Stereoselective Diels-Alder reactions promoted under continuous-flow conditions by silica-supported chiral organocatalysts

Silica nanoparticles of different morphological properties were functionalized with enantiomerically pure imidazolidinones, through different immobilization techniques; stainless-steel columns were then loaded with silica bearing chiral organocatalysts to realize chiral “homemade” reactors. The influence of the material properties and immobilization procedures on the chemical and stereochemical activities of the chiral HPLC columns was studied by performing organocatalyzed Diels[BOND]Alder reactions between cyclopentadiene and α,β-unsaturated aldehydes under continuous-flow conditions. In some cases, excellent enantioselectivities were obtained, thus showing that a catalytic reactor may work efficiently to continuously produce enantiomerically enriched cycloadducts for more than 200 h. Regeneration of the organocatalytic column was also partially accomplished, although associated with a slightly lower enantioselectivity, thus prolonging the “life” of the reactor to more than 300 h.