Reduction of cyclooctatetraene

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). 

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... 

Mixtures of 1,3,5- and 1,3,6-cyclooctatriene were obtained by partial reduction of cyclooctatetraene in ways such as protonation of cyclooctatetraene dianion3d-4 and reduction with zinc-alkali.2 5 1,3,6-Cyclooctatriene is the major product in these reductions. However, since 1,3,6-cyclooctatriene isomerizes to 1,3,5-cyclooctatriene on treatment with base, quenching cyclooctatetraene dianion with methanol and subsequent heating affords 1,3,5-cyclooctatriene in an 80% yield.3d Reduction of cyclooctatetraene with sodium hydrazide and hydrazine also produces 1,3,5-cyclooctatriene.6 Therefore, when cyclooctatetraene is available in quantity, these procedures are the methods of choice. 

Reduction of cyclooctatetraene 0.1 M (C3H7)4NC104/DMF/ mercury electrode 298 Hush (1968) 0.0087... 

Aromatic salts M2COT resulting from the two-electron reduction of cyclooctatetraene (COT) by alkali metals (Li, Na, K) have been used several times as reductants, although COT remains in solution which makes the separation difficult, for example ... 

A cell (Fig. 54) that allowed the precise control of potential and current was designed by Goldberg and Bard, who also demonstrated the advantage of combining ESR spectroscopy with electrochemical techniques such as CA, CV, and chronopotentiometry [366]. The latter approach was taken in a study of the reduction of cyclooctatetraene (COT) in which it was demonstrated that the COT radical anion is stable in the presence of tetra-butylammonium ion, which had been a matter of dispute in previous work [378] (Fig. 55). 

Methoxyazocines exhibit a two-electron polarographic wave corresponding to conversion to the lOir-electron aromatic dianion (7UA161). The HMO delocalization energy for the planar azocinyl dianion has been calculated to be -5.1 /S, compared to -3.7 /S for cyclotetraenide dianion. Voltammetric data for 2-methoxy-3,8-dimethylazocine in acetonitrile-tetramethylammonium fluoroborate have been interpreted in terms of the direct formation of the dianion at the electrode followed by rapid protonation of the strongly basic dianion (76ACS(B)773). In contrast, the reduction of cyclooctatetraene under these conditions takes place in two steps. 

When 4n 7c-electron compounds are reduced to dilithium dianions, the resulting products contain (4n -I- 2) 7t electrons and are aromatic as reflected in their NMR spectra e.g., the reduction of cyclooctatetraene gives the 10 a -electron dianion ... 

A mixture of cycloocta-l,3,5-triene and cycloocta-l,3,6-triene formed by partial reduction of cyclooctatetraene is the usual starting point for the study of cyclooctatriene complexes, and obviously complexes may be derived from both isomers. Also, the 1,3,5-isomer is in thermal equilibrium with its valence tautomer, bicyclo[4.2.0]octa-2,4-diene (XXVII), at 100° C (30), from which complexes may also be formed. Although silver nitrate... 

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 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... 

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). 

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). 

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.,... 

Electrolytic reduction of 4-cyclooctenone leads smoothly in 64 % yield to cis-bicyclo[3.3.0]octan-l-ol.106) The thermolysis of octachlorocyclooctatetraene produces perchlorinated bicyclo[3.3.0]octatrienes,107,108) several of which have been examined in detail by X-ray structural methods.109) The 1,3,5,7-tetramethyl-cyclooctatetraene dication (57), generated by oxidation of the neutral hydrocarbon with antimony pentafluoride in S02C1F at —78 °C and below, is subject to dis-rotatory cyclization above — 60°.1 °) The latter step does not follow orbital sym-... 

The iron(O) complex [Fe(terpy)(CO)2] is readily prepared by the reaction of terpy with [Fe(cot)(CO)3] (cot, cyclooctatetraene) (41,46) in contrast, [Fe(terpy)2] may be obtained as an air-sensitive, paramagnetic solid from the reduction of [Fe(terpy)2]l2 with lithium benzophenone ketyl (240). 

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