Cyclobutadiene dication

The novel four-center two-electron delocalized o-bishomoaromatic species 593, 594,599,601a, and 603 are representatives of a new class of 2jt-aromatic pericyclic systems. These may be considered as the transition state of the Woodward-Hoffmann allowed cycloaddition of ethylene to ethylene dication or dimerization of two ethylene radical cations985 (Figure 3.25, 604). Delocalization takes place among the orbitals in the plane of the conjugated system, which is in sharp contrast to cyclobutadiene dication 605 having a conventional p-type delocalized electron structure (Figure 3.25). 

Figure 3.25. Molecular orbital representations of the bonding structure of the parent bishomoaromatic dication C4H82+ (604) and cyclobutadiene dication C4H42+ (605).

The SHM permits the calculation of a kind of stabilizing energy, or, more accurately, an energy that reflects the stability of molecules. This energy is calculated by comparing the total electronic energy of the molecule in question with that of a reference compound, as shown below for the propenyl systems, cyclobutadiene, and the cyclobutadiene dication. 

Fig. 4.23 Hiickel s rule says that cyclic n systems with An + 2 n electrons ( = 0, 1, 2,. .. An + 2 = 2, 6, 10,. ..) should be especially stable, since they have all bonding levels full and all antibonding levels empty. The special stability is usually equated with aromaticity. Shown here are the cyclopropenyl cation, the cyclobutadiene dication, the cyclopentadienyl anion, and benzene formal structures are given for these species - the actual molecules do not have single and double bonds, but rather electron delocalization makes all C/C bonds the same...

Thus the stabilization energy calculation agrees with the deduction from the disposition of filled MOs (i.e. with the An + 2 rule) that the cyclobutadiene dication should be stabilized by electron delocalization, which is in some agreement with experiment [46]. 

The first singlet excitation of the hypothetical unsubstituted cyclobutadiene dication C4H4 (6a) corresponds to an electronic transition from the lowest molecular orbital into the degenerate nonbonding n MOs Similarly,... 

Thus the stabilization energy calculation agrees with the deduction from the disposition of filled MOs (i.e. with the 4n + 2 rule) that the cyclobutadiene dication should be stabilized by electron delocalization, which is in some agreement with experiment [45], More sophisticated calculations indicate that cyclic 4n systems like cyclobutadiene (where planar cyclooctatetraene, for example, is buckled by steric factors and is simply an ordinary polyene) are actually destabilized by n electronic effects their resonance energy is not just zero, as predicted by the SHM, but less than zero. Such systems are antiaromatic [17,46],... 

Figure 5. Dependence of a and jt parts of the total energy of cyclobutadiene and cyclobutadiene dication.

Pig. 2.6 Representation of pyramidane as an unhybridized C bonded to a cyclobutadiene dication [11]... 

The cyclobutadiene dication should have two electrons, both in ih, so it is predicted to have DE = 2)3, the same as cyclopropenyl cation. There was a report that the dication had been observed Olah, G. A. Mateescu, G. D. J. Am. Chem. Soc. 1970, 92,1430. 

It suggests that it is not the size of the ring but the number of electrons present in it determines whether a molecule would be aromatic or antiaromatic. In fact the molecules with An+ 2) n electrons are aromatic whereas with (An, 0) n electrons are antiaromatic. Thus, benzene, cyclopropenyl cation, cyclobutadiene dication (or dianion), cyclopentadie-nyl anion, tropylium ion, cyclooctatetraene dication (or dianion), etc. possess (4 + 2) ti electrons and hence aromatic whereas cyclobutadiene, cyclopentadienyl cation, cycloheptatrienyl anion, cyclooctatetraene (non-planar) etc. have An n electrons which make them antiaromatic . Systems like [10] annulene are forced to adopt a nonplanar conformation due to transannular interaction between two hydrogen atoms and hence their aromaticity gets reduced even if they have (An + 2)n electrons. On the other hand the steric constraints in systems like cyclooctatetraene force it to adopt a tube-like non-planar conformation which in turn reduces its antiaromaticity. Various derivatives of benzene like phenol, toluene, aniline, nitrobenzene etc. are also aromatic where the benzene ring and the n sextet are preserved. In homoaromatic " systems, like cyclooctatrienyl cation, delocalization does not extend over the whole molecule. 

Olah has reported the preparation of the cyclobutadiene dications (12a—d) under stable ion conditions.The ions have been studied by H, and n.m.r. spectroscopy. Both the and n.m.r. spectra of the phenylated dications indicate that there is substantial transfer of jc-electron density from the phenyl groups into the four-membered ring. The co-existence of halocyclobutenyl cations and the dications, in the same solution, as static species on the n.m.r. times scale,... 

The 1,2-dimethylbenzocyclobutadiene dication (316), a 6-7c-electron molecule, has been prepared by reaction of 1,2-dimethylbenzocyclobutene-l,2-diol with antimony pentafluoride in sulphuryl chlorofluoride at low temperature. The H n.m.r. spectrum of (316) showed equivalent methyl group signals at 84.42 and an aromatic proton signal at 8 9.10. The n.m.r. spectrum showed five signals at chemical shifts indicated below. The spectra are comparable with those of other cyclobutadiene dications, supporting the fully delocalized structure of (316). The ion was stable below 30 °C. Attempts to prepare the parent dication were not successful. 

Is cyclobutadiene dication (C4H4 ) aromatic according to Hiickel s rule Sketch its ir molecular orbital diagram to illustrate your answer. 

Some n.m.r. spectra for benzo- and dibenzo-cyclobutadiene dications indicate that these species are fully delocalised six- and ten-electron aromatic systems respectively. Flash-pyrolytic extrusion of nitrogen from benzocinnoline analogues has been employed in the generation of the fused cyclobutadiene derivatives (52), (53), and (54). 1,2-Dimethylbenzocyclobutadiene gives the... 

Cyclobutadiene dication, 61, is a 2jr-aromatic cation. In the carbon case, tetramethyl- and tetraphenyl-substituted derivatives of this dication were characterized in superacidic media already in 1976 [66]. A related persila-analogue, the tetrasilacyclobutadiene dication 62, was obtained only recently in an unexpected reaction between donor-stabilized silylene 63 and Cp ZrCl3. in 29% yield (Scheme 20) [67]. As an intermediate, a silicon(ll) cation, [LSi]", resulting from... 

Olah GA, Staral JS (1976) Novel aromatic systems. 4. Cyclobutadiene dications. J Am Chem Soc 98 6290... 

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