Nickel complexes, as catalysts

The alternative method uses a nickel complex as catalyst (Table 13) [91,92]. This reaction, which was carried out in a undivided cell, was found to be efficient... 

The main advantage of this procedure is that reactions occur at relatively high potential values, thus allowing the presence of sensitive functional groups on the starting material. In the presence of a nickel complex as catalyst, the process can be summarized as depicted in Scheme 3. 

The electrochemical preparation of organozinc compounds obtained from the corresponding aromatic halides and with the use of a nickel complex as catalyst is only efficient in dimethylformamide as solvent. Moreover, in most cases and as described previously, the reaction requires the presence of excess 2,2 -bipyridine (five molar equivalents with respect to nickel) to achieve the transmetallation reaction leading to the organozinc compound and to avoid the formation of biaryl, Ar-Ar (equation 53). 

The copper-catalysed, Ullman-type coupling of aryl, heteroaryl and alkenyl halides may be achieved at ambient temperature using copper(I) thiophene-2-carboxylate as catalyst.60 A new semiconducting poly(anthraquinone-l,5-diyl) with nitro groups at the 4- and 8-positions has been prepared by Ullman-type coupling using metallic copper or a zerovalent nickel complex as catalyst.61... 

Simple halides normally undergo direct electrochemical coupling in very low yields819. However, use of nickel complexes as catalysts has been used to synthetic advantage820. Also, the use of a sacrificial anode has much improved the procedure for electrochemical coupling and this process has been reviewed recently821. 

Furthermore, 2,2-difluoro-3-hydroxyesters are readily obtained from ClCFjCOOMe and carbonyl compounds by electrolysis in a one-compartment cell using a sacrificial zinc anode and a nickel complex as catalyst. The catalytic cycle for this reaction is shown in Scheme 9 with nickel zinc exchange being a key step. In this process, the CHjCyDMF solvent (9 1) system leads to suppression of undesired Claisen condensation and an increase in the yield of 2,2-difluoro-3-hydroxyester formation. It is notable that high yields are obtained even with ketones and enolizable aldehydes, which are not good participants in the Reformatsky reaction alternative for producing these substances. 

Quite early, Grignard compounds were coupled with organohalogen compounds by using nickel complexes as catalysts. In 1978 Yamamoto et al. prepared poly(/7-phenylene) from 1,4-dibromobenzene (Fig. 8b). Two years later the same authors reported the preparation of polythiophene using the same procedure but starting from 2,5-dibromothiophene. 

In the hydrocyanation of butadiene, 2 mol of HCN are added to butadiene with a nickel complex as catalyst to obtain adiponitrile directly. 

It is also possible to synthesize acyloxysilanes by reacting triorganohydri-dosilanes with carboxylic acids (eq. 10) in the presence of strong acids, aluminum, or nickel complexes as catalysts (101). 

Regarding the use of well-defined nickel complexes as catalysts for reduction of carbonyl groups, only three examples are described in the literature. In 2009, Guan and coworkers [77] described the efficiency of a nickel PCP-pincer complex performing the hydrosilylation of aldehydes. In the same year, the catalytic hydrosilylation of ketones via a transient Ni-H complex supported by a monoanionic bidentate amidophosphine ligand was reported by Mindiola [78]. Later, Jones investigated well-defined PNP nickel pincer complexes, which catalyzed the hydrosilylation of aldehydes [79] (Fig. 10.16). 

Oligomerization Dimerization or oligomerization of low molecular mass alkenes is regularly carried out industrially, with the aim of preparing linear 1-alkenes for synthetic use. In these applications, the presence of strong Lewis acids is undesirable, so neutral ionic liquids are used, exemplified by [bmim]PFg. ([bmim]+ is l-butyl-3-methylimidazolium.) This has been successful in the manufacture of hexenes from ethylene using a nickel complex as catalyst (Wasserscheid et al, 2001), with... 

Skupinski, W, and Malinowski, S., Polymer anchored Ti-allylic nickel complex as catalyst of propylene dimerization,/. Mol. Catal, 4, 95, 1978. 

In the following section, we discuss the industrial process for the manufacture of a-olefins utilizing nickel complexes as catalysts. This process is known as Shell higher olefin process or SHOP. The mechanism of oligomerization by the Ni-based catalyst is basically the same as that discussed in Section 6.3. However, in this case the ligand environment around the nickel is such that the chain length remains low, but not too low. 

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