Macrocyclic Hosts in Asymmetric Phase-Transfer Catalyzed Reactions

Abstract

The introduction and development of neutral macrocyclic hosts capable of complexing ions within their pre-organized cavity, has been of utmost importance in supramolecular chemistry. Their ability to form stable organic-soluble metal–macrocycle complexes opened up the way to their application in phase-transfer catalysis (PTC) as a viable alternative to quaternary onium salts. In particular, their conformationally rigid preorganized backbone, accommodating organic substrates in defined orientations, promotes highly efficient stereoselective reactions. This short review summarizes the applications of neutral macrocyclic hosts in stereoselective PTC, highlighting the difference and possible complementarity with quaternary ammonium salts.1 Introduction2 Crown Ethers and Related Coronands2.1 Enantioselective PTC with Chiral Macrocyclic Coronands2.2 Enantioselective PTC Reactions Catalyzed by Chiral Quaternary Ammonium Salts with Achiral Crown Ether Co-catalysts3 Cyclic Peptoids4 Miscellaneous Chiral Macrocyclic Hosts5 Summary and Outlook

Synthesis of Novel Crown Ether-Squaramides and Their Application as Phase-Transfer Catalysts

“…NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer( Bruker, Ettlingen, Germany), (500.13 MHz for 1 H, 125.76 MHz for 13 C and 50.68 MHz for 15 N) in DMSO-d 6 and were referenced to residual solvent proton signals(δ H = 2.50) and solvent carbon signals (δ C = 39.51). The approximate 15 N chemical shifts were detected by 1 H, 15 N-gs-HMBC method optimized for J(N,H) = 5 Hz long-range couplings, and 15 N chemical shifts are given on NH 3 scale.…”

“…The approximate 15 N chemical shifts were detected by 1 H, 15 N-gs-HMBC method optimized for J(N,H) = 5 Hz long-range couplings, and 15 N chemical shifts are given on NH 3 scale. The pulse programs of one-dimensional ( 1 H, 13 C and DEPTQ) and two-dimensional ( 1 H, 1 H-gs-COSY, 1 H, 13 C-gs-HSQC, 1 H, 13 C-gs-HMQC, 1 H, 13 C-gs-HMBC, 1 H, 15 N-gs-HMBC and 1 H, 1 H-gs-ROESY) measurements were utilized. All chemical shifts are reported in parts per million (ppm).…”

“…The broad signals, the non-integer integral values in the 1 H NMR spectra (seen in Supplementary Materials) of compounds and the duplication of some signals in the 1 H and 13 C NMR spectra (e.g., 1 H and 13 C NMR signals of N-methylene groups in aza-crown ether units) of compounds indicate inhibited motions in the molecule. Also, the conformational properties of the molecules resulted in many cases in which not all signals appeared in the NMR spectra due to signal broadening.…”

“…The most conspicuous benefits that help crown ether derivatives to stand out from other catalysts are as follows: they are usually more resistant to strong bases, and the cation is usually more accessible than the positively charged nitrogen or phosphorus atom in ammonium or phosphonium salts; consequently, a stronger interaction can take place with the reactive anion. Furthermore, crown ethers are more effective in extracting inorganic salts from their solid form [13].…”

Synthesis of Methyl 4,6-Di-O-ethyl-α-d-glucopyranoside-Based Azacrown Ethers and Their Effects in Asymmetric Reactions

“…Since the 1990s, asymmetric PTC has emerged as a topic of scientific interest. Consequently, different chiral catalysts derived from cinchona alkaloids, amino acids, and 1,1 -bi-2-naphthol (BINOL) alongside macrocyclic compounds have been synthesized and applied [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35].…”