Knoevenagel Condensation for Modifying the Reducing End Groups of Cellulose Nanocrystals
ACS MACRO LETTERS(2019)
摘要
Herein, we demonstrate an effective approach toward functionalization of cellulose nanocrystal (CNC) reducing ends by means of a Knoevenagel condensation reaction with a reactive beta-diketone (acetylacetone). The end-wise modification was elucidated by advanced NMR analysis, which was facilitated by dissolving the CNCs in ionic liquid electrolyte and by the concomitant assignment of a model compound derived from D-cellobiose. The diffusion-edited H-1 experiment afforded a simple method to identify the assigned model resonances in the reducing end-modified CNCs. The condensations can be carried out in aqueous bicarbonate solutions, avoiding the use of hazardous solvents. Under these preliminary aqueous conditions, end-group conversion of up to 12.5% could be confirmed. These results demonstrate the potential of beta-diketone chemistry and the Knoevenagel condensation for functionalizing cellulose reducing ends. Application of this liquid-state NMR method for confirming and quantifying reducing end conversion is also shown to be invaluable. Extension of this chemistry to other 1,3-dicarbonyl compounds and solvation conditions should allow for the topochemical and (axially) chirotopic installation of functional moieties to CNCs, paving the way to asymmetric cellulose-based nanomaterials with unique properties.
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