Synthesis, Structural Characterization, and Catalytic Evaluation of Phosphinoferrocene Ligands Bearing Extended Urea-Amide Substituents
Abstract
New phosphinoferrocene ligands bearing extended polar amidourea pendants with the general formula Ph2PfcCONHCH2CH2NHCONR2 (1; R2 = H2 (b), H/Et (c), Me2 (d), H/Ph (e)) and their model bis-amide Ph2PfcCONHCH2CH2NHCOCH3 (1a) were prepared in good yields by amidation of 1'-(diphenylphosphino)ferrocene-l-carboxylic acid (Hdpf) with the appropriate amines in the presence of peptide coupling reagents. These ferrocene-based phosphinoureas were further employed as ligands in palladium(II) complexes with h3-allyl and NC-chelating supporting ligands: viz., [PdCl(h3-C3H5)(1-kP)] (5a-e) and [PdCl(L-NC)(1-kP)] (6a-e; L-NC = [2-(dimethyl-amino-kN)methyl]phenyl-kC-1).
Both the free ligands and their Pd(II) complexes were characterized by spectroscopic methods (multinuclear NMR, IR, and MS) and by elemental analysis. The molecular structures of 1b.CH3OH, 1c, 5b,c, 6a, and two additional model complexes, [PdCl(h3-C3H5)(Hdpf-kP))] and [PdCl(h3-C3H5)(Ph2PfcCONH2-kappa P)], were determined by single-crystal X-ray diffraction analysis.
All Pd(II) complexes were evaluated as catalysts in the cross-coupling of boronic acids and acyl halides to give ketones in a toluene/water biphasic mixture. Extensive reaction studies with compound 5e, which not only exerts good catalytic activity but is also readily accessible in a defined crystalline form, demonstrated efficient coupling reactivity for unsaturated substrates such as (substituted) benzeneboronic acids and benzoyl chlorides.
The results also revealed that reaction difficulties encountered with less reactive substrates (e.g., insoluble aromatic boronic acids and all saturated aliphatic boronic acids) can be avoided by properly selecting the reaction partners, for example through transposition of substituents between reaction partners. Three representative benzophenones (4-fluoro-, 4-nitro-, and 4,4'-dinitrobenzophenone) were structurally characterized by single-crystal X-ray crystallography.