Oxidation by dioxiranes

The relative reactivity of a wide series of nucleophiles towards dioxirane, dimethyidioxirane, carbonyl oxide, and dimethylcarbonyl oxide has been examined at various levels of theory. The general trend in reactivity for oxidation by dioxirane was R2S R2SO, R3P > R3N in the gas phase, and R2S R2SO, R3N R3 (R = Me) in solution. A theoretical study of the first oxidation step of [3.2.1]-bridged bicyclic disulfides highlights a highly oriented reaction path was probably responsible for stereoselective attack on the exo face. ... 

Of the organohahdes, only the iodides are prone to oxidation by dioxirane for example, iodobenzene is oxidized by DMD to a mixture of iodosobenzene and iodylbenzene. In contrast, alkyl iodides afford labile primary oxidation products, which eliminate the oxidized iodine functionality to result in aUcenes (equation 23). In such a dioxirane oxidation, the subsequent in-situ reaction of the alkene affords the corresponding epoxides . 

Recently, substantial progress has been registered in regard to the regioselective CH oxidations by dioxiranes. Usually, the regioselectivity of the CH oxidation is mainly governed by the reactivity of the C—H bond for example, in the above-mentioned oxidation of c -decalin , the tertiary C—H bond is selectively oxidized in the presence of the secondary C—H bonds. When the reactivities are similar, the regioselectivity is determined by steric factors. For example, the preferential oxyfunctionalization of the tertiary C—H bond at the C-14 position of the steroid 19 by DMD in the presence of the other tertiary C—H bonds at the C-5, C-8 and C-9 positions is due to steric reasons (equation 30) . ... 

A concerted, spiro-structured, oxenoid-type transition state has been proposed for C-H oxidation by dioxiranes (Scheme 5). This mechanism is based mainly on the stereoselective retention of configuration at the oxidized C-H bond [20-22], but also kinetic studies [29], kinetic isotopic effects [24], and high-level computational work support the spiro-configured transition structure [30-32], The originally proposed oxygen-rebound mechanism [24, 33] was recently revived in the form of so-called molecule-induced homolysis [34, 35] however, such a radical-type process has been experimentally [36] and theoretically [30] rigorously discounted. 

Because C-H bonds are usually less reactive towards dioxirane oxidation than heteroatoms and C-C multiple bonds, it is instructive to give a few general guidelines on the compatibility of functional groups within the substrate to be submitted to oxidative C-H insertion Substances with low-valent heteroatoms (N, P, S, Se, I, etc.), C-C multiple bonds, and C=X groups (where X is a N or S heteroatom) are normally not suitable for C-H insertions, because these functionalities react preferably. Even heteroarenes are more susceptible to dioxirane oxidation than C-H bonds, whereas electron-rich and polycyclic arenes are only moderately tolerant, but electron-poor arenes usually resist oxidation by dioxiranes. N-oxides and N-oxyl radicals are not compatible because they catalyze the decomposition of the dioxirane. Oxygen insertion into Si-H bonds by dioxirane is more facile than into C-H bonds and, therefore, silanes are not compatible. Substance classes normally resistant towards dioxirane oxidation include the carboxylic acids and their derivatives (anhydrides, esters, amides, and nitriles), sulfonic acids and their de-... 

A very nice example of the oxidation by dioxirane lb where other oxidants failed is the regio- and stereoselective oxidation of C3-C4 enol double bond of quinine methide triterpenes pristimerin 76 and tingenone 77 <1996T10667>. 

In general, peroxomonosulfates have fewer uses in organic chemistry than peroxodisulfates. However, the triple salt is used for oxidizing ketones (qv) to dioxiranes (7) (71,72), which in turn are useful oxidants in organic chemistry. Acetone in water is oxidized by triple salt to dimethyldioxirane, which in turn oxidizes alkenes to epoxides, polycycHc aromatic hydrocarbons to oxides and diones, amines to nitro compounds, sulfides to sulfoxides, phosphines to phosphine oxides, and alkanes to alcohols or carbonyl compounds. 

