Structural control in palladium(II)-catalyzed enantioselective allylic alkylation by new chiral phosphine-phosphite and pyridine-phosphite ligands |
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| Institution: | 1. Department of Radiology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing 100730, China;2. PAII Inc., Bethesda, MD 20817, USA;3. Department of Radiology, Beijing United Family Hospital, Beijing 100015, China;1. Universidade Tecnológica Federal do Paraná, Department of Chemical Engineering, Av. Monteiro Lobato s/n, 84016-210, Ponta Grossa, Paraná, Brazil;2. Universidade Estadual de Ponta Grossa, Department of Chemistry, Av. General Carlos Cavalcanti, 4748, 84030-900, Ponta Grossa, Paraná, Brazil;1. British Geological Survey, Keyworth, Nottingham NG12 5GG, UK;2. Radioactive Waste Management Limited, Curie Avenue, Harwell, Didcot OX11 ORA, UK;1. University of Vienna, Faculty of Physics – Isotope Research, VERA Laboratory, Währinger Straße 17, A-1090 Vienna, Austria;2. CERN, Geneva 23, 1211 Switzerland;3. FluxSense AB, SE-412 96 Gothenburg, Sweden;4. Friedrich-Schiller-Universität, 07743 Jena, Germany;5. Department of Physics, University of Gothenburg, SE-412 96 Gothenburg, Sweden;6. ORNL Oak Ridge National Laboratory, TN, USA |
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| Abstract: | The ligands 6-(diphenylphosphanyl)methoxy]-4,8-di-tert-butyl-2,10-dimethoxy-5,7-dioxa-6-phosphadibenzoa,c]cycloheptene, 1, (S)-4-(diphenylphosphanyl)methoxy]-3,5-dioxa-4-phosphacyclohepta2,1-a;3,4a′]dinaphthalene, (S)-2, and (S)-4-(diphenylphosphanyl)methoxy]-2,6-bis-trimethylsilanyl-3,5-dioxa-4-phosphacyclohepta2,1-a;3,4-a′]dinaphthalene, (S)-3, (S)-2-(3,5-dioxa-4-phosphacyclohepta2,1-a;3,4-a′]dinaphthalen-4-yloxymethyl)pyridine, (S)-4, and (S)-2-(3,5-dioxa-4-phosphacyclohepta2,1-a;3,4-a′]dinaphthalen-4-yloxy)pyridine, (S)-5, have been easily prepared.The cationic complexes Pd(η3-C3H5)(L-L′)]CF3SO3 (L–L′=1–(S)-5) and Pd(η3-PhCHCHCHPh)(L–L′)]CF3SO3 (L–L′=(S)-2–(S)-4) were synthesized by conventional methods starting from the complexes Pd(η3-C3H5)Cl]2 and Pd(η3-PhCHCHCHPh)Cl]2, respectively. The behavior in solution of all the π-allyl- and π-phenylallyl-(L–L′)palladium derivatives 6–14 was studied by 1H, 31P{1H}, 13C{1H} NMR and 2D-NOESY spectroscopy. As concerns the ligands (S)-4 and (S)-5, a satisfactory analysis of the structures in solution was possible only for palladium–allyl complexes Pd(η3-C3H5)((S)-4)]CF3SO3, 11, and Pd(η3-C3H5)((S)-5)]CF3SO3, 12, since the corresponding species Pd(η3-PhCHCHCHPh)((S)-4)]CF3SO3, 13, and Pd(η3-PhCHCHCHPh)((S)-5)]CF3SO3, 14, revealed low stability in solution for a long time. The new ligands (S)-2–(S)-5 were tested in the palladium-catalyzed enantioselective substitution of (1,3-diphenyl-1,2-propenyl)acetate by dimethylmalonate. The precatalyst Pd(η3-C3H5)((S)-2)]CF3SO3 afforded the allyl substituted product in good yield (95%) and acceptable enantioselectivities (71% e.e. in the S form). A similar result was achieved with the precatalyst Pd(η3-C3H5)((S)-3)]CF3SO3. The nucleophilic attack of the malonate occurred preferentially at allylic carbon far from the binaphthalene moiety, namely trans to the phosphite group. When the complexes containing ligands (S)-4 and (S)-5 were used as precatalysts, the product was obtained as a racemic mixture in high yield. The number of the configurational isomers of the Pd-allyl intermediates present in solution in the allylic alkylation and the relative concentrations are considered a determining factor for the enantioselectivity of the process. |
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