Chromium -based oxidants

All of the usual chromium-based oxidation reagents that have been used for the oxidation of cyclobutanols to cyclobutanones, for example, chromium(VI) oxide (Jones reagent),302 pyri-dinium chlorochromate,304 pyridinium dichromate,307 and chromium(YI) oxide/pyridine (Collins),303 are reported to do so without any serious problems. Alternatively, tetrapropylam-monium perruthenate in the presence of A-methylmorpholine A -oxide. oxalyl chloride in the presence of triethylamine in dimethyl sulfoxide (Swern),158,309,310 or phenyl dichlorophos-phate in the presence of triethylamine and dimethyl sulfoxide in dichloromethane (Pfitzner-Moffatt),308 can be used. The Pfitzner-Moffatt oxidation procedure is found to be more convenient than the Swern oxidation procedure, especially with respect to the strict temperature control that is necessary to achieve good yields in the latter, e.g. oxidation of 1 to give 2.308... 

Chromium-based oxidants, noteworthy for their specificity and ease of use, continue to be popular. Enayatollah Mottaghinejad of Azad University of Iran, Tehran has found (Tetrahedron Lett. 2004,45, 8823) that barium dichromate, easily prepared, smoothly oxidizes alcohols to aldehydes and ketones in refluxing acetonitrile. 

Using chromium-based oxidants 2,4-Dimethylpentane-2,4-diol chromate(VI) diester, 122 Trimethylsilyl chlorochromate, 327 Using other oxidizing agents... 

Using chromium-based oxidants 2,4-Dimethylpentane-2,4-diol chromate(VI) diester, 122 Trimethylsilyl chlorochromate, 327 Using other oxidizing agents Bis(tributyltin) oxide, 41 Hydrogen hexachloroplatinate(IV)-Copper(II) chloride, 145 4-Methoxy-2,2,6,6-tetramethyl-1 -oxopiperidinium chloride, 183 Osmium tetroxide, 222 Potassium nitrosodisulfonate, 258 Samarium(II) iodide, 270 From alkenes by addition or cleavage reactions... 

An interesting consequence of the fast formation of the chromic ester is that, sometimes, chromium-based oxidants counter-intuitively are able to oxidize quicker alcohols possessing a greater steric hindrance, as the initially fonned chromic ester releases greater tension on evolving to a carbonyl. Thus, axial alcohols are oxidized quicker than equatorial ones with chromic acid.6 The reverse—a somehow expected behavior—is observed, for example in oxidations with activated DMSO.7... 

The following guidelines can help in the election of a certain chromium-based oxidant in the laboratory ... 

In 1946, Jones discovered that secondary alcohols could be efficiently oxidized to ketones by pouring a solution of chromium trioxide in diluted sulfuric acid over a solution of the alcohol in acetone.13 This procedure, which has proved to be quite safe, allows a sufficient contact of the alcohol with chromium oxide derivatives for a reaction to take place. Jones oxidation marked the beginning of the highly successful saga of chromium-based oxidants. 

Sometimes, an alcohol via the corresponding chromate ester may direct a chromium-promoted epoxidation of an aJkcne. This side reaction, which can happen with other chromium-based oxidants,83 depends on very exacting stereoelectronic factors to occur. 

Mechanistic evidences show that PDC, similar to other chromium-based oxidants, operates via an intermediate chromate ester that evolves to a carbonyl compound in the rate-determining step.125... 

Similar to other chromium-based oxidants, the action of PDC on alcohols, bearing substituents at the a position and able to support stable carbocations, may result in a carbon-carbon bond breakage from the intermediate chromium ester. 

Other Chromium-Based Oxidants 1.6.1. Chromic Acid... 

Chromium-based oxidants tend to react quicker with unsaturated alcohols, although the difference of oxidation speed with saturated alcohols is normally not sufficient for synthetic purposes. Nevertheless, the chromium-based reagent pyridinium dichromate (PDC) possesses a mildness and, therefore, a relative greater selectivity that allows its occasional employment for selective oxidations of allylic and benzylic alcohols.134... 

The periodinane (10) may also be prepared from o-iodobenzoic acid by oxidation with potassium bromate and then treatment with acetic anhydride18 (see Expt 6.36 for detailed formulation). It should be noted that the organic derivatives of pentacoordinate iodine(v) are termed periodinanes.18b This compound (the systematic name is l,l,l-triacetoxy-2,l-benzoxiodol-3(3//)-one) has found use as an oxidant of primary alcohols to aldehydes and alicyclic ketones to secondary alcohols it is claimed to have advantages over the chromium-based oxidation reagents. 

Chromium-based oxidants are probably the most widely used of all oxidizing agents. Over the years they have been continually developed and modified to overcome the typical problems that occur during oxidation and to accept wider ranges of substrates with improved selectivities. They have been accepted readily by synthesis chemists since they are easy to handle and are often off the shelf reagents . However, they are not without their problems worit-up can be problematical overoxidation can occur, and, at all times, removal of the product from toxic chromium contaminants is a concern, especially with respect to large scale preparations. In an attempt to circumvent these problems the trend has been to develop the use of catalytic and/or supported reagents. Hiis review is concerned for the most part with the ai lica-tions and limitations of more recent chromium(VI) oxidants. Several other comprehensive reviews have appeared in this area and should be consulted for more detailed descriptions of older methods, chro-mium(V) oxidants, mechanism of oxidation and for typical experimental procedures. 

Quinolinium dichromate (17) shows the solubility profile common to most chromium based oxidants—sparingly soluble in chlorinated solvents, but more soluble in more polar solvents. It has been mainly used in dichloromethane at reflux, or in DMF at 30 C to oxidize primary alcohols. Secondary alcohols are also oxidized reasonably well. 

Chromium-based oxidations aie reliable and well established, but the toxicity associated with chromium salts have meant that they are generally considered the... 

Because the use of a stoichiometric amount of chromium-based oxidant is not desirable from the standpoint of green chemistry, a number of approaches to employ catalytic amount of oxidant in combination with a stoichiometric amount of reoxidant have been reported. 

Oxidation of the above substrates were characterized by short reaction times and efficient conversion to expected products. Over-oxidation of substrates was not observed. Furthermore, the work up procedure was much easier and straightforward than other oxidation methods such as PCC, PDC, or Swem oxidation. The Dess-Martin reagent is also less toxic than the chromium-based oxidizing reagents. 

Several new chromium based oxidants have been... 

A useful application of chromium-based oxidants, especially pyridinium chlorochromate, is in the conversion of allylic tertiary alcohols to their transposed a,(3-unsaturated ketones. For example, treatment of the allylic alcohol 24 with PCC gave the a,p-unsaturated ketone 25 (6.23). The reaction is thought to proceed by rearrangement of the chromate ester of the allylic alcohol to give a new allyl chromate ester that is oxidized to the ketone. 

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