Stereoselectivity induced by support confinement effects. Aza-pyridinoxazolines: A new family of C1-symmetric ligands for copper-catalyzed enantioselective cyclopropanation reactions

Abstract

Aza-pyridinoxazoline ligands, a new class of C 1 -symmetric ligands, are described and tested in the heterogeneous enantioselective catalysis of a cyclopropanation reaction, with the aim of improving surface confinement effects by the clay support on the reaction stereoselectivity. In the case of 10 trans/cis diastereoselectivity, these surface effects lead to a systematic reversal of selectivity, ciscyclopropanes being favored. Regarding the enantioselectivities, support confinement has a positive effect in the case of major cis-cyclopropane products, leading to moderate enantioselectivity values (60% ee). A theoretical (DFT) mechanistic study is carried out to explain the origin of the enantioselectivity in homogeneous phase at a molecular level, and to get insights 15 on the geometries of the key intermediates and transition structures.

Silica “SHB” chiral Pc-L* copper complexes for halogen-free solvent cyclopropanation reactions

“…The 1 H NMR signals of the compounds described in the following have been attributed by COSY and NOESY techniques. Assignments of the resonance in 13 C NMR were made using the APT pulse sequence and HSQC and HMBC techniques. Infrared spectra were recorded on a BIO-RAD FTS-7 spectrophotometer.…”

“…The advantages of highly active and selective homogeneous catalysts coupled with the ease of separation typical of heterogeneous systems is also highly recommended to implement such catalysis. Moreover, by using nanostructured supports, like mesoporous silicas or clays, it is also possible to exploit the positive connement effects [13][14][15][16] on activity, diastereo-, and enantio-selectivity.…”

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

“…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.…”

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

“…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.…”