Nickel, tetrakis catalyst

Nickel(II) catalysts with a direct nickel(II)-polystyrene bond have been prepared by oxidative addition of tetrakis(triphenylphosphine) nickel(O) to brominated polystyrene (58) ... 

The nickel-catalyzed hydrocyanation of butadiene is a two-step process (Figure 3.32). In the first step, HCN is added to butadiene in the presence of a nickel-tetrakis(phosphite) complex. This gives the desired linear product, 3-pente-nenitrile (3PN), and an unwanted branched by-product, 2-methyl-3-butenenitrile (2M3BN). The products are separated by distillation, and the 2M3BN is then isomerized to 3PN. In the second step, 3PN is isomerized to 4PN (using the same nickel catalyst), followed by anti-Markovnikov HCN addition to the terminal double bond. The second step is further complicated by the fact that there is another isomerization product, CH3CH2CH=CHCN or 2PN, which is thermodynamically more stable than 4PN. In fact, the equilibrium ratio of 3PN/2PN/4PN is only 20 78 1.6. Fortunately, the reaction kinetics favor the formation of 4PN [95],... 

Nickel(0) catalysts have featured prominently in this field with tetrakis(tri- -tolyl phosphite)nickel(O) allowing regio-selective Markovnikov addition of hydrogen cyanide to olefins at temperatures above 50°C. Interestingly, addition of Lewis acid hydrocyanation promoters reduced the selectivity of the reaction. 

This complex is not the actual catalyst for the hydrovinylation, but needs to be activated in the presence of a suitable co-catalyst. The role of this additive is to abstract the chloride ion from the nickel centre to generate a cationic allyl complex that further converts to the catalytically active nickel hydride species. In conventional solvents this is typically achieved using strong Lewis acids such as Et2AlCl. Alternatively, sodium or lithium salts of non-coordinating anions such as tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BARF) can be used to activate hydrovinylation... 

The Lewis acidity and reactivity of these alkyl aluminum cocatalysts and activators with Lewis basic polar monomers such as acrylates make them impractical components in the copolymerization of ethylene with acrylates. To address this shortcoming, Brookhart et al. developed well-defined cationic species such as that shown in Fig. 2, in which the counterion (not illustrated) was the now-ubiquitous fluorinated arylborate family [34] such as tetrakis(pentaflurophenyl)borate. At very low methyl acrylate levels the nickel catalysts gave linear copolymers but with near-zero levels of acrylate incorporation. 

First we will describe the hydrocyanation of ethene as a model substrate. The catalyst precursor is a nickel(O) tetrakis(phosphite) complex which is protonated to form a nickel(II) hydride. Actually, this is an oxidative addition of HCN to nickel zero. In Figure 11.1 the hydrocyanation mechanism in a simplified form is given the basic steps are the same as for butadiene, the actual substrate, but the complications due to isomer formation are lacking. 

Wilkinson (9) isolated the tetrakis(trihalogenophosphine)nickel compounds Ni(PX3)4 (X= F, Cl, Br), and Behrens (10) isolated the triphenylphosphine complex Ni[P(C6H5)3]4 via [Ni(CN)4]4. With iron pentacarbonyl, isonitriles and phosphines yield (11) mono- and disubstituted derivatives, Fe(CO)4L and Fe(CO)3L2, respectively, the latter being the well-known cyclization catalyst of Reppe (7). With the same ligands, carbonyls of the chromium group afforded pentacarbonyl derivatives M(CO)5L. However,... 

This protocol is essentially Tavs method40 as reinvestigated by Balthazor and Grabiak.41 Anhydrous nickel(ll) chloride or bromide (5 mol%) may be used as the precatalyst and is reduced by triethyl phosphite to tetrakis(triethyl phos-phite)nickel(O) (which may be prepared separately and used as the catalyst in the Michaelis-Arbuzov step).41 The mechanism of the reaction has been... 

The hydronickel complex formed from tetrakis(tri-o-tolylphosphite)-nickel(O) and HCl in tetrahydrofuran is an active catalyst for the skeletal isomerization of hexa-2,4-diene as shown [162). The suggested mechanism... 

