Other Chromium-Based Oxidants

Other Chromium-Based Oxidants 1.6.1. Chromic Acid 

Chromium trioxide in aqueous solution equilibrates with a number of species, and chromic acid, being the most abundant one under acidic conditions (see page 1). Thus, a mixture of chromium trioxide and sulfuric acid is often referred to as a chromic acid solution. Such solution can also be obtained by the action of sulfuric acid on sodium dichromate (Na2Cr207) or potassium dichromate (K2Cr207). 

So far, the most common experimental conditions used for the oxidation of alcohols with chromic acid are the so-called Jones oxidation first described in 1946, in which acetone is used as co-solvent. In fact, the use of chromic acid in the oxidation of alcohols has a long tradition in organic synthesis. As soon as in the 19th century, Beckmann described335 an oxidation of alcohol with aqueous chromic acid, in which no mixing of phases was 

No use of an organic solvent is found in this oxidation, in which chromic acid is generated by the action of sulfuric acid on soditun dichromate. 

In 1961, Brown348 described the oxidation of alcohols, using a two-phase system with aqueous chromic acid and diethyl ether. Brown s oxidation349 has a work-up, facilitated by the reluctance of ether to form emulsions with materials containing chromium, and although not as popular as Jones oxidation, it is used quite often. 

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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. 

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... 

Other chromium-based reagents are also found to oxidize alcohols, following a mechanism like the one depicted above for oxidation with chromic acid.4... 

Tertiary allylic alcohols form a chromate ester that, as it lacks a hydrogen on a to the alcohol, instead of suffering a normal oxidation to ketone rearranges to an enone. This transformation, which can be brought about by other chromium-based reagents, is normally carried out with PCC when it is purposefully sought at (see page 55). 

PDC has a lesser tendency to effect oxidative transposition of allylic alcohols than other chromium-based reagents.163... 

Similar to other chromium-based reagents, kinetic evidence shows that oxidation of alcohols by PCC operates via a chromate ester intermediate that evolves to an aldehyde or ketone in the rate-determining step.194... 

This oxidative transposition of tertiary allylic alcohols into enones or enals is carried out under mild conditions and has ample application in organic synthesis. Although, it can be carried out with other chromium-based reagents (see pages 16 and 35), PCC is the reagent of choice.272... 

When the oxidation of a primary alcohol with PCC results in the formation of an aldehyde, activated with an electron withdrawing group at the a-position sometimes, a stable dimeric hemiacetal is formed that is further oxidized to a dimeric ester.331 This reaction, that can also happen with other chromium-based reagents (see page 42), can be minimized by adjusting the reaction conditions. 

A number of other chromium-based reagents have been developed for allylic oxidation for example that of stnoids by t-butyl hydroperoxide in the presence of a catalytic amount (0.0S-0.S mol equiv.) of chromium trioxide in dichloromethane solution at room temperature (equation 39). Yields vary from 32 to 69%. This modification is useful in terms of cost, operational simplicity and yields. 

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. 

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. 

As an alternative to chromium-based oxidants, chemists have developed other reagents for oxidizing alcohols, several of which are based on chlorodimethylsulfonium ion [(CH3)2SC1 ]. Most commonly, chlorodimethylsulfonium ion is generated under the reaction conditions by the reaction of dimethyl sulfoxide with oxalyl chloride. 

A wide variety of chromium oxide and Ziegler catalysts was developed for this process (61,62). Chromium-based catalysts produce HDPE with a relatively broad MWD other catalysts provide HDPE resins with low molecular weights (high melt indexes) and resins with a narrower MWD (63,64). 

Niobium appears to have a slightly beneficial effect on the oxidation resistance of nickel-chromium-base alloys, although at 1 200°C the addition of niobium to Nichrome (Table 7.18) produces a much higher rate constant. Titanium on the other hand appears to have a slightly deleterious effect on oxidation resistance at low temperatures. 

It was later shown that aziridine reacts over mixtures of zinc and chromium oxides on alumina at 400°C to give the same products as those obtained from mixtures of NH3 and acetylene [221]. Aziridine, which would form by addition of NH3 to acetylene followed by IH (Scheme 4-8), was thus postulated to be an intermediate in the formation of acetonitrile (by dehydrogenation), monoethylamine (by hydrogenation) and all other heterocyclic bases (by ammonolysis and subsequent reactions) [221]. 

In variance with other oxidants, such as the chromium-based ones, no carbon-carbon bond breakage is observed in the Pfitzner-Moffatt oxidation of this 1,2-diol. 

It is known that preparation of the oxidant salt under anhydrous conditions is explosion-prone [1], but during repetition of a supposedly safe preparative method [2], recommended for large-scale use [3], ignition and a violent explosion occurred. Use of more water to dissolve completely the chromium trioxide, and a reaction temperature below 25°C, are recommended [4], During a study by TGA of the thermal degradation of the salt, too-rapid heating of the samples led to explosions with fire [5]. See other DICHROMATE SALTS OF NITROGENOUS BASES, OXIDANTS... 

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