sacrificial zinc anode; calix[4]arenes; upper rim acylation; electrosynthesis
Abstract :
[en] A straightforward electrosynthetic method is described, which allows upper rim acylation of non-p-halogenated calix[4]-arenes. For example, a solution of tetrapropoxycalix[4]arene 4 was electrolysed in the presence of ZnBr2, in an undivided cell fitted with a sacrificial zinc anode using pure acetonitrile as solvent, yielding an organozinc species, which was then treated with acetyl chloride in the presence of a palladium catalyst to afford 5,11-diacety1-25,26,27,28-tetrapropoxycalix[4]arene 5 in ca. 35% yield after workup. (c) 2005 Elsevier Ltd. All rights reserved.
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The reactions were run as follows: in an undivided cell fitted with zinc rod (0.8 cm diameter) as anode and a stainless steel cathode were introduced freshly distilled acetonitrile (∼50 mL) containing TEAP (0.1 mol L -1) as supporting electrolyte, 0.250 g of 1 (0.37 mmol), 0.022 g (0.10 mmol) of CoBr2 and ca. 0.158 g (0.7 mmol) of ZnBr2 (formed by electroreduction of 1,2-dibromoethane in the presence of zinc anode). A constant current intensity of 0.2 A was applied. The reaction carried out at room temperature was stopped after consumption of 2 F/mol of 1. To the solution resulting from the electrolysis of 1, [PdCl2(PPh3) 2] (0.025 g, 3.56 × 10-2 mmol) and an excess of acetyl chloride (1 mL) were added and allowed to react. The solution was stirred for 2-3 h and quenched with 2 mol L-1 HCl (30 mL). After evaporation of acetonitrile under reduced pressure, dichloromethane (50 mL) was added to the residue. The resulting suspension was washed with water (3 × 250 mL) to ensure complete removal of water-soluble salts. The extracts were dried over MgSO4 upon which the solvent was removed under reduced pressure. The products were purified by flash column chromatography on silica gel using hexane/ethyl acetate 80:20 (v/v) as eluent.
Physical data for synthesised compounds: 5,17-diacetyl-25,26,27,28- tetrapropoxycalix[4]arene 2. This compound was separated chromatographically (silica gel 60, Rf = 0.3, hexane/ethyl acetate 80:20 v/v). Yield: 0.070 g (28%). 1H NMR (CDCl3, 400 MHz): 7.12 (s, 4H, arom H), 6.59 (m, 6H, ArH), 4.41 and 3.14 (AB system, 8H, 2J = 13.6 Hz), 3.81 and 3.79 (2t, 8H, OCH2), 2.39 (s, 6H, C(O)CH3), 1.83 (m, 8H, OCH2CH2), 0.94 and 0.92 (2t, 8H, CH 2CH3). 13C NMR (CDCl3, 100 MHz): 197.49 (CO), 161.01, 156.47 (2s, OC(aryl)), 135.27, 134.73, 131.21, 128.73, 128.62, 122.47 (8s, arom. C), 76.92 (s, OCH2), 76.82 (s, OCH 2), 31.02 (s, ArCH2), 26.24 (C(O)CH3), 23.37 and 23.23 (2s, CH2CH3), 10.35, 10.37 and 10.27 (2s, CH3CH2). MS (ESI): m/z 699 [M+Na]+. Anal. Calcd for C44H52O6•0.25CH3CO 2Et: C, 77.33; H, 7.79. Found: C, 77.21; H, 8.00. 5-Bromo-11,23-diacetyl-25,26,27,28-tetrapropoxycalix[4]arene 3. This compound was separated chromatographically (silica gel 60, Rf = 0.4, hexane/ethyl acetate 80:20 v/v ). Yield: 0.084 g (30%). 1H NMR (CDCl3, 400 MHz): 7.40 (s, 2H, ArH), 7.37 (br s, 2H, ArH), 6.54 (t, 1H, ArH, 3J = 7.2 Hz), 6.41 (s, 2H, H of ArBr), 6.32 (d, 2H, ArH, 3J = 7.2 Hz), 4.40 (d, 2H, ArCHaxAr, 2J = 13.6 Hz), 4.34 (d, 2H, ArCHaxAr, 2J = 13.6 Hz), 3.87 (m, 4H, OCH2), 3.70 (m, 4H, OCH2), 3.18 (d, 2H, ArCH eqAr, 2J = 13.6 Hz), 3.11 (d, 2H, ArCHeqAr, 2J = 13.6 Hz), 2.40 (s, 6H, MeC(O)), 0.96 (2 overlapping t, 6H, CH2CH3), 0.85 (t, 6H, CH2CH3). 13C NMR (CDCl3, 100 MHz): 197.52 (CO), 161.78, 155.83 and 155.12 (3s, OC(aryl)), 136.29-115.11 (arom. C), ∼76.9 (3s overlapping with CHCl3, OCH2), 76.82 (s, OCH2), 31.07 (s, ArCH2), 30.92 (s, ArCH2), 26.45 (C(O)CH3), 23.37, 23.25 and 23.17 (3s, CH2CH3), 10.54, 10.47 and 10.09 (3s, CH3CH2). MS (ESI): m/z 779 [M+Na]+. Anal. Calcd for C44H51O6Br•0.5CH 3CO2Et: C, 69.08; H, 6.93. Found: C, 69.14; H, 6.93. 5,11-Diacetyl-25,26,27,28-tetrapropoxycalix[4]arene 5. The synthesis of this compound was carried without addition of CoBr2. The compound was separated chromatographically (silica gel 60, Rf = 0.4, hexane/ethyl acetate 80:20 v/v). Yield: 0.095 g (35%) (note, beside the starting compound other acylated products were detected, but these could not be isolated as pure compounds). 1H NMR (CDCl3, 400 MHz): δ 7.14 (s, 4H, m-ArH of OArC(O)Me), 6.52-6.50 (m, 4H, m-ArH of OAr), 6.43 (t, 2H, p-ArH, 2J = 7.4 Hz), 4.41, 4.38, 4.36 (3d, 1H:2H:1H, A part of ArCH 2Ar groups, 2J = 13.2, 13.6 and 14.0 Hz), 3.18, 3.14 and 3.08 (3d, 1H:2H:1H, B part of ArCH2Ar groups, 2J = 14.0, 13.6 and 13.2 Hz), 3.88-3.72 (m, 8H, OCH2), 2.28 (s, 6H, C(O)Me), 1.86-1.77 (m, 8H, OCH2CH2), 1.18-0.90 (2 overlapping t, 6H, CH2CH3). 13C{1H} (CDCl 3, 100.63 MHz): δ 197.44 (s, C(O)Me), 161.11 and 156.52 (2s, arom. Cq-O), 135.65-122.01 (arom. C's), 76.80 and 76.71 (2s, OCH 2), 31.09, 31.03 and 30.93 (3s, ArCH2Ar), 26.36 (s, C(O)CH3), 23.30 and 23.24 (2s, CH2CH3), 10.31 and 10.27 (2s, CH2CH3). MS (ESI): m/z 699.5 [M+Na] +. Anal. Calcd for C44H52O6•0. 25CHCl3: C, 75.20; H, 7.45. Found: C, 75.21; H, 7.40.
