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N-Heterocyclic carbenes as ligands in palladium-catalyzed Tsuji–Trost allylic substitution
Institution:1. Institut de Chimie Moléculaire et des Matériaux d''Orsay, Equipe de Chimie Bioorganique et Bioinorganique, Univ. Paris-Sud 11, F-91405 Orsay Cedex, France;2. Dipartimento di Bioscienze, Università degli Studi di Parma, Parco Area delle Scienze 11A, 43124 Parma, Italy;3. Institut de Biochimie et Biophysique Moléculaire et Cellulaire (UMR8619), Univ. Paris-Sud 11, F-91405 Orsay Cedex, France;4. Dipartimento di Chimica, Università degli Studi di Parma, Parco Area delle Scienze 17A, 43124 Parma, Italy;5. Ecole Normale Supérieure, Département Chimie, 24 rue Lhomond, F-75005 Paris France;6. Sorbonne Universités, UPMC Univ Paris 06, LBM, 4 Place Jussieu, 75005 Paris, France;7. Laboratoire des BioMolécules, UMR7203, CNRS, France
Abstract:A Pd(0)-catalyzed allylic substitution (i.e., Tsuji–Trost reaction) using N-heterocyclic carbene as a ligand was investigated. It has been proven that an imidazolium salt 2d having bulky aromatic rings attached to the nitrogens in its imidazol-2-ylidene skeleton is suitable as a ligand precursor and that a Pd2dba3–imidazolium salt 2d–Cs2CO3 system is highly efficient for producing a Pd–NHC catalyst in this reaction. Allylic substitution using a Pd–NHC complex differed from that using a Pd–phosphine complex as follows: (1) the reaction using a Pd–NHC complex required elevated temperature (50 °C or reflux in THF), (2) allylic carbonates were inert to a Pd–NHC complex, and (3) nitrogen nucleophiles such as sulfonamide and amine did not react with allylic acetate. It was also found that allylic substitution with a soft nucleophile using a Pd–NHC catalyst proceeds via overall retention of configuration to give the product in a stereospecific manner, the stereochemical reaction course obviously being the same as that of the reaction using a Pd–phosphine complex.
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