Cycloheptatrienes-norcaradiene valence tautomerism

Cycloheptatriene gives (181) on reaction with bis(trifluoromethyl)thioketen, via an ene reaction, a cycloheptatriene-norcaradiene valence tautomerism, and an intramolecular Diels-Alder reaction. Cyclo-octa-1,5-diene gives (182) and (183). 

The molecules taking part in a valence tautomerization need not be equivalent. Thus, NMR spectra indicate that a true valence tautomerization exists at room temperature between the cycloheptatriene 110 and the norcaradiene (111). In this case one isomer (111) has the cw-l,2-divinylcyclopropane structure, while the other does not. In an analogous interconversion, benzene oxide and oxepin exist in a tautomeric equilibrium at room temperature. 

Although the structural elements supporting cyclopropyl homoaromaticity and nobond homoaromaticity are now generally understood, it is not clear under what conditions a homoconjugated molecule will prefer to occupy a single minimum or to adopt classical forms connected by a valence tautomeric equilibrium. Of course, one can explain that the norcaradiene/cycloheptatriene system is characterized by a valence tautomeric equilibrium while the homotropenylium cation possesses a single minimum PES. This has simply... 

Photo-oxygenation of 1,3,5-cycloheptatrienes 39 which can undergo valence tautomerization to norcaradiene isomers, leads to the bicyclic endoperoxides 40 and/or tricyclic derivatives 41 depending on the steric and electronic nature of the substituents (Sch. 21) [33]. 

Kraka and Cremer have calculated the and C NMR chemical shifts and magnetic susceptibility as a function of interaction distance of both the cycloheptatriene and norcaradiene systems They point out that both the magnetic susceptibility and the shift difference between the endo and exo protons at C(7) are at a maximum at the transition state for the valence tautomeric rearrangement between the two systems. The transition state is characterized by a C(l)-C(6) distance of 1.864 A and an almost complete equalization of C—C bond lengths, bond orders, atomic charges and shifts of C(2)—C(5). 

A wide range of 3,3-disubstituted cyclopropenes, e.g. the dimethyl and diphenyl derivatives, as well as spiro[2.4]hept-l-ene or 6,6-dimethyl-4,8-dioxaspiro[2.5]oct-l-ene, can thus be reacted with a variety of mono- and disubstituted alkynes. Usually, the chemoselectivity of this cy-clocotrimerization reaction is remarkably high. The norcaradiene derivatives initially obtained are in equilibrium with the valence tautomeric cycloheptatrienes. The equilibrium ratio of the valence tautomers is strongly dependent on the position and kind of substituents, especially those in the 7-position of the newly formed norcaradienes and cycloheptatrienes. When one or two phenyl groups are present in this position, only norcaradiene products can be detected by and NMR spectroscopy at room temperature, whereas in the case of methyl substituents, both valence tautomers are formed in almost equal amounts. When cyclic alkynes, such as cyclooctyne, are employed in the reaction, only norcaradienes are formed regardless of the substituents present in the cyclopropene cosubstrate. ... 

Thermal conversion of cycloheptatriene 30 to 31 and 32 takes place by [l,5]-alkyl shift followed by electrocyclic ring opening of norcaradienes. This phenomenon is known as valence tautomerism of norcaradiene [17]. 

---