Oxidation pyridinium chloro chromate

Oxidation of primary alcohols to aide hydes (Section 15 10) Pyridinium di chromate (PDC) or pyridinium chloro chromate (PCC) in anhydrous media such as dichloromethane oxidizes primary al cohols to aldehydes while avoiding over oxidation to carboxylic acids... 

Primary alcohols are oxidized to either aldehydes or carboxylic acids, depending on the reagents chosen and the conditions used. One of the best methods for preparing an aldehyde from a primary alcohol on a small laboratory scale, as opposed to a large industrial scale, is to use pyridinium chloro-chromate (PCC, CsH NCrO Cl) in dichloromethane solvent. 

Oxidation of 2,3-O-eyclohexylidcne-D-ribose by pyridinium chloro-chromate gave 2,3-O-cyclohexylidene-D-ribonono-1,4-lactone in 55-60% yield.23... 

Oxidation of alcohols is normally carried out with Cr(VI) reagents (Chapter 24) but these, like the Jones reagent (Na2Cr2C>7 in sulfuric acid), are usually acidic. Some pyridine complexes of Cr(Vl) compounds solve this problem by having the pyridinium ion (p Ta 5) as the only acid. The two most famous are PDC (Pyridinium DiChromate) and PCC (Pyridinium Chloro-Chromate). Pyridine forms a complex with CrO but this is liable to burst into flames. Treatment with HC1 gives PCC, which is much less dangerous. PCC is particularly useful in the oxidation of primary alcohols to aldehydes as overoxidation is avoided in the only slightly acidic conditions (Chapter 24). 

Reduction of ot-keto epoxides. Epoxidation of cyclic allylic alcohols results mainly in the epoxide syn to the hydroxyl group (4, 76). Schlessinger et al. have reported a method for isomerization of the alcohol group by oxidation to the ketone and reduction to the anh-alcohol. In a model case, 2->4, pyridinium chloro-chromate buffered with sodium acetate was found to be the most satisfactory oxidant. Stereoselective reduction to 4 was found to be a more difficult problem, but eventually triisobutylaluminum was found to effect this reduction in high yield. 

Another pyridine-chromium trioxide complex, pyridinium chloro-chromate, CsHsNHCrOjCl (PCC), is prepared by adding pyridine to a solution of chromium trioxide in 6 M hydrochloric acid [605. This complex is superior to Collins reagent in that much a smaller excess is needed, with the ratio of the substrate to the oxidant being 1 1.5-2 (equation 211). 

The tritylsulfenamide can be prepared from an amine and the sulfenyl chloride (Na2C03, THF, H2O or Pyr, CH2CI2, 64-96% yield) it is cleaved by hydrogen chloride in ether or ethanol (0°C, Ih, 90% yield), " CuClj (THF, EtOH, 58-67% yield), MeaSil (77-96% yield),I2 (0.1 M, THF, collidine, HjO, 97% yield), BusSnH, 115°C, toluene, 5 min, 82% yield. " The tritylsulfenamide is stable to lA HCl, base, NaCNBHs, L1A1H4, tn-chloroperoxybenzoic acid, pyridinium chloro-chromate, Jones reagent, Collins oxidation and Moffat oxidation. The stability of this group is largely due to steric hindrance. 

The same allylic alcohols (142) undergo regiospecific oxidative cleavage with pyridinium chloro-chromate at the ethylenic bond to give, after saponification, a-hydroxyacids (145). Extension of this reaction to a -unsaturated ketones (146), obtained by oxidation of secondary alcohols with silver carbonate on celite, affords a-ketoacids (147) <88TL626l>. 

It has been prepared (Ullrich and Grosch, 1988b) by oxidation of 2-nonynol with pyridinium chloro-chromate, followed by hydrogenation in the presence of palladium on calcium carbonate. The formed... 

Oxidation photocatalyzed by polyoxometalates [66k] has been applied to the fimctionalization of 1,8-cineole (structure IX-10) [661], widely distributed in the plant kingdom. The photooxygenation of IX-10 gave a mixture of ketones and alcohols which were transformed by the subsequent action of pyridinium chloro-chromate into 5- and 6-keto derivatives in the ratio IX-11 IX-12 = 2.5 1. A laser flash photolysis study of the mechanism has been carried out for the deca-tungstate anion catalyzed reaction [66m]. 

The chain extension of carbanions by three carbon atoms by [1,4] addition to acrylate esters is attractive in theory, but beset with practical difficulties. It has now been shown that at low temperatures ( —40°C) Grignard reagents in the presence of CuCl give acceptable yields (40—80%) of Michael adducts with ethyl acrylate. Enol ethers can be directly oxidized to esters in high yield by pyridinium chloro-chromate. Thus, ethyl vinyl ether is converted into ethyl acetate and 2,3-dihydrofuran to butyrolactone. 

It was possible to transfer the nitrile group from acetone cyanohydrin to the C,9 aldehyde (508) (Van Scott and Ruey, 1980), the a-hydroxynitrile (509) being obtained. This was hydrolyzed with hydrochloric acid to give the hy-droxycarboxylic acid (510), which was then oxidized with pyridinium chloro-chromate. 

Oxidation of a primary alcohol with pyridinium chloro-chromate or hypochlorous acid (11.5). 

Preparation of PEG aldehyde proved problematic primarily because of the pronounced ability of PEG to form complexes with metallic oxidizing agents. An initial preparation with pyridinium chloro-chromate gave aldehyde product, which could be purified with variable success, by chromatography on alumina. Oxidation with DMSO-acetic anhydride and substitution on the diethyl acetal of bromoacetaldehyde (eq. 8) provided two excellent methods for preparation of the PEG aldehyde. 

Due to the relative instability of silylated corticosteroids, alternatives have been developed by Her and Watron [52] and Courtheyn et al. [47]. For dexamethasone and structurally related analytes, an oxidation reaction with pyridinium chloro-chromate has proven to be useful in several laboratories. Later, the procedure was improved by changing the oxidizing reagent to dichromate and reducing the reaction time. Furthermore, the application of a hydrolysis step permitted triamcinolone acetonide to be included in the analytical procedure [46]. As described above, special attention should be given to the identification of dexamethasone and betamethasone because of their similar mass spectra and inadequate peak separation. 

Alcohols are also oxidized with a milder oxidizing agent consisting of pyridinium chloro-chromate (PCC) in methylene chloride (CH Cy as solvent. PCC is made by dissolving CrO in HCl and then adding pyridine to obtain a sohd, which is isolated and then dissolved in methylene chloride. The reagent is anhydrous, so a primary alcohol can be converted to an aldehyde without further oxidation to a carboxylic acid. 

The reaction was further extended to intramolecular Diels-Alder reaction, and cis-hexahydro-lH-indene 90 was synthesized from diene 89 and terminal alkyne in one operation. The possible intermediate 92 was spontaneously converted into 90. Deprotection of the silyl group followed by pyridinium chloro chromate (PCC) oxidation gave indanone 91 (Scheme 6.23) [28]. 

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. 

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