SELECTIVE CATION REARRANGEMENTS

In 2001, Albrecht Berkessel and Nadine Vogl reported on the Baeyer-Villiger oxidation with hydrogen peroxide in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as solvent in the presence of Brpnsted acid catalysts such as para-toluenesulfonic acid (equation 85) . Under these conditions cyclohexanone could be selectively transformed into the corresponding lactone within 40 min at 60 °C with a yield of 92%. Mechanistic investigations of Berkessel and coworkers revealed that this reaction in HFIP proceeds by a new mechanism, via spiro-bisperoxide 234 as intermediate, which then rearranges to form the lactone. The study illustrates the importance of HFIP as solvent for the reaction, which presumably allows the cationic rearrangement of the tetroxane intermediates. 

Alkyl and hydrogen shifts are normally fast on the NMR time scale down to -150°C. Selected degenerate rearrangements have been discussed in the preceding paragraph. Additional examples include the 2-butyl cation (34), in which the secondary-secondary hydrogen shift is faster than 104 sec 1at -130°C (AG <5.5 kcal/... 

In reactions that include C—H bond activations, the fate of the reactive intermediates (cations, radical cations, or radicals) formed by an enzyme varies greatly depending on its identity. Reactions after the initial C—H cleavage step may include radical recombinations, radical or cationic rearrangements, and many others. However, the selectivity of C—H oxidation in enzymes is typically determined by the arrangement of the substrate in the enzymatic active site. As such, substrate over-oxidations or nonselective C—H oxidations are rarely observed. ... 

Scheme 4.1 includes examples of oxymercuration reactions. Entries 1 and 2 illustrate the Markovnikov orientation under typical reaction conditions. The high exo selectivity in Entry 3 is consistent with steric approach control on a weakly bridged (or open) mercurinium ion. There is no rearrangement, indicating that the intermediate is a localized cation. 

Butene as the feed alkene would thus—after hydride transfer—give 2,2,3-TMP as the primary product. However, with nearly all the examined acids, this isomer has been observed only in very small amounts. Usually the main components of the TMP-fraction are 2,3,3-, 2,3,4-, and 2,2,4-TMP, with the selectivity depending on the catalyst and reaction conditions. Consequently, a fast isomerization of the primary TMP-cation has to occur. Isomerization through hydride- and methyl-shifts is a facile reaction. Although the equilibrium composition is not reached, long residence times favor these rearrangements (47). The isomerization pathways for the TMP isomers are shown schematically in Fig. 3. 

The experiments indicate that WD can have a major effect on reactions that occur at the mineral-water interface. This process can rearrange clay 2 1 layers into more stable configurations, can substantially alter smectite selectivity for competing exchange cations, and can weather sparingly-soluble K-minerals in the presence of smectite. 

Thus, fluorination of 1,3-dienes proceeds through an allylic ion, while weakly bridged halonium ions are the intermediates in chlorination and bromination of dienes (vide infra). Furthermore, starting from the experimental evidence that 13 is produced under kinetic conditions and not from subsequent rearrangement of the 1,2- and 1,4-adducts, the authors suggested that 13 arose from rearrangement of the allyl cation intermediate, 17. Consistent with an open ion pair intermediate is also the stereoselective formation of the threo isomer from both 1,3-pentadienes, as well as the preference for the addition to the 1,2-bond observed in the reaction of both isomeric pentadienes. This selectivity may indeed... 

The thermodynamic stabilities of phenonium ions have been determined based on bromide-transfer equilibria in the gas phase and, depending on the substituents, the bridged species (1) has been proposed as an intermediate or transition state on the potential-energy surface for the 1,2-aryl rearrangement of triarylvinyl cations (see Scheme 1). Phenonium ion (3) has been presented as an intermediate to account for the fact that lactonization of methyl 4-aryl-5-tosyloxy hexanoate (2) produces y-lactone (4) selectively under thermodynamic conditions, but affords 5-lactone (5) preferentially under kinetic conditions. It has been shown that anodic oxidation of frany-stilbene in alcohols in the presence of KF or BU4NBF4 is accompanied by its electro-oxidative rearrangement into diphenylacetaldehyde acetals. The mechanism outlined in Scheme 2 has been proposed" for the transformation. 

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