Efficient homogeneous-like enantioselective catalysis of bulky chiral catalyst: Covalent immobilization of chiral spirocyclic phosphoric acid to polystyrene brushes grafted on SiO2 nanospheres

Abstract

To solve mass transfer limitation and multi-step immobilization of bulky chiral phosphoric acids in heterogeneous asymmetric catalysis, this paper develops a direct immobilization of (R)-6,6'-di(9-anthryl)spirobiindane phosphoric acid ((R)-AnSPA) via Friedel-Crafts alkylation onto SiO2-grafted poly(p-vinylbenzyl chloride) (PVBC) brushes (PVBC@SiO2) and hollow mesoporous PVBC nanospheres (HMPNs), affording supported organocatalysts (AnSPA#PVBC@SiO2 and AnSPA@HMPNs), respectively. Owing to the good dispersibility of PVBC brushes in organic solvents, the PVBC brush-anchored (R)-AnSPA with an open-ended structure enables the enantioselective desymmetrization of oxetanes with mercaptobenzothiazoles to perform in a homogeneous-like manner, leading to the improved mass transfer of reactants due to alleviative steric hindrance. Compared to homogeneous (R)-AnSPA, the PVBC brush-anchored (R)-AnSPA shows the decrements in yield and enantioselectivity below 2%. However, the HMPNs-anchored (R)-AnSPA shows the larger decrements in yields (3‒6%) and enantioselectivities (3‒7% ee). Furthermore, AnSPA#PVBC@SiO2 has a good reusability for in enantioselective desymmetrization with no significant decreases in yield and enantioselectivity. Particularly, the deactivated AnSPA#PVBC@SiO2, caused by the hydrolysis of phosphate ester in (R)-AnSPA, can be easily renovated to its fresh state by dealing with POCl3. Overall, the direct immobilization of (R)- AnSPA to PVBC brushes and renovation of deactivated (R)-AnSPA provide an effective strategy for achieving a high sustainability of expensive chiral spirocyclic phosphoric acids in large-scale synthesis.