Nickel complexes cyclohexene

Tolman has shown that the equilibrium constants for the reactions of 38 substituted ethylenes with Ni[P(0-o-tolyls)]3 in benzene, to form (ENE)bis-(tri-o-tolylphosphite)nickel complexes, is sensitive to the ethylene s structure, eqn. (2) (ref. 7). Values of Ki at 25° vary from 10 for cyclohexene to... 

Cyclization of 1,6-enynes3 (cf. 13, 91 14, 299). Cyclization of these enynes catalyzed by palladium or nickel complexes generally leads to five-membered ring products. However, cyclization catalyzed by Wilkinson s catalyst generally leads to methylene-2-cyclohexenes. Substitution on either of the terminal groups suppresses this cyclization, which probably involves insertion of Rh(I) in the C—H bond of the terminal alkyne. 

In contrast with the results obtained with simple allqfl halides, benzyl bromide leads to the formation of 77 and the ketone 78 in variable ratios (Scheme 26). A similar result has been reported in the reactions between the oxidative addition product of Ni(COD)bpy or Ni(COD)TMEDA with cw-4-cyclohexen-l,2-dicarboxylic anhydride and alkyl iodidesWith allyl bromide as the electrophile, ketone 79 is the only product isolated. However, when the reaction is performed with isolated nickelacycle 66 in the absence of Ni(CO)2Me2Phen, allylated alanine 80 is formed exclusively (60% yield) (Scheme 26). These results show that the carbonyl nickel complex is not inert because with certain reagents it transfers CO to the nickeMactone 66. Alternatively, the formation of ketones in these reactions could be explained by alkylation of the primary oxidative addition product or by carbonylation of allyl or benzyl bromide to give acyl bromides which react with 66 to give the observed products. However, this last reaction pathway seems unlikely because acetyl or benzoyl chloride do not react with in situ generated nickelacycle 66. 

Diazomethane is also decomposed by N O)40 -43 and Pd(0) complexes43 . Electron-poor alkenes such as methyl acrylate are cyclopropanated efficiently with Ni(0) catalysts, whereas with Pd(0) yields were much lower (Scheme 1)43). Cyclopropanes derived from styrene, cyclohexene or 1-hexene were formed only in trace yields. In the uncatalyzed reaction between diazomethane and methyl acrylate, methyl 2-pyrazoline-3-carboxylate and methyl crotonate are formed competitively, but the yield of the latter can be largely reduced by adding an appropriate amount of catalyst. It has been verified that cyclopropane formation does not result from metal-catalyzed ring contraction of the 2-pyrazoline, Instead, a nickel(0)-carbene complex is assumed to be involved in the direct cyclopropanation of the olefin. The preference of such an intermediate for an electron-poor alkene is in agreement with the view that nickel carbenoids are nucleophilic 44). 

Catalytic homogeneous hydrogenation of cyclohexene has been claimed for simple systems such as nickel(II) acetylacetonate [39] or a nickel-chloride complex with two monodentate amines [40]. The latter complex was used as comparison for a heterogeneous catalyst obtained by impregnation of the complex on y-alu-mina [40]. SCRs of 100 were used at 30 atm. H2 and temperatures up to 100°C, resulting in conversions of only 20-35% after 1 h. 

Ni(II) complexes of cyclam and oxocyclam derivatives catalyze the epoxidation of cyclohexene and various aryl-substituted alkenes with PhIO and NaOCl as oxidants, respectively. In the epoxidation catalyzed by the Ni(II) cyclam complex using PhIO as a terminal oxidant, the high-valent nickel- complexes (e.g., LNiin-0, LNi=0, LNiin-0-... 

Reaction (21) occurs with cyclohexene and both cis- and trans-2-butene, without isomerization of the alkenes,139 while the reduction of Ni(acac)2 with Al(alkyl)3 in the presence of PCy3 and 2-butene (cis and trans) affords the complex bis(tricyclohexylphosphine)(l-butene)nickel(0)>... 

Stolzenberg and coworkers have used electrogenerated nickel(I) tetrapyrrole complexes for the catalytic reduction of dichloromethane and methyl iodide [364], alkyl halides [365-367], and aryl halides [367], and Lexa and coworkers [368] have discussed the catalytic reduction of frm75 -l,2-dibromocyclohexane to cyclohexene by electrogenerated nickel(I), cobalt(I), and iron(I) porphyrin complexes. 

