Podand‐Based Dimeric Chromium(III)–Salen Complex for Asymmetric Henry Reaction: Cooperative Catalysis Promoted by Complexation of Alkali Metal Ions

Abstract

A new kind of podand-based dimeric salen ligand was synthesized, and its association with potassium cations was investigated by (1) H NMR spectroscopy. The corresponding Cr(III) -salen dimer was assembled by a supramolecular host-guest self-assembly process and was then used as a catalyst in highly efficient and enantioselective asymmetric Henry reactions. Regulation by KBArF (BArF =[3,5-(CF3 )2 C6 H3 ]4 B) led to remarkable improvements in yield (by up to 58 %) and enantioselectivity (for example, from 80 % ee to 96 % ee).

Enantioselective ring opening of epoxides with TMSN3 by macrocyclic oligomer-supported Cr(III)–salen catalysts

“…Flash column chromatography was performed using silica gel 60 Å (230-400 mesh) from Dynamic Absorbents Inc. 1 H NMR spectra were recorded at 25°C on a Bruker Avance 250 (250 MHz) spectrometer. 13 C NMR spectra were obtained at 25°C on a Bruker Avance 250 (60 MHz) spectrometer. All chemical shifts are reported in parts per million (ppm) with reference to solvent residual peaks.…”

“…The salen ligand 10 was coupled with 8 or 9 in the presence of EDC [N-(3-dimethylaminopropyl)-N 0 -ethylcarbodiimide] to yield the monomeric ligand 11 or 12, respectively, which underwent ring-expanding olefin metathesis using Grubbs' third initiator 13 to generate the corresponding macrocyclic oligomer 14 or 15. The formation of the macrocycles was confirmed by 1 H NMR spectroscopy, 13 C NMR spectroscopy and MALDI-TOF mass spectrometry. MALDI-TOF mass spectra show that 4, 14 and 15 are a mixture of macrocycles of different ring sizes, ranging from dimer to decamer, nonamer or octamer, respectively.…”

“…After filtering and removing the solvent under reduced pressure, the crude product was subjected to flash column chromatography to afford the target product. 13 The appropriate salen ester monomer 11 or 12 (1 equiv) was weighed into a round bottom flask. The round bottom flask was sealed with a septum and flushed with argon.…”

“…7,8 Azidotrimethylsilane (TMSN 3 ) is a common nucleophile used in both reactions. To promote the cooperativity of Cr(III)-salen moieties and also to develop more practical and recyclable catalytic systems, substantial efforts have been made to immobilize Cr(III)-salen onto a variety of supports including oligomers, [8][9][10][11][12][13] polymers, [14][15][16] dendrimers 17 and silica. 18,19 Non-covalent encapsulations of Cr(III)-salen in mesoporous silica, 20 micelle 21 and zeolite 22 have also been reported.…”

Supramolecularly fine-regulated enantioselective catalysts

“…31 The designed catalyst was derived from enantiopure ligand L9 and contained two chromium(III)-salen moieties (salen = 2,2'-((1E,1'E)-(ethane-1,2-diylbis(azanylylidene))bis(methanylylidene))diphenol; see the red fragment in Fig. 10) separated by a polyether chain.…”