Cycloheptatriene valence isomerism

Similar reactions under basic conditions are mechanistically ambiguous. They can either be regarded as norcaradiene to cycloheptatriene valence isomerizations or as cyclopropyl to allyl rearrangements promoted by exocyclic proton abstraction. ... 

Cycloheptatrienes are in many cases in rapid equilibrium with an isomeric bicy-clo[4.1.0]heptadiene. The thermodynamics of the valence isomerism has been studied in a number of instances, and some of the data are given below. Calculate the equilibrium constant for each case at 25°C. Calculate the temperature at which K= for each system. Are the signs of the enthalpy and entropy as you would expect them to be Can you discern any pattern of substituent effects from the data ... 

Oxepin and its derivatives have attracted attention for several reasons. Oxepin is closely related to cycloheptatriene and its aza analog azepine and it is a potential antiaromatic system with 871-elcctrons. Oxepin can undergo valence isomerization to benzene oxide, and the isomeric benzene oxide is the first step in the metabolic oxidation of aromatic compounds by the enzyme monooxygenase. 

The triazolinedione adds to cycloheptatriene and cyclooctatetraene to yield the valence-isomeric adducts 265 and 266, respectively (equations 143 and 144)136. 

Recently, a new reactivity index has been proposed (80H(14)1717> which predicts accurately the site selectivity of photocyclization of substituted cycloheptatrienes to their bicyclic valence tautomers. Unfortunately, application of the method to substituted lH-azepines is far less successful. For example, for 2-methyl-l-methoxycarbonyl-lH-azepine (37 R = 2-Me) AGrs values for C-2—C-5 and C-4—C-7 cyclization are calculated as 0.093 and 0.040 kJ mol-1, respectively, i.e. predicting the 1-methyl isomer (39) as the major product. Experimentally, however, the reverse is true, the yields being 93.5% for 3-methyl (38 R = Me) and 6.5% for 1-methyl (39 R = Me). The corresponding photoinduced valence isomerizations of 1-benzazepines to 3,4-benz-2-azabicyclo[3.2.0]hepta-3,6-dienes (38a) have been recorded (80JOC462). These isomerizations have also been achieved thermally in the presence of silver ion (80TL3403). 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

The valence isomeric relationship in the cycloheptatriene/norcaradiene system has been known for over ninety years. ... 

The photochemistry of other cyclic conjugated trienes (e.g., 1,3,5-cyclooctatriene and substituted benzenes) will be discussed and their valence isomerization reactions will be compared with those of substituted cycloheptatrienes. The photochemistry of acyclic conjugated trienes is necessarily more complex than that of cyclic trienes due to the possibility of cis-trans isomerization. The presence of several geometric isomers in a reaction mixture opens up the possibility of other modes of reaction. These transformations will be briefly summarized. Nonconjugated acyclic and cyclic trienes will not be considered in this article. 

Figure 4. Exo-endo equilibration via facile norcaradiene-cycloheptatriene valence bond isomerization (64) and cycloheptatriene ring inversion (65).

A series of ab initio calculations at MP2/6-31GV/RHF/6-31G and UMP2/6-31GV/UHF/6-31G levels of theory were carried out to investigate ben-zynium-methylide structure 8.14 and the biradical structure 8.15 as potential intermediates. Species 8.14 may be a transition state in the valence isomerization of norcaradiene into cycloheptatriene, 8.15 may be an intermediate on the way to nor-caradiene. 

Bicyclic—monocyclic valence isomerization of a radical dianion in the bicyclo[3 1.0]hexanesemidione system has been demonstrated. Symmetrical 1,4-semldiones formally derived from cycloheptatriene and cyclobctatetraene perfer to exist in the bicyclic (4.1.0 and 4.2.0) structures. Bicyclic—monocyclic valence isomerization in the bicyclo[4.1.0]hept-3-ene-2,5-dione system occurs more readily for the dianion than for the radical anion. Several radical anions derived from the (CH)6 io annulenes are reported. In the case of the 1,2-oxygenated derivatives of (CH)io the dianions or radical anions are stable, but the diones undergo valence isomerization and under oxida-time conditions are converted to 4-hydroxynaphthalene-1,2-semiquinone. Enediol dianions in the bicyclo [2.2 l]hepta-2,5-dione and l-carbQalkoxybicyclo[3 2.1] octa-2,6-diene systems have been observed to undergo... 

Valence Isomerism ° Norcaradienes. The dicyano-substituted compound (U5) (see p. 38) exists only as the norcaradiene, and at 110°C it undergoes thermolysis to a benzylfulvene rather than valence isomerization. Protonation of the cycloheptatriene (330) at — 60°C gives an equilibrium in which only the norcaradiene (331) is detectable by n.m.r. spectroscopy. The n.m.r. 

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

Cycloheptatriene represents a challenging substrate for cycloaddition. It can react either in a [2 + 4 ]- or [2 + 6]-cycloaddition to yield isomeric endoperoxides 53 and 54, respectively (Scheme 9.8). Moreover, it may give a 1,2-dioxetane (55) in a [2 + 2] process. Norcaradiene, its valence isomer, may also yield a [2 + 4] adduct (56). All four products were detected in the transformation of the nonsubstituted molecule.421,422... 

Products. The valence tautomerization reaction (18), which gives rise to bicyclo [3.2.0] hepta-2,6-diene, was first reported by Dauben and Cargill. The reaction has a quantum yield of 0.01 or less in the vapor phase and is a minor mode of isomerization. In the condensed phase, this reaction accounts for much of the 1,3,5-cycloheptatriene that is photolyzed although the quantum yield is of the same order (0.03). A second isomerization process gives rise to toluene. The quantum yield... 

An experimental distinction among these three alternatives proved to be difficult, particularly as norcaradienes and cycloheptatrienes usually interconvert easily via valence bond isomerization (norcaradiene - cycloheptatriene EA = 6.5 kcal/mol log A = 11.8) (64) and cycloheptatriene ring inversion (1,3,5-cyclo-heptatriene EA - 6.3 kcal/mol) (Figure 4 ref. 65). Therefore it was necessary to resort to such devices as introducing chirality into the molecules by way of an additional stereochemical label. 

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