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RESEARCH PRODUCT

Synthesis and Catalytic Use of Polar Phosphinoferrocene Amidosulfonates Bearing Bulky Substituents at the Ferrocene Backbone

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International audience; Anionic phosphinoferrocene amidosulfonates bearing sterically demanding t-butyl substituents in positions 3 and 3' of the ferrocene scaffold, viz. rac-(Et3NH)[Fe(η5-tBuC5H3PR2)(η5-tBuC5H3C(O)NHCH2SO3)] (R = phenyl and cyclohexyl) were synthesized by amidation of the corresponding phosphinocarboxylic acids, [Fe(η5-tBuC5H3PR2)(η5-tBuC5H3CO2H)]. These ditopic polar phosphinoferrocenes and their non-t-butylated analogues have been used as lig-ands to prepare zwitterionic (η3-allyl)palladium(II) complexes [Pd(η3-C3H5){Fe(η5-R'C5H3PR2)(η5-R'C5H3C(O)NHCH2SO3)}] (R' = H, tBu, R = Ph, Cy). Depending on the isolation procedure and crystallization condi-tions, some complexes were isolated in two isomeric forms which differed by the coordination of the amidosulfonate pendant group, where either amide or sulfonated oxygen ligated the Pd(II) centre. The preference for coordination of the amide or sulfonate oxygen atoms has been explained by interplay of electrostatic and solvation effects and further supported by DFT calculations. The (η3-allyl)Pd(II) complexes have been applied as defined pre-catalysts for Pd-catalysed C–H arylation of unprotected indole with aryl iodides in polar solvents. Under the optimized reaction condi-tions at 100°C in water, C2-arylation proceeded selectively with various aryl iodides to produce the respective 2-arylindoles in acceptable yields at low catalyst loading (1 mol.% Pd) and in the absence of any phase transfer agent. The catalyst possessing t-butyl groups at the ferrocene core and an electron-rich dicyclohexylphosphino group exhibited the best catalytic performance.