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Synlett 2006(17): 2767-2770  
DOI: 10.1055/s-2006-950256
LETTER
© Georg Thieme Verlag Stuttgart · New York

A Facile Strategy to a New Fluorous-Tagged, Immobilized TEMPO Catalyst Using a Click Reaction, and Its Catalytic Activity

Alexandru Gheorghea, Erick Cuevas-Yañeza, Joachim Hornb, Willi Bannwarthb, Banda Narsaiah*c, Oliver Reiser*a
a Institut für Organische Chemie der Universität Regensburg, Universitätsstr. 31, 93053 Regensburg, Germany
Fax: +49(941)9434121; e-Mail: Oliver.Reiser@chemie.uni-regensburg.de;
b Institut für Organische Chemie und Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstr. 21, 79104 Freiburg, Germany
c Indian Institute of Chemical Technology, Division of Fluoro Organics, Uppal Road, Hyderabad 500007, AP, India
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Publikationsverlauf

Received 4 August 2006
Publikationsdatum:
09. Oktober 2006 (online)

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Abstract

A new fluorous-tagged TEMPO catalyst was prepared by a facile approach using the copper-catalyzed azide-alkyne cycloaddition as the ligation method. A fluorous F17-CLICK-TEMPO showed excellent activity and selectivity in the oxidation of alcohols to aldehydes using bleach as the terminal oxidant. Moreover, the catalyst could be easily recovered using silica gel 60 and recycled in four cycles without loss of activity.

Key words

catalyst immobilization - fluorous - oxidation - click chemistry - TEMPO

15

Propargyl Ether TEMPO 2: To a stirring suspension of NaH (60% in mineral oil, 830 mg, 20.75 mmol) in dry DMF (10 mL), 4-hydroxy TEMPO (1, 3.0 g, 17.44 mmol) was added portionwise at 0 °C and stirred at r.t. for 50 min. Propargyl bromide (1.85 mL, 21.0 mmol) was added dropwise at 0 °C over a period of 45 min. The resulting mixture was stirred at r.t. overnight. The reaction mixture was poured into ice water (80 mL) and was extracted with EtOAc (4 × 25 mL). The combined organics were washed with brine (2 × 30 mL), dried over MgSO4, filtered and evaporated under reduced pressure. The residue was purified by column chromatography (silica gel, hexanes-EtOAc, 9:1) to give the title compound 2 (2.85 g, 78%) as an orange solid. 1H NMR and 13C NMR spectra were identical to those previously reported. [10]

17

1-Azidoperfluorodecane (4): 2-(n-perfluorooctyl) ethyl iodide (3) was reacted with sodium azide in acetone-H2O (5:1) under reflux conditions for 7 h, this gave 2-(n-perfluorooctyl) ethyl azide (4) in quantitative yield. 1H NMR and 13C NMR spectra were identical to those previously reported. [16]

18

F 17 -CLICK-TEMPO 6: To a stirred mixture of 1-azido-perfluorodecane (4, 1.19 g, 2.43 mmol) and propargyl ether TEMPO 2 (0.95 g, 4.54 mmol) in degassed THF (15 mL), CuI (27 mg, 6 mol%) was added. The resulting mixture was stirred under a nitrogen atmosphere at r.t. for 1 day. Then, the solvent was removed in vacuo and the residue was purified by column chromatography (silica gel, hexanes-EtOAc, 5:1 → 1:1) to give some of the recovered TEMPO 2 (0.4 g, 42%) and the title compound 6 (1.35 g, 80% based on 4, 73% based on consumed 2) as a light orange solid. Mp 105-107 °C; IR (KBr): 2984, 1468, 1371, 1410, 1177, 1142, 1084, 1045, 989, 957, 661 cm-1; MS (PI-EI, 70 eV): m/z 699 [M+]; HRMS: m/z [M + H+] calcd for C22H24F17N4O2: 699.1628; found: 699.1620.

20

General Procedure for the Oxidation of Alcohols by F 17 -CLICK-TEMPO 6/Bleach: Alcohol (1.0 mmol) in 2 mL CH2Cl2, KBr (24 mg, 0.2 mmol) in H2O (2 mL) and
F17-CLICK-TEMPO 6 (7 mg, 1 mol%) were added to a round-bottom flask. The reaction mixture was stirred at 0 °C before addition of NaOCl (0.8 mL, 10%) and NaHCO3 (40 mg, 50 mg·mL-1, bleach). The resulting mixture was stirred at 0 °C for 15 min. The reaction was quenched by the addition of H2O (5 mL) and the organic layer was extracted with CH2Cl2 (2 × 5 mL). The combined organic layers were dried over anhydrous Na2SO4 and concentrated in vacuo to give crude product, which was placed on a short bed of silica and eluted with CH2Cl2 in order to obtain pure aldehyde.

24

Aerobic Oxidation of 4-Bromobenzyl Alcohol by F 17 -CLICK-TEMPO 6; Recycling Experiments: 4-Bromo-benzyl alcohol (2.0 g, 10.7 mmol) in CH2Cl2 (20 mL), KBr (257 mg, 2.16 mmol) in H2O (2 mL) and F17-CLICK-TEMPO 6 (75 mg, 1 mol%) were added to a round-bottom flask. The reaction mixture was stirred at 0 °C before addition of NaOCl (8.6 mL, 14 mmol), NaHCO3 (430 mg, 50 mg·mL-1, bleach), stirring then continued at 0 °C for 15 min. The reaction was quenched by addition of H2O (10 mL) and the organic layer was extracted with CH2Cl2 (2 × 10 mL). The combined organic layers were dried over anhydrous Na2SO4 and concentrated to give crude product, which was placed on a short bed of silica (5 g) and eluted with CH2Cl2 in order to obtain 4-bromobenzylaldehyde. The catalyst was recovered from the column by elution with Et2O and used in the next cycle.