A number of olefins are converted in the presence of tetrakis(tri-o-tolyl phos-phite)nickel(O) into the corresponding nitriles. These additions yield the terminal nitriles predominantly [15]. Systematic investigations were performed on the hydrocyanation of olefins containing the norbomene skeleton 9 as a basic structure. Table 1 demonstrates the development of catalysts to gain stereocontrol of product formation. 

Oxatrimethylenemethanepalladium complexes can also be generated by oxidative addition of palladium(O) to 5-methylene-l,3-dioxolan-2-ones and subsequent decarboxylation. Again, reaction with norbornene, norbornadiene and dicyclopentadiene yields polycyclic cyclopropyl ketones in medium to high yield (Table 19). In this case, tetrakis(triphenylphosphane)pal-ladium(O) was the best catalyst found, whereas tris(dibenzylideneacetone)palladium(0)-chloro-form/triphenylphosphane (see above) and bis(cycloocta-l,5-diene)nickel/triphenylphosphane (used in stoichiometric amounts) proved less efficient. 

In Nolte and Drenth s nickel catalyzed system, the polymerization was believed to be initiated by a nucleophilic attack by the alcohol used as a solvent or the halide on the starting complex on the coordinated isocyanides. Successful asymmetric polymerization was achieved using a dicationic tetrakis(isocyanide)nickel(II) complex 46 with enantiopure primary amines, which served as a chiral nucleophile in the initial step (Scheme 35) [58, 59]. In a typical experiment, a catalyst prepared from (f-BuNC)4Ni(II)(C104)2 (46a) (1 mol%) and an optically active amine (1 mol%), was used for polymerization of isocyanides with, or without a solvent, such as n-hexane, in... 

The classical methodology of the Ullmann biaryl synthesis was significantly improved by Semmelhack et al. using zero-valent nickel complexes [28]. Aryl iodides were coupled in high yield using bis(1,5-cyclooctadiene)nickel(0) as catalyst. Yields were further improved by the application of tetrakis(triphenylphosph-ine)nickel(0) [29] (Scheme 40). 

Alkynylzinc chlorides are readily available by treatment of lithium acetylide (6, 324) or an ethynyllithium with anhydrous zinc chloride in THF. Tetrakis-(triphenylphosphine)nickel(O) (6, 570) is a much less efficient catalyst than the Pd(0) system. Dienes and diynes are formed in only traces (<5%). 

Dienes. The (E)-l-alkcnylzirconium compounds, obtained by hydrozirconation of 1-alkynes (6, 177-178), undergo cross coupling with alkenyl halides to form conjugated dienes (cf. the similar reaction of alkcnylalanes with alkenyl halides, 7, 95-96). In the present synthesis tetrakis(triphenylphosphine)-palladium (6, 571-573) can usually serve as catalyst. Nickel catalysts are less effective than Pd catalysts in this case. ... 

Alkenyl-alkenyl cross-coupling. Baba and Negishi have prepared a catalyst from this Pd(II) complex and 2 equiv. of diisobutylaluminum hydride that promotes this coupling reaction. Tetrakis(triphenylphosphine)palladium(0) is inactive, as is material prepared in situ from palladium chloride, triphenylphos-phine and HAKr-CtHg) . A nickel catalyst prepared from Ni(acac)2, PfCnHsja, and diisobutylaluminum hydride is somewhat less efficient. The coupling Involves (E)-alkenylalanes (4, 158, 159) and alkenyl halides. The products are (E,E)- and (E,Z)-dienes. 

A review has appeared on the synthesis and reactivity of (sUylamino) and (silyl-anilino)phosphines. The S5mthesis, derivatisation and coordination behaviour of the morpholine (69) and piperazine (70) derivatives have been described. They are formed from the appropriate amine with diphenylchlorophosphine, and both react with sulfur or selenium to give the chalcogenide, whilst reaction of (69) with paraformaldehyde leads to insertion of methylene into the P N bond to give the phosphine oxide. The reaction between ethylenediamine and diphenylchlorophosphine can lead to the bis-, tris- and tetrakis(diphenylphosphino)ethylene-diamines, depending upon the stoichiometry. The first two are oxidised in situ whist the latter, (71), is stable. A nickel complex of (71) has been shown to be active as a catalyst for ethylene pol5nnerisation. ... 

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