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On the Nature of the Intermediates and the Role of Chloride Ions in Pd-Catalyzed Allylic Alkylations: Added Insight from Density Functional Theory

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journal contribution
posted on 18.12.2008, 00:00 by Peter Fristrup, Mårten Ahlquist, David Tanner, Per-Ola Norrby
The reactivity of intermediates in palladium-catalyzed allylic alkylation was investigated using DFT (B3LYP) calculations including a PB-SCRF solvation model. In the presence of both phosphine and chloride ligands, the allyl intermediate is in equilibrium between a cationic η3-allylPd complex with two phosphine ligands, the corresponding neutral complex with one phosphine and one chloride ligand, and a neutral η1-allylPd complex with one chloride and two phosphine ligands. The η1-complex is unreactive toward nucleophiles. The cationic η3-complex is the intermediate most frequently invoked in the title reaction, but in the presence of halides, the neutral, unsymmetrically substituted η3-complex will be formed rapidly from anionic Pd(0) complexes in solution. Since the latter will prefer both leaving group ionization and reaction with nucleophiles in the position trans to phosphorus, it can rationalize the observed “memory effect” (a regioretention) in the title reaction, even in the absence of chiral ligands.