestations of antiaromaticity. A recent experimental measurement by photoacoustic calorimetry of the energy change in the formation of cyclobutadiene shown in eq 1 permits derivation of AHf for llofll4 ll kcal/mol. " The destabilization... 

Dibenzoannelation of cyclobutadiene results in biphenylene (50), a long-known isolable hydrocarbon with an archival gas-phase enthalpy of 417.9 3.3 kj/mol. That its enthalpy of formation is essentially the same as the monobenzoannelated cyclobutadiene speaks to the antiaromaticity of the latter. An ab initio study of benzocyclobutadiene and biphenylene found that the intrinsic destabilization energy is ca. 33 kJ/mol greater in the former compound." This does not mean biphenylene is not destabilized relative to benzenoid expectations. The destabilization of biphenylene of more than 200 kJ/mol relative to triphenylene or biphenyl as explained earlier is huge, although not as large as for cyclobutadiene itself. The benzene rings clearly ameliorate the antiaromaticity of cyclobutadiene. 

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). 

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