Of cyclooctatetraenes

FIGURE 11 12 Molecular geometry of cyclooctatetraene The ring is not planar and the bond distances alternate between short double bonds and long single bonds... 

Cyclization with various nickel complex catalysts gives up to 97% selectivity to a mixture of cyclooctatetraene derivatives, with only 3% of benzene derivatives. The principal isomer is the symmetrical l,3,5,7-cyclooctatetraene-l,3,5,7-tetramethanol (29). 

CyclooctatetraenylCompounds. Sandwich-type complexes of cyclooctatetraene (COT), CgH g, are well known. The chemistry of thorium—COT complexes is similar to that of its Cp analogues in steric number and electronic configurations. Thorocene [12702-09-9], COT2Th, (16), the simplest of the COT derivatives, has been prepared by the interaction of ThCl [10026-08-1] and two equivalents of K CgHg. Thorocene derivatives with alkyl-, sdyl-, and aryl-substituted COT ligands have also been described. These compounds are thermally stable, air-sensitive, and appear to have substantial ionic character. 

G. Schroder, Cyclootatetraene, Verlag Chemie, Weinheim, 1965 G. I. Fray and R. G. Saxton, The Chemistry of Cyclooctatetraene and Its Derivatives, Cambridge University Press, Cambridge, 1978. 

Display the electrostatic potential map for the lower-energy form of cyclooctatetraene. Where is the highest concentration of negative charge ... 

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

The interest in azacyclooctatetraenes is mainly due to their structural similarity to cycloocta-tetraene.1 3 Most synthetic efforts have concentrated on a comparison of the unusual chemical and physical properties of cyclooctatetraene with its aza analogs. Besides studies concerning the aromaticity, the primary focus of much work has concerned the stability of the eight-membered ring and its valence isomerization. 

Two major review articles deal generally with aza analogs of cyclooctatetraenes,4 5 while specific classes of compounds arc presented in this section. 

Semi bull valcncs are well-known intermediates in the synthesis of cyclooctatetraenes, 1,5-diazo-cines and 1,3,5,7-tetrazocines. There are also some examples for the synthesis of azocines, although so far only for W-substituted lactam derivatives, e.g. 1.25,26... 

The parent system oxonin (1) is generated by benzophenone sensitized irradiation with a Hanovia light source3 5 or direct irradiation6 of the monoepoxide of cyclooctatetraene (9-oxabicyclo[6.1.0]nona-2,4,6-triene, 6). 

The existence of a metalated epoxide was first proposed by Cope and Tiffany, to explain the rearrangement of cyclooctatetraene oxide (8) to cydoocta-l,3,5-trien-7-one (11) on treatment with lithium diethylamide. They suggested that lithiated epoxide 9 rearranged to enolate 10, which gave ketone 11 on protic workup (Scheme 5.4) [4],... 

Few 1 -benzothiophene-S-oxides 218 were obtained in moderate yields by treatment of 1-arylacetylenes 219 with sulfur dioxide and benzene in the presence of antimony pentafluoride250 (equation 127). A series of cyclic sulphoxides have been prepared by hydrolysis of the corresponding alkoxy sulphonium salts 220251-254 (equation 128). Syn-sulphoxide 221 was obtained in a low yield (15-20%) in the reaction of the dianion of cyclooctatetraene 222 with thionyl chloride255 (equation 129). 

Fig. 2. Correlation of the molecular orbitals of cyclooctatetraene with those of pentalene...

Figure 4.37 Synthesis trees for Reppe synthesis of cyclooctatetraene (a) treating each molecule of acetylene as a separate input (b) treating all 4 acetylene molecules as a single input.

Cope, A.C. Overberger, C.G. (1948) CycUc Polyolefins. I. Synthesis of Cyclooctatetraene from Pseudopelletierine. Journal of the American Chemical Society, 70, 1433-1437. 

Figure 5.4. Potential energy surfaces for out-of-plane bending of cyclooctatetraene and its negative ion. The arrow indicates a vertical transition upon photodetachment to the transition state of the neutral.

Again, the bicyclic valence isomer coexists in sufficient concentration, that the bicyclic peroxide 19 was readily accessible in ca. 20% yield. Alternatively, the thermally labile bicyclic valence isomer of cyclooctatetraene, namely bicyclo[4.2.0]-octa-2,4,7-triene, was converted into the corresponding endoperoxide on low temperature singlet oxygenation and reduced with diimide to yield 19. 

