Nickel complexes isomerization rates

The reaction of planar Ni ([14]aneN4) + represented as shown in (2.10) with a number of bidentate ligands (XY) to produce c -octahedral Ni ([14]aneN4) XY + is first-order in nickel complex and [OH ] and independent of the concentration of XY.In the preferred mechanism, the folding of the macrocycle (base-catalyzed tmns — cis isomerization) is rate determining, and this is followed by rapid coordination of XY ... 

With the addition of 1,3-butadiene, the initially yellow hydride solutions turn red with the formation of relatively stable l-Me-it-allyl-nickel complexes, and olefin isomerization activity stops. By measuring the rate of formation of the rc-allyl complexes in the presence of added P(OEt)3, it was possible to measure the rate constant for dissociation of L from HNiL4 and show that this is the rate-determining step (42). 

With a DELOXAN supported palladium complex catalyst, DELOXAN HK I, the linoleate selectivity is further increased. In comparison to the commercial batch hydrogenation with a nickel on kieselguhr catalyst, the DELOXAN supported palladium complex catalyst in combination with sc CO2 as a solvent gives higher space-time-yields, a higher linoleate selectivity and a significantly decreased cis/trans isomerization rate. 

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... 

It has been observed that the isomerization of 1-olefins by palladium complexes shows simultaneous appearance of all the possible internal isomers at comparable rates (55, 59) rather than the stepwise formation shown in the nickel systems. It has been proposed that perhaps the reaction proceeds by some type of 7r-allyl mechanism (60). Two mechanistic schemes have been proposed for the isomerization of olefins by iron pentacarbonyl (61, 62). One is analogous to that of the nickel isomeriza-tions, and the other involves a 7r-allyl intermediate (Fig. 25). 

Nickel-catalyzed hydrocyanation of a-olefins t)q)ically produces the terminal nitrile as the more abundant, but not exclusive isomeric product. In contrast, nickel-catalyzed hydrocyanation of vinylarenes typically generates the branched product. This branched selectivity arises fijom the stability of -phenethyl complexes, as is shown in more detail in Section 16.2.5 on as)unmetric hydrocyanation. The relative rates for hydrocyanation follow the trend ethylene > styrene > propene 1-hexene > disubstituted olefins. Examples of these reactions and selectivities for formation of the linear and branched products are shown in Scheme 16.1. ... 

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