Radialene

B-N 140pm. Structurally, this cyclohexaborane derivative resembles the radialenes, particularly the isoelectronic [Ce(=CHMe)6] in which the Ce ring likewise adopts the chair conformation. 

Electron-hopping is the main charge-transport mechanism in ECHB materials. There is precedence in the photoconductivity Held for improved charge transport by incorporating a number of redox sites into the same molecule. A number of attempts to adapt this approach for ECHB materials have been documented. Many use the oxadiazole core as the electron-transport moiety and examples include radialene 40 and dendrimer 41. However, these newer systems do not offer significant improvements in electron injection over the parent PBD. 

Keywords Acetylene chemistry. Cross-coupling reactions, Cyclo[n]carbons, Expanded radialenes. Molecular scaffolding. Nanostructures, Perethynylated chromophores, Poly(triacetylene), Tetraethynylethene. 

Fig. 5. Insertion of buta-l,3-diynediyl fragments (a) into radialenes [108] produces carbon-rich expanded radialenes and (b) into dendralenes [109] produces expanded dendralenes...

The physical properties of the expanded radialenes were greatly enhanced upon donor functionalization, leading to the stable derivatives 76-78 with fully planar conjugated rr-chromophores [110]. These compounds exhibit large third-order nonlinear optical coefficients, can be reversibly reduced or oxidized, and... 

Fig. 6. Electronic absorption spectra of donor-functionalized expanded radialenes 76-78 in CHCI3 at 298 K...

In contrast, the synthesis of tetraethynylethene (TEE, C10H4) was described in 1991 and, since then, a rich variety of cyclic and acyclic molecular scaffolds incorporating this carbon-rich molecule as a construction module have been prepared. The majority of these compounds, such as the expanded radialenes or the oligomers and polymers of the poly(triacetylene) type, are highly stable and... 

Diels-Alder cycloadditions of sila- and germa[3]radialenes 541 with MTAD or PTAD provide the corresponding products 542 (Equation 73) <19930M1996, 1994AG723>. Similarly, phospha derivative 543 with MTAD gives 544 (Equation 74). On the other hand, the same reaction of cyclic 545 with MTAD is slower and the formed reaction mixture contains only small amount of a product probably analogous to 544 and compound 546 as the major product (Equation 75) <2000JA12507>. 

The compound 251 decarbonylates on photolysis to bis(4-hydroxyaryl) acetylene 253, which is easily oxidized to the quinonoid cumulene 254. This is also obtained by thermal decarbonylation of the product of oxidation of cyclopropenone 251, the diquinocyclopropanone 252. Likewise, the blue derivative of 3-radialene 256 (a phenylogue of triketo cyclopropane) is formed from tris-(4-hydroxyaryl) cyclopropenium cation 255 by oxidation34. ... 

Exocyclic double bonds at cyclic systems, which contain cross-conjugated double bonds, cannot be considered as a subgroup of radialenes and shall therefore be treated separately, although many of the structural features are comparable. However, in these systems the exocyclic and endocyclic double bonds are competing with each other as sites for Diels-Alder reactions, cycloadditions and electrophilic attacks. The double bond character of both, as measured by its distance, can provide some evidence for the selec-tivities. If no strain and conjugation are expected, the double bonds should be comparable... 

This comparison demonstrates that the exocyclic double bond length is little affected by the cyclic strain, which was also found for the radialenes see Section n.E. However, significant deviations were found for conjugated systems, and the same holds for the linear and branched dienes and polyenes. 

Radialenes are a class of compounds that have only relatively recently been synthesized and described160-162. They may also be described as all-ew-methylene-cycloalkanes, and the first four members of this group of molecules, which we for the convenience of the reader, will refer to by the number of ring atoms, are presented in Figure 9. 

Two questions related to the structure of radialenes are of special interest ... 

Which factors determine the conformation of the radialene rings ... 

The reason for the minimum energy conformer of 6 cannot be as simple as that proposed for 5, as the former is far more puckered than what is necessary for minimizing the H- - -H nonbonded repulsions. Valence angle strain is another factor that might be important in this case. The similarity to the chair conformer of cyclohexane is striking, although the calculated [6]radialene conformation is less puckered. 

I. 355(4) A, 1.358(3) A. The exocyclic CC double bonds in 8 appear to be significantly longer than in 7. This is, however, not surprising, as the exocyclic double bonds in 8 are cross-conjugated with the ethynyl substituents. The experimental results available for [3]radialenes are in good agreement with the calculated results for the parent compound (Table 20). 

Considerably more structure data are available for [4]radialenes than for their smaller homologs168-177. The structure of the parent molecule 4 has not been determined yet, but its vibrational spectrum is in agreement with a planar molecule of symmetry178. Most [4]radialene structures are, however, found to be puckered 9, 22.1°168 10, 26.5°172 ... 

II, 19.2°173 12, 34.7°174. These include, for example the molecules shown in Figure 11. The nonplanarity of these [4]radialene molecules is obviously caused by nonbonded... 

These molecules also have large substituents, and it might seem surprising that the radialene rings avoid puckered conformations in these species. The nonbonded repulsions are, however, reduced in these molecules because of external ring closures (13) or because two of the exocylic CC double bonds involve cumulated double bond systems (14 and... 

FIGURE 11. Structures of substituted puckered [4]radialenes (distances in A)... 

The parent [5]radialene (5) has so far evaded preparation. The decamethyl derivative is, however, known, and this molecule is found to have a half-chair conformation, with approximately C2 symmetry179. There are, however, observations indicating that... 

The [6]radialenes are normally observed to have chair conformations164,168 181,182, although a twist-boat conformation has been observed for a very highly substituted... 

The latter two molecules are, however, special cases, where forces other than those inherent in the [6]radialene system are determining the preferred conformation. Hexakis(ethylidene)cyclohexane is the only radialene molecule where structure results obtained in the solid state181, as well as in the gas phase182, are available for comparison (Table 21). 

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