Synthesis, Structural Characterization, and Catalytic Evaluation of Phosphinoferrocene Ligands Bearing Extended Urea-Amide Substituents

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-1-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 η3-allyl and NC-chelating supporting ligands: viz., [PdCl­(η3-C3H5)­(1-κP)] (5a–e) and [PdCl­(LNC)­(1-κP)] (6a–e; LNC = [2-(dimethyl­amino-κN)­methyl]­phenyl-κC1).
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(η3-C3H5)­(Hdpf-κP)] (5f) and [PdCl­(η3-C3H5)­(Ph2PfcCONH2-κP)] (5g), 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.