Cyclobutadienes, and

Cava, M. P. Mitchell, M. J. 1967, Cyclobutadiene and Related Compounds, Academic Press New York London... 

During our discussion of benzene and its derivatives it may have occurred to you that cyclobutadiene and cyclooctatetraene might be stabilized by cyclic rr electron delocal ization m a manner analogous to that of benzene... 

The same thought occurred to early chemists However the complete absence of natu rally occurring compounds based on cyclobutadiene and cyclooctatetraene contrasted starkly with the abundance of compounds containing a benzene unit Attempts to syn thesize cyclobutadiene and cyclooctatetraene met with failure and reinforced the grow mg conviction that these compounds would prove to be quite unlike benzene if m fact they could be isolated at all... 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Benzene cyclobutadiene and cyclooctatetraene provide clear examples of Huckel s rule Benzene with six tt electrons is a An + 2) system and is predicted to be aromatic by the rule Square cyclobutadiene and planar cyclooctatetraene are An systems with four and eight tt electrons respectively and are antiaromatic... 

The general term annulene has been coined to apply to completely conjugated mono cyclic hydrocarbons with more than six carbons Cyclobutadiene and benzene retain then-names but higher members of the group are named [jcjannulene where x is the number of carbons m the ring Thus cyclooctatetraene becomes [8]annulene cyclodecapentaene becomes [10] annulene and so on... 

Section 11 18 Although cychc conjugation is a necessary requirement for aromaticity this alone is not sufficient If it were cyclobutadiene and cycloocta tetraene would be aromatic They are not... 

For some systems a single determinant (SCFcalculation) is insufficient to describe the electronic wave function. For example, square cyclobutadiene and twisted ethylene require at least two configurations to describe their ground states. To allow several configurations to be used, a multi-electron configuration interaction technique has been implemented in HyperChem. 

Fig. 1.12. Energy level diagrams for cyclobutadiene and benzene, illustrating the application of Frost s circle.

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. 

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

The pattern of orbital energies in Figure 11.13 provides a convincing explanation for why benzene is aromatic while square cyclobutadiene and planar- cyclooctatetraene are not. We start by counting tt electrons cyclobutadiene has four, benzene six, and cyclooctatetraene has eight. These tt electrons are assigned to MOs in accordance with the usual rules—lowest energy orbitals first, a maximum of two electrons per orbital. 

McWeeny, R., Proc. Roy. Soc. (London) A227, 288, The valence-bond theory of molecular structure. III. Cyclobutadiene and benzene." f. 

Hiberty, P. C. The Distortive Tendencies of Delocalized n Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153, 27-40 (1990). 

In some cases, double bonds add to triple bonds to give cyclobutenes, apparently at about the same rate that they add to double bonds. The addition of triple bonds to triple bonds would give cyclobutadienes, and this has not been observed, except where these rearrange before they can be isolated (see 15-63) or in the presence of a suitable coordination compound, so that the cyclobutadiene is produced in the form of a complex (p. 60). 

For other reactions between cyclobutadienes and triple bonds to give Dewar benzenes, see Wingert, H. Regitz, M. Chem. Bet, 1986, 119, 244. 

Intramolecular cycloadditions between cyclobutadiene and an oxygen-tethered unactivated alkene (alkyne) offers an attractive route to benzo[c]furans (Scheme 26, <96JA9196>). 

This, the ozonide of cyclobutadiene (and also a likely peroxide of furan), may be stored in liquid nitrogen. It can explode with considerable force on warming. 

Recently, synthetic routes to longer ladderanes with C-C double bonds in the terminal rungs have been improved the [3]-, [5]-, and [7]ladderane derivatives were synthesized by the repeated cycloaddition of cyclobutadiene derivatives,30 and the [w]ladderane derivatives (w = 3,4,5,6,7,9) were synthesized by the successive and alternate cycloaddition with cyclobutadiene and dimethyl acetylenedicarboxylate.31 X-ray crystallographic analysis of the [5]ladderane derivative31 shows a corrugated backbone without twist, in contrast with saturated and unsubstituted ladderanes.22,23... 

During all these studies on cyclobutadienes and tetrahedranes formed via carbenes as transient species we wondered whether matrix isolation IR spectroscopy might be a good tool for the direct observation of cyclopropenylidene (2) and trimethylenemethane (3). This is indeed the case. 

Monoiodination of a zirconacyclopentadiene with one equivalent of iodine followed by the addition of one equivalent of CuCl gives the dimer of the cyclobutadiene and the Diels—Alder product in the presence of methyl maleate. This indicates the formation of a l-iodo-l,3-dienyl copper compound and the subsequent elimination of Cul to give a cyclobutadiene equivalent. Direct reductive elimination of zirconacydopentadienes affording cyclobutadienes has not yet been observed. 

U. H. F. Bunz, G. Roidl, M. Altmann, V. Enkelmann, K. D. Shimizu, Synthesis and Structural Characterization of Novel Organometallic Dehydroannulenes with Fused CpCo-Cyclobutadiene and Ferrocene Units Including a Cydic Fullerenyne Segment , J. Am Chem Soc 1999,121, 10719-10726. 

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