Surface confinement effects in enantioselective catalysis: Design of new heterogeneous chiral catalysts based on C1-symmetric bisoxazolines and their application in cyclopropanation reactions

Electrostatic immobilization of bis(oxazoline)-copper complexes on mesoporous crystalline materials: Cation exchange vs. incipient wetness methods

“…After the addition, the reaction was left to stir for 30 min and then analyzed by CG [5][6][7][8][9].…”

“…This method has been commonly used in the case of immobilization on laponite clay [5][6][7][8][9][10][11][12][13]. The bis(oxazoline)-copper(II) triflate complex is stirred with the support in a solvent with a high dielectric constant, able to dissolve the sodium triflate generated in the exchange process, which should then subsequently eliminated by filtration.…”

“…The yield produced by the supported complex ( Table 2) was much lower than that of the homogeneous complex, and the enantioselectivity was significantly lower. If this result were caused by a confinement effect, as happened in the case of laponite [5,9], a significant variation in cis/trans diastereoselectivity would be expected. Since this is not the case, a more obvious cause for the reduction in enantioselectivity would be there being an almost complete decomplexation of the chiral ligand from the copper during the cation-exchange process, which results from a low stability of the complex.…”

“…Non-covalent immobilization methods [4] using already existing unmodified ligands and complexes, significantly reduce the synthetic cost and effort of immobilization. Among the different methods utilizing non-covalent support-catalyst interactions, electrostatic immobilization is very useful and practical to immobilize charged complexes, as our group has demonstrated in the case of copper (II) complexes with different types of chiral ligands, such as bis(oxazolines) [5], azabis(oxazolines) [6], quinolineoxazolines [7], and pyridine-azaoxazolines [8]. The most successful support has been laponite, which is a synthetic clay with the intrinsic advantages of having small particles, and reliably consistent composition.…”

“…The most successful support has been laponite, which is a synthetic clay with the intrinsic advantages of having small particles, and reliably consistent composition. Importantly, the electrostatic immobilization on this support leads to important effects on diastereo-and enantioselectivities of cyclopropanation [5,9] and other reactions [10][11][12][13], which have not been detected with other types of support. The lamellar nature of the clays, that provides a flat surface of contact for the complex, has been proposed to be the origin of this effect.…”