Benzvalene

The first study was made on the benzene molecule [79], The S ISi photochemistry of benzene involves a conical intersection, as the fluorescence vanishes if the molecule is excited with an excess of 3000 crn of energy over the excitation energy, indicating that a pathway is opened with efficient nonradiative decay to the ground state. After irradiation, most of the molecules return to benzene. A low yield of benzvalene, which can lead further to fulvene, is, however, also obtained. 

In a photochemical experiment, irradiation of benzene leads to Sj, which connects to the ground-state surface via the conical intersection shown. Benzene, the much more stable species, is expected to be recovered preferentially, but the prebenzvalene structure which hansfomis to benzvalene is also fomied. Another possible route from the prebenzvalene, along a different coordinate, will lead to fulvene [90, p.357] after a hydrogen-atom transfer from... 

Figure 39. Benzene to benzvalene reaction, (a) Assuming that the prebenzvalene structure is a transition state. The two benzvalene isomers are anchors, (b) Assuming that prebenzvalene is an intermediate, A two-anchor loop results, compare Figure 12.

A. Obtain the energies of benzene (1), Dewar benzene (2), benzvalene (3), and prisrnane (4). all of which have the empirical fonnula in either the... 

Butyne trimerizes in the presence of aluminum chloride to give hexamethyl Dewar-benzene (W. Schafer, 1967). Its irradiation leads not only to aromatization but also to hexa-methylprismane (D.M. Lemal, 1966). Highly substituted prlsmanes may also be obtained from the corresponding benzene derivatives by irradiation with 254 nm light. The rather stable prismane itself was synthesized via another hydrocarbon, namely benzvalene, a labile molecule (T. J. Katz, 1971, 1972). 

A drawback to the Durham method for the synthesis of polyacetylene is the necessity of elimination of a relatively large molecule during conversion. This can be overcome by the inclusion of strained rings into the precursor polymer stmcture. This technique was developed in the investigation of the ring-opening metathesis polymerization (ROMP) of benzvalene as shown in equation 3 (31). 

Copolymerizations of benzvalene with norhornene have been used to prepare block copolymers that are more stable and more soluble than the polybenzvalene (32). Upon conversion to (CH), some phase separation of nonconverted polynorhornene occurs. Other copolymerizations of acetylene with a variety of monomers and carrier polymers have been employed in the preparation of soluble polyacetylenes. Direct copolymeriza tion of acetylene with other monomers (33—39), and various techniques for grafting polyacetylene side chains onto solubilized carrier polymers (40—43), have been studied. In most cases, the resulting copolymers exhibit poorer electrical properties as solubiUty increases. 

Irradiation of benzene and certain of its derivatives results in bond reorganization and formation of nonaromatic products. Irradiation of liquid benzene with light of 254-nm wavelength results in the accumulation of fulvene and a very small amount of tricy-clo[3.1.0.0 ]hex-3-ene, also known as benzvalene. The maximum conversion to this product in liquid benzene is about 0.05%. The key intermediate is believed to be a biradical formed by 1,3-bonding. 

Because of the low photostationary concentration of benzvalene, photolysis is not an efficient way of accumulating this compound. The highly reactive molecule can be trapped, however, if it is generated in the presence of other molecules with which it reacts. Irradiation of benzene in acidic hydroxylic solvents gives products formally resulting from 1,3-bonding in the benzene ring and addition of a molecule of solvent ... 

These compounds are not direct photoproducts, however. The compounds of structure 9 arise by solvolysis of benzvalene, the initial photoproduct. Products of type 10 are secondaiy photoproducts derived from 9. ... 

Bubbling ozone into a pentane solubon ot hexakjs(trifluoromethyl)benzvalene gives a pale yellow ozonide, which is stable at room temperatures [27] (equation 18)... 

Strained pertrifluoromethyl-substituted valence tautomers of aromatic systems, such as tetrakis(trifiuoromethyl)Dewar thiophene [87] and hexalas(tnfluorQ-methyl)benzvalene [Diels-Alder reactions with various cyclic and acyclic dienes (equations 76 and 77). 

In 1966, a photochemical rearrangement by ultraviolet (UV) irradiation of hexafluorobenzene to hexafluorobicyclo[2 2 0]hexa-2,5-diene was achieved Since then, many reactions analogous to the valence tautomerism of benzene and bicy clo[2 2 0]hexadiene (1 e, Dewar benzene), as well as of fluonnated benzvalene and... 

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

Photolysis of 2,3,5,6-tetrakis(trifluoromethyl)-l,4-di hoshabenzene gives 1,3,4,6 tetrakis(tnfluoromethyl)-2,5-diphosphatncyclo[3 1 0 0 ]hex-3-ene, an ana logue of benzvalene containing phosphorus atoms in the ring system [267, 16S] (equation 40)... 

The stability of benzvalene may be discussed by dividing the molecule into two parts, the tetramethine part and the dimethine part, as illustrated below. 

Benzvalene (61)(87> and the benzenonium ion (62) leading to cation (63)(89> have been proposed as intermediates in these reactions ... 

The isolation of benzvalene (61) from the irradiation of benzene at 254 nm and the observation that this compound produces the expected bicyclic ethers when treated with acidified methanol lend credence to the intermediacy of (61).(90> Photolysis of benzene in acetic acid was found to result in formation of acetates (64)—(67), with the product composition changing with time ... 

Thus, 4,5-benzo-3-thiatricyclo[4.1.0.02 7]heptene (46), which is regarded as a valence isomer of benzo[8]thiepin (4), appears to be an attractive precursor to 4. The most obvious method for the synthesis of 46 would be the application of the elegant Katz method 37> to the benzvalene synthesis. The well established success of the preparation of 43 36) by the Katz method makes this a likely route to 46. 

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... 

Benzene is commonly used as solvent in photochemical reactions nevertheless one has to take into consideration that arenes do undergo photorearrangements 330a,b). Benzene isomers as Dewar benzene331 (3.20) or benzvalene 332) (3.21) have been prepared by photolysis of benzenes. 

Benzoylquinolinium chloride, 27 185 Benzthiazide, 5 168 Benzvalene, copolymerization with norbornene, 7 515... 

Noranda submerged tuyere process, 16 146 Noranda sulfur recovery process, 23 575 NOR arrays, 22 258 Norbornene, copolymerization with benzvalene, 7 515... 

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