Bullvalene, valence tautomerism

Bullvalene and hypostrophene are members of a group of compounds all of whose formulas can be expressed by the symbol (CH)iq. " Many other members of this group are known. Similar groups of (CH) compounds exist for other even-numbered values of n . " For example, there are 20 possible (CH)g compounds, and hve possible (CH)g compounds, all of which are known benzene, prismane (p. 220), Dewar benzene (p. 1641), bicyclopropenyl, ° and benzvalene. An interesting example of a valence tautomerism is the case of 1,2,3-tri-tert-butylcyclobutadiene (p. 74). There are two isomers, both rectangular, and NMR spectra show that they exist in a dynamic equilibrium, even at — 185°C. ... 

There are many examples of valence tautomerism in the literature. An interesting example is the isomerization of bullvalene. 

The structural peculiarity of trilithium heptaphosphide is its fluctuating anion.On account of the structural similarity to the hydrocarbon bullvalene, the P7 ion is capable of a degenerate Cope rearrangement and at room temperature has to be described by 1680 valence-tautomeric forms of equal structure. 

Valence tautomerism of the bullvalene skeleton appears to be implicated in the rearrangement processes that occur during the formation of potential bullvalenyl-carbinyl carbocations. " Thus, for example, solvolysis of the esters [(154) and (155)] in aqueous acetone affords the alcohol (156) as the exclusive product. This result is consistent with the mode of attack pictured in (154), or the equivalent alternative formulation which places the carbinyl ester function at the cyclopropane bridgehead position [i.e. the valence tautomer of (154)]. In the case of the positionally isomeric... 

Another type of tautomerism, intramolecular in nature, is called valence tautomerism (or valence isomerization). Valence tautomers rapidly interconvert with one another, but the tautomeric forms differ principally by the positions of covalent bonds rather than by the positions of protons. There are many examples of valence tautomerism in the literature. An interesting example is the isomerization of bullvalene, an interesting compound with threefold symmetry. At low temperatures (below -85°C), the proton NMR spectrum of bullvalene consists of four complex multiplets (each... 

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