Cyclooctadiene complexes with nickel

One other reaction deserves mention. From bis(cyclooctadiene)nickel and butadiene (31), and in the presence of an isocyanide (RNC, R = cyclohexyl, phenyl, tcrt-butyl) two organic oligomeric products are obtained, 1 -acylimino-11 -vinyl-3,7-cycloundecadiene and 1 -acylimino-3,7,11 -cyclo-dodecatriene. In each, one isocyanide has been incorporated. An analogous reaction with carbon monoxide had been reported earlier. The proposed mechanism of these reactions, via a bis-7r-allyl complex of nickel, is probably related to the mechanism described for allylpalladium complexes above. 

The synthetic route represents a classical ladder polymer synthesis a suitably substituted, open-chain precursor polymer is cyclized to a band structure in a polymer-analogous fashion. The first step here, formation of the polymeric, open-chain precursor structure, is AA-type coupling of a 2,5-dibromo-1,4-dibenzoyl-benzene derivative, by a Yamamoto-type aryl-aryl coupling. The reagent employed for dehalogenation, the nickel(0)/l,5-cyclooctadiene complex (Ni(COD)2), was used in stoichiometric amounts with co-reagents (2,2 -bipyridine and 1,5-cyclooctadiene), in dimethylacetamide or dimethylformamide as solvent. 

A remarkably stable, deep red Ni° stannylene complex, [Ni(1068)4l, has been prepared by the reaction of [Ni(l,5-cyclooctadiene)2] with (1068) in toluene at —78 °C. 70 In spite of the bulkiness of (1068) and the known tendency of analogous Ni° phosphine complexes to dissociate in solution, [Ni(1068)4] remains intact in solution and, moreover, melts at 178-180 °C without decomposition. X-ray crystallography shows tetrahedral geometry about the nickel atom, with Ni—Sn bond lengths of 2.3898(2)-2.399(2) A. 

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). 

The best known example is the cyclization of butadiene and acetylene 121 14°). Butadiene forms cyclooctadiene and cyclododecatriene by the catalytic action of nickel, iron, and other metal complexes. By an experiment using an iron complex with deuterated butadiene, it was proved that no hydrogen shift takes place in the cyclization reaction 70>. 

Closely related to the dimerization of biphenylene (36) to tetraphen-ylene (37, Scheme XV) is the dimerization of an aryl-substituted cyclobutadiene to octadienes or cyclooctadienes by way of nickel complexes. A useful source of the cyclobutadiene group is its air-stable complex with NiBr2. Reduction of this complex with tert-butyllithium (electron-transfer agent) gives the tetraphenylcyclobutadiene-nickel(0)-triphenylphosphine complex (38), which isomerizes to the nickelole (39). The dimerization of 39 leads to 40, whose protonation yields the octadiene. Alternatively, at higher temperatures, 40 can extrude Ni(0) to produce 41 (26, Scheme XVI). 

Mechanistic studies from Semmelhack s group showed that in the first step aryl halide undergoes oxidative addition to the low valent metal complex with formation of organometallic intermediate in a higher oxidation state [1,2]. For example, bis(l,5-cyclooctadienyl)nickel(0), Ni(COD)2, reacts with aryl halide (I) to give arylnickel(II) halide (VIII), which further reacts with another aryl halide molecule to form diarylnickel(IV) halide (IX). Each oxidative addition step includes substitution of ligands at metallic centre. 1,5-Cyclooctadiene (COD) dissociates from the nickel to form a coordinatively unsaturated metallic-centre, which does react with aryl halide. Biaryl II is formed by reductive elimination step from IX with liberation of nickel(II) halide [1,2], Scheme 1. 

Similar to the iron chemistry (compare Chapter 2.3), also nickel complexes allow the reaction of one molecule of butadiene with two molecules of CO2 yielding a,u-dicarboxylic acids [48]. In the reaction of butadiene and CO2 in the presence of nickelbis(cyclooctadiene) and tetramethylethylenediamine first a nickelamonocarboxylate is formed (Figure 19). By further treatment with carbon dioxide and by addition of pyridine a nickeladicarboxylate complex is obtained in yields up to 72 %. Decomposition of the complex with methanol/hydrochloric acid gives cis-dimethyl-3-hexenedioate. 

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). 

The thermal polymerization of butadiene yields, according to Ziegler et al., a mixture of vinylcyclohexene with at most 15% of cyclooctadiene (95, 96). In 1954 Reed (97) discovered the catalytic cyclodimerization of butadiene to cycloocta-1,5-diene with Reppe catalysts, with a 30-40% conversion at 120-130° C. Wilke et al. recently synthesized a very efficient class of catalyst. If nickel-acetylacetonate is treated with metal alkyls (especially aluminum alkyls) in the presence of electron-donating compounds (mainly cycloolefins), new tt complexes of nickel are obtained which catalyze the cyclo-oligomerization of butadiene (98, 99). Using cycloocta-1,5-diene as the olefinic component, the well-crystallized, faintly yellow bis(cycloocta-... 

Nickel compounds can also be employed as catalysts [161-170]. A three-component system consisting of nickel naphthenate, triethyl-aluminum, and boron trifluoride diethyletherate is used technically. The activities are similar to those of cobalt systems. The molar Al/B ratio is on the order of 0.7 to 1.4. Polymerization temperatures range from -5 to 40 °C. On a laboratory scale the synthesis of 1,4-polybutadiene with allylchloronickel giving 89% cis, 7.7% trans, and 3.4% 1,2-structures is particularly simple [8]. In nickel compounds with Lewis acids as cocatalysts, complexes with 2,6,10-dodecatriene ligands are more active than those with 1,5-cyclooctadiene (Table 4) [171]. 

Bazan reported on the synthesis of pseudo-tetrablock copolymers comprised of ethene and 5-norbomen-2-yl acetate, using the initiator system (LiPr2)Ni(q -CH2Ph) (PMes) [(LiPr2) = 77-(2,6-diisopropylphenyl)-2-(2,6-diisopropylphenylimino) propanamide] and 2.5 equivalents of Ni(cod)2 [bis(l,5-cyclooctadiene)nickel [121, 130, 131]. Square-planar nickel complexes with anionic P,0-chelate ligands were used by Goodall and coworkers for the co- and terpolymerization of norbomene and 5-norbomene-2-carboxylic acid ethyl ester with ethene [126]. 

Nickel carbonyl is an extremely toxic substance, but a number of other nickel reagents with generally similar reactivity can be used in its place. The Ni(0) complex of 1,5-cyclooctadiene, Ni(COD)2, can effect coupling of allylic, alkenyl, and aryl halides. 

Among many examples of -orbital interaction, only the following two are selected to illustrate the feature of HO—LU conjugation. One is the cyclooctadiene-transition metal complex ">. The figure indicates the symmetry-favourable mode of interaction in a nickel complex. The electron configuration of nickel is (3d)8 (4s)2. The HO and LU of nickel can be provided from the partly occupied 3d shell from which symmetry-allowed occupied and unoccupied d orbitals for interaction with cyclo-octadiene orbitals are picked up. 

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