Oxidative rearrangements skeletal

Oxidative rearrangement. Corey and co-workers made the suprising discovery that the 3/8,1 la-dihydroxy-A "-pentacyclic triterpenoid (1) on treatment in methylene chloride with a solution of 30% hydrogen peroxide and p-toluenesulfonic acid in t-butanol affords, after acetylation, the 1 la,12 -epoxide, with a rearranged skeletal... 

The bromination and oxidation of securinine have been reported (Scheme 87). Oxidative difunctionalization of 2-amino-4//-pyrans occurs with iodobenzene diacetate and A -chlorosuccinimide reagents. An oxidative rearrangement of [7]helicene by heating with iron(n) chloride and aluminium(III) chloride involves skeletal rearrangement with the loss of helicity (Scheme 88). ... 

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

Both benzothieno[3,2-b]pyridine 5-oxide (31) and thieno[3,2-b 4,5-b ]dipyridine 5-oxide (32) exhibit competitive loss of oxygen either as an atom or as carbon monoxide after initial skeletal rearrangement, e.g. to sulfenate esters (equation 10)18b. These results together with some data for Y-oxides indicate that the presence of an intense [M — 16] + peak is not diagnostic for the latter only. 

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

The structures of the radical anions were confirmed by the following experiment (Scheme 9). The reduction of the ladder polysilanes was monitored by UY-visible-NIR spectroscopy. When the absorption of the ladder polysilanes was completely replaced by the absorption of the radical anions, the sealed tube was opened. The radical anions were immediately oxidized, and the starting ladder polysilanes were recovered in high isolated yields. It is reasonable to conclude that the radical anions of the ladder polysilanes retain the ladder structure, and the Si-Si bond cleavage or skeletal rearrangement does not occur. 

Skeletal catalysts can lose activity over time. This phenomena has been attributed to several causes depending on the application and that include surface fouling with by-products, surface oxidation, and structure rearrangement. 

Steric factors are often responsible for skeletal isomerization in ion-radical states. The simple example in Scheme 6.31 illustrates the effect of steric congestion on activation energy of this kind of isomerization and depicts the transition of 2,2,3,3-tetramethylmethylenecyclopropane into 1,1,2,2-tetramethyltrimethylenemethane cation-radical. The rearrangement is brought about by one-electron oxidation of the substrate and represents an entirely barrierless process. Interestingly, methylenecy-clopropane (bearing no methyl groups) is protected from such a spontaneous collapse by a barrier of 7.4 k J mol l (Bally et al. 2005). 

Non-Kolbe reactions are often favoured by skeletal reaiTangements which generate a more stable carbonium ion. Reaction of the cyclic ketal 22 is driven by formation of a carbonium ion stabilised by the oxygen substituent [114]. Reactions of nor-bomanecarboxylic acids are driven by the norbomane carbonium ion rearrangement [115, 116], Oxidation of adamant-1-ylacetic acid in methanol affords 1-methoxyhomoadamantane via a skeletal rearrangement [117],... 

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