Solid-phase synthesis of a peptide-based P,S-ligand system designed for generation of combinatorial catalyst libraries

An efficient methodology for the solid-phase synthesis of diverse combinatorial peptide-based P,S-ligand libraries based on a modular approach was developed. Chiral thioethers were introduced into a series of peptide scaffolds using commercially available Fmoc-protected cysteine derivatives, and secondary amines were incorporated into the peptide backbones by reductive alkylation using readily available Fmoc-protected amino aldehydes. Phosphinylation of the secondary amines of the scaffolds, applying two different reagents, yielded two different types of ligands. Subsequent complexation with palladium afforded six- or seven-membered chelates, respectively. The selectivity, in an asymmetric allylic substitution reaction, of the two different types of chelates, derived from the same peptide scaffold, was complementary in all cases studied, affording the product as opposite stereoisomers with up to 60% ee. These results hold great promise for the identification of highly selective catalysts upon screening of larger P,S-based catalyst libraries.