Supramolecular bidentate rhodium(I) or iridium(I) phosphine and oxazoline amino acid bioconjugates as selective catalysts for enantioselective reactions

Abstract

This publication describes monodentate phosphine and oxazoline ligands attached to an amino acid ester and the application of their supramolecularly assembled rhodium(I) or iridium(I) complexes in asymmetric catalysis. The major feature of these complexes is the transmission of chirality from distant hydrogen bonded amino acids to the prochiral catalytic metal center ("backdoor induction"). The in situ generated homoleptic and heteroleptic rhodium(I) or iridium(I) precatalysts were studied by NMR, UV-VIS and CD spectroscopy as well as X-ray single crystal diffraction. In asymmetric hydrogenation of methyl α-acetamidocinnamate, rhodium(I) and iridium(I) complexes afforded complete conversions with enantioselectivities up to 85 %, while iridium complexes proved to be more sensitive to the variation of reaction conditions, including catalyst loading, metal to ligand ratio and temperature. The hydrogenation of four other dehydroamino acid substrates resulted in similar conversion and selectivity as obtained with methyl α-acetamidocinnamate. The influence of the phosphine/oxazoline heteroleptic mixtures in catalysis was studied using both rhodium(I) and iridium(I) complexes. Finally, a homoleptic phosphine rhodium(I) complex was successfully applied in asymmetric hydroformylation of styrene and 1-octene with complete conversions and selectivity up to 40% e.e. for the branched styrene product.