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Synlett 2009(20): 3291-3294  
DOI: 10.1055/s-0029-1218375
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Biomimetic Oxidation of Alcohols Catalyzed by TEMPO-Functionalized Polyethylene Glycol and Copper(I) Chloride in Compressed Carbon Dioxide

Cheng-Xia Miao, Liang-Nian He*, Jin-Quan Wang, Jian Gao
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, P. R. of China
Fax: +86(22)23504216; e-Mail: heln@nankai.edu.cn;
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Publication History

Received 1 September 2009
Publication Date:
18 November 2009 (online)

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Biomimetic Oxidation of Alcohols Catalyzed by TEMPO-Functionalized Polyethylene Glycol and Copper(I) Chloride in Compressed Carbon DioxideSynlett 2009; 2009(20): e7-e7
DOI: 10.1055/s-0029-1218549

Abstract

Recyclable TEMPO-functionalized polyethylene glycol [PEG6000-(TEMPO)2] in combination with cuprous chloride were developed for biomimetic oxidation of a series of benzylic, allylic, heterocyclic alcohols, and 2-phenylethanol into the corresponding aldehydes or ketones in high selectivity and in moderate to high conversion in compressed CO2, which enhanced the catalytic activity as well as improved the selectivity.

Key words

alcohol oxidation - TEMPO - carbon dioxide - polymer - copper(I) chloride

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Representative Procedure for the Aerobic Oxidation of Alcohols A mixture of substrate (1.93 mmol), PEG6000-(TEMPO)2 (0.3045 g, 2.5 mmol%), and CuCl (9.6 mg, 5 mmol%) was placed in a 25 mL autoclave equipped with an inner glass tube. 2 MPa CO2 and 1 MPa O2 were introduced into the autoclave, and the reactor was heated to the reaction temperature. Then final pressure was adjusted to the desired pressure at the reaction temperature by introducing amount of CO2. The mixture was stirred continuously for the designed reaction time. After cooling, products were then extracted by Et2O and analyzed by gas chromatography with a Shimadzu GC-2014 equipped with a capillary column (RTX-5, 30 m × 0.25 µm) using a flame-ionization detector. The residue was purified by column chromatography on silica gel (200-300 mesh, eluting with 20:1 PE-EtOAc) to afford the desired product. The structure and purity of products were further identified using NMR (Bruker 300 or 400 MHz), GC-MS (HP G1800A), HPLC-MS (LCQ Advantage), GC, and HPLC by comparing retention times and fragmentation patterns with those of authentic samples. Safety warning: Experiments using large amounts of compressed gases, especially molecular oxygen and supercritical fluids, are potentially hazardous and must only be carried out by using the appropriate equipment and under rigorous safety precautions. In particular, CO2 is introduced into the substrate-loaded reactor before oxygen is added.

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The Typical Procedure for the Recycling of TEMPO-Functionalized PEG (A) The reaction mixture was extracted with compressed CO2 (202.7 bar, 50 ˚C) to afford the corresponding carbonyl compound. The TEMPO-functionalized PEG phase containing CuCl was reused without further purification
or activation.
(B) Extract procedure with Et2O: after addition of Et2O (3 × 10 mL) to the resulting mixture upon completion of reaction, the PEG phase was solidified when cooled to
-10 ˚C to -20 ˚C, and followed by simple decantation of the ether phase containing oxidized products. Subsequently, the PEG phase was dried under vacuum for next run. We conducted further oxidation by addition of successive portions of the alcohol and run the reaction under identical reaction conditions.