Cyclobutadiene. antiaromaticity reactivity

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. ... 

Removal of one electron should make no difference to the relative stabilities of polyene molecule ions or even electron polyene fragments as compared to their neutral counterparts, e.g. butadiene and the allyl radical should have the same relative stabihties as the butadiene molecule ion, and the allyl cation. Removal of one electron will, however, alter the stabihties, and thus the reactivities of cychc polyenes. The molecule ions of aromatic hydrocarbons will be substantially less aromatic then their neutral counterparts. Correspondingly the molecule ions of antiaromatic hydrocarbons will not be as antiaromatic as their neutral analogs, e.g. cyclobutadiene + should be relatively more stable than cyclobutadiene. The largest charge effects in hydrocarbons will be observed in nonaltemant ) monocychc hydrocarbons. The cyclopropenium ion 7 and the tropillium ion 2 are both strongly aromatic as compared to their neutral analogs. Consequently CsHs is a very common ion in the mass spectra of hydrocarbons while cyclopropene is not a common product of hydrocarbon pyrolysis or photo-... 

The structural similarities between S-alkylthiophenium salts and thiophene sulfoxides have prompted the suggestion that both classes are antiaromatic (70JA7610, 70JCS(C)1764). Thiophene sulfoxides are exceptionally reactive, undergoing spontaneous Diels-Alder dimerization unless stabilized by bulky substituents at the 2- and 5-positions. This type of reactivity is reminiscent of cyclobutadiene. In stark contrast to the sulfoxides, S-alkylthiophenium salts show no tendency to take part in Diels-Alder reactions either in an intra- or intermolecular sense. Pentamethylthiophenium hexafluorophosphate (11, X = PFg ) was shown to be completely unreactive toward both electron-rich and electron-deficient dienophiles under conditions where thiophene sulfoxides undergo efficient cycloaddition reactions. 

Difficulties with isomerization of bicyclo[2.1.0]pent-2-ene were noted and overcome by the authors who also note that antiaromatic destabilization of bicyclo[2.1.0]pent-2-ene is manifested both in its thermochemistry and in its augmented dienophilic reactivity relative to cyclopentadiene. They refer to bicyclo[2.1.0]pent-2-ene as pseudobicyclo-butadiene because a cyclopropane ring replaces the double bond in cyclobutadiene. The hydrogenation product is cyclopentane. 

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... 

Azete is iso- r-electronic with cyclobutadiene and is therefore the simplest antiaromatic het-eroannulene. It would be expected to be thermally unstable and extremely reactive, and as yet the parent compound has not been synthesized. In 1973, tris(dimethylamino)azete was described. In 1986, Regitz succeeded in synthesizing tri- er -butylazete by thermolysis of 3-azido-l,2,3-tri- er -butylcyclopropene [6] ... 

This time the n energy of double union is less than that for single union so cyclobutadiene should be less stable than butadiene. This is because the NBMO coefficients at the ends of the allyl radical have opposite signs. The first-order perturbations are out of phase with one another and so cancel. We therefore predict cyclobutadiene to be as much less stable than a corresponding open-chain polyene as benzene is more stable. Cyclobutadiene should be antiaromatic. Cyclobutadiene is indeed extraordinarily reactive it dimerizes immediately when it is formed, even in dilute solution at low temperatures. 

Cyclobutadiene has four tt electrons and is antiaromatic. The tt electrons are localized into two double bonds rather than delocalized around the ring, as indicated by an electrostatic potential map. Cyclobutadiene is highly reactive and shows none of the properties associated with aromaticity. In fact, it was not even prepared until 1965. 

Other theoretical treatments which have been applied to considerations of antiaromaticity include spin-coupled theory, topological methods,and quantum statistical definitions. The relevance of spin-coupled theory has been described The comparison between the (spin-coupled) descriptions of cyclobutadiene, benzene, and cyclooctatetraene clearly indicates that the reason for the lower stability and higher reactivity of antiaromatic systems is due to a simultaneous unfavorable coupling of the spins of all valence orbitals to triplet pairs, which discourages bonding interactions and suggests diradical character. ... 

Reactivity was an early criterion for aromaticity and was one of the first applied in studies of antiaromaticity. This includes both qualitative and quantitative studies of reactivity in forming or destroying antiaromatic systems, for example, in the resistance to forming systems such as cyclobutadiene, and their high chemical reactivity if made. 

The strained cyclobutene shows an increase of 42.7 1.7 kJ/mol upon benzoannelation. " As mentioned earlier, the very endothermic enthalpy of formation of cyclobutadiene - is evidence of considerable instability of this species. Kinetic reactivity accompanies this thermodynamic instability, and so it is not surprising that this species is not isolable under ambient conditions. Benzoannelation results in benzocyclobutadiene (53), a somewhat more stable species, both thermodynamically and kinetically, for which recent gas-phase ion measurements result in an enthalpy of formation of 406 17 kj/mol. That the enthalpy of formation of cyclobutadiene becomes less positive on benzoannelation supports the conclusion for considerable antiaromatic character of cyclobutadiene. 

The special stability and reactivity associated with cyclic delocalization is not unique to benzene and polycyclic benzenoids. Thus, we shall see that other cyclic conjugated polyenes can be aromatic, but only if they contain (An + 2) tt electrons (n = 0, 1, 2, 3,. . . ). In contrast, An tt circuits may be destabilized by conjugation, or are antiaromatic. This pattern is known as Hiickel s rule. Nonplanar systems in which cyclic overlap is disrupted sufficiently to impart alkene-like properties are classified as nonaromatic. Let us look at some members of this series, starting with 1,3-cyclobutadiene. 

Cyclobutadiene, a An tt system (n = 1), is an air-sensitive and extremely reactive molecule in comparison to its analogs 1,3-butadiene and cyclobutene. Not only does the molecule have none of the attributes of an aromatic molecule like benzene, it is actually destabilized through tt overlap by more than 35 kcal moF (146 kJ moF ) and therefore is antiaromatic. As a consequence, its structure is rectangular, and the two diene forms represent isomers, equilibrating through a symmetrical transition state, rather than resonance forms. 

Let us now examine tlie properties of the next higher cyclic polyene analog of benzene, 1,3,5,7-cyclooctatetraene, another 4n tt cycle (n = 2). Is it antiaromatic, like 1,3-cyclobutadiene First prepared in 1911 by Willstatter, this substance is now readily available from a special reaction, the nickel-catalyzed cyclotetramerization of ethyne. It is a yellow liquid (b.p. 152 °C) that is stable if kept cold but that polymerizes when heated. It is oxidized by air, eatalytically hydrogenated to cyclooctane, and snbject to electrophilic additions and to cycloaddition reactions. This chemical reactivity is diagnostic of a normal polyene (Section 14-7). 

The comparison between the SC descriptions of cyclobutadiene, benzene, and cyclooctatetraene clearly indicates that the reason for the lower stability and higher reactivity of antiaromatic systems is due to a simultaneous unfavorable coupling... 

Cyclobutadiene is a key compound in the study of antiaromaticity since it is the smallest neutral example and it is planar. Its chemistry has been the subject of several reviews. It was first observed in an argon matrix, being formed by the photolysis of a-pyrone. Subsequently, it was prepared from a variety of other precursors. It is highly reactive, and it readily dimerizes when the matrix softens and molecular diffusion becomes important. The dimerization process has been studied theoretically. Although cyclobutadiene cannot be isolated in the pure form, it can be stabilized by the formation of metal complexes. 

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