Design and characterization of novel dirhodium coordination polymers – the impact of ligand size on selectivity in asymmetric cyclopropanation

Abstract

Three chiral dirhodium coordination polymers Rh₂–Ln (n = 1–3) have been synthesized via ligand exchange between dirhodium trifluoroacetate Rh₂(TFA)₄ and differently sized chiral dicarboxylic acids derived from L-tert-leucine. SEM images indicate that the Rh₂–Ln (n = 1–3) polymers have a lamellar structure. XPS data demonstrate that the oxidation state of rhodium in the dirhodium nodes is maintained during the synthesis of the polymers. The coordination polymers have been further characterized by FTIR, ¹H → ¹³C CP MAS NMR and ¹⁹F MAS NMR spectroscopy to prove the formation of polymers via ligand exchange. Although the quantitative ¹⁹F MAS NMR spectra reveal incomplete ligand substitution in the coordination polymers, these catalysts show excellent activity and selectivity in the asymmetric cyclopropanation reaction between styrene and diazooxindole. In particular, the enantioselectivity has been significantly improved compared with previously designed dirhodium coordination polymers, which were synthesized from aromatic dicarboxylic acids derived from L-phenylalanine. Meanwhile, the dirhodium polymers can be easily recycled five times without significant reduction in their catalytic efficiency.