Nickel complexes keto form

When we first contemplated thermochemical products available from Glu, a search of the literature revealed no studies expressly directed at hydrogenation to a specific product. Indeed, the major role that Glu plays in hydrogenation reactions is to act as an enantioselectivity enhancer (17,18). Glu (or a number of other optically active amino acids) is added to solutions containing Raney nickel, supported nickel, palladium, or ruthenium catalysts and forms stereoselective complexes on the catalyst surface, leading to enantioselective hydrogenation of keto-groups to optically active alcohols. Under the reaction conditions used, no hydrogenation of Glu takes place. 

Ketz et al. synthesised a keto functionalised imidazolium salt using the standard protocol by reacting the appropriate N-aryl substituted imidazole with the respective keto halide [95], Their intention was to use the corresponding nickel(II) carbene enolate complex in olefin polymerisation reactions similar to the phosphino enolates in the SHOP process [1 ]. It proved difficult to prepare the intended precatalyst with only one carbene enolate ligand attached to the nickel centre. Initially, the homoleptic complex with two carbene enolate ligands was formed. The authors pointed out that the high proportion of n-olefins is unusual. 

Since the oxygen atoms as well as the alpha carbon on the /3-keto-enolate ring can function as bases to coordinate with metal ions (5, or to hydrogen bonding solvents (14)f it should be recognized that /3-keto-enolate complexes may have a stereochemistry quite different from that expected—based on the stoichiometry of the complex. However, by a suitable choice of a /3-ketoenol ligand, such as 2,2,6,6-tetramethyl-3,5-heptanedione (dipivaloylmethane), H-DPM, intermolecular effects can be reduced or eliminated with the result that the metal complex formed has the stereochemistry expected for the monomeric acetylacetonate complex of that metal ion. For example, nickel(II) acetylacetonate is trimeric as a solid and in solution (11), but the dipivaloylmethane complex is a monomeric, undoubtedly planar species, in solution, as well as in the solid state (37), This stereochemistry occurs with the monomeric acetylacetonate in dilute solution or in the vapor phase, as indicated by absorption spectra and electron diffraction (SO). 

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