Synthesis of Pyridinocrownophanes Exhibiting High Ag+-Affinity

Field: Chemistry (Physical Sciences)

Crownophanes possessing a pyridine moiety were prepared by means of intramolecular [2 + 2] photocycloaddition of the styrene derivatives.The crown compounds having six to ten ethereal oxygens selectively extracted Ag + with high efficiency in the liquid-liquid extraction.The high extractability and efficiency were maintained even at low pH region of the extraction system.From ESI-MS analysis, it was found that the crownophanes formed some kinds of complexes having 1:1 and 2:1 stoichiometry (host/guest) with Ag + and Pb 2+ in MeCN-H 2 O homogeneous system.Nitrogen atom has been found to show high affinity toward heavy metal cations such as Ag + and Pb 2+ ions when it is employed as dentate(s) of a crown ether or ligating site(s) of its in side chain(s). 1,24][5][6][7] Recently, we have conveniently and efficiently prepared crownophanes bridged by cyclobutane ring by means of intramolecular [2 + 2] photocycloaddition of styrene derivatives with oligo(oxyethylene) linkage. 8We have also prepared lariat crownophanes having pyridine residues in the side chains by the photocycloaddition, followed by attachment of side arms, and clarified some interesting complexation behavior with heavy metal cations. 9,10For example, one of the lariat crownophanes showed high extractability toward Ag + ion even at high proton concentration (pH<1). 9In this paper, we would like to describe the synthesis and complexing properties of crownophanes possessing pyridine moiety in the polyether linkage (pyridinocrownophanes). EXPERIMENTALApparatus.IR spectra were obtained on a Hitachi 270-50 spectrophotometer. 1 H NMR spectra were taken at 500 MHz using tetramethysilane as an internal standard.Elemental analysis was carried out in Technical Research Center for Instrumental Analysis, Gunma University.Electrospray ionization mass spectra (ESI-MS) were obtained on a following conditions: A sample solution was sprayed at a flow rate of 2 μL min -1 at the tip of needle biased by a voltage of 4.5 kV higher than that of a counter electrode.Reagents.Toluene and THF were purified by distillation over Na after prolonged reflux under a nitrogen atmosphere.Guaranteed reagent grade CH 2 Cl 2 was distilled before use.2,6-Bis[(tosyloxy)methyl)]pyri-dine was prepared by reported method. 11Reagent grade dibenzopyridino-18crown-6 (5) was used without further purification.Paracrownophane (6) was prepared by the method reported previousy. 8AgNO 3 , Pb(NO 3 ) 2 , Cu(NO 3 ) 2 , Mn(NO 3 ) 2 , Zn(NO 3 ) 2 , Ni(NO 3 ) 2 , Co(NO 3 ) 2 , and Fe(NO 3 ) 3 were of commercially available highest grade and used after vacuum drying.All aqueous solutions were prepared with distilled, deionized water. Preparation of oligoethyleneglycol mono(p-bromophenyl) ethers. 2-(p-Bromo-phenoxy)ethanol (1a) was prepared as follows: A mixture of p-bromophenol (25.0 g, 0.145 mol) and 2-chloroethanol (116.3 g, 1.45 mol) in 10% aqueous NaOH solution (600 mL) was stirred under a nitrogen atmosphere at rt.After 24 h, the mixture was extracted with CH 2 Cl 2 (3 x 300 mL).The CH 2 Cl 2 extracts were combined, washed with 10% aqueous NaOH (2 x 500 mL) and with water (3 x 500 mL), dried over anhydrous MgSO 4 .After the solvent was evaporated, the residue was recrystallized from hexane-ethanol to give 1a as white crystals (30.1 g, 96%).mp 52.0-53.0o C. 1 H NMR (CDCl 3 ) δ=7.39 (2H, ABq, J=6.7 Hz), 6.82(2H, ABq, J=6.7 Hz), 4.06 (2H, m), 3.96 (2H, m).Oligoethylene glycol mono(p-bromophenyl) ether was prepared in a following manner: A mixture of 4bromophenol (20.00 g, 0.115 mol), oligoethylene glycol monotosylate (0.058 mol), pulverized K 2 CO 3 (23.91g, 0.173 mol), and DMF (250 mL) was stirred under a nitrogen atmosphere at 50 o C for 24 h.The