活性MnO2 氧化

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，通常脂肪族的醇氧化反应进行的非常的慢，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，常用的溶剂有芳香性物质、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进行反应。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。由于是非均相催化剂，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，反映本身非常的简便。

活性二氧化锰的制备：

高锰酸钾(960g)的水溶液(6000mL)加热搅拌、在一小时中同时慢慢加入硫酸锰(II)四水和物(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，碾成粉末状备用(920g)。

反应实例

Preparation of (4S)-(−)-tert-butyldimethylsiloxy-2-cyclopenten-1-one （Organic Syntheses, Coll. Vol. 9, p.136; Vol. 73, p.44）

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).

Preparation of 3-Butyroyl-1-methylpyrrole (Organic Syntheses, Coll. Vol. 7, p.102; Vol. 62,p.111)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.