Nickel hydride catalyst

Branched a-alkenes, most notably propylene, can insert in either a Markovnikov (secondary or 2,1) fashion or anti-Markovnikov (primary or 1,2) fashion (equation 5). Depending on the regioorientation of the next inserting monomer, this can lead to head-to-taU sequences (the most frequent in polypropylene), head-to-head or tail-to-tail. An illustration of the possible structures is shown in the catalytic dimerization of propylene by nickel catalysts (Figure 1). n-Hexenes, methylpentenes, and 2,3-dimethylbutene can be formed by the nickel hydride catalyst... 

When a nickel hydride catalyst is generated, e.g. in the reaction of dibromobis(tributylphos-phane)nickel(II) and butyllithium, 1 -methylene-2-vinylcyclopentane (17) is formed in 91 % yield as the sole product in alcoholic solvents. ... 

The greatest potential for industrial applications may lie with nickel hydride catalysts. Homogeneous catalysts may be prepared from Ni[P(OR3)3]4 or by reduction of NiS2(P/ 3)2 with alkylaluminum com-pounds. " Carbon skeleton rearrangements are known with these complexes. For example, cis-1,4- hexadiene is converted to trans- 2- methyl-1,3-pentadiene and 2,4-hexadienes at room temperature " ... 

The dimerization of alkenes has been extensively studied by Wilke (Scheme 67).P1 hi the tail-to-tail dimerization of methyl aaylate, cationic nickel hydride species have heen proposed as the active catalysts. The mechanism of this process proceeds hy alkene hydro-metalation, insertion of a second equivalent of methyl aaylate, and then fl-hydride elimination to release the product and generate the nickel hydride catalyst. 

RajanBabu has developed an asymmetric protocol for the heterodimerization of vinyl-arenes and ethenej The use of Hayashi s novel, weakly chelating phosphine 91 is critical to the success of this asymmetric reaction (Scheme 68). 1,6-Dienes (e.g., 92) also undergo direct cycloisomerization in the presence of bis[allyl(bromo)nickel] to afford meth-ylenecyclopentane products (e.g., 93 Scheme 69). The scope of the intramolecular process allows preparation of a variety of carbocyclic and heterocyclic ring systems. A reaction mechanism involving in situ generation of a nickel hydride catalyst, alkene hydro-metalation, cyclization, and p-hydride elimination has been proposed. ... 

A square-planar nickel hydride complex is suggested as the catalytic species [589]. In the first step, the nickel hydride catalyst adds across the double bond of propylene to give two intermediates, namely, a propyl nickel and isopropyl nickel complex. Both of these intermediates can react further with propylene by insertion of the double bond into the nickel-carbon bond, resulting in formation of four more intermediates. ( -Elimination of nickel hydride from these intermediates produces the possible products of propylene dimerization, namely, 4-methyl-1-pen-tene, cis- and trans-4-methyl-2-pentene, 2,3-dimethyl-l-butene, n-hexene, 2-hexene, and 2-methyl-l-pentene. Terminal unbranched olefins are rapidly isomerized under the influence of catalyst by a process of repeated nickel hydride addition and elimination to the internal olefins. Therefore, under ordinary reaction conditions the yield of 4-methyl-l-pentene is low. 

A short survey of information on formation, structure, and some properties of palladium and nickel hydrides (including the alloys with group IB metals) is necessary before proceeding to the discussion of the catalytic behavior of these hydrides in various reactions of hydrogen on their surface. Knowledge of these metal-hydrogen systems is certainly helpful in the appreciation, whether the effective catalyst studied is a hydride rather than a metal, and in consequence is to be treated in a different way in a discussion of its catalytic activity. 

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... 

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. 

The proposed mechanism of the hydrovinylation is supported by available evidences, but so far no study has established clearly all the reaction intermediates. RajanBabu has proposed a mechanism of nickel-catalyzed hydrovinylation, which seems to be one of the most efficient processes, involving a cationic nickel hydride species 144 complexed with a weakly coordinated counterion (Scheme 40). The active catalyst species can be generated through... 

