A redox-reconfigurable catalyst derived from L-methionine and incorporating catalytic urea groups has been synthesized. This copper complex catalyzes the enantioselective addition of diethyl malonate to trans-β-nitrostyrene. Either enantiomer of the product can be predetermined by selection of the oxidation state of the copper ion. Enantiomeric excesses of up to 72% (S) and 70% (R) were obtained in acetonitrile. The ability of the catalyst to invert enantiomeric preference was reproduced with several different solvents and bases. Facile interconversion between the Cu(2+) and Cu(+) redox states allowed easy access to both active helical forms of the complex and, therefore, dial-in enantioselectivity.
General Methods. All reagents were purchased from commercial suppliers (Aldrich or Fisher) and used without further purification. Cyclobis(paraquat-p-phenylene) hexafluorophosphate S1 (CBPQT • 4PF 6), 1,5-bis[2-(2-hydroxyethoxy)ethoxy)] naphthalene S2 (6a), 1,5-bis[2-(2-(2-hydroxyethoxy)ethoxy)ethoxy]naphthalene S3 (6b), 1,5-bis[2-(2-(2-(2hydroxyethoxy)ethoxy)ethoxy)ethoxy]naphthalene S3 (6c) and mono-tosylates S4,S5 of 6a (S1) and 6b were prepared according to literature procedures. Preparation and characterization of catenanes 1b • 4PF 6 and 2b • 4PF 6 , and their precursors 7b, 8b, and 9b was reported in a previous publication. S6 Thin layer chromatography (TLC) was performed on silica gel 60 F 254 (E. Merck). Column chromatography was performed on silica gel 60F (Merck 9385, 0.040-0.063 mm). Melting points were recorded on an Electrothermal 9100 instrument in open capillary tubes and are uncorrected. Routine nuclear magnetic resonance (NMR) spectra were recorded at 25 °C on a Bruker Avance 400 and 500 spectrometers, with working frequencies of 400 and 500 MHz for 1 H, and 100 and 125 MHz for 13 C nuclei respectively. VT-NMR spectra were recorder on a Bruker Avance 500 spectrometer and temperature-calibrated using neat MeOH (for T < 295 K) and ethylene glycol (for T > 295 K). Chemical shifts are reported in ppm relative to the signals corresponding to the residual non-deuterated solvents (CDCl 3 : δ 7.26 ppm, CD 3 CN: δ 1.93 ppm, DMF-d 7 : δ 8.01 ppm). All 13 C spectra were recorded with the simultaneous decoupling of proton nuclei. Fast atom bombardment mass spectra were obtained on a JEOL JMS-600H high resolution mass spectrometer equipped with a FAB probe. Scheme S1. Synthesis of DNP Azide Derivative 7a. A solution of tosylate S4 S1 (1.37 g, 2.78 mmol) and NaN 3 (904 mg, 13.9 mmol) in dry DMF (15 mL) was heated at 80 °C for 20 h. After cooling, the solvent was removed in
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.