Chiral phosphinoferrocene carboxamides with amino acid substituents as ligands for Pd-catalysed asymmetric allylic substitutions. Synthesis and structural characterisation of catalytically relevant Pd complexes
Abstract
An extensive series of chiral amino acid amides prepared from 1'-(diphenylphosphino)ferrocene-1-carboxylic acid (Hdpf) or its planar-chiral isomer, 2-(diphenylphosphino)ferrocene-1-carboxylic acid, have been tested as ligands for Pd-catalysed asymmetric allylic substitution reactions. In alkylation of 1,3-diphenylallyl acetate as a model substrate with dimethyl malonate the ligands performed well in terms of both reaction rate and enantioselectivity, achieving up to 98% ee.
In contrast, the reactions of the same substrate with other nucleophiles proceeded either slowly and with poor ee's (amination with benzylamine) or not at all (etherification with benzyl alcohol). In order to rationalise the influence of the ligand structure on the reaction course, three model complexes, viz. [(h3-methallyl)PdCl(L-kP)], [(h3-methallyl)Pd(L-k2O,P)]ClO4 and [(h3-methallyl)Pd(L-kP)2]ClO4 have been prepared from the achiral amide Ph2PfcCONHCH2CO2Me (L; fc = ferrocene-1,1'-diyl) and structurally characterised.