氯铬酸吡啶鎓（PCC）参与的醇氧化反应—反应机理与实例

PCC参与的醇氧化反应

图 1

氯铬酸吡啶鎓（PCC）是一种选择性氧化剂，与高锰酸钾（KMnO₄）或铬酸（H₂CrO₄）等强氧化剂不同，PCC能够进行可控的氧化反应，使反应停在特定氧化态。这种选择性使PCC成为将伯醇转化为醛、以及将仲醇转化为酮的不可或缺的工具。

图 2

图 2 给出了 PCC 氧化醇的两个反应实例：向醇底物中加入 1 当量 PCC，即可得到对应的氧化产物；反应副产物为四价铬物种与盐酸吡啶鎓，在图中以灰色标示。

1. 反应机理

那么PCC氧化反应是如何进行的？

PCC 对醇的氧化本质是消除反应，具体机理如图3所示。

图 3

首先，氧原子进攻铬原子，形成 Cr−O 键；随后发生质子转移，羟基上带正电荷的质子转移至铬的一个氧原子上，氯离子离去。

接下来碱夺取与氧原子相连碳原子上的质子，C−H 键的电子对重排构建C=O 键，同时 Cr−O 键断裂，六价铬 Cr (VI) 被还原为四价铬 Cr (IV)。

在这个反应过程中，若体系含水，水可与生成的醛发生加成得到醛水合物（如图4所示）。若体系中存在第二当量 PCC，该水合物会被进一步氧化。酮则不会发生该副反应，原因是酮羰基相连的碳原子上无氢原子。

图 4

2. PCC的制备

图 5

将三氧化铬（1eq）加入到6M盐酸（1.1eq）中得到氯铬酸溶液，0℃下加入吡啶（1eq）得到橙黄色的PCC固体。

3. 反应实例

图 6

OrganicSyntheses, Vol. 80, p.1

A 500-mL, round-bottomed flask equipped with a 4.5cm, egg-shaped Teflon-coated magnetic stir bar is charged with 130 mL of CH2Cl2, the alcohol prepared in Step A (10.4 g, 40.0 mmol), and 15 g of freshly powdered 3 Å molecular sieves. Pyridinium chlorochromate (21.5 g, 100 mmol) is added portionwise over 10 min and the resulting mixture is stirred at room temperature for 15 hr.

Ether (200 mL) is added slowly with vigorous stirring and the solution is filtered under vacuum through a pad of 35 g of Celite. The solids remaining in the reaction flask are transferred to the Celite pad by scraping with a spatula and washing with three 50-mL portions of ether. The resulting cloudy brown filtrate is concentrated by rotary evaporation at room temperature to give a brown solid.

To this solid is added 25 mL of 1:1 ether:hexane and the solids are scraped with a spatula. The mixture is then poured onto 60 g of Whatman 60 Å (230-400 mesh) silica gel packed in a 4-cm diameter chromatography column and the liquid is adsorbed onto the silica gel by gravity. The material remaining in the flask is further washed with 1:1 ether:hexane and transferred onto the silica gel; this process is repeated until all the material has been loaded onto the silica gel.

The ketone is eluted using 500 mL of 1:1 ether:hexane and the eluent is concentrated by rotary evaporation to afford the crude ketone as a white solid. This material is dissolved in 40-45 mL of boiling hexane. Upon cooling the solution to room temperature, the ketone begins to crystallize. The flask is then cooled to −25°C for 2 hr.

The resulting solids are collected by filtration, washed with three 25-mL portions of cold (−25°C) hexane, and dried to afford 8.84-9.08 g, (86-88%) of the ketone as a white solid.

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参考文献：

1.《基础有机化学》邢其毅等

2.《Organic Chemsitry》clayden等.

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