活性二氧化锰氧化

二氧化锰在有机化学中十分有用，被用于氧化物的二氧化锰的形态不一，因为二氧化锰有多个结晶形态，化学式方面可以写成MnO2-x(H2O)n，其中x介于0至0.5之间，而n可以大于0。二氧化锰可在不同pH下的高锰酸钾(KMnO4)和硫酸锰(MnSO4)的反应之中产生。啡色的二氧化锰沉淀物很活泼。最有效的有机溶剂包括芳香性物质、氯化碳、醚、四氢呋喃和酯类等。活性MnO2广泛用于氧化α,β-不饱和基团(三键，双键、芳香环)的醇，可选择性氧化烯丙式醇，条件温和，不会引起双键的异构化MnO2的活性及溶剂的选择对反应至关重要，常用的溶剂有二氯甲烷、乙醚、石油醚、己烷、丙酮等。极性溶剂中反应较慢，一般用二氯甲烷和过量的二氧化锰。由于是非均相反应，反应的后处理只需要简单地过滤。中性条件的缘故，不发生副反应。

反应实例

A 500-mL, round-bottomed flask, equipped with a Teflon-coated magnetic stirring bar, is charged with the 11.2 g of allylic alcohol and 300 mL of dichloromethane, and the resulting vigorously stirred solution is treated with 33 g of active manganese dioxide (380 mmol). Additional 2–5 g lots of the oxidant are added every 2–3 hr until the reaction is complete. The reaction mixture is vacuum-filtered through a pad of diatomaceous earth, and the pad is washed with 200 mL of dichloromethane. The resulting clear filtrate is concentrated carefully using a rotary evaporator, and the residual oil is purified by bulb-to-bulb distillation at 0.3 mm (pot temperature 100 °C) affording 8.43–8.71 g (87–90%) of enone as a pale yellow oil that solidifies when cooled below 15°C. Crystallization of the crude product from pentane at −70 °C gives (4S)-(−)-tert-butyldimethylsiloxy-2-cyclopenten-1-one as colorless needles having mp 32–33 °C, [α]D23 −65.1° (CH3OH, c 0.94).

【Organic Syntheses, Coll. Vol. 9, p.136; Vol. 73, p.44】

A 100-mL, one-necked, round-bottomed flask is fitted with an efficient reflux condenser and arranged for magnetic stirring and heating. The flask is charged with 50 mL of pentane and 2.0 g (13 mmol) of alcohol. To the rapidly stirred solution is added 16 g (180 mmol) of activated manganese (IV) oxide in small portions over 5 min. The solution is heated at reflux for 18 hr and then an additional 8 g (90 mmol) of activated manganese (IV) oxide is added in portions. After being heated at reflux for 24 hr, the reaction mixture is filtered through a 2-cm Celite filter pad. The filtered manganese oxides are thoroughly washed with about 200–300 mL of dichloromethane. Evaporation of solvent from the combined filtrates leaves 1.4–1.6 g of a light yellow oil. Bulb-to-bulb distillation at 100°C/0.1 mm gives 1.27–1.40 g (8.4–9.3 mmol, 64–71% yield) of an oil 3-Butyroyl-1-methylpyrrole.

【Organic Syntheses, Coll. Vol. 7, p.102; Vol. 62, p.111】

【Zh. Org. Khim., 1971, 7, 21210 (CA 1972, 76, 13712)】

【Chem. Ber., 1977, 110, 3664】

【J. Chem. Soc., 1963, 4983】

【Synthesis, 1988, 715】

活性二氧化锰的制备方法

将高锰酸钾(960g)溶于水溶液(6000mL)中，加热搅拌，同时缓慢加入四水合硫酸锰(II)(1110g)的水溶液(1500mL)以及氢氧化钠水溶液(40%, 1170mL)。能看到二氧化锰以茶色沉淀析出。继续搅拌一小时，离心机分离，收集固体，水洗至洗液到无色为止。得到的固体在空气中(100-120℃)干燥，碾成粉末备用(920g)。

【J. Chem. Soc. 1952, 1094】

实验技巧

※ 不同的公司的二氧化锰活性纯度不同。

※失活的主要原因是二氧化锰能结合一分子的结晶水，处理方法是碾碎后两百度以上高温长时间加热，能恢复活性。