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Keywords:

  • Phosphanylferrocene ligands;
  • Amino acids;
  • Amides;
  • Copper;
  • Asymmetric synthesis;
  • Conjugate addition

Abstract

A series of chiral phosphanylferrocenecarboxamides was prepared by treatment of either 1′-(diphenylphosphanyl)ferrocene-1-carboxylic acid (Hdpf) or its planar-chiral 1,2-isomers with amino acid methyl esters in the presence of peptide coupling agents. The compounds were characterised by spectroscopic methods, and the crystal structure of one representative was determined by X-ray diffraction. Catalytic testing of these donors in Cu-catalysed asymmetric conjugate additions of diethylzinc to chalcones revealed that the reaction outcomes were highly sensitive to the ligand structure and the reaction conditions (copper source and solvent), whereas the chalcone substituents (Me, MeO, or Cl in positions 4 or 4′) had a less pronounced influence. Compounds based on Hdpf proved to be better ligands than their planar-chiral analogues. Under optimised conditions, the reactionwith L-valine–Hdpf conjugate, (S)-Ph2PfcCONHCH(CHMe2)CO2Me (fc = ferrocene-1,1′-diyl) and unsubstituted chalcone gave the alkylation product with complete conversion (20 °C/4 h) and in 87 % ee. The catalytic behaviour of the amidophosphane ligands was correlated to the results of amodel coordination study and the crystal structure of [Cu(Ph2PfcCONHCH2CO2Me)2](CF3SO3)·2CHCl3.