Cyclooctatetraene reduction

Nickel plays a role in the Reppe polymeriza tion of acetylene where nickel salts act as catalysts to form cyclooctatetraene (62) the reduction of nickel haUdes by sodium cyclopentadienide to form nickelocene [1271 -28-9] (63) the synthesis of cyclododecatrienenickel [39330-67-1] (64) and formation from elemental nickel powder and other reagents of nickel(0) complexes that serve as catalysts for oligomerization and hydrocyanation reactions (65). 

The NMR spectrum indicates a planar aromatic structure. It has been demonstrated that the dianion is more stable than the radical anion formed by one-electron reduction, since the radical anion disproportionates to cyclooctatetraene and the dianion ... 

Cyclooctatetraene provides a significant contrast to the preference of aromatic hydrocarbons for one-electron reduction. It is converted to a diamagnetic dianion by addition of two electrons. It is easy to understand the ease with which the cyclooctatetraene radical accepts a second electron because of the aromaticity of the 10-7t-electron aromatic system which results (Section 9.3). 

The 1.4-dihydro-l,4-diazocines prepared from iyn-benzene diimines (Section 1.4.1.2.) can be transformed to other derivatives by exchange of the substituents at nitrogen. For this purpose, the dipotassium salt of 1,4-diazocine is generated and then reacted with appropriate electrophiles. For example, reduction of the bistosyl derivative 3 gives a relatively stable dianion, a lOrr-electron system analogous to cyclooctatetraene dianion, which on protonation clearly gives the parent l,4-dihydro-l,4-diazocine (4, E = H) as the only product. 

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

Cyclooctatetraene and some of its derivatives are electrochemically reducible in dry degassed DMF containing BU4NCIO4 as the supporting electrolyte. The first reduction peak potentials which are required to form the corresponding anion radical are shown in Table 824, though a further reaction of the intermediates is not known. 

An alternative approach (Scheme 2) to polyoxyfunctionalised azepines (eg. 9) involves cyclooctatetraene as a starting material via its la, 2a, 5a, 6a -diepoxy-3(3,4(S-diol 7, and subsequent amine nucleophilic attack to give 8 followed by ozonolysis with reductive work up to afford 9 <00TL5483>. 

The most widely studied examples are cyclooctatetraene (COT, 1) and its derivatives. In such conventional aprotic solvents as DMF, dimethyl sulfoxid (DMSO), or acetonitrile containing tetraalkylammonium salts, two distinct one-electron reduction waves are observed at approximately —1.64 V and —1.80 V vs. saturated calomel electrode (SCE), with separations... 

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). 

Reduction of nitrobenzene (Grant and Streitwieser 1978, Todres et al. 1985) and 4-methoxy-nitrobenzene (Todres et al. 1985) by uranium, thorium, and lanthanum-di(cyclooctatetraene) complexes leads to azo compounds. Scheme 1.8 illustrates these reductive reactions using the di(cyclooctatetraene)-uranium complex as an example. 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

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). 

The diamagnetic complex is not reduced further by the cyclooctatetraene dianion. This prevents the conversion of the azoxybenzene anion-radicals into azodianions. Potassium cation plays an important role in this limitation of the reduction process, which, generally, proceeds readily (the... 

However, the analogs containing the nitro group exhibit different behavior in reactions at the surface of the electrode and on reduction by the cyclooctatetraene dianion. The difference is depicted in Scheme 2.22 (Todres 1980). 

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... 

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... 

D. A. Hrovat, J. H. Hammons, C. D. Stevenson, and W. T. Borden, Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-dg and Cyclooctatetraene-dg, J. Am. Chem. Soc. 1997,119, 9523. B3LYP/6-31+G calculations on the title compounds and on the radical anions formed from them show that the very large difference between the equilibrium isotope effects, found by Stevenson, is due to an inverse isotope effect on the planarization of the COT ring. This explanation was subsequently confirmed by KIE measurements, carried out by C. D. Stevenson, E. C. Brown, D. A. Hrovat, and W. T. Borden, Isotope Effects on the Ring Inversion of Cyclooctatetraene, J. Am. Chem. Soc. 1998, 120, 8864. 

By exciting the red-orange cyclooctatetraene dianion 1 in the presence of cyclooctatetraene in our photoelectrochemical cell (n-TiC>2/NH3/Pt), we were able to observe photocurrents without detectable decomposition of the anionic absorber (2). Presumably, a rapid dismutation of the photooxidized product inhibited electron recombination, producing a stable hydrocarbon whose cathodic reduction at the counter electrode regenerates the original mixture essentially quantitatively (eqn 3). 

Linear sweep voltammetry of silylated cyclooctatetraene shows that silylation decreases the first reduction potential but increases the second one (Table 16). The effect on the second reduction potential seems to be due to the stabilization of the dianion by the djr-pjr electron-withdrawing effect of the silyl group69. 

Benzene and cyclooctatetraene (COT) derivatives are formed by [2+2+2] and [2+2+2+2] cycloadditions of alkynes. At first the metallacyclopropene 107 and metallacyclopentadiene 108 are formed. Benzene and COT (106) are formed by reductive elimination of the metallacycloheptatriene 109 and the metallacyclononate-traene 110. Formation of benzene by the [2+2+2] cycloaddition of acetylene is catalysed by several transition metals. Synthesis of benzene derivatives from... 

The reduction of Zr(OR)4 compounds with Et2AlH in the presence of cyclooctatetraene (COT) produces the dihydride (COT)ZrH2. This reacts with protonic hydrogens, liberating hydrogen but since the dihydrides in general act as efficient hydrogenating catalysts for alkenes, the final hydrocarbon product formed is essentially a mixture of cyclo-octatriene and cyclooctadiene (110), e.g.,... 

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