Pyridinium dichromate oxidation solvents

General Considerations. The following chemicals were commercially available and used as received 3,3,3-Triphenylpropionic acid (Acros), 1.0 M LiAlH4 in tetrahydrofuran (THF) (Aldrich), pyridinium dichromate (Acros), 2,6 di-tert-butylpyridine (Acros), dichlorodimethylsilane (Acros), tetraethyl orthosilicate (Aldrich), 3-aminopropyltrimethoxy silane (Aldrich), hexamethyldisilazane (Aldrich), tetrakis (diethylamino) titanium (Aldrich), trimethyl silyl chloride (Aldrich), terephthaloyl chloride (Acros), anhydrous toluene (Acros), and n-butyllithium in hexanes (Aldrich). Anhydrous ether, anhydrous THF, anhydrous dichloromethane, and anhydrous hexanes were obtained from a packed bed solvent purification system utilizing columns of copper oxide catalyst and alumina (ether, hexanes) or dual alumina columns (tetrahydrofuran, dichloromethane) (9). Tetramethylcyclopentadiene (Aldrich) was distilled over sodium metal prior to use. p-Aminophenyltrimethoxysilane (Gelest) was purified by recrystallization from methanol. Anhydrous methanol (Acros) was... 

Adogen has been shown to be an excellent phase-transfer catalyst for the per-carbonate oxidation of alcohols to the corresponding carbonyl compounds [1]. Generally, unsaturated alcohols are oxidized more readily than the saturated alcohols. The reaction is more effective when a catalytic amount of potassium dichromate is also added to the reaction mixture [ 1 ] comparable results have been obtained by the addition of catalytic amounts of pyridinium dichromate [2], The course of the corresponding oxidation of a-substituted benzylic alcohols is controlled by the nature of the a-substituent and the organic solvent. In addition to the expected ketones, cleavage of the a-substituent can occur with the formation of benzaldehyde, benzoic acid and benzoate esters. The cleavage products predominate when acetonitrile is used as the solvent [3]. 

Oxidations with pyridinium cUorochromate PCC and pyridinium dichromate PDCY Oxidations with PCC and PDC of secondary hydroxyl groups of sugars and nucleosides is slow and incomplete. The reaction is markedly catalyzed by 3 A molecular sieves. Celite, alumina, and silica are not effective. CH2C12 is the most satisfactory solvent oxidations are slower in CICH2C H2CI and C6llf). The rate of oxidation increases in the order 5A< 10A<4A<3A. 

This oxidant is a bright-orange solid that is soluble in organic solvents, and very convenient to store and manipulate, because of its lack of hydro-philicity. Pyridinium dichromate (PDC), which is normally used in dichlor-omethane at room temperature, is a very efficient oxidant able to transform alcohols in aldehydes and ketones in high yield. The absence of water in the reaction media prevents the over-oxidation of aldehydes into carboxylic acids. 

Pyridinium dichromate (14) is an isolable, stable orange solid that can be simply and safely prepared. PDC had been used previously, but it was Corey and coworkers who demonstra the wide applicability of this mild and selective oxidant in organic synthesis. PDC is very soluble in solvents such as DMF, water and DMSO, but sparingly soluble in chlorinated hydrocarbons and acetone. It is normally used either as a solution in DMF or as a suspension in dichloromethane (Table 12). 

Pyridinium dichromate (PDC) The problem caused by the acidic nature of PCC is largely removed by using the neutral reagent PDC . For example, geraniol (7.5) was oxidized to geranial (7.6) in 92% yield by PDC in DMF (dimethylformamide) solvent. 

Imidazolium dichromate is a selective oxidant for allylic and benzylic hydroxy groups. (Allylic alcohols are oxidized faster than benzylic alcohols.) The selectivity over saturate alcohols is similar to that of 4-(dimethylamino)pyridinium chlorochromate. DMF is recommended as the solvent for oxidations, since it appears that the choice of solvent is critical to obtaining high yields. This reagent has also been observed to cause some ( )/(Z)-isomerization during the oxidation of allylic alcohols. 

In the following experiments cyclohexanol is oxidized to cyclohexanone using pyridinium chlorochromate in dichloromethane. The progress of the reaction can be followed by thin-layer chromatography. On a larger scale this reaction would be carried out using sodium dichromate in acetic acid because the reagents are less expensive, the reaction is faster, and much less solvent is required. 

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