Resting State and Kinetic Studies on the Asymmetric Allylic Substitutions Catalyzed by Iridium-Phosphoramidite Complexes.
The resting state of the iridium catalyst in the asymmetric allylation of amines and kinetic data on the overall catalytic cycle are reported. The resting state is a complex of the metallacyclic catalyst with the olefinic unit of the allylamine product. The species containing the branched allylamine N-phenyl-1-phenylprop-2-enylamine and a related ethylene complex have been isolated in pure form. Binding constants of enantiomeric allylamines to the Ir(I) fragment were measured. The ratio of binding affinities is within experimental error of the ratio of rates for formation of the two enantiomeric amine products in the catalytic cycle. The rates of the catalytic reaction are first-order in allylic carbonate, amine, and catalyst and inverse-first order in product. This combination of data, along with the observation that the resting state is stable toward 1.5 equiv of allylic carbonate, implies that the reaction of the iridium(I) species with the allylic carbonate is reversible and endergonic, and that the allyl intermediate reacts with the nucleophile to form product. Finally, the simple synthesis of the metallacyclic iridium ethylene complex gives rise to a single-component, highly active, and enantioselective catalyst for the allylic amination.
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