PDC(pyridinium dichromate)氧化

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将吡啶加入到三氧化铬的水溶液中有亮黄色的PDC固体生成,PDC溶于有机溶剂,在空气中室温下,易于储存和操作。在二氯甲烷的悬浮溶液中可以将醇氧化为醛酮( 与PCC氧化类似,惰性气体保护避免接触空气中的水。)。PDC用DMF当溶剂可以将伯醇氧化为酸。另外3Å分子筛,HOAc,PPTS,PTFA,Ac2O可以加快PDC的氧化速度。反应结束可以用亚硫酸钠快速淬灭反应。

PDC的氧化能力较PCC强,其氧化作用一般在中性条件下进行,而PCC则需在酸性中进行。因此,对酸不稳定的化合物用PCC氧化时,必须在醋酸钠存在下进行。

单单只是用PCC或PDC的话,伴随着反应的进行,会生成出黑褐色油状残渣。生成物被这些残渣所包裹,导致产率降低。像反应溶液中加入适量硅藻土,硅胶或者是分子筛,能够改善这种情况。


氧化的一般操作步骤:

Approximately,1.1–7 equivalents of solid PDC are added over a ca. 0.01–0.30 M solution of thealcohol in dry DCM. The resulting suspension is stirred at roomtemperature. Approximately, 0.5–4 g of activated MS-preferably finely ground 3ÅMS-per mmol of alcohol can be added in order to accelerate the oxidation. Thereaction can also be accelerated by the addition of ca. 0.9–4 equivalents of dryAcOH or 0.75–12 equivalents of acetic anhydride. The simultaneous use ofmolecular sieves and an organic acid has a synergistic accelerating effect. Theaddition of ca. 0.5–2.50 g of Celite or Florisil (magnesium silicate) per mmol of alcohol canfacilitate the work-up. Celite or Florisil can be added either at the beginningof the oxidation or ca. 30 min before the work-up. Two alternative work-ups canbe carried out.

Work-upA:

The most common workup. Et2Ois added and the precipitate is decanted and washed with ether. The collectedorganic phases are filtered through a pad of Celite, silica or Florisil.Alternatively, decanting the precipitate can be avoided and the mixture,resulting from the addition of ether, can be directly filtered through a pad ofsilica, Celite or Florisil. When the reaction is carried out under diluteconditions, the addition of diethyl ether can be avoided. The organic phase isconcentrated giving a residue that may need chromatographic purification. Whenthe reaction is

carried out in the presence of addedCelite or Florisil, a similar work-up is made in which the Celite or Florisilis filtered.

Work-upB:

Et2O is added and theresulting mixture is washed with aqueous phases. The aqueous phases used canbe: plain water, aqueous saturated NaHCO3 solution, diluted HCl or brine.The collected organic phases are dried (MgSO4 or Na2SO4)and concentrated, giving a residue that may need chromatographic purification.


反应实例


【Terpstra, J. W.; Van Leusen, A. M. J. Org. Chem. 1986, 51, 230–208】




J. Org. Chem. 1992, 57, 6173–6177】




中间先生成了半缩醛中间体,接着氧化得到内酯。

Tetrahedron: Asymmetry 2003, 2567–2571】



参考文献

一、Name Reactions (A Collection of Detailed Reaction Mechanisms), Jie Jack Li, PCC oxidation,page 337.

二、common oxidation reagents, Yue Xu, Sundia Meditech

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