Naked nickel

Figure 10.32. Typical "naked" nickel and palladium catalysts...

By naked -nickel is meant a nickel complex from which all the bonded ligands are easily displaced by butadiene. 

The formation of CDT is suppressed if ethylene as well as butadiene is brought into contact with a naked-nickel catalyst. Depending on the reaction conditions, the product is a mixture of m,tram-1,5-cyclodecadiene (CDD) and 1,tram-4,9-decatriene (DT) (90). With equal concentration of butadiene and ethylene the co-oligomerization occurs some six times faster than the cyclotrimerization of butadiene to CDT. 

Catalyst, naked -nickel. C2H4 C4H6 = 1 1. c Remainder CDT with traces of VCH and COD. 

Dependence of Product Distribution on C2H4 C4H6°, Using Naked -Nickel Catalyst at 39°-41°C... 

A reasonable mechanism for the co-oligomerization of butadiene with ethylene on a naked-nickel catalyst is shown in Eq. (49). Interaction of an ethylene molecule with the bis(7r-allyl) C8 chain produces a C,0 chain, containing both an alkyl- and a 7r-allylnickel group (XLVI). Coupling of the alkyl bond with the terminal atom of a m-Tr-allyl group or the terminal... 

Ligands with good acceptor character (e.g., triphenylphosphite, Table IX) stabilize the m-7r-allyl, cr-alkyl intermediate (XLVIb) and only CDD is formed. The stabilization introduced by triphenylphosphite is so effective that even at elevated temperatures (60° C) practically no DT results, while a naked-nickel catalyst at the same temperature produces DT and CDD in about equal proportions. 

Butadiene and butyne, in a 2-to-l ratio, react with both the naked-nickel and nickel-ligand catalyst to form 4,5-dimethyl-cb,m,tram-1,4,7-cyclodecatriene (DMCDeT) (94). The yield with naked-nickel, however, never exceeds 25% and will not be discussed further. 

I. Butadiene Styrene = 2 1 20 C in autoclave, naked-nickel catalyst. 

The products formed by the co-oligomerization of acrylic esters with butadiene (102,106) provide useful information concerning the nature and configuration of the intermediates involved. Naked-nickel, methyl acrylate, and butadiene do not react together.7 However, reaction does occur if the nickel-ligand system is used. The formation of the Diels-Alder adduct between the diene and olefin (a cyclohexene derivative) can be suppressed by adding the reactants dropwise to the catalyst (Table XVI footnote C). 

At normal temperatures methyl crotonate does not react with butadiene in the presence of either naked-nickel or the nickel-ligand catalyst. Moreover, since no oligomerization of the butadiene occurs, it is probable that the formation of a stable nickel complex renders the catalyst inactive. 

A parallel may be drawn between the formation of CDD and DT from ethylene and butadiene using a naked-nickel catalyst and the formation of trialkylamine and octatriene. In both cases low temperature favors the formation of a C—C or C—N bond (i.e., formation of CDD or R3N), whereas at higher temperatures (< 60°) the hydrogen-transfer reaction becomes predominant (i.e., formation of DT and w-octatriene). 

In principle pentadienyls can bond to transition elements in at least three basic ways, tj3, and tjs (Fig. 1). These can be further subdivided when geometrical factors are considered. If r 5 coordination could be converted to rj3 orr/1, one or two coordination sites could become available at the metal center, and perhaps coordinate substrate molecules in catalytic processes. Little is known about the ability of pentadienyl complexes to act as catalysts. Bis(pentadienyl)iron derivatives apparently show naked iron activity in the oligomerization of olefins (144), resembling that exhibited by naked nickel (13). The pentadienyl groups are displaced from acyclic ferrocenes by PF3 to give Fe(PF3)5 in a way reminiscent of the formation of Ni(PF3)4 from bis(allyl)nickel (144). 

