Nickel cyclooctadiene complex

Several authors developed the method further of Ni(0)-mediated couplings to generate several PPP derivatives (9, 13, 14. They described homocouplings of various 1,4-dihalobcnzene derivatives by means of nickel(lI)chloridc/triphenylpho-sphine/zinc or the niekel(0)/cyclooctadiene complex. 

Some of the most interesting work on nickel(O) complexes has been carried out by Otsuka et al. (107, 110). These workers have succeeded in obtaining a complex, [Ni(CNBu )2],. This complex is prepared from bis(l,5-cyclooctadiene)nickel and the isocyanide, carefully restricting the amount of the latter to 2 moles per mole of nickel [Eq. (28)]. 

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. 

Nickel-triarylphosphite complexes catalyze the dimerisation of butadiene to cyclooctadiene. Cyclododecatriene is an unwanted by-product, which results from trimerization catalyzed by the same catalyst. Table 3.2 shows the product yields using various ligand-metal complexes (the remainder in each case is a tarry polymeric material). 

Scheme 7.3 Preparation of (fl//-franj-l,5,9-cyclododecatriene)nickel(0) (12) and its reactions with all-cis-C12H18 to the isomer 13 and with 1,5-cyclooctadiene to the bis(diolefin) nickel(0) complex 14...

The hydrosiiyiation of butadiene with HSiMe3 catalyzed by nickel(O) complexes, such as Ni(PR3)2(CO)2, Ni(COD)2, Ni(CO)4, and Ni(CH2=CHCN)PPh3, gives a mixture of but-2-enyltrimethylsilane and octa-2,6-dienyltrimethylsilane together with a considerable amount of cyclooctadiene. Product ratio is affected by the donor ligand employed. ... 

Nickel(O) complexes rapidly oligomerize aliene selectivity depends on the detailed ligand environment . Allylnickel complexes are isolable from these reactions and are involved in the catalytic cycle. Treatment of bis(cyclooctadiene)Ni at — 30°C with, sequentially, aliene and triphenylphosphine affords ... 

Complexes formed from tantalum(V) chloride or niobium(V) chloride, alkynes and zinc undergo analogous reactions. Mixtures of phenols are obtained from cyclobutenones and alkynes in the presence of nickel(cyclooctadiene)2 at 0°C. Thus 4-methyl-3-phenylcyclobut-2-enone and 4-methylpent-2-yne yield 593 and 594 ". ... 

As can be seen from these examples, a great number of exceptional carboxylic acids can be prepared starting from CO2 with the aid of nickel complexes, but up to the present only stoichiometrically. However, if phenyl isocyanate is used which has a structure very analogous to carbon dioxide, a catalytic reaction with ethene occurs in the presence of nickel(0) complexes [17]. When a mixture of phenyl isocyanate, nickelbis(cyclooctadiene) and triphenylphosphine is initially treated with ethene under pressure (3 bar) at -50°C and then heated for 2 d at 60 C, acrylanilide is obtained after protonation in about 180 % yield with respect to nickel(0). This corresponds to 1,8 catalytic cycles (Figure 7). 

The high reactivity of nickel-diene complexes, which renders them very useful in catal5d-ic applications, makes their isolation quite difficult in most cases. The few cases in which nickel-1,3-diene complexes (e.g., 4) have been isolated generally involve displacement of a ligand with a low binding constant such as cyclooctadiene. This should, in theory, be possible using nickel(II) salts reduced in situ, although bis(q -cycloocta-l,5-diene)nickel(0) or (Ti -cyclododeca-l,5,9-triene)nickel(0) (Scheme 3 cdt = cyclododeca-l,5,9-triene) are typically employed.l 1... 

Some olefin complexes may conveniently be obtained by electrochemical reduction methods. Cyclooctadiene and cyclododecatriene nickel(O) complexes such as Ni(COD)2 and Ni(CDT) may be prepared by electrochemical reduction of nickel acetylacetonate in the presence of butadiene in acetonitrile, dimethoxymethane, or DMF solutions. Complexes [Fe(CO)4 (alkene)] are reduced electrochemically to lie compounds [Fe(CO)3 (alkene)] . In the case of complexes [Fe(CO)3 (f/Miene)] (diene = BD, cyclohexadiene), during reduction, lie anions [Fe(CO)3 (diene)] are formed in which one olefin bond is decomplexed. ... 

The nickel(O) complexes are effective in cyclooligomerization of butadienes. The complexes XII and XIII have been demonstrated to be intermediates in the catalytic oligomerization of butadiene the ligand displacement reaction of XII gives a mixture of 1,2-divinylcyclobutane, 4-vinylcyclohexene, and 1,5-cyclooctadiene (Brenner et al., 1969 Heimbach and Wilke, 1969), whereas complex XIII affords 1,5,9-cyclododecatriene (Wilke, 1963 Bogdanovic et al., 1969). Similarly, a number of bis(it-cyclo-... 

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. 

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. 

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. 

Although the copper mediated Ullmann reaction is a well known method for biaryl synthesis, drastic conditions in the range of 150-280 °C are required. Zerovalent nickel complexes such as bis(l,5-cyclooctadiene)nickel or tetrakis(triphenylphosphine)nickel have been shown to be acceptable coupling reagents under mild conditions however, the complexes are unstable and not easy to prepare. The method using activated metallic nickel eliminates most of these problems and provides an attractive alternative for carrying out aryl coupling reactions(36,38). 

The crystal structures of two nickel-complexes which contain cis.ds-cyclooctadiene as chelate ligands are known (89). For complex formation the 2v -symmetric boat-form is most favourable. The energetical compromise in the complexes is therefore such that the... 

Another type of dimerization was observed by Japanese authors198. In the presence of Ni°, compounds like bis(l,5-cyclooctadiene) nickel(0), diphenyl and di-n-propyl cyclopropenone, and cyclohepteno cyclopropenone are transformed to tetra-substituted p-benzoquinones (261/262) by formal (2 + 2) or (3 + 3) cycloaddition of two cyclopropenone moieties effected by metal complexing. 

It is generally assumed that the Lewis acid in 3 decreases the charge on the metal, i.e., increases its electrophilicity. The removal of charge from the nickel allows additional electron donors to coordinate to the nickel atom, and reaction with, for example, 2 moles of carbon monoxide or 1 mole of 1,5-cyclooctadiene (COD) gives the insoluble, catalytically inactive and presumably ionic complexes 7 and 8. In contrast, 7r-allyl-nickel halides (1) add only 1 mole of carbon monoxide while they do not react with COD (52). 

Among transition metal complexes used as catalysts for reactions of the above-mentioned types b and c, the most versatile are nickel complexes. The characteristic reactions of butadiene catalyzed by nickel complexes are cyclizations. Formations of 1,5-cyclooctadiene (COD) (1) and 1,5,9-cyclododecatriene (CDT) (2) are typical reactions (2-9). In addition, other cyclic compounds (3-6) shown below are formed by nickel catalysts. Considerable selectivity to form one of these cyclic oligomers as a main product by modification of the catalytic species with different phosphine or phosphite as ligands has been observed (3, 4). 

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