Cyclo nickel

COT is prepared by the polymerization of ethyne at moderate temperature and pressure in the presence of nickel salts. The molecule is non-planar and behaves as a typical cyclic olefin, having no aromatic properties. It may be catalytically hydrogenated to cyclo-octene, but with Zn and dil. sulphuric acid gives 1,3,6-cyclooclairiene. It reacts with maleic anhydride to give an adduct, m.p. 166 C, derived from the isomeric structure bicyclo-4,2,0-octa-2,4,7-triene(I) ... 

Polymerization of alkynes by Ni" complexes produces a variety of products which depend on conditions and especially on the particular nickel complex used. If, for instance, O-donor ligands such as acetylacetone or salicaldehyde are employed in a solvent such as tetrahydrofuran or dioxan, 4 coordination sites are available and cyclotetramerization occurs to give mainly cyclo-octatetraene (cot). If a less-labile ligand such as PPhj is incorporated, the coordination sites required for tetramerization are not available and cyclic trimerization to benzene predominates (Fig. A). These syntheses are amenable to extensive variation and adaptation. Substituted ring systems can be obtained from the appropriately substituted alkynes while linear polymers can also be produced. 

Coordination-catalyzed ethylene oligomerization into n-a-olefins. The synthesis of homologous, even-numbered, linear a-olefins can also be performed by oligomerization of ethylene with the aid of homogeneous transition metal complex catalysts [26]. Such a soluble complex catalyst is formed by reaction of, say, a zero-valent nickel compound with a tertiary phosphine ligand. A typical Ni catalyst for the ethylene oligomerization is manufactured from cyclo-octadienyl nickel(O) and diphenylphosphinoacetic ester ... 

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. 

One other reaction deserves mention. From bis(cyclooctadiene)nickel and butadiene (31), and in the presence of an isocyanide (RNC, R = cyclohexyl, phenyl, tcrt-butyl) two organic oligomeric products are obtained, 1 -acylimino-11 -vinyl-3,7-cycloundecadiene and 1 -acylimino-3,7,11 -cyclo-dodecatriene. In each, one isocyanide has been incorporated. An analogous reaction with carbon monoxide had been reported earlier. The proposed mechanism of these reactions, via a bis-7r-allyl complex of nickel, is probably related to the mechanism described for allylpalladium complexes above. 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

Among many examples of -orbital interaction, only the following two are selected to illustrate the feature of HO—LU conjugation. One is the cyclooctadiene-transition metal complex ">. The figure indicates the symmetry-favourable mode of interaction in a nickel complex. The electron configuration of nickel is (3d)8 (4s)2. The HO and LU of nickel can be provided from the partly occupied 3d shell from which symmetry-allowed occupied and unoccupied d orbitals for interaction with cyclo-octadiene orbitals are picked up. 

The interaction of HO of cyclo-octadiene with unoccupied d orbital of nickel. 

Structure-Reactivity Relationships in the Cyclo-Oligomerization of 1,3-Butadiene Catalyzed by Zerovalent Nickel Complexes... 

The catalytic cyclo-oligomerization of 1,3-butadiene mediated by transition-metal complexes is one of the key reactions in homogeneous catalysis.1 Several transition metal complexes and Ziegler-Natta catalyst systems have been established that actively catalyze the stereoselective cyclooligomerization of 1,3-dienes.2 Nickel complexes, in particular, have been demonstrated to be the most versatile catalysts.3... 

The catalytic cyclo-oligomerization of 1,3-butadiene was first reported by Reed in 1954 using modified Reppe catalysts.4 Wilke et al., however, demonstrated in pioneering, comprehensive and systematic mechanistic investigations, the implications, versatility and the scope of the nickel-catalyzed 1,3-diene cyclo-oligomerization reactions.3,5... 

Although the pioneering, systematic, and comprehensive experimental work of Wilke et al3,5 has led to a thorough understanding of the nickel-catalyzed cyclo-oligomerization reaction of 1,3-butadiene, there are still some essential mechanistic details that are not yet firmly established (vide infra). In the following account, we summarize recent progress in the... 

CATALYTIC REACTION CYCLES FOR THE NICKEL-CATALYZED CYCLO-OLIGOMERIZATION OF 1,3-BUTADIENE... 

The cyclo-oligomer products are formed in final reductive elimination steps commencing from the octadienediyl-Ni11 and dodecatrienediyl-Ni11 complexes for the C8- and Ci2-cyclo-oligomer production channels, respectively. Reductive elimination is accompanied with a formal electron redistribution between the nickel and the organyl moieties, which will be analyzed in Section 5.4. 

Scheme 4. Condensed free-energy profile (kcalmol-1) of the complete catalytic cycle of the C8-reaction channel of the nickel-catalyzed cyclo-oligomerization of 1,3-butadiene for catalyst IV with L = P(OPh)3. The favorable [Ni°(p2-tr

A. Cyclo-Oligomerization with Zerovalent PR3/P(0R)3-Stabilized Nickel Complexes as the Catalyst... 

Scheme 6. Interplay of the C8- and C -production channels for the cyclo-oligomerization of 1,3-butadiene with zero valent PR3/P(OR)3-stabilized nickel complexes as the catalyst. Free energies (AG, AGJ in kcalmol-1) are given relative to the favorable rf-synrfiC A-cis isomer of 2a for catalysts bearing strong a-donor ligands namely I (L = PMe3), III (L = PPrj), VI (L = PBU3), and -acceptor ligands namely V (L = P(OMe)3), IV...

For PR3/P(OR)3-stabilized nickel complexes, there are two borderline cases known from the experimental investigation of Heimbach et al. 1 which, unlike the usual behavior, redirect the cyclo-oligomerization reaction into the Ci2-cyclo-oligomer production channel. Catalysts bearing either strong a-donor ligands that must also introduce severe steric pressure (e.g., PBu Pr2) or sterically compact n-acceptors (like P(OMe)3) are known to yield CDT as the predominant product. From a statistical analysis it was concluded,8a,8c that the C8 Ci2-cyclo-oligomer product ratio is mainly determined by steric factors (75%) with electronic factors are less important. 

In this account we have presented a consistent and theoretically well-founded view of the catalytic structure-reactivity relationships for the nickel-catalyzed cyclo-oligomerization of 1,3-butadiene, which represents one of the key reactions in homogeneous catalysis that, furthermore, has... 

II. Catalytic Reaction Cycles for the Nickel-Catalyzed Cyclo-Oligomerization... 

The patent literature contains several references to the use of sulfoxide complexes, usually generated in situ, as catalyst precursors in oligomerization and polymerization reactions. Thus, a system based upon bis(acrylonitrile)nickel(0> with added Me2SO or EtgSO is an effective cyclotrimerization catalyst for the conversion of butadiene to cyclo-1,5,-9-dodecatriene (44). A similar system based on titanium has also been reported (407). Nickel(II) sulfoxide complexes, again generated in situ, have been patented as catalyst precursors for the dimerization of pro-pene (151) and the higher olefins (152) in the presence of added alkyl aluminum compounds. 

Raney nickel at 250° and 100-200atm afforded 88% yield of ethyl 3-cyclo-hexylpropanoate [1068], and hydrogenation of the same compound over copper chromite at 250° and 220atm gave 83% yield of 3-phenylpropanol [7057] (p. 158). 

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