Mapping the Surface Groups of Amine-Rich Carbon Dots Enables Covalent Catalysis in Aqueous Media

Carbon nanodots stand as the missing link between the molecular and the nanoscale world, owing to the unique molecular-like behavior emerging from their synthetic precursors. A converging set of analytical and spectroscopic data yields a precise inventory of the surface reactive groups of amine-rich carbon dots (NCDs-1). As a result, NCDs-1 provide a multi-functional nano-platform that is able to covalently activate carbonyl groups, form iminium-ions and enamine intermediates, and efficiently promote diverse aminocatalytic transformations in water. Remarkably, the catalytic activity of carbon dots can also govern the stereoselectivity in the bond-forming event. Indeed, the use of chiral carbon dots (NCDs-7) as catalysts affords the final aldol products with significant enantiomeric excess. The successful implementation of carbon nanostructures into chemical roles so far restricted to molecular systems opens new avenues for advanced applications where the nanoscale and the molecular realms will merge and complement each other. What is the breakthrough expected for the next-generation nanomaterial? The programming of its structure and functions using “green” synthetic and up-scaling protocols? Low toxicity and application in advanced technology platforms? In this vision, carbon nanodots are a potential game-changer among all other forms of nanomaterials. Carbon nanodots are obtained under hydrothermal conditions from a blend of molecular reagents whose properties turn out to be encoded within the final material. Herein, we show that amine-rich carbon nanodots display a prominent molecular behavior in terms of (1) acid-base properties, (2) distribution of amino domains with different chemical environments, probed by 19F-tagging NMR spectroscopy, (3) formation of enamine and iminium catalytic intermediates, and (4) asymmetric catalysis. Carbon nanodots can, therefore, bridge the gap between homogeneous and heterogeneous catalysis, by transferring key molecular prerogatives at the horizon of nanoscale materials. A detailed characterization of nitrogen-rich carbon dots (NCDs) has been instrumental in unlocking their potential as novel colloidal organocatalysts in water media. As a result of their multi-functional properties, diverse aminocatalytic transformations of carbonyl compounds are efficiently mediated by NCDs under iminium-ion and enamine activation pathways. These include conjugate additions and aldol reactions occurring with remarkable efficiency (up to 96% yield) and complete diastereocontrol (d.r. >20:1) and are amenable to asymmetric catalysis applications in the presence of chiral NCDs.