Synthetic applications of other decarbonylation reactions are found in the conversion of cyclooctatetraene to barrelene 250), with the photodecarbonyla-tion of a Diels-Alder adduct as key step (2.31) and the preparation of tetrathioesters from 1,3-dithioles (2.32) 251). The most remarcable application of such a reaction up to date is the synthesis of tetra t.butyltetrahedrane from a tricyclic ketone precursor (2.33) 252). 

Additions of benzene, not to olefins but to acetylenes have been utilized in the synthesis of cyclooctatetraenes (4.38)443). 

TABLE 8. Reduction peak potentials for some derivatives of cyclooctatetraene... 

Scheme 56 Mo-promoted cyclization of cyclooctatetraene and its co-cyclization with alkynes. 

Fig. 4. Calculated eneigies (kcal/mole) of planar forms of cyclooctatetraene with alternating and equal bond lengths relative to the tub form...

Dick turned up some interesting chemistry of caprolactam and its O-alkyl imino ethers. He and collaborators went on to explore the chemistry of allene, for example, its reactions with acetylene, carbon monoxide, and tetrafluo-roethylene. He did extensive work on the chemistry of cyclooctatetraene and of ferrocene. In the cyanocarbon area he collaborated on studies of the anion radical of tetracyanoethylene, that is, tetracyanoethylene bearing an extra electron. He was author or coauthor of 45 papers and 16 U.S. Patents that came out of the Central Research Department. 

The numerous transformations of cyclooctatetraene 189 and its derivatives include three types of structural changes, viz. ring inversion, bond shift and valence isomerizations (for reviews, see References 83-85). One of the major transformations is the interconversion of the cyclooctatetraene and bicyclo[4.2.0]octa-2,4,7-triene. However, the rearrangement of cyclooctatetraene into the semibullvalene system is little known. For example, the thermolysis of l,2,3,4-tetra(trifluoromethyl)cyclooctatetraene 221 in pentane solution at 170-180 °C for 6 days gave three isomers which were separated by preparative GLC. They were identified as l,2,7,8-tetrakis(trifluoromethyl)bicyclo[4.2.0]octa-2,4,7-triene 222 and tetrakis(trifluoromethyl)semibullvalenes 223 and 224 (equation 71)86. It was shown that a thermal equilibrium exists between the precursor 221 and its bond-shift isomer 225 which undergoes a rapid cyclization to form the triene 222. The cyclooctatetraenes 221 and 225 are in equilibrium with diene 223, followed by irreversible rearrangement to the most stable isomer 224 (equation 72)86. 

The interaction of cyclooctatetraene as a dienophile with the diazadiene, 3,6-bis(trifluo-romethyl)-l,2,4,5-tetrazine 226, is accompanied by nitrogen elimination and gives rise to the 1,1-adduct 227. The latter displays interesting thermal rearrangements depending on the solvent polarity and temperature (equation 73)87. In toluene solution a [l,3]-carbon... 

In the same work, encumbered stilbenes were found to have much higher intrinsic barriers than the hydrocarbons listed in the caption of Fig. 3. This is due to a strong internal reorganization around the central C—C bond (Dietz and Peover, 1968). Similar effects have been found in the reduction of cyclooctatetraene and related systems (see Evans and O Connell, 1986, and references cited therein). 

Much yet remains to be done as regards the choice of the pendent radicals. The ring-opening metathesis polymerization of cyclooctatetraenes (Gins-burg et ai, 1989 Klavetter and Grubbs, 1988) has not yet been applied to the construction of high-spin polyacetylenes. 

The behavior of the same azoxybenzene is studied in homogeneous conditions— when the dipotassium salt of cyclooctatetraene dianion (CgHgKj) acts as a dissolved electrode. In this case, the reduction of azoxybenzene stops at the very first stage, that is, after the transfer of one electron only (Todres et al. 1975). The initial one-electron reduction produces the azoxybenzene anion-radicals, which are not reduced further despite the presence of residual electron donor in the solution. The ESR method does not reveal these anion-radicals although one-electron oxidation by phenoxyl radicals quantitatively regenerates azoxybenzene and produces the corresponding potassium phenolate molecules in a quantitative yield. Treatment with water leads to a 100% yield of azobenzene (Scheme 2.14). 

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