Bis --allyl nickel

The last two examples strongly suggest the possibility that these cyclization reactions proceed through bis(allyl)nickel intermediates,1 particularly since... 

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 RMgX is added to a suspension of the Ni salt in ether at low T. When the reaction is complete, the solvent is evaporated and the product isolated by crystallization from pentene or by sublimation. The resultant bis(allyl)nickel derivatives have limited thermal stability decomposition occurs between 20°C and — 30°C depending on the nature of the substituents (see Table 4). A procedure is available for Ni(i -C3H5)2 Alternatively, the... 

Soluble single species catalysts are also known, such as the bis(7r-allyl nickel halides) [7]. These can be prepared separately or in situ by reacting bis-allyl nickel (which is an ologomerization catalyst for butadiene but does not give high molecular weight polymer) with an equimolar quantity of nickel halide, and thus bears some resemblance to the catalysts from titanium subhalides and alkyl titanium halides. It is of interest to note that the active species is the monomeric form of the initiator as TT-complex with butadiene (XII) [61]. 

The dihydrodiboraanthracene 3 easily forms the dianion 32-, which reacts with the corresponding organometal halides to give the bis(allyl-nickel) triple-decker 21 and the trinuclear Cp Co complex 22 (where Cp ... 

E22.4 (a) (Ni(i -C3Hs)2l- Bis(allyl)nickel(0) has the structure shown below. Each ally ligand, which is planar, is a four-... 

Bis(allyl)nickel (Figure 24.21) is one of the best known allyl complexes, but it is pyrophoric, and decomposes above 293 K. Bulky substituents can be used to stabilize analogues of Ni(ri -C3H5)2. Thus, Ni ri -l,3-(Me3Si)2C3H3 2 is kineti-cally stable in air for up to several days, and decomposes only when heated above 373 K. 

Some Ziegler-Natta catalysts can be adopted to produce polyethylene homopolymers that are branched and possess low density. One example is a catalyst that is prepared by in situ reactions of bis(l,5-cyclooctadiene)nickel or bis(// -allyl)nickel with amino-bis-(imino) phosphorane. These catalysts, however, are not used for commercial production of low-density polyethylene. 

A detailed mechanistic study of the coupling of jt-allyl complexes and organic halides has been carried out.l A mechanism involving the establishment of a pre-equilibrium between bis(allyl)nickel complexes and monoallyl halo-bridged dimers is proposed. A single-electron-transfer mechanism then initiates cross coupling via nickel(I) intermediates. 

Bis(allyl)nickel and the corresponding palladium and platinum compounds... 

The nickel catalyst is obtained by reduction of a nickel(II) compound such as the acetylacetonate in the presence of butadiene, or simply by exchange of butadiene and bis( -allyl)nickel or bis(cycloocta-l, 5-diene)nickel. In the absence of added ligands, butadiene is rapidly and catalytically converted at low temperatures into 1,5,9-cyclododecatriene, mainly the trans, trans, trans isomer. The trimerization bears the mark of a template process, three butadiene molecules linking very specifically around a naked nickel centre. Detailed studies show that the process is not synchronous but occurs in a stepwise fashion (Fig. 12.5). 

Bis(allyl)nickel complexes react with alkyl derivatives of 2,3-dihydro-l,3-diborole to give the first n -allyl tetradecker complexes (14) to be isolated. ... 

C7Hi2Ni Sn[CH(SiMe3)2l2 which, like (1), reacts with butadiene to give the bis(allyl)nickel complex Ni(T 3-C3H4CH2)2Sn[CH(SiMe3)2]2 -... 

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