Pyridinium chlorochromate oxidation solvents

Q Primary alcohols can be oxidized to give aldehydes (Section 17.7). The reaction is often carried out using pyridinium chlorochromate (PCC) in dichloro-methane solvent at room temperature. 

The pyridinium chlorochromate (PCC) oxidations of pentaamine cobalt(III)-bound and unbound mandelic and lactic acids have been studied and found to proceed at similar rates.Free-energy relationships in the oxidation of aromatic anils by PCC have been studied. Solvent effects in the oxidation of methionine by PCC and pyridinium bromochromate (PBC) have been investigated the reaction leads to the formation of the corresponding sulfoxide and mechanisms have been proposed. The major product of the acid-catalysed oxidation of a range of diols by PBC is the hydroxyaldehyde. The reaction is first order with respect to the diol and exhibits a substantial primary kinetic isotope effect. Proposed acid-dependent and acid-independent mechanisms involve the rapid formation of a chromate ester in a pre-equilibrium step, followed by rate-determining hydride ion transfer via a cyclic intermediate. PBC oxidation of thio acids has been studied. ... 

A better reagent for the limited oxidation of primary alcohols to aldehydes is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HC1. PCC oxidizes most primary alcohols to aldehydes in excellent yields. Unlike most other oxidants, PCC is soluble in nonpolar solvents such as dichloromethane (CH2C12), which is an excellent solvent for most organic compounds. PCC can also serve as a mild reagent for oxidizing secondary alcohols to ketones. 

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. 

Oxidations with pyridinium chlorochromate PCC and pyridinium dichromate PDC. 4 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 C1CH2CH2C1 and C6H5. The rate of oxidation increases in the order 5A<10A<4A<3 A. 

The diastereomeric excess could be increased to > 99.5% by employing the bulky chiral auxiliary (3S,55)-2,6-dimethylheptane-3,5-diol which prevents the solvent effect by suppressing the formation of substrate - reagent - solvent complex. The 2,6-dimethylheptane-3,5-diol moiety was readily removed by oxidation with pyridinium chlorochromate followed by treatment with potassium carbonate in methanol to give the bridged cyclopropanol compound. An example is the cyclopropanation of 56 to give predominantly 57 with > 95% de 57 was converted to (15,65 )-bicyclo[4.1.0]heptan-l-ol. ... 

To a stirred solution of 100 mg (0.38 mmol) of 1.1.10c in 5 mL of methylene chloride were added 500 mg of pyridinium chlorochromate (PCC) on aluminium oxide (6.1 mmol PCC/7.5 g AI2O3). The reaction mixture darkened from orange to black and after 24 h additionally 500 mg of PCC/alox were added. After completion (62 h, TLC control) the mixture was filtered through a small silica gel column and washed with 50 mL of CH2CI2. The solvent was removed under reduced pressure, yielding 79 mg (80%) of a colorless viscous oil, [a]j3 = -70.1° (1.0, MeOH). 

The oxidation reaction can be stopped at the aldehyde if the reaction is carried out with pyridinium chlorochromate (PCC), because PCC is used in an anhydrous solvent. If water is not present, the hydrate cannot be formed. 

New oxidation reactions of organic substances and the reasons for their acceleration under microwave irradiation have recently been investigated. In particular, in a search for highly efficient oxidation procedures, Chakraborty and Bordoloi [53] used pyridinium chlorochromate (PCC) under the action of microwave irradiation for oxidation of protected a-glucofuranose 62 to the corresponding ketone 63 (99%) much more quickly (10 min) than using the conventional technique (4 h under reflux) (Scheme 12.29) and with an easier work-up procedure. They found the oxidation can also be performed with moist PCC under solvent-free conditions and with the same yield. 

Alcohols are oxidized by pyridinium chlorochromate (PCC) to the corresponding aldehydes or ketones. They are not further oxidized to the corresponding carboxylic adds because the reaction was done in organic solvents, not in water. If water existed, the carbonyls would form aldehyde hydrates or ketone hydrates, which are then oxidized to acids. 

A number of selective oxidizing agents, such as pyridinium chlorochro-mate, are frequently used for this purpose. With this reagent, the oxidation stops at the aldehyde stage, because the oxidation, as pointed out earlier, is conducted in a nonaqueous solution. Thus, decanal can be obtained from 1-decanol using this reagent (methylene chloride solvent) in 92% yield. Pyridinium chlorochromate (PCQ is a solid, yellow salt prepared from chromium trioxide as shown here ... 

A specific oxidizing agent for the conversion of primaiy alcohols to aldehydes is pyridinium chlorochromate, abbreviated as py CrOsCl A Generally, the oxidation is run in methylene chloride solvent. For example. 

One reason for the success of oxidation with pyridinium chlorochromate is that the oxidation can be carried out in a solvent such as CH2CI2, in which PCC is soluble. Aldehydes themselves are not nearly so easily oxidized as are the aldehyde hydrates, RCH(OH)2, that form (Section 16.7A) when aldehydes are dissolved in water, the usual medium for oxidation by chromium compounds ... 

Pyridinium chlorochromate has also been used (Chakraborty and Bordoloi, 1999) under microwave irradiation for the oxidation of alcohols to the corresponding carbonyl functions with an efficient and mild methodology. Microwave irradiation of alcohols with silica supported active manganese dioxide in solvent-free condition provides rapid and selective oxidation of alcohols to the corresponding carbonyl compounds (Varma et al., 1997). 

Following earlier studies of the oxidation of formic and oxalic acids by pyridinium fluoro-, chloro-, and bromo-chromates, Banerji and co-workers have smdied the kinetics of oxidation of these acids by 2, 2Tbipyridinium chlorochromate (BPCC) to C02. The formation constant of the initially formed BPCC-formic acid complex shows little dependence on the solvent, whilst a more variable rate constant for its decomposition to products correlates well with the cation-solvating power. This indicates the formation of an electron-deficient carbon centre in the transition state, possibly due to hydride transfer in an anhydride intermediate HCOO—Cr(=0)(0H)(Cl)—O—bpyH. A cyclic intermediate complex, in which oxalic acid acts as a bidentate ligand, is proposed to account for the unfavourable entropy term observed in the oxidation of this acid. 

The kinetics of oxidative deoximation of aldo- and keto-oximes by 2,2/-bipyridinium chlorochromate (back to the parent carbonyl compounds) have been studied in DMSO, where the reaction is found to be first order in both oxime and oxidant.89 The aldoximes proved more reactive, and rates correlated well with the Pavelich-Taft dual substituent equation. Following extension of the study to hindered cases, and to 18 other solvents (analysed by Taft and Swain multi-parametrics), a cyclic intermediate is proposed for the rate-determining step. The same reaction order behaviour is found using the pyridinium version, and again electronic, steric, and solvent effects were examined.90... 

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