Cyclobutadiene dimerisation

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... 

The case of this dimerisation can be related to the closeness in energy of cyclobutadiene HOMO and LUMO. In contrast, the tritertiary butyl cyclobutadiene in which the bulky t-butyl groups hinder dimerisation can be isolated at room temperature. 

The reactivity of the incarcerated cyclobutadiene was probed by warming the NMR sample to 220°C for 5 min. This resulted in the formation of free cyclooctatetraene (6.108), clearly from ejection of the cyclobutadiene from the cavity and its subsequent dimerisation via 6.107. Reaction with 02 (which is able to enter the cavity of 6.101 gave incarcerated malealdehyde (6.109). 

Unlike ordinary alkenes, ketenes do 2 + 2 cycloadditions with themselves—the dimerisation above—and with other alkenes.1 Reaction of dichloroketene with cyclobutadiene 11 to give the... 

The double-addition products of type XVI are surprisingly stable. Complex 36 i, e.g., can be crystallized from CH2Cl2/MeOH in the presence of hydrochloric acid without substantial decomposition. The most striking property of 36i is the conversion into the cyclobutadiene complex 33 (R = Ph), which is more conveniently prepared from 28a and PhC2Ph at 150 °C. As already mentioned, the reaction proceeds by a stepwise mechanism through complexes XIV—XVI. This reaction offers a new, facile preparation of 33. The cyclo-dimerisation of alkynes other than diphenylacetylene could not been substantiated, however. 

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