Norcaradiene tautomerism

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. 

The cyclohepta[e][l,2,4]triazinone (382) reacted with cyclopropene to give compound (383), which showed no tendency to tautomerize to the azaheptalene derivatives (383a) or (383b) (81UP21900). Reaction of (382) with 1-methylcyclopropene affords the condensed norcaradiene (384), which tautomerizes very slowly, particularly in the presence of silica gel, to the methylene derivative (385). The azanorcaradiene (384) reacts with a second molecule of 1-methylcyclopropene to form the polycyclic compound (386). 

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... 

Reactions of triazinotropone 542 (Scheme 143) with cyclopropenes gave norcaradiene derivatives 543a,b that did not rearrange to the tautomeric 1-azaheptalene system, but rather gave azepine 544 (83LA1845). 

Another example of bromotropone contraction is derived from Nozoe s benzotropazine chemistry [Scheme 146 89H(29)1459]. Condensation of tropone 552 with further o-aminothiophenol (117b) gave benzenoid compound 554 instead of the expected tris(benzazino)tropylidene. Presumably, this condensation takes place via initial cyclization (to benzothiazine), then cyclocondensation (to spirobenzothiazoline), followed by tautomerization (to norcaradiene 553), ring contraction, and dehydrogenation. 

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. ... 

Similarly, thermolysis (150°C) of 3-diazodecafluoropentane in the presence of benzene yielded 7,7-bis(pentafluoroethyl)cyclohcpta-l,3,5-triene in 22% yield, rather than the valence tautomeric norcaradiene derivative. ... 

For photochemically generated (2-biphenylcarbonyl)phenylcarbene, several competing intramolecular reactions are observed, namely cyclopropanation of an aromatic nucleus (followed by norcaradiene to cycloheptatriene tautomerization), Wolff rearrangement, C-H insertion, and a carbene-to-carbene rearrangement (see Houben-Weyl, Vol. E19b, pl282). 

Photolysis of (aryl)(phosphoryl)diazomethane derivatives 1 in benzene gave mixtures of 1 1 (2) and 1 2 fraw.v-adducts (3), which were separated by column chromatography. Note that the copper-catalyzed reactions proceeded in poorer yield. Careful NMR studies have shown that the norcaradienes 2 are in valence tautomeric equilibria with the corresponding cyclohep-tatrienes 4, and that the phosphoryl group is always in the exo position. ... 

Figure 5. Walk rearrangement of chiral tropilidenes proceeding with inversion in each step (si process) (66, 70). The term tropilidene is used as a general designation for a cyclohepta-triene-norcaradiene valency tautomeric pair. Since only relative configurations of tropili-denes have been assigned, each configuration depicted in this and the following figures may actually correspond to the opposite sign of optical rotation.

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