Crystal nickel enolates

Transmetallation of the lithium or potassium enolates is also a reliable method for the preparation of palladium and nickel enolates, as illustrated in Scheme 2.50. Clear evidence for the C-bound structure of enolates 172 and 173 thus prepared was provided by NMR spectroscopy and - for nickel enolate 172 (M = Ni, L = Cp ) - by a crystal structure analysis. The reaction of C-bound nickel and palladium enolates 172 and 173 with aldehydes is much more sluggish and much less uniform than the analogs of that of the polar main-group metals. In addition to P-hydroxyketones or esters, products resulting from a Tishchenko reaction were also observed [164b]. 

Also in the enolates of group 10 metals, the different metalla tautomers were postulated, isolated, and characterized. Generally, it is assumed that in particular palladium - due to its low oxophilicity - favors the C-bound mode, but for this metal too, the O-bound tautomer has been detected. In an early study on nickel and palladium enolates 37, the C-bound structure was assigned on the basis of and C NMR and IR spectroscopy and confirmed for the nickel ester enolate (L = Cp, R = OCMe3) by a crystal structure [73]. The metallacyclic nickel enolates 38, on the other hand, are O-bound tautomers, as unambiguously shown by a crystal structure of the complex 38a, wherein the nickel adopts a slightly distorted square planar coordination. A slow equilibration was observed between the O-bound tautomer 38b and the C-bound isomer 39, with AG = 25.3 kcal mol . Remarkably, the isomerization rate was substantially... 

Keim and co-workers have found that the treatment of Ni(COD)2 with both triphenylphosphine and the phosphorane, Ph3PCHC(0)Ph, affords a nickel(II) chelate complex formally derived from the enolate of Ph2PCH2C(0)Ph (Figure 7.10). ° This crystalline compound, which can be conveniently prepared on a large scale, has been characterized by single crystal X-ray diffraction. Much like o-diphenylphosphinobenzoate, the novel enolate ligand functions as an anionic P-O donor. What is particularly intriguing is that its nickel complex also efficiently catalyzes the formation of linear a-olefins from ethylene. 

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