Peroxy trifluoroacetic acid

Surprisingly, there are very few examples of successful fV-oxidation of pyrazoles. Simple fV-alkylpyrazoles generally do not react with peracids (B-76MI40402,77JCS(P1)672). The only two positive results are the peracetic acid (hydrogen peroxide in acetic acid) transformation of 1-methylpyrazoIe into 1-methylpyrazole 2-oxide (268) in moderate yield and the peroxy-trifluoroacetic acid (90% hydrogen peroxide in trifluoroacetic acid) transformation of 5-amino-l-methylpyrazoIe into l-methyl-5-nitropyrazoIe 2-oxide (269). 

The procedure described here for the preparation of peroxy-trifluoroacetic acid in methylene chloride has been carried out by the submitters several hundred times without incident and is believed to be the best available. However, it has been pointed out that suspensions of 90% hydrogen peroxide in methylene chloride can be detonated by impact under certain conditions. Accordingly, the use of the recommended safety screen is imperative, and the preparation should not be scaled up without special precautions. The homogeneous solution of peroxytrifluroacetic acid, once obtained, is undoubtedly much safer to handle than the suspension of hydrogen peroxide in methylene chloride. The latter suspension is not transferred, however, and exists for only a brief time period during the preparation. 

Oxidation of aminopyridines with Caro s acid gives nitropyridines the use of peroxy-trifluoroacetic acid leads to nitropyridine JV-oxides (60JOC1716). Oxidation with alkaline hypochlorite converts aminopyridines into symmetrical azopyridines, and azoxypyridines are formed by oxidation with persulfate (59YZ549). 

Biological oxidation of a 2-aminoimidazole gives poor yields (<38%), and none at all with l-alkyl-2-aminoimidazoles. Nor will oxidation with peroxy-trifluoroacetic acid work It is, however, satisfactory for the oxidation of 4-aminoiniidazoles (which are usually rather unstable compounds). ITie most common way of making 2-nitroi midazoles is from the diazonium fluoro-borates subjected to the Gattermann reaction (see Section 7.3). Yields vary from 20 to 50% [6, 7], and again are dependent on the availability of the 2-aminoimidazoles (see Section 8.2.2). 

Among the indirect methods of preparing aliphatic nitro compounds are the reaction of various alkyl halides with silver nitrite or sodium nitrite, the oxidation of oximes and amines with peroxy-trifluoroacetic acid, and the potassium permanganate oxidation of tertiary amines. 

On the basis of theoretical studies by Bach and co-workers,17 it was found that the nucleophilic 71-bond of the alkene attacks the 0-0 cr-bond in an Sn2 fashion with displacement of a neutral carboxylic acid. There are, however, some mechanistic anomalies. For example, a protonated peracid should be a much more effective oxygen transfer agent over its neutral counterpart, but experiments have shown only modest rate enhancements for acid catalysed epoxidation. Early attempts to effect acid catalysis in alkene epoxidation where relatively weak acids such as benzoic acid were employed proved unsuccessful.18 The picture is further complicated by contradictory data concerning the influence of addition of acids on epoxidation rates.19 Trichloroacetic acid catalyses the rate of epoxidation of stilbene with perbenzoic acid, but retards the rate of a double bond containing an ester constituent such as ethyl crotonate.20 Recent work has shown that a seven-fold increase in the rate of epoxidation of Z-cyclooctene with m-chloroperbenzoic acid is observed upon addition of the catalyst trifluoroacetic acid.21 Kinetic and theoretical studies suggest that the rate increase is due to complexation of the peroxy acid with the undissociated acid catalyst (HA) rather than protonation of the peroxy acid. Ab initio calculations have shown that the free energy of ethylene with peroxy-formic acid is lowered by about 3 kcal mol-1 upon complexation with the catalyst.21... 

Esters of phenols are obtained by treatment of aromatic hydrocarbons with peroxy acids [292], organic peroxides [1119, 1120], and lead tetraacetate or tetrakis(trifluoroacetate) [435, 449]. Benzene treated with tri-... 

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