Cycloheptatriene, isomerization

It is evident that substituents play an important role in the photo-chemically induced cydization and migration reactions of conjugated trienes. Cycloheptatrienes isomerize to other trienes by migration and the direction of migration is frequently controlled by substituents present in the ring. The direction of cydization to the bicyclo[3.2.0]-heptadiene system is also dependent upon substituents. The relative competition of these two types of reactions has been shown to depend on the type of substituent present in the ring system. Most evidence indicates that these reactions are most likely excited singlet state reactions. 

The results of figure 7.3 might suggest that the 2n — 1 vibrational frequencies are completely equivalent to AS. This has indeed been suggested by Troe (1988) who claimed that for the case of the cycloheptatriene isomerization to toluene, it makes little difference whether a single frequency is adjusted (as in the above calculation), or whether all frequencies are simply multiplied by a common factor. The k E) curve is the same over ten orders of magnitude in the calculated rates. However, Troe s calculation is for a case in which the molecule and TS frequencies are rather similar and in which the different TS frequencies are also similar. We show in figure 7.4 what happens for a dissociation in which the transition state is loose and in which the... 

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. 

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.). 

The facile thermal isomerization (17) of norbornadiene derivatives [71]-[77] to cycloheptatrienes in polar solvents has been explained in terms of the initial heterolytic cleavage of the strained C(l)-C(7) bond (Hoffmann and Hauser, 1965 Lemal et al., 1966 Hoffmann, 1971, 1985 Lustgarten and Richey, 1974 Hoffmann et al., 1986 Bleasdale and Jones, 1993). The resulting zwitterion intermediates are stabilized by the cation-stabilizing groups attached to C(7) and the cyclohexadienyl-type delocalization of the negative charge. 

With l,3>5-cycloheptatriene 2 can be trapped to yield four isomeric [2+2] adducts and the exo/endo isomeric [6+2] compound 16. Heating this mixture to 110°C leads to the complete transformation of the silacyclobutanes into 16 via a dipolar intermediate. The attempted synthesis of the diphenyl derivative of the [2+2] products leads to the stereospecific formation of endo-Yl which could be characterized by X-ray diffraction analysis [4]. 

Starting from 2,4,6-octatriene and pivaldehyde, the conjugated homoallylic alcohol 8 is obtained as the sole product. Cycloheptatriene-derived complexes react with aldehydes and C02 to afford mixtures of the isomeric 1,3- and 1,4-cycloheptadienyl carbinols or acids, respectively. Interestingly, analogous reactions with methyl chloroformate or dimethyl carbamoyl chloride produce the conjugated dienyl ester 9 or amide 10 as unique products [19,20]. 

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

It was shown that [1,5]-hydrogen shift occurs in this case about 30 times more quickly than that in the cycloheptatriene, while the butadienylcyclopropane rearrangement proceeds 3 x 10 9 slower than the Cope rearrangement of the isomeric 2,5-diene 11054. 

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... 

The molecular ions of toluene and cycloheptatriene are capable of mutual isomerization prior to dissociation. [49,53,56] It has been known for long that hydrogen loss from toluene molecular ions takes place as if all eight positions within these ions were equivalent, and this has been interpreted in terms of complete hydrogen scrambling prior to dissociation. [57] The extent of isomerization becomes apparent from the close similarity of the toluene and cycloheptatriene El mass spectra... 

Mormann, M. Knck, D. Protonated 1,3,5-Cycloheptatriene and 7-Alkyl-1,3,5-Cycloheptatrienes in the Gas Phase Ring Contraction to the Isomeric Alkylben-zenium Ions. J. Mass Spectrom. 1999, 34, 384-394. 

As to the cation-radical version of this isomerization, there are testimonies on the transition of the norcaradiene carcass into the cycloheptatriene skeleton. Calculations at the B3LYP level shows that cycloheptatriene cation-radical is more stable than norcaradiene cation-radical by ca. 29 kJ mol (Norberg et al. 2006). Hydrocarbon ion-radicals with strained ring structures have a tendency to undergo facile rearrangement to enforce the unpaired electron delocalization and release their strain energy. 

The precursors were obtained from a mixture of several isomeric cycloheptatrienes formed from 2-phenoxy- or 2-phenylthiophenyl car-benes by intramolecular carbene insertion. 

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). 

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... 

Exercise 27-13 The esr spectrum shown in Figure 27-18 is a first-derivative curve of the absorption of a radical produced by x irradiation of 1,3,5-cycloheptatriene present as an impurity in crystals of naphthalene. Sketch this spectrum as it would look as an absorption spectrum and show the structure of the radical to which it corresponds. Show how at least one isomeric structure for the radical can be eliminated by the observed character of the spectrum. 

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