Chromium reagents dimethyl sulfoxide

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... 

The above discussion has concentrated upon the reagents used, but it is equally of value to comment on the substrate, particularly in reactions for which other oxidation methods have been reported to fail. A good example is the oxidation of the iron-carbonyl complex (31) to the ketone (32 equation 14). The use of dimethyl sulfoxide activated with sulfur trioxide-pyridine complex gave a 70% yield of the product, in contrast to the use of the Pfitzner-Moffatt procedure (dimethyl sulfoxide-DCC) or the chromium... 

Although many oxidizing reagents remove the chromium tricarbonyl group, benzylic alcohols can be oxidized to benzaldehydes using dimethyl sulfoxide with acetic anhydride, trifluoroacetic anhydride, or sulfurtrioxide with minimal decomplexation. Asymmetric oxidation of aUcylthio-substituted complexes can be achieved using titanium tetraisopropoxide and an optically active tartrate ester (Scheme 108). Dimethyloxirane can also be used to oxidize sulfides to sulfoxides. 

In contrast, the chemistry of the oxidation of a primary alcohol to an aldehyde differs sharply from the oxidation of an aldehyde to a carboxylic acid (case (b)). Advantage, in this case, must be taken of the difference in the mechanisms of these steps. Among the reagents which can effectively oxidize alcohols and remain rather inert toward aldehydes are pyridinium chlorochro-mate (a chromium trioxide-hydrogen chloride complex of pyridine) or dimethyl sulfoxide-Lewis acid. 

Because of the toxicity of chromium-based reagents, other methods for the oxidation of alcohols have been developed. One of the most widely employed methods, called the Swem oxidation, uses dimethyl sulfoxide [(CH3)2SO], oxalyl chloride [(COCl)2], and triethylamine. Since the reaction is not carried out in an... 

In polar media, NBS effectively oxidizes primary and secondary alcohols to carbonyl compounds via hypobromite or alkoxysuccinimide intermediates. Although this transformation is more commonly effected by the use of chromium reagents or activated Dimethyl Sulfoxide, the most notable application of NBS and related reagents lies in its selectivity for the oxidation of axial vs. equatorial hydroxy groups in steroid systems (see N-Bromoacetamide). Often, a single secondary alcohol may he converted to the ketone in the presence of many other alcohol groups. 

Several neutral oxidation reactions do not involve chromium(VI), but readily convert an alcohol to a ketone or an aldehyde. One such reaction uses a reagent derived from the reaction of dimethyl sulfoxide (DMSO, 37) and oxalyl chloride (38 see Chapter 16, Section 16.7). When 2-butanol is mixed with these reagents at -60°C, dimethyl sulfide (MeSMe) is observed as a product and 2-butanone (43) is isolated in 78% yield. ... 

Oxidations. WChlorosuccinimide-dimethyl sulfide (NCS-DMS) is one of several reagents for converting alcohols to alkoxydtmethylsulfonium salts, which in the presence of base convert to the corresponding carbonyl compounds via intramolecular proton transfer and loss of dimethyl sulfoxide. These oxidations are among the mildest and most selective for conversions of alcohols to aldehydes and ketones. This reaction does not suffer the overoxidation to acids or the carbon-carbon bond cleavages which are often encountered in chromium(VI) or manganese(VII) oxidations (eq 1). ... 

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. 

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