Finally, when methylene cyclohexan is submitted to the action of NaH-f-AmONa—Ni(OAc)2 reduction yield is only 75% and 1-methyl cyclohexene appears. This isomerization as well as the reductions performed strongly suggest21,241 the presence of nickel hydride species in the complex reducing agent. 

Olefin hydrocyanation using palladium catalysts has been less well studied than with nickel. Nevertheless, zerovalent complexes of palladium, particulrly triarylphosphite complexes, hydrocyanate a wide range of olefins in useful yields (see Table 1). Early work reported the merit of excess phosphorus ligand to promote the reaction, and further paralleling the observations with nickel, Lewis acids have been used to improve catalytic activity. However, addition of ZnClj fails to improve nitrile product yield . Asymmetric induction in hydrocyanation results in optical yields of 30% in the synthesis of exo-2-cyanonorbomane using the chiral ligand DIOP, and studies on the stereochemistry of HCN and DCN addition to terminal alkenes and a substituted cyclohexene with the same catalyst have been reported. ... 

Other methods have been used for the decomposition of carbonyl precursors. For example, an early report on nickel particles preparation through UV irradiation of Ni(CO)4 was published by Tanner et al. Triangular particles of ca. 6 nm mean size were produced in this way. Similarly, the irradiation of a solution of the carbonyl polyoxoanion rhodium complex [Rh(C0)2-P2WisNb3062] in the presence of H2 and cyclohexene involves the formation of a black precipitate containing Rh(0) nanoclusters with a size in the range l.O. Onm. ... 

Ni(II) complexes of cyclam and oxocyclam derivatives catalyze the epoxidation of cyclohexene and various aryl-substituted alkenes with PhIO and NaOCl as oxidants, respectively. In the epoxidation catalyzed by the Ni(II) cyclam complex using PhIO as a terminal oxidant, the high-valent nickel-oxo complexes (e.g., LNi -0, LNi=0, LNi -0-I-Ph, or LNi -0-Ni L) have been proposed as the active oxidant (92). In the reaction, E olefins are more reactive than the corresponding Z isomers, and a strong correlation was observed between the electron-donating effect of the para substituents in styrene and the initial reaction rate (91). Isotope labeling studies have shown that the epoxide oxygen is derived from PhIO. 

The results of the study on the hydrogenation of different functional groups were summarized [194]. Here complexes of rhodium, palladium and nickel fixed on balls of densely cross-linked macroporous polystyrene of HAD-4 grade, on which an-thranilic acid residues were bonded, were used as catalysts. Special attention was paid to the kinetic study of cyclohexene hydrogenation by rhodium(+) derivatives and to elucidate the reaction mechanism using D2. The influence of diffusion restrictions on the reaction rate was discussed, in particular, that of the transport of hydrogen to pores and through the gas-liquid interface. 

Phthalocyanlnes. Gebler (18) has reported the attachment of a variety of metal phthalocyanines to both 8% and 20% dlvlnylbenzene polystyrene copolymer beads. The attachment of the phthalocyanine unit was either ly a sulfonamide or a sulfone linkage. Nickel, vanadyl, cobalt, iron and manganese complexes were formed in this way. Since solution aggregation accounts for a diminution of the catalytic activity, it was anticipated that polymer immobilization would Increase reactivity. Such an effect was not observed and little advantage over the homogeneous catalysts could be observed in the oxidation of cyclohexene. Oxidations of thiols by immobilized phthalocyanines have been reported (19-20) by both Schutten and Brouwer. 

Other Metal Complexes Apart from metal complexes derived from BINOL, other metal complexes, such as the lithium-aluminum amiuo diol complex, " aluminum and nickel salen complex, ruthenium diamine complex, and ruthenium phosphinite diamine complex were also found applicable for the asymmetric Michael addition of 1,3-dicarbonyl compound to cyclic enone. All these metal complexes afforded about 90% of asymmetric induction in the Michael reaction of 2-cyclohexen-l-one and malonate. 

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