Synthesis of Chiral Allylic Esters by Using the New Recyclable Chiral Heterogeneous Oxazoline-Based Catalysts

Abstract

A new class of recyclable supported chiral heterogeneous ligands has been synthesized by the reaction of functionalized mesoporous SBA-15 with aliphatic- and aromatic-substituted chiral amino oxazoline ligands. The obtained chiral heterogeneous ligands were characterized by several techniques such as Fourier transform infrared, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and BET–BJH. The application of these new heterogeneous ligands in copper-catalyzed asymmetric allylic oxidation of olefins by using perester showed better yields up to 95% and better enantioselectivities up to 96% compared to the corresponding homogeneous catalysts. These findings can be considered as an important step in the advancement of green chemistry. Investigation of the recyclability of the catalysts confirmed that they were easily recovered and reused eight times without significant losses in reactivity, yield, and enantioselectivity.

Preparation and DFT studies of chiral Cu (I)-complexes of biphenyl bisoxazolines and their application in enantioselective Kharasch–Sosnovsky reaction

“…Under a nitrogen atmosphere, at room temperature, a 10 mL flame dried schlenk flask was charged with dried acetonitrile (2 mL), Cu(CH 3 CN) 4 PF 6 (10 mg, 0.027 mmol) and chiral ligand 1a (14 mg, 0.032 mmol) and stirred for 2 h. Then, phenyl hydrazine (5 μL, 0.05 mmol) and HZSM-5 (5 mg) were added. After a few minutes, cyclohexene (2.5 mmol, 0.25 mL) was added slowly, and the reaction mixture was cooled to 0 °C, and tert-butyl-p-nitrobenzoperoxoate 7a 15,18,[50][51][52] (0.85 mmol, 0.203 g) was added portionwise, and then stirred at 0 °C until complete disappearance of 7a (TLC). After that, 5 mL 10% NH 4 OH was added to the mixture and extracted with EtOAc (3 × 5 mL).…”

“…Column chromatography of the obtained residue on silica gel afforded (S)-2-cyclohexenyl-p-nitrobenzoate as a white solid (98%, 93% ee). The bisoxazoline ligand was also recovered in 92% yield 15,18,[50][51][52][76][77][78][79][80] .…”

“…Previous studies have shown that this asymmetric transformation is considerably improved by inorganic porous additives 15,18,50 . Therefore, based on the above-mentioned and in continuation of our research group studies to tune the best condition 15,18,[49][50][51][52] , enantioselective allylic C-H bond oxidation of alkenes was evaluated with different substituted t-butyl perbenzoates as…”

“…Similarly, ligand 1a gave better results than 1b, and also p-nitroperbenzoate 7a was the best oxidant (Table 2 www.nature.com/scientificreports/ entries 1-9). This discrepancy in the enantioselectivity may be attributed to the interaction between generated allyl radicals and the phenyl substituents in ligand 1a at the transition states 51,66 . In case of acyclic olefins, the reaction times were longer, and yields and ee values were also inferior, although the best results were obtained in the presence of p-iodoperbenzoate 7b (Table 2, entries 10-12).…”

Chiral Pseudohomogeneous Catalyst Based on Amphiphilic Carbon Quantum Dots for the Enantioselective Kharasch–Sosnovsky Reaction

“…Since the allylic oxidation reaction (Kharasch–Sosnovsky reaction) products in the chiral form, chiral allylic esters and chiral allylic alcohols, play an important role as key intermediates for the total synthesis of a variety of natural products and biologically significant compounds, this reaction has garnered the attention of numerous chemists. − Therefore, an important part of allylic oxidation reactions is the development of effectual chiral catalysts. − Herein, chiral PHCs as an interesting category of chiral catalysts with unique characteristics were prepared, and after in situ immobilization of copper salts on them, they were used in the enantioselective allylic C–H bond oxidation of alkenes in the presence of different peresters in order to prepare chiral allylic esters (Scheme ).…”