Noncovalent immobilization of chiral Lewis acids on single-walled carbon nanotubes as a tool for synthetic organic aquachemistry

Polycyclic aromatic hydrocarbons (PAHs) were employed as anchoring tags to noncovalently immobilize Lewis acids onto single-walled carbon nanotubes (SWNTs). This heterogeneous system demonstrated remarkable performance in asymmetric catalysis, particularly in water devoid of organic solvents or surfactants, outperforming other carbonaceous π-materials in activity, stereoselectivity, and reusability. The use of large-diameter SWNTs modified with 4-fluorophenyl groups further enhanced catalytic activity. Notably, Sc(PyS)₃-SWNT combined with a chiral modifier retained high performance without the covalent anchorage, even after multiple reuse cycles. No leaching of scandium or the chiral modifier was observed, consistent with TGA results, despite washing with solvents in which the chiral modifier is highly soluble. XPS analysis demonstrated that among the supports examined, SWNTs exhibited the most significant electron donation to the Sc and S centers, underscoring the exceptional electronic interaction responsible for the stable immobilization. Despite its high surface area and adsorption capacity, activated carbon showed poor performance and significant leaching, suggesting that electron donation from SWNTs stabilizes flexible, solution-like conformations of the Lewis acid complex, mitigating the unavoidable attenuation of Lewis acidity more effectively than other supports. This strategy also mitigates deactivation risks posed by nucleophiles such as amines, thiols, and free N-H indoles, which typically displace chiral ligands. Thus, a robust, non-covalent immobilization platform has been established, capable of delivering high activity, selectivity, and durability even in water, which is traditionally challenging for Lewis acid catalysis. This approach offers a promising pathway toward more sustainable and environmentally conscious asymmetric synthesis.