Transition metal catalysts with nickel

Chemical transformations through transition metal-catalysed C-H bond activation have attracted much attention owing to the remarkable potential for atom economy and environmental sustainability. Among a range of transition-metal catalysts used, nickel in combination with various ligands plays an important role in performing the transformation. 

In addition Ziegler-Natta polymerization reactions have also shown some success when carried out in ionic liquids. The most common production methods for this form of polymerization involve the use of triethylaluminium catalysts at ca. 100°C and 100 atmospheres pressure. Advances have been developed through the use of organometallic transition metal catalysts, typically nickel or titanium. Given the solvent characteristics of ionic liquids it should be possible to effectively immobilize such catalysts in an ionic liquid solvent. Indeed, Carlin and Wilkes have reported the Ziegler-Natta polymerization of ethene in an ionic liquid solvent. In these reactions an acidic [Cj-mimJCl-AlClj ionic liquid solvent was used to support dichlorobis(Ti -cyclopentadienyl)titanium(IV) with an alkyl-chloroaluminium(III) co-catalyst. 

In contrast to the transition metal catalysts above, nickel complexes have been studied intensely for the polymerization of CHD and its derivatives. jr-Allylnickel-based complexes were employed by Dolgoplosk et al. for CHD polymerization. They used n-alkenylnickel halides, for example, K-metallyl nickel dichloride and it-allyl nickel dibromide, combined with electron acceptors such as chloranil (tetrachloro-/ -quinone) or nickel trichloroacetate. Unfortunately, the true propagating species of the above catalytic systems are not clear, but they were moderately active for CHD polymerization and the polymer obtained appeared to have a predominantly 1,4-linked structure. However, the stereoregularity of the polymer was not very high based on its Tm (270 °C). 

The cycloaddition between norbornadiene (23 in Scheme 1.12) and maleic anhydride was the first example of a /mmo-Diels-Alder reaction [55]. Other venerable examples are reported in Scheme 1.12 [56]. Under thermal conditions, the reaction is generally poorly diastereoselective and occurs in low yield, and therefore several research groups have studied the utility of transition metal catalysts [57]. Tautens and coworkers [57c] investigated the cycloaddition of norbornadiene and some of its monosubstituted derivatives with electron-deficient dienophiles in the presence of nickel-cyclo-octadiene Ni(COD)2 and PPhs. Some results are illustrated in Tables 1.4 and 1.5. 

Transition metals can display selectivities for either carbonyls or olefins (Table 20.3). RuCl2(PPh3)3 (24) catalyzes reduction of the C-C double bond function in the presence of a ketone function (Table 20.3, entries 1-3). With this catalyst, reaction rates of the reduction of alkenes are usually higher than for ketones. This is also the case with various iridium catalysts (entries 6-14) and a ruthenium catalyst (entry 15). One of the few transition-metal catalysts that shows good selectivity towards the ketone or aldehyde function is the nickel catalyst (entries 4 and 5). Many other catalysts have never been tested for their selectivity for one particular functional group. 

Alloying the nickel of the anode to improve tolerance for fuel contaminants has been explored. Gold and copper alloying decreases the catalytic activity for carbon deposition, while dispersing the anode with a heavy transition metal catalyst like tungsten improves sulfur resistance. Furthermore, ceria cermets seem to have a higher sulfur tolerance than Ni-YSZ cermets [75],... 

Nickel and other transition metal catalysts, when modified with a chiral compound such as (R,R)-tartaric acid 5S), become enantioselective. All attempts to modify solid surfaces with optically active substances have so far resulted in catalysts of only low stereoselectivity. This is due to the fact that too many active centers of different structures are present on the surface of the catalysts. Consequently, in asymmetric hydrogenations the technique of homogeneous catalysis is superior to heterogeneous catalysis56). However, some carbonyl compounds have been hydrogenated in the presence of tartaric-acid-supported nickel catalysts in up to 92% optical purity55 . 

Alkenes. Most Group VIII metals, metal salts, and complexes may be used as catalyst in hydrosilylation of alkenes. Platinum and its derivatives show the highest activity. Rhodium, nickel, and palladium complexes, although less active, may exhibit unique selectivities. The addition is exothermic and it is usually performed without a solvent. Transition-metal complexes with chiral ligands may be employed in asymmetric hydrosilylation 406,422... 

Hydrothermal methods, for molecuarlar precursor transformation to materials, 12, 47 Hydrotris(3,5-diisopropylpyrazolyl)borate-containing acetylide, in iron complex, 6, 108 Hydrotris(3,5-dimethylpyrazolyl)borate groups, in rhodium Cp complexes, 7, 151 Hydrotris(pyrazolyl)borates in cobalt(II) complexes, 7, 16 for cobalt(II) complexes, 7, 16 in rhodium Cp complexes, 7, 151 Hydrovinylation, with transition metal catalysts, 10, 318 Hydroxides, info nickel complexes, 8, 59-60 Hydroxo complexes, with bis-Cp Ti(IV), 4, 586 Hydroxyalkenyl complexes, mononuclear Ru and Os compounds, 6, 404-405 a-Hydroxyalkylstannanes, preparation, 3, 822 y-Hydroxyalkynecarboxylate, isomerization, 10, 98 Hydroxyalkynes, in hexaruthenium carbido clusters, 6, 1015 a-Hydroxyallenes... 

Vinylphosphonates are useful reagents but simple vinyl halides do not undergo the Michaelis-Arbuzov reaction except in the presence of a transition metal catalyst [Ni(II) or Cu(I), cf. Protocol 4] so vinylphosphonates are usually synthesized from other functionalized phosphonates or by the palladium-catalysed Michaelis-Becker reaction (cf. Protocol 8).38 Similarly, simple aryl halides undergo the Michaelis-Arbuzov reaction only under special conditions palladium or nickel species (Protocol 4) are suitable catalysts. Indeed these and other catalysts have been applied to the Michaelis-Arbuzov reaction of various substrates, though they are generally essential only with vinyl and aryl halides, as described herein.39... 

---