Benzvalene isomerization

A route to processible polyacetylene, devised initially using classical initiators (Scheme 1i) 576-578 has been developed using well-defined molybdenum initiators to prepare conjugated polymers.579-585 They have also been employed to prepare polyacetylene via the polymerization of cyclooctate-traene, COT,586 and by the isomerization of poly(benzvalene).587 588 Substituted, and hence soluble, polyacetylene derivatives may be synthesized by polymerizing monosubstituted COT substrates.589-591... 

Two other azepine syntheses involving the interconversion of valence isomers are worthy of note, namely the isomerization of the novel 7-azatetracycloheptane (261), prepared from benzvalene as indicated in Scheme 34, to l-phenyl-l//-azepine (79TL1553) and the rearrangement of the syn isomer of the 2-azatricycloheptane (70 H in place of D), which at 120 °C yields 1 -ethoxycarbonyl-2,3-dihydro- 1H-azepine by a [ 2 +v4t] concerted process. As expected, the anti isomer, for which a concerted process would require a geometrically unfavoured [ 2, + w4a] process, rearranges only at 350 °C, and then by a non-concerted polar reaction (73JA7320). 

Although valence isomerization reactions of aromatic compounds have found little by the way of practical application, they are a fascinating area for mechanistic and theoretical study. The details are not completely dear, but it seems that, for benzene itself, benzvalene arises from the lowest excited singlet state, perhaps by way of a biradical intermediate (3.32) that could also be a precursor to fulvene bicyclohexadiene is probably produced from the second excited singlet state. For some other aromatic compounds the electronic nature of 5, and S2 may be reversed, or at least the states are much closer in energy, so that the preference for benzvalene or bicyclohexadiene formation under conditions of long-wavelength irradiation can be rationalized. 

As already mentioned, the triplet state of benzene was identified by the cis-trans isomerization of the 2-butenes. Nevertheless, Wilzbach and Kaplan in the liquid phase, following work of Srinivasan,42 find a photochemical adduct of benzene to olefins. This adduct they find to be an adduct of benzvalene and not of benzene itself.414 There is also evidence of formation of a small amount of this adduct even in the vapor phase.35 Benzene exposed to ultraviolet radiation in a flowing system diluted with nitrogen and condensed shows compound formation with trifluoroethanol, CF3CH2OH. Again the adduct is one of benzvalene.410... 

As will be seen in a later section, substituted benzenes rearrange photochemically. Thus o-xylene isomerizes to m-xylene and 1,3,5-tri-isobutyl benzene isomerizes to the 1,2,4- and the 1,2,3-triisobutyl benzenes.410 Such isomerizations conceivably could proceed through free-radical intermediates but Wilzbach and Kaplan and their coworkers have shown that the ring carbon to which the moving substituent is attached also changes position with the substituent. These authors offer the very reasonable explanation that the formation of benzvalene followed by rupture of bonds other than the new ones just formed could lead to rearrangements of the type in question. It should be noted that both benzvalene and prismane could serve as intermediates in this way but that Dewar benzene could not. 

The behavior of substituted benzenes does not afford adequate proof that benzene would behave in a similar way. There are two lines of evidence that it does (/) 1,3,5-trideuterobenzene isomerizes to benzenes with different deuterium distributions43 (the status of this work is at present not entirely satisfactory) and (2) benzvalene has been identified by nuclear magnetic resonance in liquid benzene exposed to ultraviolet light.41... 

Foote, Mallon, and Pitts48 report a gas-phase isomerization of benzene at 1849 A with a quantum yield of approximately unity, and tentatively identify the isomer as benzvalene although they suggest the... 

Shifts of atoms or groups in fluorine-containing aromatic or heteroaromatic compounds are covered in this section. Shifts due to valence isomerization (benzene-Dewar benzene-prismane-benzvalene), however, are covered in Section 5.3.4.3. A major review1 on polyfluoroaromatic and heteroaromatic compounds has been written by G.M. Brooke, one of the principal contributors in this area, covering the literature until the end of 1995 (see pp 56-59 of ref 1 for the section on rearrangement reactions). 

Benzene/Dewar Benzene/Prismane/Benzvalene Valence Isomerization... 

Progress in the areas of photoscrambling and valence bond isomerization reactions in aromatic156 and heteroaromatic compounds157 has led to the study of new cyclic systems with strained olefinic double bonds.158 Dewar benzene, Dewar thiophene, benzvalene, and their heteroaromatic counterparts substituted with fluoro and/or perfluoroalkyl substituents have been synthesized and their reactions with phenyl azide have been investigated. 

Sequential addition methods were utilized to synthesize block copolymers of polybenzvalene with polynorbornene to yield block copolymers of PA and pol-ynorbornene after isomerization [43]. (It should be noted that both benzvalene and polybenzvalenes are sensitive to shock and mechanical stress causing ring strain-promoted explosions in the precursor materials.)... 

The field of metallabenzenes is under active investigation, with numerous reports being published each year. A brief listing of references that have not been cited above is given according to the metal ruthenabenzenes [300-302], platina-benzenes [303,304], iridabenzenes [305], osmabenzenes [306], Haley and coworkers observed that in some instances the formation of metallabenzenes from vinylcyclo-propenes competes with valence-isomeric benzvalenic structures [307, 308] (for a systematic survey of valence isomers of annulenes an earlier three-volume book may be consulted [309]). 

Ultraviolet irradiation causes changes in carbon skeleton of many compounds. A classical example is conversion of hexafluorobenzene to hexafluoro Dewar benzene [759], and isomerization of benzene to three isomers. The same is true of hexakis(trifluoromethyl)benzene, which rearranges to the perfluorinated isomers of hexamethyl Dewar benzene (F), prismane (G), and benzvalene (H) [140]. 

In that the various components of benzvalene all have rather low ionization potentials, it is not surprising that benzvalene does also. The experimental value, 8.10 eV, is some 25 kcal mol" lower than that of benzene. Benzvalene radical cation is thus less stable than benzene radical cation by 67 — 25 = 42 kcal mol However, like the neutral benzvalene, benzvalene radical cation does not immediately isomerize in the gas phase. Little is known about the ion chemistry of substituted benzvalenes. The in situ produced cation radicals of 3,4-dimethylbenzvalene resulted in initial isomerization to that of the 5,6-isomer prior to eventually forming the cation radicals of o- and m-xylene. The radical ion chemistry of benzvalene is thus conceptually inseparable from that of yet other cyclopropane with additional strain. 

Figure 5.12. Qualitative state diagram for the fluorescence quenching of benzene by radiationless transition into one of the higher vibrational levels of the isomeric benzvalene. The back reaction is a hot ground-state reaction.

Isomerization reactions of benzene that result in scrambling of hydrogen and carbon atoms presumably involve a benzvalene intermediate and thermal or photochemical rearomatization depending on which of the CC bonds indicated in Scheme 37 by a heavy or a broken line is cleaved, either the rearranged product is formed or the reactant is recovered (Wilzbach et al., 1968). 

Katz s procedure may widely be applied some examples are given in Burger s review 36). Starting from dicyclopenta(a,d)benzene dianiOn, a bisbenzvalenobenzene was prepared as described by scheme (35) 37>. The bisbenzvalene is isomerized in different steps to anthracene by catalysis with silver ions while the benzvalene part is converted to a methylenecyclopentadiene by thermolysis. 

Non-sensitized isomerization of unsubstituted Dewar benzene and benzvalene was found to proceed in high quantum yield (45, 46) 48). 

The reactions of benzvalenes are classified into three categories isomerization, reactions at the double bond and those of the bicyclobutane part. 

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