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| - New Insights into the Mechanism of Palladium-CatalyzedAllylic Amination
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| - A comparative investigation into palladium-catalyzed allylic amination of unsubstituted aziridinesand secondary amines has been carried out. The use of NH aziridines as nucleophiles favors formation ofvaluable branched products in the case of aliphatic allyl acetates. The regioselectivity of this reaction isopposite to that observed when other amines are used as nucleophiles. Our study provides evidence forthe palladium-catalyzed isomerization of the branched (kinetic) product formed with common secondaryamines into the thermodynamic (linear) product. In contrast, the branched allyl products obtained fromunsubstituted aziridines do not undergo the isomerization process. Crossover experiments indicate thatthe isomerization of branched allylamines is bimolecular and is catalyzed by Pd0. The reaction has significantsolvent effect, giving the highest branched-to-linear ratios in THF. This finding can be explained by invokingthe intermediacy of σ-complexes, which is consistent with NMR data. The apparent stability of branchedallyl aziridines towards palladium-catalyzed isomerization is attributed to a combination of factors that stemfrom a higher degree of s-character of the aziridine nitrogen compared to other amines. The reaction allowsfor regio- and enantioselective incorporation of aziridine rings into appropriately functionalized building blocks.The resulting methodology addresses an important issue of forming quaternary carbon centers next tonitrogen. The new insights into the mechanism of palladium-catalyzed allylic amination obtained in thisstudy should facilitate synthesis of complex heterocycles, design of new ligands to control branched-to-linear ratio, as well as absolute stereochemistry of allylamines.
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