Metal cyclooctatetraene

Cyclooctatetraene can be obtained on an industrial scale by metal carbonyl catalyzed thermal tetramerization of acetylene. If cyclooctatetraene is UV-irradiated at low temperature in the presence of acetone, it is reversibly rearranged to form semibullvalene (H.E. Zimmerman, 1968, 1970). 

Cuprous salts catalyze the oligomerization of acetylene to vinylacetylene and divinylacetylene (38). The former compound is the raw material for the production of chloroprene monomer and polymers derived from it. Nickel catalysts with the appropriate ligands smoothly convert acetylene to benzene (39) or 1,3,5,7-cyclooctatetraene (40—42). Polymer formation accompanies these transition-metal catalyzed syntheses. 

Problem 15.7 Cyclooctatetraene readily reacts with potassium metal to form the stable cycio-octatetraone dianion, C8H82. Why do you suppose this reaction occurs so easily What geometry do you expect for the cyclooctatetraene dianion ... 

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],... 

Connelly et al. (1979) investigated the reactions of arenediazonium salts with cyclooctatetraene (cot) and some of its transition metal compounds. Free cyclooc-... 

Among the compounds that form complexes with silver and other metals are benzene (represented as in 9) and cyclooctatetraene. When the metal involved has a coordination number >1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these eomplexes are called carbonyl groups. Thus, benzene-chromium tricarbonyl (10) is a stable compound. Three arrows are shown, since all three aromatic bonding orbitals contribute some electron density to the metal. Metallocenes (p. 53) may be considered a special case of this type of complex, although the bonding in metallocenes is much stronger. 

As is the case for [2 + 2] cycloaddition reactions (15-61), certain forbidden electrocyclic reactions can be made to take place by the use of metallic catalysts." An example is the silver ion-catalyzed conversion of tricyclo[4.2.0.0. ]octa-3,7-diene to cyclooctatetraene " ... 

The anion from one electron transfer is probably an intermediate. Simple alkenes do not form such stable dianions, but some conjugated derivatives do form dianions as with cyclooctatetraene forming dianion with alkali metals. 

Cyclooctatetraene)metal complexes (13) were some of the first recognized fluxional organometallic complexes. The ( j4-cyclooctatetraene)Mn(CO)3 anion [13, ML3 =... 

The crystal structure of (ij4-cyclooctatetraene)(hexamethylbenzene)ruthenium (16) indicates bonding as a tetrahapto ligand60. For this complex and similar iron-, ruthenium- and osmium-(ij4-cyclooctatetraene)(arene) complexes, their XH and 13C NMR spectra exhibit only a single signal for the cyclooctatetraene ligand at temperatures as low as —145 °C. Using this temperature, the barrier-to-metal migration is estimated to be <6.6 kcal mol 1. 

Cyclooctatetraene was reduced electrochemically to cyclooctatetraenyl dianion. In DMF the product is mostly (92%) 1,3,5-cyclooctatriene at —1.2 V. If the potential is lowered the main product is 1,3,6-cyclooctatriene. Previous experiments, in which the anion radical was found to be disproportionated, were explained on the basis of reactions of the cyclooctatetraene dianion with alkali metal ions to form tightly bound complexes, or with water to form cyclooctatrienes. The first electron transfer to cyclooctatetraene is slow and proceeds via a transition state which resembles planar cyclooctatetraene102. 

Several lanthanide cyclooctatetraene complexes have been synthesized, including both the divalent metal complexes Eu(CgHg) and Yb(CgH8) 44), and the trivalent complexes [M(CgH8)2] (M =Ce, Pr, Nd, SmorTb) 35). The former were preprired by direct reaction of the metal with cyclooctatetraene in liquid ammonia. No structural data exist for these divalent compounds, but they probably involve some kind of bridging interaction since, even with coordinated solvent, one COT... 

A comparison of the metal-carbon bond lengths, ionic radii and formal coordination numbers of these compounds is summarized in Table 2. The formalism used in estabhshing coordination number assumes that a -cyclooctatetraene ligand is a 5 electron-pair donor. The ionic radii have been adjusted for both the charge of the central metal and coordination number (50). When the ionic radius for a given coordination number is not available, it has been estimated by interpolation from radii of other coordination numbers. It will be seen that the differ-... 

In systems of conjugated double bonds catalytic hydrogenation usually gives a mixture of all possible products. Conjugated dienes and polyenes can be reduced by metals sodium, potassium, or lithium. The reduction is accomplished by 1,4-addition which results in the formation of a product with only one double bond and products of coupling and polymerization. Isoprene was reduced in 60% yield to 2-methyl-2-butene by sodium in liquid ammonia [357]. Reduction of cyclooctatetraene with sodium in liquid ammonia gave a... 

A planar molecular structure in the solid state was observed for dilithiated dibenzo[a, e]cyclooctatetraene by Rabideau and coworkers. Dilithiated 233 is formed in THF by the reaction of compound 232 with an excess of metallic lithium at room temperature (Scheme 81). In the solid state, two lithium-TMEDA units are capping both sides of the central eight-membered ring plane . 

The synthesis of bis(rj8-cyclooctatetraene)uranium(IV) (uranocene)J from uranium tetrachloride and (cyclooctatetraene)dipotassium was first published in 1968.1 The method reported here is a modification of that procedure and is suitable for a large variety of cyclooctatefraene complexes.2-4 BisO 8-cyclo-octatetraene)uranium(IV) has also been prepared by the reaction of uranium tetrafluoride with (cyclooctatetraene)magnesium in the absence of solvent.5 Direct reaction of finely divided uranium metal with cyclooctatetraene vapors at 150° also gives some uranocene.5 However, both methods give low yields. 

Pairs of radical ions of like charge also react by electron transfer (i.e., they disproportionate). One classic example involves reduction of tetraphenylethylene and subsequent ET between two tetraphenylethylene anions. A more recent interesting example is that of cyclooctatetrene radical anion 148 . Alkali metals readily reduce the nonplanar cyclooctatetraene, generating a persistent planar radical anion... 

According to the generalized Woodward-Hoffmann rule, the total number of (4q + 2)s and (4r)0 components must be odd for an orbitally allowed process. Thus, Eq. (14) is an allowed, and Eq. (13) a forbidden sigmatropic rearrangement. The different fluxional characteristics of tetrahapto cyclooctatetraene (52, 138) and substituted benzene (36, 43, 125) metal complexes may therefore be related to orbital symmetry effects. 

Tetrahaptocyclohexatriene- and cyclooctatetraene-metal-tricarbonyl complexes undergo some unusual cyclo-addition reactions with tetracy-anoethylene in which the dienophile undergoes a 1,3 addition across the coordinated ligand and causes a rearrangement of the metal-carbon bonds, for example (99),... 

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