Benzvalene route

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... 

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... 

In the photochemistry of benzene, the so-called channel 3 represents a well-known decay route along which fluorescence is quenched above a vibrational excess of 3000 cm [57], The decay takes place through a prefulvenic conical intersection characterized by an out of plane bending [52,58] and results in the formation of benzvalene and fulvene. The purpose of this study is to find distinct radiationless decay pathways that could be selected by exciting specific combinations of photoactive modes in the initial wavepacket created by a laser pulse. For this, we carry out quantum dynamics simulations on potential energy surfaces of reduced dimension, using the analysis outlined above for the choice of the coordinates. 

The route shown succeeded in making the desired diazene 23 but the yield in the intramolecular carbene addition step was poor and initial attempts to obtain prismane by deazetization of compound 23 failed.A much more successful synthesis made use of the reaction of A-phenyl-l,2,4-triazole-3,5-dione with benzvalene. This gave an adduct which was simply converted to the same diazene 23 and it was subsequently shown that a low yield (ca. 6%) of prismane (24) could be obtained from the photolysis. 

The first azaphosphaprismanes and -benzvalenes were also prepared by the above-mentioned route. The combination tri-tert-butylazete (25)/tert-butylphosphaacetylene (9 a) is initially responsible for the formation of the azaphospha-Dewar-benzene 26 (Scheme 6-6) again the reacting centers are determined by the steric demands of the two reaction partners [39]. 

The mechanism of formation of benzvalene from cyclopentadienyl-lithium and dichloromethane has been studied in detail. On employing dideuteriodichloro-methane, the deuterium label is found to be stereospecifically located at the C-1 position of the product, in contrast to earlier reports. These data are compatible with two routes (i) chlorocyclopropanation of the cyclopentadienyl-lithium and subsequent nucleophilic displacement of the chloro-substituent and (ii) attack of the cyclopenta-dienyl anion on the dichloromethane to produce a cyclopentadienyl carbenoid A distinction between these two routes comes from a study of the reaction with indenyl-lithium. The exclusive formation of 1- and 2-deuterionapthalene (0.6 1) as byproducts is compatible only with the carbenoid path (Scheme 6) in which 1,2-cheletropic addition affords benzobenzvalene uniquely labelled at C-1, as is observed. 

Sensitized [2 + 2] photocycloadditions of alkynes to the olefmic bond in hexakis-(trifluoromethyl)benzvalene have been achieved, and are regarded as a convenient route to the Cg unit. The benzene-sensitized irradiation of cis, trans-cyclodeca-1,5-diene yields a mixture of products containing cis- and trans-l,2-divinylcyclo-hexane, cis,cis-cyclodeca-1,5-diene, and the product of crossed [2 -I- 2] addition... 

Swager et al. [214] demonstrated an alternative precursor route to polyacetylene in which the production of volatile byproducts is avoided. The method involves the ring-opening polymerization of benzvalene and the subsequent catalytic isomerization to polyacetylene of low crystallinity (Fig. 5). 

An atom-efficient route to polyacelylene from benzvalene... 

A novel route to bicyclo[2,l,l]hexanes by additions to the now readily available benzvalene (63) has much promise. Addition of bromine affords (64) quantitatively. This remarkable specificity implies the high reactivity of the electron-rich central bond (no observed addition to the double bond). 

---