Synthesis, Structural Characterization and Catalytic Evaluation of Anionic Phosphinoferrocene Amidosulfonate Ligands
Abstract
Triethylammonium salts of phosphinoferrocene amidosulfonates with electron-rich dialkyphosphino substituents, R2PfcCONHCH2SO3(HNEt3) (4a-c), where fc = ferrocene-1,1 '-diyl, and R = i-Pr (a), cyclohexyl (Cy; b), and t-butyl (c), were synthesized from the corresponding phosphinocarboxylic acids-borane adducts, R2PfcCO2H.BH3 (1a-c), via esters R2PfcCO2C6F5.BH3 (2a-c) and adducts R2PfcCONHCH2SO3(HNEt3).BH3 (3a-c). Compound 4b was shown to react with [Pd(mu-Cl)(eta-C3H5)]2 and AgClO4 to afford the zwitterionic complex [Pd(eta(3)-C3H5)(Cy2PfcCONHCH2SO3-kappa O-2,P)] (5b), in which the amidosulfonate ligand coordinates as a chelating donor making use of its phosphine moiety and amide oxygen.
The structures of 3b.CH2Cl2, 4b and 5b.CH2Cl2 were determined by single-crystal X-ray diffraction analysis. Compounds 4a-c and their known diphenylphosphino analogue, Ph(2)PfcCONHCH2SO3(HNEt3) (4d), were studied as supporting ligands in Pd-catalyzed cyanation of aryl bromides with K4[Fe(CN)6] and in Suzuki-Miyaura biaryl cross-coupling performed in aqueous reaction media under mild reaction conditions.
In the former reaction, the best results were achieved with a catalyst generated from [PdCl2(cod)] (cod = eta(2):eta(2)-cycloocta-1,5-diene) and 2 equiv. of the least electron-rich ligand 4d in dioxane-water as a solvent. In contrast, the biaryl coupling was advantageously performed with a catalyst resulting from palladium(II) acetate and ligand 4a (1 equiv.) in the same solvent.