Oxonium intermediates

Entry 4 shows that reaction of a secondary 2-octyl system with the moderately good nucleophile acetate ion occurs wifii complete inversion. The results cited in entry 5 serve to illustrate the importance of solvation of ion-pair intermediates in reactions of secondary substrates. The data show fiiat partial racemization occurs in aqueous dioxane but that an added nucleophile (azide ion) results in complete inversion, both in the product resulting from reaction with azide ion and in the alcohol resulting from reaction with water. The alcohol of retained configuration is attributed to an intermediate oxonium ion resulting from reaction of the ion pair with the dioxane solvent. This would react until water to give product of retained configuratioiL When azide ion is present, dioxane does not efiTectively conqiete for tiie ion-p intermediate, and all of the alcohol arises from tiie inversion mechanism. ... 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

The reaction starts of with a protonation - use the catalyst. Resist the urge to protonate the 4-hydroxyl, but go for the one at position 1 that has the added functionality of the hemiacetal linkage. It is going to be the more reactive one. Protonation is followed by loss of water as leaving group. The intermediate oxonium cation shown is actually a resonance form of the simpler carbocation now you can see the role of the adjacent oxygen. The reaction is completed by attack of the nucleophile, the 4-hydroxyl of another molecule. This is not special, but is merely another version of the hemiacetal synthesis done in part (a). 

Bohmeioser and Jenkner1841 noted that 1,4-dioxan formation r.m involve participation by one of two isolable intermediate oxonium complexes. The first is a fairly stable bisoxonium complex of , 4-clioxiui itself, whereas the seoond appears to be a very labile bisoxonium... 

Formation of the inner oxonium salt may he preceded by an intermediate oxonium state (Eq. 953) of the type considered above Cleavage of the oxonium ling is then accomplished by nucleophilic... 

Terpenoid alcohols, such as 25, are cyclized in superacids (FSO3H/SO2) under amixture of kinetic and thermodynamic control. Intermediate oxonium species were identified by 13C NMR59. 

In many cases, the main step in the syntheses of trialkyloxonium salts is the alkylation of a dialkyl ether with a reactive intermediate oxonium ion formed in situ. Thus, the most widely used method for preparing trialkyloxonium tetrafluoroborates by the reaction of epichlorohydrin and BF3 is based on the intermediacy of the inner oxonium salt 3291 95 97 [Eq. (4.19)]. 

The method has been extended by the use of reactive intermediate oxonium ions such as sulfonyl cation-ether adducts, dialkyloxycarbenium salts with ether, and so on. The more reactive dialkyl halonium ions also readily alkylate dialkyl ethers to the corresponding oxonium ions."... 

Treatment of 1,2-C-methylene carbohydrates, prepared by cyclopropanation of unsaturated sugars, with Lewis acids and trapping of the intermediate oxonium ion with a nucleophile is a convenient route to 2-substituted-2,3,6,7-tetrahydrooxepines <95TL683i>. Thus rearrangement of (55) with TMSOTf in acetonitrile in the presence of MeCN affords (56). The trapping nucleophile may be a substituent of the original carbohydrate, as demonstrated by the conversion of (57) into the bridged bicyclic oxepine (58). 

The strongly electrophilic sulfur trioxide is then added to the hydroxyl group and the intermediate oxonium ion is decomposed to sulfate ester and proton ... 

The intermediate oxonium ion.is delocalized and achiral. If a single enantiomer of the starting material is used, racemic product is formed through this achiral intermediate. Attack at one carbon atom gives one enantiomer attack at the other gives the mirror image. 

A closely related reactive oxonium ion can be prepared by Lewis-acid-catalysed breakdown of the corresponding acetal. Alternatively, especially if the acetal is at least partly a silyl acetal, the same oxonium ion can be produced in situ using yet more silicon in the form of TMSOTf as the Lewis acid catalyst. All these intermediate oxonium ions act as powerful electrophiles towards allyl silanes producing homoallylic alcohols or ethers. 

When a glycosidase enzyme cleaves an O-glycoside, we should expect a simple general acid-catalysed first step followed by fast addition of water to the intermediate oxonium ion, essentially the same mechanism as is shown by the chemical reaction (Chapter 13). 

The first step of acid-catalyzed ether cleavage is protonation of the ether oxygen to give an intermediate oxonium ion, which collapses to form an alcohol and a tertiary carbocation. The carbocation then loses a proton to form an alkene, 2-methylpropene. This is an example of E elimination. The acid used for cleavage is often trifluoroacetic acid. 

Formation of a Carbocation. These can be formed in a number of ways (see p. 247), but one of the most common methods when a rearrangement is desired is the acid treatment of an alcohol to give 2 from an intermediate oxonium ion. These two steps are of course the same as the first two steps of the SnIcA or the El reactions of alcohols. 

The CH2CH2OH group in the starting material suggests the addition of PhLi or PhMgBr to eth. oxide. The next reaction is like acetal formation with the intermediate oxonium ion capture chloride. 

On the other hand, in the case of pentopyranose, the 1,4-anfi-adduct was preferentially obtained (O Scheme 29), due to the preferred conformation of the intermediate oxonium cation derived from the glycal [46,51a]. 

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