Cyclooctatetraene-nickel complex

The discovery by Reppe that ethyne is converted catalytically into its tetramer, cyclooctatetraene by nickel(II) cyanide aroused much interest, as it provided a direct route to a compound which had previously been obtained only after a long multi-stage synthesis. Many nickel complexes catalyse the cyclooligomerization of ethyne. Ni(CN) and Ni(CH2=CHCN)2 in benzene yield cyclooctatetraene in about 70% yield at 80-120°C/10-25 atm. Other products include benzene and polymer. Tetramer formation is suppressed by coordinating solvents such as pyridine, vinylacetylene (but-l-en-5-yne) and polymer being produced instead. 

It was found that if bis(duroquinone)-nickel is thermally decomposed in the presence of cyclooctatetraene, cyclooctatetraene-duroquinone-nickel, (XXII), is obtained (56). A more convenient synthesis was found shortly thereafter in a one-step reaction of duroquinone with nickel carbonyl in the presence of the polyolefin. Using the same procedure, analogous complexes with various other olefins were made, the properties of which are listed in... 

Stoichiometric yields (Fig. 7). The reaction of acetylene with bis(acrylo-nitrile)-nickel is not catalytic. During reaction, the tr-bonded acrylonitrile molecules are displaced by acetylene molecules. The final intermediate complex would have the composition Ni(C2H2)4 and could decompose into nickel and cyclooctatetraene. The nickel atom evidently cannot be resolvated by acetylene and the reaction comes to an end. 

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). 

Upon treatment with suitable cobalt complexes, methylbutynol cyclizes to a 1,2,4-substituted benzene. Nickel complexes give the 1,3,5-isomer (196), sometimes accompanied by linear polymer (25) or a mixture of tetrasubstituted cyclooctatetraenes (26). 

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). 

Cyclooctatetraene (continued) with iron, 12 267-273 with nickel, 12 307, 308 with palladium, 12 315 with platinum, 12 319 with rhodium, 12 304 with ruthenium, 12 280 with silver, 12 340, 342, 343, 346 complexes of, 4 81 iron complexes of, 4 89 Cyclooctatriene complexes... 

Cyclotetramerization to form cyclooctatetraene occurs only with nickel.46,63 68 The best catalysts are octahedral Ni(II) complexes, such as bis(cyclooctatetraene) dinickel.46 Internal alkynes do not form cyclooctatetraene derivatives but participate in cooligomerization with acetylene. Of the possible mechanistic pathways, results with [l-13C]-acetylene81 favor a stepwise insertion process or a concerted reaction, and exclude any symmetric intermediate (cyclobutadiene, benzene). The involvement of dinuclear species are in agreement with most observations.46,82-84... 

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethylbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages. 

Alkynes can be selectively dimerized, cyclotrimerized, or polymerized with a large variety of transition metal and lanthanide catalysts nickel also catalyzes the cyclote-tramerization of HC=CH to cyclooctatetraene. Very electrophilic complexes such as Cp 2LnR and Group 4 compounds,137 as well as 18-electron species such as Cp RuH3(L) and Ru(Tp)Cl(PPh3)2, catalyze the linear dimerization of terminal alkynes 138... 

A number of metal carbonyls and cyanides, particularly those of nickel and iron, form 7r-complexes with alkynes. These systems behave cat-alytically in the carbonylation of acetylene and in the formation of trimers (benzene) and tetramers (cyclooctatetraene). 

Stable olefin-Ni(O) complexes are formed also with 1,5-cycloocta-diene (COD) and cyclooctatetraene (COT), by displacement of cyclo-dodecatriene from (Ci2Hig)Ni (608) or by reduction of nickel acetyl-acetonate (73). The COD complex has also been produced (419) by treating anhydrous NiClg with an excess of iso-C3H7MgBr and COD in ether under UV irradiation. 

Schrauzer and Thyret have described (528, 529, 531) the synthesis of olefin-Ni(O) complexes containing a quinone, in particular, duro-quinone, as a ligand. The red, diamagnetic duroquinone complexes are obtained by reaction of nickel carbonyl with the quinone in excess olefin. They are stable in air and soluble in polar organic solvents and water. Those olefins which form the coiiqilex contain essentially parallel double bonds, e.g., norbornadiene, dicyclopentadiene, 1,5-cycloocta-diene, 1,3,5-cyclooctatriene, or cyclooctatetraene. 

Acrylonitrile and related compounds displace all the carbonyl groups from nickel carbonyl to form [(RCH CHCN)2Ni], in which the nitrile bonds through the olefinic double bond 222, 418). The bis(acrylonitrile) complex catalyzes many reactions, including the conversion of acrylonitrile and acetylene to heptatrienenitrile and the polymerization of acetylene to cyclooctatetraene 418). Cobalt carbonyl gave a brown-red amorphous material with acrylonitrile, which had i cn absorptions typical of uncoordinated nitrile groups, but interestingly, the presence of C=N groups was also indicated 419). In acidic methanol, cobalt carbonyl converts a,j8-unsaturated nitriles to saturated aldehydes 459). 

Cyclooctatetraene also forms interesting complexes with cobalt, rhodium, nickel, and other transition metals, but these will not be elaborated on here. It should also be mentioned that other eight-mem-bered ring systems, such as 1,5-cyclooctadiene, 1,3,5- and 1,3,6-cyclo-octatrienes, etc., form a variety of metal, t complexes. The most recent survey of cyclooctatetraene and related metal, t complexes is the review by Fischer and Werner (100) as well as earlier reviews by these authors (99) f and Bennett (11). 

Reppe s cyclooctatetraene synthesis also appears to be ir complex concerted (41). The formation of Binor-S of (XI), and of CaHg thus may be formally related, tt complex, multicenter processes. Accordingly, the reaction of acetylene with Zn[Co(CO)4]2 produced small amounts of cyclooctatetraene (52). This is the first reported case in which cyclooctatetraene was formed using a catalyst containing metals other than nickel, and on a binuclear catalyst center. 

A number of metal complexes catalyses specific alkyne polymerizations, giving rise to four-, six- or eight-membered carbocyclic rings. The first work in this area was the nickel-catalysed formation of cyclooctatetraene (40) from acetylene by the group of Reppe ", but since then formation of cyclic systems from acetylenes has been found to be also catalysed by molybdenum, cobalt, iridium and tantalum. ... 

Two complexes of zero-valent nickel with cyclooctatetraene, Ni(CgH8)2 and [NiCgHg] , have been reported briefly (J24). 

Dicyclopentadiene has two cyclopentene rings with different reactivities. In the reaction with nickel complexes and carbon dioxide only the more reactive norbornene ring couples with CO2, whereas the unstrained ring proved to be unreactive [14,16]. The last example in Figure 6 is cyclooctatetraene, which stands in a temperature-dependent equilibrium with bicyclo[4.2.0]octatriene. Both isomers undergo an oxidative coupling with nickel and CO2. Decomposition by hydrochloric acid leads to cycloocta-2,4,6-triene-carboxylic acid and bicyclo-[4.2.0]octa-2,4-diene-7-carboxylic acid [14]. 

Chelating olefins such as cycloocta-1,5-diene, cyclooctatetraene, or dicyclopentadiene have not yielded isolable complexes by direct reaction with nickel carbonyl, probably for reasons outlined in Section III, B. From this it should not be concluded that such complexes are incapable of existence. Using reactions in which nickel carbonyl is not the reactant it has been possible to prepare not only complexes of cyclooctadiene and cyclooctatetraene, but also of simple monoolefins (Section IX). 

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