Cyclobutadiene properties

Colourless liquid with alkenic properties. Many substituted derivatives are known, the preferred method of preparation being the addition of an alkyne to a cyclobutadiene. 

In Fig. 9.1, orbitals below the dashed reference line are bonding orbitals when they are filled, the molecule is stabilized. The orbitals that fall on the reference line are nonbonding placing electrons in these orbitals has no effect on the total bonding energy of the molecule. The orbitals above the reference line are antibonding the presence of electrons in these orbitals destabilizes the molecule. The dramatic difference in properties of cyclobutadiene (extremely unstable) and benzene (very stable) is explicable in terms of... 

The term annulene was coined to refer to the completely conjugated monocyclic polyenes. The synthesis of annulenes has been extended well beyond the first two members of the series [4]annulene (cyclobutadiene) and [6]annulene (benzene). The generality of the Hiickel rule can be tested by considering the properties of members of the annulene series. 

It is of interest to consider at this point some of the specific molecules in Scheme 9.2 and compare their chemical properties with the calculated stabilization energies. Benzo-cyclobutadiene has been generated in a number of ways, including dehalogenation of... 

Cyclo butadiene is highly reactive and shows none of the properties associated with aromaticity. In fact, it was not even prepared until 1965, when Rowland Pettit of the University of Texas was able to make it at low temperature. Even at —78 °C, however, cyclobutadiene is so reactive that it dimerizes by a Diels-Alder reaction. One molecule behaves as a diene and the other as a dienophile. 

The kinetic stabilities and the donor-acceptor properties of cyclic conjugated molecules [68] have been described (Scheme 12) in the theoretical subsection (Sect. 2.2.2) to be controlled by the phase property. There is a parallelism between the thermodynamic and kinetic stabilities. An aromatic molecule, benzene, is kinetically stable, and an antiaromatic molecule, cyclobutadiene, is kinetically unstable (Scheme 13). 

Cyclobutadiene owes its observed instability much to the kinetic property. Cyclobutadiene dimerizes in the argon matrix above 35 K [69] and only exists for 2-10 ms under low pressure [70, 71], However, cyclobutadiene is stabilized by bulky substituents (Scheme 30). The ferf-butyl derivative was quantitatively prepared even at a high temperature (130 °C) [72]. Yellow crystals of the cyclobutadiene fused by two seven-membered rings did not decompose below 240 °C [73]. 

The same procedure was later effectively used by Masamune1Sb 18 to prepare tert-butylated cyclobutadienes. Regitz19 widely employed the Masamune cyclopmpenylcarbene mute in his studies on the properties of the tert-butylester of tri-rert-butylcyclobutadiene carboxylic acid. We also used several differently... 

Fig. 9 illustrates that the two acetylenic systems become nearly parallel at C1-C6 distances close to 3 A where the constructive overlap of the re-orbital with one of the re -nodes is compensated by a destructive overlap with the other rc -node (Fig. 9, bottom). From a conceptual point of view, the properties of the in-plane re-system at the 3 A threshold bear a striking resemblance to the interaction of the two re-bonds in D2h cyclobutadiene where the re-re interaction is zero and the re-re repulsion is considerable, thus accounting for the extreme instability of this antiaromatic molecule.41 Even more relevant is a comparison with the TS of the symmetry forbidden thermal [2S + 2S] cycloaddition (Fig. 10) which prompted us to call this region antiaromatic .42... 

Bunz et al. explored the possibility of doping PPE chains covalently with small amounts of fluorescence-quenching cyclobutadiene complexes, in order to endow their optical properties to the base polymer, PPE [80]. Due to their extensive experience of cyclobutadiene complexes in polymer synthesis [81], the authors prepared several polymers PAE-CoCpl-5 (Table 4) containing different contents of CoCp complexes. The quantum yields were determined by simple comparison of the intensities of the emitted light to that of a standard... 

Just as the unusual stability and reactivity of benzene are placed into their proper context by comparison with cyclobutadiene and cyclooctatetraene39, the 4 -electron homo-logues of benzene, it is instructive to compare the formally homoantiaromatic bicyclo [3.1.0]hexenyl/cyclohexadieny 1 cation systems with the homocyclopropenium and homo-tropenylium ions (Scheme 14). Such a comparison not only puts in context the properties of the latter two homoaromatic cations, but also reveals a different mode of cyclopropyl conjugation that occurs in the 4 -electron systems. 

More than 100 years ago, Kekule recognized the possible existence of other-conjugated cyclic polyalkenes, which at least superficially would be expected to have properties like benzene. The most interesting of these are cyclobutadiene, 23, and cyclooctatetraene, 24 ... 

Cyclobutadiene, 23, eluded Kekule, Willstatter, and a host of other investigators for almost 100 years. As more work was done, it became increasingly clear that the molecule, when formed in reactions, was immediately converted to something else. Finally, the will-o -the-wisp was captured in an essentially rigid matrix of argon at 8°K. It was characterized by its spectral properties (not by combustion analysis). On warming to even 35°K, it dimerizes to yield 26 ... 

It is in this area that qualitative MO procedures have great success because there are general characteristics of the 77 molecular orbitals of mono-cyclic, conjugated polyene systems that predict differences in the properties of cyclobutadiene, benzene, cyclooctatraene, and other similar compounds that are not obvious from the simple VB method. 

Benzoquinones owe their unusual properties as a,/3-unsaturated ketones to the ease by which they are transformed to stable aromatic systems. How would these properties change if the quinone were derived from nonaromatic structures, such as cyclobutadiene, cyclooctatetraene, or pentalene There is no final answer to this question because few such substances have been prepared, the best known so far being the mono- and diphenylcyclobulenediones ... 

The reader will also be aware that not all cyclic conjugated molecules for which such equivalent structures may be written share with benzene these special properties. For example, cyclobutadiene (28) eluded synthesis for many years when finally prepared, it and its simple derivatives proved to be extremely... 

The very delicate ji a-balance poses an opportunity to study delocalized antiaromatic species and to gain new insight into the resonance energy of antiaromatics and its manifestations, e.g., in the magnetic properties. The calculations of van Wiillen and Kut-zelnigg185 show that AU, cyclobutadiene has a positive... 

Scheme 33. Generation of the Twin States by Mixing of the Kekule Structures for Benzene (a) and Cyclobutadiene (b) as Archetypal Aromatic and Antiaromatic Species, (c) Transformation Properties of Ki and K2 with Respect to Symmetry Operations of the Dgh and Dih Point Groups...

Parts b and c of Figure 18 show the results of a recent computational study by Zilberg and Haas188 210 aimed at probing the properties of the ground Big and excited 1 Aig twin states of cyclobutadiene and... 

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. 

CONTENTS Preface, Brian Halton. Cydopropene-Vinylcar-bene Isomerization and Some Applications in Synthesis, John Warkentin and John McK. R. Wollard. Preparation and Subsequent Reaction Reactions of Halo- and Alkoxyhalocyclopro-panes, Leiv K. Sydnes and Einar Bakstad. The Phenylenes Synthesis, Properties and Reactivity, K.PeterC. Vollhardtand Debra L. Mohler. Cyclobutadienes and Azacyclobutadienes in the Synthesis of Valence Isomers of Six-Membered Aromatic and Heteroaromatic Compounds, Manfred Regitz, Heinrich ... 

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