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Activated manganese dioxide (MnO2) reliably oxidizes acetylenic, allylic, and benzylic alcohols to aldehydes and ketones. Saturated primary and secondary alcohols are also oxidized, albeit more slowly. The two main concerns are the activity of the manganese dioxide and the slow filtration of salts after the reaction. Activated MnO2 is available commercially or may be prepared. To a solution of 15.3 g (37.5 mmol) of the alcohol in 150 mL of hexanes was added 60 g of activated MnO2. The reaction mixture was stirred at 22 °C overnight and filtered, and the solid residue was washed with 30% EtOAc in hexanes solution. The combined filtrates were dried (Na2SO4) and concentrated in vacuo. The residue was purified by chromatography on SiO2 (EtOAc:hexanes, 1:10) to give 13.7 g (90%) of the ketone as a colorless oil. Reference: Wipf, P.; Xu, W. J. Org. Chem. 1996, 61, 6556–6562. Chromium-based oxidations are reliable and well established, but the toxicity associated with chromium salts have meant that they are generally considered the second choice. For a review of chromium–amine complex oxidations, see Luzzio, F. A. Org. React. 1998, 53, 1-221. To a mixture of pyridinium chlorochromate (PCC 339 mg, 1.57 mmol), ammonium acetate (215 mg, 2.62 mmol), and 4 Å molecular sieves (610 mg) in CH2Cl2 (33 mL) was added a solution of the alcohol (208 mg, 1.05 mmol) in CH2Cl2 (14 mL) under argon at 0 °C over a period of 10 min. After the mixture had been stirred at room temperature for 3 h, diethyl ether (200 mL) was added and the mixture was filtered through a short pad of Florisil. The filtrate was washed successively with water (100 mL) and brine (100 mL), dried with Na2SO4, and concentrated. The residue was purified by chromatography on silica gel (hexane 70%, Et2O 30%) followed by distillation to give the aldehyde as a colorless oil (132 mg, 63%). |
