A magnetically recyclable heterogeneous BINOL organocatalyst for the asymmetric aldol reaction

Abstract

A chiral organocatalyst derived from (R)-1,1′-bi-2-naphthol (BINOL) was developed for immobilisation onto magnetic iron oxide particle (MIOP). This heterogeneous organocatalyst was characterised using infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA) and scanning electron microscope (SEM). The performance of BINOL-grafted MIOP (BINOL–MIOP) was then evaluated using aldol reaction between benzaldehydes and cyclic ketones. Comparative studies between homogeneous versus heterogeneous aldol reaction revealed the similar reactivity for both reaction systems. The reaction system mediated by BINOL–MIOP was versatile to produce aldol adducts in moderate-to-good yields (45–99%) from different benzaldehydes and cyclic ketones. In addition, more syn adducts were produced in most cases. Up to 35% ee was observed in anti adducts, despite that a higher 50% ee of anti adduct was observed in the homogeneous reaction system. This observation was supported by the results obtained from the molecular modelling, which revealed the reduced selectivity in the heterogeneous system that was possibly caused by the torsional angle distortion of BINOL after immobilisation. In contrast to the free-BINOL, the distorted-BINOL exhibited lower tendency to form a complex with aldehyde, thereby reducing the selectivity that the free-BINOL could deliver. In addition, the reaction system mediated by BINOL–MIOP was exhibiting an excellent reusability for up to 10 cycles of reactions.