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

Anionic phosphinoferrocene amidosulfonates bearing sterically demanding tert-butyl substituents in positions 3 and 3′ of the ferrocene scaffold, viz. rac-(Et3NH)­[Fe­(η5-tBuC5H3PR2)­(η5-tBuC5H3C­(O)­NHCH2SO3)] (R = phenyl, cyclohexyl), were synthesized by amidation of the corresponding phosphinocarboxylic acids, [Fe­(η5-tBuC5H3PR2)­(η5-tBuC5H3CO2H)]. These ditopic polar phosphinoferrocenes and their non-tert-butylated analogues have been used as ligands 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 conditions, some complexes were isolated in two isomeric forms which differed in the coordination of the amidosulfonate pendant group, where either amide or sulfonated oxygen ligated the Pd­(II) center. The preference for coordination of the amide or sulfonate oxygen atoms has been explained by the interplay of electrostatic and solvation effects and further supported by DFT calculations. The (η3-allyl)­PdII complexes have been applied as defined precatalysts for Pd-catalyzed C–H arylation of an unprotected indole with aryl iodides in polar solvents. Under the optimized reaction conditions at 100°C in water, C2-arylation proceeded selectively with various aryl iodides to produce the respective 2-arylindoles in acceptable yields at a low catalyst loading (1 mol % Pd) and in the absence of any phase transfer agent. The catalyst possessing tert-butyl groups at the ferrocene core and an electron-rich dicyclohexylphosphino group exhibited the best catalytic performance.