Nickel alkyl

Hydrogen reduction of nickel alkyl, Ni(C5H5)2 at 200°C, or nickel chelate, Ni(C5HFg02)2 at 250°C. 

Mention was made earlier about insertion reactions into nickel alkyl bonds 108, 164), and about polymerizations of oleiins by isocyanide nickel complexes 31,174). 

Evidence has been collected over the years which strongly indicates that the active species in the oligomerization reactions are nickel-hydride and nickel-alkyl complexes. [This is not necessarily true for catalysts prepared from nickel(II) compounds and organoaluminum compounds having low Lewis acidity, e.g., (C2H5)2A10C2H5 (44).] The majority of the evidence is circumstantial and is discussed below. 

A large number of (mostly zero-valent) nickel-alkene complexes has been reported. Although these complexes have not been recently reviewed, their general properties and structures were expertly described in 1982 [21]. A complete overview of the reported nickel-alkene and nickel-alkyl complexes is beyond the scope of this section, in which a selection of nickel-alkene and nickel-alkyl complexes is described, mostly related to possible intermediates in hydrogenation catalysis. 

Figure 9.3 pictures the oligomerisation reaction Ni is an abbreviation for the nickel-ligand moiety, kg stands for the rate of the growth reaction, and kt for the rate of the termination reaction. These rate constants are the same for all intermediate nickel alkyls, except perhaps for the first two or three members of the sequence owing to electronic and steric effects. Interestingly, a simple kinetic derivation leads to an expression for the product distribution. One can... 

The nickel(II) macrocycles catalyze the decomposition of alkyl halides (187, 188) and the chemistry, which involves nickel-alkyl complexes, has been investigated recently (189) in some detail. 

It is to be noted that the monomer can only be inserted into a primary nickel-alkyl bond at the end of the growing chain, the insertion is regioselective and only CV-C2 coupling of the growing chain with the next monomer molecule occurs, and the nickel species migrates along the polymer main chain (isomerisation) between two insertions. It should be added that, during the isomerisation, transfer reactions can take place, but not insertions. 

The dimerization of propylene carried out by IFP is called the DIMEROSOL process and involves the use of nickel catalysts. This is shown in Fig. 7.7. Complexes 7.20 and 7.21 are the anti-Markovnikov and Markovnikov insertion products into the Ni-H bond. Structures 7.23(A) and (B) are intermediates derived from 7.21 by inserting the second propylene molecule in a Markovnikov and anti-Markovnikov manner, respectively. Similarly 7.22(A) and (B) are intermediates from 7.20 by the insertion of the second propylene molecule. These lour nickel-alkyl intermediates by /3-elimination give six alkenes. Under the process conditions these alkenes may undergo further isomerization. 

Imidazole- and benzimidazole-2-thiols usually exist largely as the thione tautomers. The thiol (thione) group is susceptible to alkylation (especially in alkaline media), and can be oxidized to sulfide, disulfide and sulfonic acid. This oxidation can often be carried out quite selectively by careful choice of oxidizing agent. The sulfur function can be removed with nitric acid, iron(III) chloride, hydrogen peroxide or, most commonly, Raney nickel. Alkyl- and arylthio groups can be oxidized to sulfoxide or sulfone. 

In an experiment at room temperature using D2SO4 in CH3OD, 39% of the original 1-butene was isomerized in 15 s. There was some rfi-1-butene (0.5%), but more than 99% of the 2-butene products were undeuter-ated. The ratio of isomerization to deuteration is about 170. In spite of this, we are confident that the reaction involves olefin insertion into a nickel hydride to form a nickel alkyl intermediate, followed by de-insertion of... 

Metal complexes are known to insert CO into carbon-sulfur bonds [118], even catalytically [119], Stoichiometric precedents exist for the formation offhioesters from nickel-alkyls, CO, and thiols [120], For example, NiMe2(bipy) reacts with thiols to afford mefhylnickel(ll) fhiolates, which carbonylate to afford acetyl-nickel(ll) fhiolates. These acetylnickel(II) thiolates reductively eliminate fhioester in the presence of CO [121], More biologically relevant is the reactivity of nickel acyls toward fhiolates, which gives fhe thioester concomitant with reduction to Ni(0) (Eq. 12.10) [122]. Thiolates are known to reduce Ni(II) to Ni(0) under an atmosphere of CO [123]. 

The mechanism of Ni-catalyzed ethylene oligomerization involves both nickel hydride and nickel alkyl species. The mechanism is known in the literature as the metal-hydride mechanism, Cossee-Arlman mechanism, or ethylene insertion - -hydride elimination mechanism and results in a Schulz-Flory distribution of the oligomerization products. The mechanism is depicted in Figure 6.16.4. Note that two other coordination sites at the nickel are occupied by one bidentate ligand or two monodentate ligands (see Section 2.4 for details) that have been omitted in Figure 6.16.4 for clarity. 

Nickel Alkyl and Aryl Complexes in Oxidation States 1, iii, and iv 117... 

Table 6 Nickel alkyl and aryl complexes containing anionic chelating ligands, Ni(R)(chelate)(L)...

The study of G-X (X = 0, N, S) coupling reactions is attracting much interest, especially in view of their potential incorporation in catalytic processes that have found ample application in organic synthesis, such as the synthesis of aromatic amines.Most nickel alkyl-alkoxo or alkyl-amido complexes are reluctant to undergo thermal C-N or C-O coupling, but this can be induced by oxidation with O2, [FeCp2], or I2. In the case... 

Although / -elimination is a very common process in nickel alkyl chemistry, the direct observation of this reaction and the characterization of the metal-containing products are rare, since both the starting alkyl and the resulting hydride are usually unstable species. The decomposition of the 1-hexyl anilinotropolone compound shown in Scheme 48 takes place... 

Nickel alkyl and aryl complexes in olefin polymerization and oligomerization catalysis... 

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