Cyclobutadiene anti-aromaticity

Figure 3.15). The special case of cyclobutadienes (anti-aromatic in the...

Common examples of systems often mistaken as being aromatic (because of their alternating double and single bonds) are cyclobutadiene and cyclo-octatetraene (shown in Figure 6-9). In the case of cyclobutadiene, 4n + 2 = 4, giving n = 0.5, while for cyclooctatetraene, 4n + 2 = 8, so that n = 1.5. In these two compounds, n is not an integer, so these systems are anti-aromatic (nonaromatic). Anti-aromatic systems (non-Hilckel systems) are less stable than aromatic or normal systems. 

Fig. 6 Reaction energy profile for reactions 34a/34b (A), 35a/35b (B), and 36a/36b (C). (A) and (B) Aromatic stabilization of the transition state is greater than that of benzene or cyclopentadienyl anion, respectively. (C) Anti-aromatic destabilization (positive ASE) of the transition state is less than that of cyclobutadiene the high barrier results from the additional contribution by angular and torsional strain at the transition state.

The bond length variation in the substituted benzodicyclobutadiene shown in Fig. 14.3.6(d), as determined from X-ray analysis, can be fairly well accounted for by resonance between the canonical formulas la and lb. The central six-membered ring contains a pair of extremely long C(sp2)-C(sp2) bonds at 154.0(5) pm, which significantly exceed the reference bond distance of 149.0(3) pm observed for tris(benzocyclobutadieno)benzene [Fig. 14.3.6(e)], The bond length pattern in the latter compound indicates that its structure is better described by the radiallene formula Ha in preference to formula lib, which contains anti-aromatic cyclobutadiene moieties. 

The theoretical models start with Kekule s [44] description of benzene, as having two structures. Later Hiickel [45,46] discovered his [4 +2] and [4n] rules, and was able to account for the stability of benzene ([4 +2]) and the instability of cyclobutadiene and cyclo-octatetraene (both [4 ]). The [4 +2] compounds were called aromatic after benzene, while the [4n compounds were given the designation anti-aromatic. 

Fig. 9 Encapsulation within a hemicarcerand allows cyclobutadiene, an anti-aromatic, highly strained and reactive molecule, to be isolated...

To explain the increase in the rate of the cyclobutene opening 6.292 —> 6.293, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the anti-aromatic Hiickel cyclobutadiene that we saw in Fig. 1.38. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.42, where we can see that a... 

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. 

Then the methyl coefficient in Eqs. (29) and (30) is equal to unity, and the coefficients of the residue can be found by the method described on p. 59. Eventually we come to the same conclusions as before benzene is more stable than hexatriene (Fig. 20a) and consequently is aromatic while butadiene is more stable than cyclobutadiene (Fig. 20 ) and consequently the latter is anti-aromatic. 

The cyclobutenium di-cation, 59, is formally derived from the (as yet unisolated) anti-aromatic cyclobutadiene by the removal of two n-electrons. This di-cation, like the cyclopropenyl cation, represents a member of the aromatic sub-group possessing just two 7t-electrons. The tetramethyl derivative of 59, 64 (see Table 14), has been studied 206), and its n.m.r. spectrum appears to possess the expected aromatic characteristics. The methyl groups resonate at t 6.32 in SbFs SO2 solutions and at t 5.89 in SbF5—SO2CIF. A reasonable model for this... 

Cyclobutadiene, C4H4, is anti-aromatic (i.e. it does not have 4n + 2 TT-electrons) and readily polymerizes. However, it can be stabilized by coordination to a low oxidation state metal centre. Yellow crystalline (ri -C4H4)Fe(CO)3 is made by reaction 23.115 and its solid state structure (Figure 23.25a) shows that (in contrast to the free ligand in which the double bonds are localized) the C—C bonds in coordinated C4H4 are of equal length. 

Thermochemistry of cyclobutadiene Enthalpy of formation, ring strain, and anti-aromaticity... 

Whereas aromatic systems are defined by a positive resonance energy, anti-aromatic systems are characterized by a negative resonance energy. As a rule, antiaromatic compounds are unstable and contain 4n 7i-electrons in a cyclic planar, completely conjugated arrangement. Cyclobutadiene belongs to this category and is stable only in a solid matrix at very low temperatures (20 K). 

Figure 2.2 Energy diagram for the anti-aromatic compound cyclobutadiene (formulated as a biradical and diene)...

The anti-aromaticity of cyclobutadiene (46) has made this molecule a topic of choice in research by matrix isolation, where it occupies an important chapter. It was initially obtained by matrix photodecomposition of a-pyrone by a prolonged irradiation at 20.4 K in argon matrix via a series of intermediates (Scheme 6.21). The peaks marked in the unexpectedly simple spectrum obtained were attributed to such species (Fig. 6.30). To finally establish the structure of this molecule, rectangular or square, many further experiments were required, in particular the generation from a different precursor and testing the effect of specific deuteration in the spectrum [8]. 

Experimental measurements place delocalized cyclobutadiene approximately 150 kJ/mol (36 kcal/mol) higher in energy than a structure with noninteracting double bonds both square cyclobutadiene and planar cyclooctatetraene are antiaromatic. Anti-aromatic molecules are destabilized by delocalization of their tt electrons and cyclobutadiene and cyclooctatetraene adopt stmctures that minimize the delocalization of these electrons. 

List four things about cyclobutadiene that make it anti-aromatic. 

Cyclobutadiene is one of the few molecules that is unable to avoid being anti-aromatic. Explain. 

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