In addition to metal catalyzed oxygenation of nonactivated alkane C-H bonds, oxofunctionalization of C-H bonds can also occur in water by using dioxiranes.20 Alkylketones and alkylketoesters could be regioselectively oxidized at the 5-position of the aliphatic chain by dioxiranes generated in situ by oxone in a mixture of H20/MeCN... 

Aliphatic primary amines are known to be oxidized by dimethyl dioxiranes to various products such as oximes, nitroso dimers, nitroalkanes, nitrones and oxazrridines under various conditions depending upon the oxidation reaction . In contrast, when secondary amines lacking a-hydrogens are allowed to react with Oxone and PTC in buffered acetone solution at 0 °C, nitroxides are obtained in good yields in a few minutes (equation 61) . 

The dioxirane epoxidation of a prochrral alkene will produce an epoxide with either one new chirality center for terminal alkenes, or two for internal aUcenes. When an optically active dioxirane is nsed as the oxidant, expectedly, prochiral alkenes should be epoxi-dized asymmetrically. This attractive idea for preparative purposes was initially explored by Curci and coworkers in the very beginning of dioxirane chemistry. The optically active chiral ketones 1 and 2 were employed as the dioxirane precursors, but quite disappointing enantioselectivities were obtained. Subsequently, the glucose-derived ketone 3 was used, but unfortunately, this oxidatively labile dioxirane precursor was quickly consumed without any conversion of the aUcene . After a long pause (11 years) of activity in this challenging area, the Curci group reported work on the much more reactive ketone... 

Substrates with doubly bonded nitrogen-atom functionalities, e.g. the C=N-R (imino, oxime) group, are usually cleaved by dioxirane to give the corresponding carbonyl product" . A particular case represents the DMD oxidation of the nitronate ions, generated from nitroalkanes" or nitroarenes. For example, the nitronate anion 16 (equation 13) affords initially the cyclohexadienone on oxidation with DMD, which subsequently tau-tomerizes to the phenol as the final product. An exception is the DMD oxidation of an... 

Trivalent phosphorus compounds are more readily oxidized than the corresponding nitrogen derivatives on account of their higher nucleophilicity however, the oxidation of such highly reactive substrates by dioxiranes has been sparsely studied. Only about a handful of examples are available in the literature, such that little may be said about the general trends in reactivity and selectivity. 

Also, potassium superoxide (KO2) decomposes DMD in acetone solution to release singlet oxygen, as has been detected by the characteristic infrared chemiluminescence . Furthermore, a catalytic amount of n-Bu4NI decomposes TFD into oxygen gas and triflu-oroacetone in high yield . Analogous to the Caroate decomposition by ketones, also the catalytic decomposition of peroxynitrite by ketones, e.g. methyl pyruvate, is rationahzed in terms of peroxynitrite oxidation by in-situ-generated dioxirane. ... 

Little has been reported on the oxidation of selenium functionalities by dioxiranes. One case is the oxidation of selenophenes by DMD " at subambient temperatures. Good yields of either the selenophene 1-oxide or 1,1-dioxide may be achieved, which depends on the amount of DMD used (Scheme 12) . These results are similar to those already presented for the sulfur congeners. ... 

In regard to the stereoselectivity of the insertion process, Murray and coworkers have shown that the CH oxidation of substituted cyclohexanes by dioxiranes is, like the already discussed epoxidation, highly stereo-controUed . A specific case is c -decalin, which gives only the cis alcohol, as exemplarily displayed in equation 27. A similar stereoselective retention of configuration was also obtained for frawi-decalin and cis- and frawi-dimethylcyclohexanes"°. In fact, complete retention of configuration was demonstrated in the CH oxidation of chiral alkanes ". For example, the optically active (f )-2-phenylbutane was converted by either DMD or TFD" to (5 )-2-phenylbutan-2-ol (equation 28) without any loss of the enantiomeric purity (ep) in the product. 

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