Design of a Heterogeneous Catalyst Based on Cellulose Nanocrystals for Cyclopropanation: Synthesis and Solid-State NMR Characterization

Heterogeneous dirhodium(II) catalysts based on environmentally benign and biocompatible cellulose nanocrystals (CNC-Rh-2) as support material were obtained by ligand exchange between carboxyl groups on the CNC surface and Rh-2(OOCCF3)(4), as was confirmed by solid-state F-19 and C-13 NMR spectroscopy. On average, two CF3COO- groups are replaced during ligand exchange, which is consistent with quantitative analysis by a combination of F-19 NMR spectroscopy and thermogravimetry. CNC-Rh-2 catalysts performed well in a model cyclopropanation reaction, in spite of the low dirhodium(II) content on the CNC surface (0.23 mmolg(-1)). The immobilization through covalent bonding combined with the separate locations of binding positions and active sites of CNC-Rh-2 guarantees a high stability against leaching and allows the recovery and reuse of the catalyst during the cyclopropanation reaction.