Norbomadiene and ethene can be codimerised to give vinylnorbomene, which is isomerised to ethylidenenorbornene with the same nickel or titanium hydride catalyst (Figure 9.14). 

Nickel 2,6,10-dodecatrien -1,12-diyl, as catalyst for butadiene polymerization, 23 303 Nickel formate as nickel catalyst, 32 226-229 Nickel hydride... 

Tamao and Ito proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of 1,7-diynes initiated by oxidative addition of the silane to an Ni(0) species to form an Ni(ii) silyl hydride complex. Gomplexation of the diyne could then form the nickel(ii) diyne complex la (Scheme 1). Silylmetallation of the less-substituted G=C bond of la, followed by intramolecular / -migratory insertion of the coordinated G=G bond into the Ni-G bond of alkenyl alkyne intermediate Ila, could form dienylnickel hydride intermediate Ilia. Sequential G-H reductive elimination and Si-H oxidative addition would release the silylated dialkylidene cyclohexane and regenerate the silylnickel hydride catalyst (Scheme 1). 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

Nickel,40 41 like almost all metal catalysts (e.g., Ti and Zr) used for alkene dimerization, effects the reaction by a three-step mechanism.12 Initiation yields an organometallic intermediate via insertion of the alkene into the metal-hydrogen bond followed by propagation via insertion into the metal-carbon bond [Eq. (13.8)]. Intermediate 11 either reacts further by repeated insertion [Eq. (13.9)] or undergoes chain transfer to yield the product and regenerate the metal hydride catalyst through p-hydrogen transfer [Eq. (13.10)] ... 

The Metathesis of Unsaturated Hydrocarbons Catalyzed by Transition Metal Compounds J. C. Mol and J. A. Moulijn One-Component Catalysts for Polymerization of Olefins Yu. Yermakov and V. Zakharov The Economics of Catalytic Processes J. Dewing and D. S. Davies Catalytic Reactivity of Hydrogen on Palladium and Nickel Hydride Phases... 

A nickel hydride complex, NiHCl(diphenylphosphinoethane), catalyses the tandem isomerization-aldolization reaction of allylic alcohols with aldehydes.156 The atom- (g) efficient process proceeds at or below ambient temperature with low catalyst loading, and works well even for bulky aldehydes. Magnesium bromide acts as a co-catalyst, and mechanistic investigations suggest that a free enol is formed, which then adds to the aldehyde in a hydroxyl-carbonyl-ene -type reaction. 

The initiation step in polymerisation with a catalyst derived from the [(MeAll)(Cod)Ni]+[PF6] complex and P(Chx)3 involves the secondary insertion of styrene into a Ni H bond in the cationic nickel hydride species formed in situ ... 

Allyl complexes of nickel with monodentate phosphines, e.g., (22-XXXIII), are highly active catalysts for the dimerization of propene.130 Nickel hydride species... 

The system is halide-free and the actual catalyst is probably a nickel hydride with the extremely toxic and volatile Ni(CO)4 as precursor. No detailed mechanistic studies have been published, but a possible scheme for the reaction is in Figure 8. 

Reduction of aqueous or ethanolic solutions of inorganic salts with sodium or potassium borohydride is now the most useful procedure for catalyst activation. Reduction is fast and efficient at room temperature, and, particularly in the case of nickel, a catalyst more active than Raney nickel can be obtained.66 Instead of borohydride, silicon hydrides such as tribenzylsilane have been used for the reduction to produce active platinum catalysts.75... 

The mechanism of hydrocyanation by nickel catalysts should proceed through a nickel hydride addition on the double bonds. The nickel hydrides should result from the oxidative HCN addition to the metal, or from the above Lewis acid-assisted dissociation of HCN. The oxidative HCN addition to low-valent metal complexes has been demonstrated, particularly by NMK spectroscopy with Ni(0)(P(OF.t)3 4. 

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