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.