Diradicals cyclobutadiene

Like cyclobutadiene, azete is very unstable, but it is known in the form of certain derivatives, such as 2-phenylbenzo[/)]azete. This compound can be synthesized in 64% yield by passing a stream of 4-phenyl-1,2,3-benzotria-zoline vapour at low pressure through a tube heated to 420-450 °C. Nitrogen is lost from the starting material, possibly generating a diradical that spontaneously cyclizes to form 2-phenylbenzo[ ]azete (Scheme 8.1). 

Hiickel s rule has been abundantly verified [17] notwithstanding the fact that the SHM, when applied without regard to considerations like the Jahn-Teller effect (see above) incorrectly predicts An species like cyclobutadiene to be triplet diradicals. The Hiickel rule also applies to ions for example, the cyclopropenyl system two n electrons, the cyclopropenyl cation, corresponds to n 0. and is strongly aromatic. Other aromatic species are the cyclopentadienyl anion (six n electrons, n = 1 Hiickel predicted the enhanced acidity of cyclopentadiene) and the cyclohep-tatrienyl cation. Only reasonably planar species can be expected to provide the AO overlap need for cyclic electron delocalization and aromaticity, and care is needed in applying the rule. Electron delocalization and aromaticity within the SHM have recently been revisited [43]. 

The electronic configuration in Figure 16-7 indicates that cyclobutadiene should be unstable. Its highest-lying electrons are in nonbonding orbitals (ir2 and ir3) and are therefore very reactive. According to Hund s rule, the compound exists as a diradical (two unpaired electrons) in its ground state. Such a diradical is expected to be extremely reactive. Thus, molecular orbital theory successfully predicts the dramatic stability difference between benzene and cyclobutadiene. 

Pyrolysis can be carried out either using flow- conditions in a stream of dry nitrogen at atmospheric pressure or under vacuum. The yields of acetylene are 82-90%. Symmetrical acetylenes are not detected which indicates that nitrogen extrusion from pyridazines leads to a diradical intermediate rather than cyclobutadienes or tetrahedral species. 

The procedure followed in Sample Problem 17.1 also illustrates why cyclobutadiene is antiaromatic. Having the two unpaired electrons in nonbonding MOs suggests that cyclobutadiene should be a highly unstable diradical. In fact, antiaromatic compounds resemble cyclobutadiene because their HOMOs contain two unpaired electrons, making them especially unstable. 

This approach would predict a diradical for cyclobutadiene (one electron in each 1-node orbital). Although cyclobutadiene itself is very reactive (P. Reeves, T. Devon, and R. Pettit, J. Am. Chem. Soc., 1969, 91, 5890), complexes containing derivatives of cyclobutadiene are known. Cyclobutadiene itself is rectangular rather than square (D. W. Kohn and P. Chen, J. Am. Chem. Soc., 1993,115, 2844) and at 8 K it has been isolated in a solid argon matrix (O. L. Chapman, C. L. McIntosh, and J. Pacansky, J. Am. Chem. Soc., 1973, 95, 614 A. Krantz, C. Y. Lin, and M. D. Newton, ibid., 1973, 95, 2746). 

Figure 16-8 shows that the first 3 pairs of electrons are in three bonding molecular orbitals of cyclooctatetraene. Electrons 7 and 8, however, are located in two different nonbonding orbitals. As in cyclobutadiene, a planar cyclooctatetraene is predicted to be a diradical, a particularly unstable electron configuration. 

Cyclobutadiene According to the molecular orbital picture, square planar cyclobn-tadiene shonld be a diradical (have two unpaired electrons). The fonr TT electrons are distribnted so that two are in the lowest energy orbital and, in accordance with Hund s rule, each of the two equal-energy nonbonding orbitals is half-filled. (Remember, Hnnd s rnle tells ns that when two orbitals have the same energy, each one is half-hlled before either of them reaches its full complement of two electrons.)... 

Huckefs rule has been abundantly verified [17] notwithstanding the fact that the SHM, when applied without regard to considerations like the Jahn-Teller effect (see above) incorrectly predicts 4n species like cyclobutadiene to be triplet diradicals. The Hiickel rule also applies to ions for example, the cyclopropenyl with system two n electrons, the cyclopropenyl cation, corresponds to n = 0, and is strongly aromatic. 

We have also studied the automerization barrier in cyclobutadiene, where the transition structure has a diradical character [75] and the singlet-triplet gaps in alkyl-carbenes [76]. 

Six of the eight ir electrons of cyclooctatetraene occupy three bonding orbitals. The remaining two tt electrons occupy, one each, the two equal-energy nonbonding orbitals. Planar cyclooctatetraene should, like square cyclobutadiene, be a diradical. 

As far as the mechanism for those processes is concerned, taking into account (i) Viehe s work on t-butylacetylene (Viehe et al., 1964), (ii) the mildness of the trimerization conditions, (iii) the symmetry restrictions for the relevant conversions, (iv) the steric repulsions among perchlorotriphenyl groups and (v) the presumed electronic stability of the intermediates, it is postulated that a dimeric head-to-head diradical is formed first. This then cyclizes to give two bicyclic diradicaloid structures possessing minimal steric repulsions among the substituents (cyclobutadiene and quasi-tetrahedrane structures). By addition of a third molecule of perchlorophenylacetylene, only the 1,2,3- and the 1,2,4-isomers would result (107) (Ballester et al.. 

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