Electrophilic addition rearrangement

Hydroboration-oxidation (Sections 6.11-6.13) This two-step sequence achieves hydration of alkenes in a stereospecific syn manner, with a regiose-lectivity opposite to Markovnikov s rule. An organoborane is formed by electrophilic addition of diborane to an alkene. Oxidation of the organoborane intermediate with hydrogen peroxide completes the process. Rearrangements do not occur. 

Evidence for the Mechanism of Electrophilic Additions Carbocation Rearrangements... 

What evidence is there to support the carbocation mechanism proposed for the electrophilic addition reaction of alkenes One of the best pieces of evidence was discovered during the 1930s by F. C. Whitmore of the Pennsylvania State University, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HC1 with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the "expected" product, 2-chloro-3-methylbutane. 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

In the first step an S03 molecule is inserted into the ester binding and a mixed anhydride of the sulfuric acid (I) is formed. The anhydride is in a very fast equilibrium with its cyclic enol form (II), whose double bonding is attacked by a second molecule of sulfur trioxide in a fast electrophilic addition (III and IV). In the second slower step, the a-sulfonated anhydride is rearranged into the ester sulfonate and releases one molecule of S03, which in turn sulfonates a new molecule of the fatty acid ester. The real sulfonation agent of the acid ester is not the sulfur trioxide but the initially formed sulfonated anhydride. In their detailed analysis of the different steps and intermediates of the sulfonation reaction, Schmid et al. showed that the mechanism presented by Smith and Stirton [31] is the correct one. 

The electrophilic addition of bromine to benzonorbornadiene was first reported by Wittig and Knauss to yield a dibromide (ref. 3). At about the same time, the related addition of bromine to a diacetoxy derivative (substituted in the benzene ring) of 1, which proceeded with rearrangement, was described (ref. 4). 

When carbocations are involved as intermediates, carbon skeleton rearrangement can occur during electrophilic addition reactions. Reaction of f-butylethylene with hydrogen chloride in acetic acid gives both rearranged and unrearranged chloride.5... 

In the synthesis in Scheme 13.46, a stereoselective aldol addition was used to establish the configuration at C(2) and C(3) in Step A. The furan ring was then subjected to an electrophilic addition and solvolytic rearrangement in Step B. 

Acids that have weakly nucleophilic anions, e.g. HS04e from dilute aqueous H2S04, are chosen as catalysts, so that their anions will offer little competition to H20 any R0S03H formed will in any case be hydrolysed to ROH under the conditions of the reaction. Rearrangement of the carbocationic intermediate may take place, and electrophilic addition of it to as yet unprotonated alkene is also known (p. 185). The reaction is used on the large scale to convert cracked petroleum alkene fractions to alcohols by vapour phase hydration with steam over heterogeneous acid catalysts. Also under acid catalysis, ROH may be added to alkenes to yield ethers, and RCOzH to yield esters. 

Glycals can be glycosylated not only directly via electrophilic addition, cyclo addition, nucleophilic addition and rearrangement reactions but also indirectly by conversion into a range of other glycosyl donors. One of the most important classes of these glycal-derived donors, the 1,2-anhydro sugars, is discussed in detail in Section 1.3. 

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