Efficient homogeneous-like enantioselective catalysis of a bulky chiral catalyst: covalent immobilization of chiral spirocyclic phosphoric acid onto polystyrene brushes grafted on SiO2 nanospheres.

Abstract

To solve the 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 proceed in a homogeneous-like manner, leading to the improved mass transfer of reactants due to alleviative steric hindrance. Compared to the homogeneous (R)-AnSPA, the PVBC brush-anchored (R)-AnSPA shows decrements in yield and enantioselectivity below 2%. However, the HMPN-anchored (R)-AnSPA shows larger decrements in yields (3-6%) and enantioselectivities (3-7% ee). Furthermore, AnSPA#PVBC@SiO₂ exhibits good reusability 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 onto PVBC brushes and renovation of deactivated (R)-AnSPA provide an effective strategy for achieving high sustainability of expensive chiral spirocyclic phosphoric acids in large-scale synthesis.