Allyl complexes dynamic equilibria

Similar to the octadienediyl-Ni" complex, the various decatrienyl-Ni" configurations 5-7 are likely to be in a dynamic equilibrium. The q -n-allyl form 5, which is stabilized by the coordinated olefmic double bond, is prevalent, while 6 and 7, with a q -o-allyl group, are found to be thermodynamically disfavored [ 11 ]. The investigation of the interconversion between the initially formed anti-q -decatrienyl-Ni" form and its syn-q -allyl counterpart will be focused only on the decatrienyl-Ni" isomers with an inner trans double bond that have been shown to be formed almost exclusively along 2—>5 (see previous section). 

The allylic conversion processes clearly represent the most feasible ones among all the critical elementary steps along the Cg-channel, involving the [NiII(octadienediyl)L] complex. Consequently, the several forms as well as the different stereoisomers are in a dynamic, pre-established equilibrium, that can likely be assumed as always being attained. 

Among the several configurations of the crucial [Nin(octadienediyl)L] complex, all of which are in equilibrium, the p3, 1 1) species 2a and the bis(p3) species 4a are predicted to be prevalent. The odonor/71-acceptor ability of the ancillary ligand is shown to predominantly determine the position of the kinetically mobile 2a 4a equilibrium. The conversion of the terminal allylic groups via allylic isomerization and/or allylic enantioface conversion are indicated to be the most facile of all the elementary processes that involve the [NiII(octadienediyl)L] complex. Consequently, the several octadienediyl-Ni11 configurations and their stereoisomers are likely to be in a dynamic pre-established equilibrium, that can be assumed to be always present. 

A wide number of chiral palladium complexes have been used in the context of DKR. In 1999, Cook et al. reported the palladium-mediated synthesis of chiral vicinal diamines from chiral oxazolidinones. The process involved successive oxidative insertion, loss of CO2 and subsequent cyclisation at the amide oxygen atom. The intermediate re-allyl palladium complexes underwent a rapid equilibration. Moreover, the intermediate oxazoline was also ionised by the palladium catalyst and was in equilibrium with the 7t-allylpalladium complexes, giving rise to thermodynamically controlled product ratios. These dynamic intermediates could be trapped with phthalimide under kinetic control to afford enantio- and diastereoselectively the corresponding syn-ch x 1,2-diamines (Scheme 2.46). 

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