The thermally sensitive (cod)2Ni has shown unusual reactivity and resembles naked nickel in its chemistry. The chemistry of Ni atoms and (cod)2Ni is often very similar. The following examples demonstrate the high reactivity of (cod)2Ni ... 

Soon after the initial discovery of this trimerization reaction, Wilke and coworkers found that the versatility and reactivity of such catalyses is enhanced when homogeneous zerovalent nickel catalyst are being used [6b, 13]. Catalysts of this type can either be generated from Ni complexes with ligands that can easily be substituted by BD (eg., Ni[CDT], Ni[COD]2) ( naked Nickel [6b]), or from Ni complexes that are reduced in the presence of BD (almost any reducing agent will serve) [6] a typical example is [Ni(acac)2]-Al(OEt)Et2. Condensed nickel vapor has also been shown to be active [13]. 

In the presence of naked nickel , methylenecyclopropane can be codimerized with alkyl acrylates, alkyl crotonates and alkyl maleates 175-186-187) giving Type B cycloadducts in moderate to excellent yields (Eq. 80). 

The result of the reaction with MCP and dimethyl bicyclo[2.2.1]hept-2-ene-2,3-dicarboxylate vide supra) is in contrast to the outcome of most nickel-catalyzed [3 + 2] cycloadditions in the absence of phosphanes ( naked nickel catalysts) which employ unsaturated esters as cosubstrates. Normally, products arising from formal proximal cleavage of the three-membcred ring are formed in these cases. All of these reactions proceed readily at slightly above room temperature (20 40°C) and are highly regio- and stereoselective. 

Compared with the naked nickel reactions, reactions with phosphane-modified nickel catalysts require higher temperatures (80-100 C) in order to proceed at a reasonable rate and usually exhibit a decreased stereoselectivity. Despite these obvious drawbacks, such catalyst systems are advantageous in the case of highly electron-deficient alkenes, such as ( )-but-2-enal. ° or dialkyl fumarates, which can be readily employed as cycloaddition substrates only with modified nickel catalysts (vide supra). 

Diphenylmethylene)cyclopropane (7) reacts with a variety of alkenes, including cyclopentene and vinyl compounds, in the presence of palladium(O) catalysts, but higher reaction temperatures are required. Whereas a 68% yield of 2-(diphenylmethyIene)octahydropentalene [8 R , R = —(CHjjj—] is obtained from cyclopentene with a palladium(O) catalyst at 130°C, naked nickel at 120°C only induces allylic alkylation of the cyclopentene to yield 70% of 3-(2-... 

With (l-methylethylidene)cyclopropane (1) and a naked nickel catalyst, cyclodimers are formed with methyl acrylate in a combined yield of 59% and an isomeric ratio (2/3) of 96 4. When the reaction is performed in the presence of tris(2-phenylphenyl) phosphite as modifying ligand, the combined yield increases to 89% but the selectivity drops to 85 15. ... 

Fig. 4.5 Proposed active site (from naked nickel).

Fig. 4.6 Typical temperature profile of a bulk norbornene polymerization using naked nickel (early RIM experiment).

Catalyst 1, [(z7 -crotyl)(cycloocta-l,5-diene)nickel]hexafluorophosphate ( naked nickel ), was prepared following a procedure developed for the methallyl isomer by Tkatchenko and coworkers [35, 37]. Thus bis(cyclo-l,5-octadiene)nickel was reacted with crotyl bromide to yield the bromide-bridged (// -crotyl)nickel dimer. Abstraction of the bromide by thallium hexafluorophosphate in the presence of cyclo-1,5-octadiene yielded 1. The structure of 1 in solution is consistent with its solid state X-ray crystal structure [38]. 

Naked nickel (1) is a very effective catalyst for norbornene polymerization. Addition of 1 to 1,2-dichloroethane solutions of norbornene (norbornene Ni molar ratio of 2000 1) gave conversions of norbornene homopolymer typically exceeding 95% after about 1 h. 

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