Substituted cyclooctatetraene

Paquette and coworkers reported the electrochemical behaviour of silyl-substituted cyclooctatetraene [72]. Two waves are observed with polarography in hexamethylphosphoramide (HMPA) solutions. The silyl substitution causes a... 

Substituted polyacetylenes may be produced through the ring-opening metathesis polymerization of substituted cyclooctatetraenes.127... 

The mass spectrum of cyclooctatetraeneiron tricarbonyl shows stepwise loss of the three carbonyl groups from the molecular ion, followed by elimination of acetylene giving C6H6Fe+, and further breakdown gives Fe+ (119). The mass spectra of two substituted cyclooctatetraene complexes (see Table VIII) show the molecular ion and stepwise loss of the carbonyl groups (83). 

TABLE 38. Calculated energies of the ring inversion transition state (A.Eri) shown in Figure 45 for substituted cyclooctatetraenes. a... 

An interesting extension of cyclooctatetraene polymerisation is the polymerisation of trimethylsilyl-substituted cyclooctatetraene to give soluble polyacetylene substituted with one trimethylsilyl group every eight carbon atoms [162], Such materials have been used to construct solar cells [165],... 

This strategy has also been extended to [6 + 2]-cycloaddition [61]. Thus, substituted cyclooctatetraenes can be prepared in two steps employing a Cr-(0)-promoted [67t + 27t] thiepin dioxide-alkyne cycloaddition, followed by photoactivated sulfur dioxide extrusion (Scheme 9.37) [62]. 

Arylsilanes serve as a typical example of this system. The reduction potentials of arylsilanes are slightly less negative than those of the parent aromatic hydrocarbons [199-203]. This seems to be attributed to the dj -pj interaction between the aromatic ring and the silicon atom. The electrochemical behavior of silyl-substituted cyclooctatetraene is interesting [204]. The second reduction potential becomes less negative by the silyl substitution. The stabilization of the dianion (aromatic 10 7r-system) by dj -p interaction seems to be responsible for this phenomenon. 

Gyclooctatrienyne zirconocene complexes of type 704 can be prepared by /3-hydrogen elimination from zirconocene biscyclooctatetraenyl complexes (Scheme 171).528 These complexes are fluxional by a ring inversion process, with activation barriers in the range typical of cyclooctatetraenes. An X-ray crystal structure (R = Ph) reveals a boat conformation of the cyclooctatrienyne ring with no significant flattening when compared with cyclooctatetraene, thus the structure more closely resembles substituted cyclooctatetraenes than that expected of a cyclooctatrienyne. 

These two energies are, respectively, comparable to the experimental activation energies for conformation inversion of the tub conformer and bond shifting, suggesting that the two planar structures might represent the transition states for those processes. The have been measured for several substituted cyclooctatetraenes. According to... 

Cyclooctatetraenes. (Z,Z)-1,3-Dienyl-1,4-dicopper species are formed on treatment of zirconacyclopentadienes with CUCI2. Further reaction with NBS results in the substituted cyclooctatetraenes. 

The latter process must presumably involve a planar delocalised intermediate state. The former process may well also pass through a planar intermediate state but it could alternatively be a stepwise process. Investigations with some substituted cyclooctatetraenes indicate that process Clower activation energy and takes place more rapidly than process (5) [11]. Steric as well as electronic factors are involved and care must be taken in providing definitive interpretations of these results. 

Using alkoxy-substituted cyclooctatetraenes, A/T for the ring flip and bond switch have been placed in the range 10.9-12.1 kcal/mol and 14.9-15.8 kcal/mol, respectively.These data imply an unfavorable delocalization energy of about 4 kcal/mol... 

The related compounds from alkyl substituted cyclooctatetraenes can be best prepared by refluxing bis(alkyl[8Jannulene)thorium(IV) with excess thorium tetrachloride in THF or DME until the yellow color of the thorocene disappears. [8]Annulenethorium(IV) dichlorides can also be prepared by reaction of thorocene with anhydrous hydrogen chloride.23... 

Jozefiak, T. H., Ginsburg, E. J., Gorman, C. B., Grubbs, R. H., and Lewis, N. S., Voltammetric characterization of soluble polyacetylene derivatives obtained from the ring opening metathesis polymerization (romp) of substituted cyclooctatetraenes, J. Am. Chem. Soc., 115, 4705-4713 (1993). 

Various routes to polyacetylene and derivatives. A. Direct Ziegler-Natta polymerization of acetylene as developed by Shirakawa. B. Feast s retro Diels-Alder, precursor polymer route. C. Ring-opening metathesis pol)rmerization (ROMP) of a substituted cyclooctatetraene as developed by Grubbs. 

Consider the halogen-substituted cyclooctatetraenes with general formula CsHs- X, n = 0,1,2,..., 8. Write down which H atoms are to be replaced to obtain the isomers which have the following symmetries (i) Ci (ii) Csj (iii) Cij (iv) Civj (v) S4 (vi)Di. reference W.-K. Li, Identification of molecular point groups. J. Chem. Educ. 70,485-7 (1993). 

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