Cyclooctatetraenes formation

Schrauzer et al. suggested that for cyclooctatetraene formation these four coordination positions are occupied by acetylenes which are thus automatically arranged (XCIX) in such a fashion that on coupling they give the cyclooctatetraene (C) and that a cyclobutadiene is not an intermediate. 

The first example of cyclooctatetraene formation from metallacyclopenta-dienes was reported by Takahashi and coworkers [81]. Sequential treatment of zirconacyclopentadiene with two equivalents of CuCl and one equivalent of NBS afforded cyclooctatetraenes in good yields (Eq. 73). 

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. 

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). 

Formation of living polymers is not restricted to norbornene. For example, Grubbs successfully polymerized cyclooctatetraene to polyacetylene, and demonstrated the living nature of this polymer by forming block polymers with cyclooctadiene 19). 

The hydrogen-labeled benzobarrelene indicates that the formation of benzo-cyclooctatetraene from the direct photolysis takes place predominantly via benzovinyl bridging<44) j... 

When starting this chapter we promised ourselves and the reader not to consider benzene and its derivatives. Cyclooctatetraene [or more properly (Z,Z,Z,Z)-l,3,5,7-cyclooctatetra-ene, 147] is generally recognized as a polyene and so this latter compound would appear to belong here. How can we do one and not the other Therefore, in this concluding section of the chapter, we briefly discuss the enthalpies of formation of some of the... 

Except for one recent example, all iridium-catalyzed allylic substitution reactions have been performed under an inert atmosphere with dry solvent and reagents. The iridium metalacycle is sensitive to protonation, which opens the metalacycle and results in the formation of a less-active complex containing a K -phosphoramidite ligand. A paper by Helmchen et al. addressed this issue [107]. Nearly all iridium catalysts used for allylic substitution consist of an iridium fragment chelated by COD. In the presence of a catalyst containing dibenzo[a,c]cyclooctatetraene (dbcot) in place of COD, allylic substimtion reactions occur in air with results that are comparable to those of reactions performed under an inert atmosphere (Scheme 35). 

Let us compare anion-radicals with dianions, which are definitely stronger bases. For example, the cyclooctatetraene dianion (CgHg ) accepts protons even from solvents such as dimethylsulfoxide (DMSO) and V,V-dimethylformamide. The latter is traditionally qualified as an aprotic solvent. In this solvent, the cyclooctatetraene dianion undergoes protonation resulting in the formation of cyclooctatrienes (Allendoerfer and Rieger 1965) + 2H+ CgHjo. It is seen that... 

Di(thiocyano)thiophene also presents a different behavior at a mercury-dropping electrode and in the case of treatment by dipotassium salt of cyclooctatetraene dianion THF is the same solvent for both cases (Todres et al. 1979). The reaction between di(thiocyano) derivative and CjHgKj taken in equimolecular amounts leads to the formation of potassium salt of 2-mercapto-5-thiocyanothiophene (potassium mercaptide), potassium cyanide, and cyclooctatetraene (see Scheme 2.17). Potassium mer-... 

A single-electron transfer from cyclooctatetraene dipotassium (C8H8K2) to 2- and 4-nitrostilbenes in THF leads to the formation of paramagnetic potassium salts of the anion-radicals. In this solvent, the salts exist as coordination complexes (Scheme 3.43). 

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... 

Dienes. (+ )-Camphor-derived 1 H-, 2,4-triazole-3,5(2//,4//)-dione 24 was used to determine the absolute configuration of chiral cyclooctatetraenes of type 23 and ent-23 by cycloadduct formation (compare with 25 and 26) and X-ray analysis (see also p 446)166. 

The species (CH)8Mg, or we should say its THF 2.5-solvate, is readily formed from cyclooctatetraene and Mg. The NMR spectrum shows eight equivalent ring atoms and so suggests either the cyclooctatetraene dianion and Mg salt or a putative (and highly fluxional) solvated magnesacyclopentene (or more properly magnesabicyclonon-atriene). However, there is no structural data for the rf open sandwich species and the enthalpy of formation of this simple and sensible half-sandwich, or tight ion pair, cannot even be estimated. 

Both compounds were first synthesized in this way by Reppe Ni(II) salts were shown to be active in the formation of benzene,61 whereas cyclooctatetraene was isolated as the main product with zerovalent phosphinonickel complexes.62 No styrene was, however, obtained in the tetramerization, although it was the sought-after product by Reppe. 

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