Cyclobutadiene barrier

Cycloaddition reactions, 162-165, 197-198 component analysis, 168 Diels-Alder, 162, 198 ethylene + ethylene, 198 orbital correlation diagram, 198 stereochemistry, 162-163 Cycloalkanols, synthesis, 277 Cyclobutadiene barrier, 91 ground state, 91 point group of, 5 self-reactivity, 97 SHMO, 151 structure, 309-310 Cyclobutane... 

Cyclobutadiene barrier, 91 ground state, 91 point group of, 5 self-reactivity, 97 SHMO, 151 structure, 309-310... 

Tetra(tert-butyl)tetrahedrane converts into tetra(tert-butyl)cyclobutadiene only when heated up to 140°C in vacuum. A barrier of 170 kJ mol separates these two isomers (Heilbronner et al. 1980). In the cation-radical state, the tetrahedrane structure converts into the cyclobutadiene structure without heating (Bock et al. 1980, Fox et al. 1982). From Scheme 6.34 it can be seen that by the action of aluminum chloride on methylene chloride, tetrahedrane forms the cation-radical of its isomer—the cyclobutadiene cation-radical and not the cation-radical of the same skeleton. The latter is more stable than the former because of more effective delocalization of the unpaired electron and positive... 

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.

Benzene and other aromatics alike are stable molecules, while cyclobutadiene and other antiaromatic molecules are unstable molecules.27-76 Similarly, allylic species are stable intermediates and possess significant rotational barriers. It may appear as a contradiction that, for example, the tr-component of benzene can be distortive but it still endows the molecule with special stability or that the distortive jr-component of allyl radical can lead to a rotational barrier. We would like to show in this section that these stability patterns derive from the vertical resonance energy which is expressed as a special stability because for most experimental probes (in eluding substitution reactions) the o-frame restricts the molecule to small distortion167 in which the vertical resonance energy is still appreciable, as shown schematically in Figure 5. 

Figure 8. VB diagram (ref 37) describing the barrier for dimerization of cyclobutadiene. The barrier is small since the excitation gap, G, is small.

The case of cyclobutadiene may at first seem to contradict the above assertion that if a central atom A is connected to two atoms B the force constants must be the same, giving rise to equal bond lengths. Cyclobutadiene is rectangular rather than square and so one bond from a carbon is single, and one is double, say the bonds designated here C1-C2 and C1-C3 1 and 2 are distinct molecules separated by a barrier [1] ... 

For conjugated (diene)metal complexes, the 7r-molecular orbitals of cyclobutadiene consist of a degenerate eg pair (Figure 3). Because of this orthogonality, there is not a single electronically preferred conformei and barriers for rotation of the cyclobutadiene ligand are generally low . For (cyclobutadiene)Fe(CO)3, in the solid state, NMR spin relaxation data indicate that there are two inequivalent lattice sites. For each of these inequivalent lattice sites, barriers for rotation of the cyclobutadiene have been measured to be 3.63 and... 

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