Bonds oxyfunctionalization

Direct insertion into an X—H a bond constitutes the highlight of dioxirane chemistry . Besides the insertion of a dioxirane oxygen atom into an alkane acH bond, for practical purposes a most valuable oxyfunctionalization, also the more facile insertion into the asiH bond is known, a convenient and chemoselective method of preparing silanols. 

For the oxidation of alkanes, the reactivity order follows the sequence primary < secondary < tertiary < benzylic < allylic C—H bonds. The readily accessible and economical DMD is suitable for most substrates, although this oxyfunctionalization may... 

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 novel approach to achieve high regioselectivity was recently developed by Yang and coworkers, in which the in-situ-generated dioxirane functionality, contained within the substrate, oxidizes the secondary 5c-h bond rather than the more reactive tertiary yc-H bond, due to a favorable concerted transition state (equation 31). A limitation of this method, however, is that the ketone unit has to be incorporated into the substrate molecule at the appropriate position, normally a rather unpractical and cumbersome task. Moreover, such a process is necessarily stoichiometric in practice and, thereby, it lacks appeal since nowadays catalytic enantioselective oxyfunctionalizations are in vogue ... 

The CH oxidation is also useful in the synthesis of special synthetic targets that are difficult to be achieved by means of conventional oxidants. An early example is the conversion of adamantane to its tetrahydroxy derivative by the exhaustive and selective oxyfnnc-tionalization of all four tertiary C—H bonds, which are contained within the adamantane skeleton. More recently, Curd and coworkers prepared fenestrindane monoalcohol from fenestrindane in good yield by direct TFD oxidation (equation 34), unquestionably a valuable oxyfunctionalization in view of the mild oxidation conditions. ... 

As already briefly mentioned, the oxygen-atom insertion into Si—H bonds of silanes constitutes a selective method for the chemoselective preparation of silanols, which has been much less studied compared to the CH oxidation. This unique oxyfunctionalization of silanes is also highly stereoselective (equation 35) since, like the CH insertions, it proceeds with complete retention of configuration. A novel application of the SiH insertion process is the synthesis of the unusual iron complex with a silanediol functionality, in which selectively both Si—H bonds of the silicon atom proximate to the iron ligand are oxidized in the silane substrate (equation 36). ... 

A new convenient polymer modification for the conversion of the Si—H to Si—OH by the selective oxidation of the Si—H bond by dimethyldioxirane has been described. The oxyfunctionalization of the silane precursor polymers proceeded rapidly and quantitatively and can be applied to the synthesis of a wide variety of novel silanol polymers with specific properties from the corresponding precursor polymers containing Si—H functional groups. Control over the properties of these silanol polymers, such as reactivity and self-association of silanols, was realized through the placement of different substitute groups bonded directly to the silicon atom and by the variation of silanol composition in a copolymer. These novel silanol polymers with a... 

With either isolated or in-situ-generated DMD or TFD, numerous substrates with different types of C-H bond have been oxyfunctionalized. Some typical examples, mostly hydroxylations, are summarized in Table 1. 

The chemoselectivity of the dioxirane oxyfunctionalization usually follows the reactivity sequence heteroatom (lone-pair electrons) oxidation > JT-bond epoxida-tion > C-H insertion, as expected of an electrophilic oxidant. Because of this chemoselectivity order, heteroatoms in a substrate will be selectively oxidized in the presence of C-H bonds and even C-C double bonds. In allylic alcohols, however, C-H oxidation of the allylic C-H bond to a,/ -unsaturated ketones may compete efficaciously with epoxidation, especially when steric factors hinder the dioxirane attack on the Jt bond. To circumvent the preferred heteroatom oxidation and thereby alter the chemoselectivity order in favor of the C-H insertion, tedious protection methodology must be used. For example, amines may be protected in the form of amides [46], ammonium salts [50], or BF3 complexes [51] however, much work must still be expended on the development of effective procedures which avoid the oxidation of heteroatoms and C-C multiple bonds. 

Direct oxidation of benzene to phenol is of great interest not only for its industrial importance, but also from a purely scientific point of view. Apart from many earlier reports [35] on the oxidation of benzene to phenol by hydroxyl radicals generated by the reaction of Fe2+ salt (Fenton reagent) with H202 not much is known about the homogeneously catalysed oxyfunctionalization of aromatic C-H bonds. The lack of studies is largely attributable to the fact that the activation of the C-H bond in benzene is difficult owing to its resonance stability and the reactivity of phenol, which is consecutively oxidized to quinones and other by-products. 

Scheme 1 shows the various types of oxyfunctionalizations which dimethyl-dioxirane is cabable of performing. These include epoxidations of n systems, oxidations of heteroatoms, and insertion into a bonds. Here we will restrict... 

The unusual reactivity of dioxiranes is impressively exhibited in their ability to insert into C — H bonds (Scheme 7) [28]. Thus, tertiary alkanes are oxidized to their respective alcohols [29]. In the example shown, the insertion took place with complete retention of configuration at the chirality center. 1,3-Dicarbonyl derivatives [30] are hydroxylated with high efficiency, but more than likely the intermediary enol is being oxyfunctionalized. Secondary alcohols are transformed into ketones, a specific example is the oxidation of the epoxy alcohol in the rosette [31], In an attempt to epoxidize the hydroxy acrylic ester [22], the epoxy 1,3-dicarbonyl product was obtained, although in low yield in accord with its rather reluctant nature towards oxidation. 

Oxyfunctionalization. Oxygen insertion into unactivated secondary and tertiary C-H bonds of protonated alkylamines can be very efficient with this dioxirane. Hydrogen bonding between the dioxirane and the NHj moiety is important in determining the regiochemistry of the functionalization. 

The reagents are available in two steps frt SbFj [to obtain RfN=C(F)Rf ] and then wnl Oxyfunctionalization of hydrocarbons. tertiary C-H bonds by these neutral and si ceeds smoothly. 

Few studies refer to the oxidation of aromatics. The hydroxylation of benzene to phenol [81] and the oxidation of alkylaromatics to arylcarboxylic acids [82] have been claimed. The oxyfunctionalization of saturated C-H bonds has not been reported. 

Oxyfunctionalization of unactivated C-H bonds. 2,2,3,3-Tetramethylbutane affords a primary alkyl trifluoroacetate in 99% yield when it is oxidized with MefCFjlCOj in the presence of (CFjC0)20 in dichloromethane at 0°. Esters show a remarkable regioselectivity in their oxidation. ... 

Direct Functionalization of C-H Bonds by Dimethyl-dioxirane. The efficient oxyfunctionalization of simple, unactivated C-H bonds of alkanes under extremely mild conditions undoubtedly is one of the major highlights of dioxirane chemistry. ... 

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