Active oxonium

Pure enantiomer optically active alcohol is protonated to optically active oxonium ion. 

The alkyne is protonated by the acid, to give a resonance stabilised cation intermediate, which is intercepted by water to give an enolic intermediate. Tautomerisation gives an activated oxonium cation, which is attacked by water. This initiates ester hydrolysis, leading to expulsion of ethanol, giving the carboxylic acid product. 

Carbonyl groups readily interact with Lewis acids to generate an activated oxonium ion, which activates the carbonyl group to nucleophilic addition by a suitable nucleophile. 

The authors believe that the transformation outlined above proceeds via the mechanism depicted in Scheme 35 where an oxonia-Cope rearrangement applied to 134 is followed by an intramolecular allylsilane addition to activated oxonium ion 135. The stereoselectivity in the generation of 132 is believed to be a result of chair transition states in both the oxonia-Cope reaction and the allylsilane cyclization. 

Similar to oxonium ions, our studies of sulfonium ions also showed protosolvolytic activation in superacids to give sulfur superelectrophiles. The parent sulfonium ion (HjS ), for example, gives H4S (diprotonated hydrogen sulfide) in superacids. 

The plausible cause of shape selectivity to DME in H-mordenite is the presence of the active protons wittun the side-pockets of mordenite that are accessible only to methanol. The protonated methanol molecule, a methyl oxonium ion, undergoes rear-attack by a second methanol molecule entering the side-pocket from the main channel... 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

Alkylation with Oxonium Salts Oxonium salts are the most efficient alkylating agents and can react with both activated and nonactivated AN, including nitromethane and 2-nitropropane (Scheme 3.4). 

The bioactivity of imino sugars is due to the fact that they inhibit glycosidases. Once protonated, the imino sugar mime the oxonium ion intermediate of the reaction catalysed by those enzymes (Fig. 32), and therefore binds the active site. 

The activation of NPGs during a glycosylation reaction (Scheme 5.7a) depends on electrophilic addition to the olefin (—>111), followed by intramolecular displacement by the anomeric oxygen to form the oxonium species IV. Trapping with a glycosyl... 

Common Anomeric Groups, Common Activators Both conformational and electronic factors have been exploited to obtain selectivity in one-pot glycosylations using common anomeric groups and activators. Selectivity results from differences in the stability of oxonium ion intermediate between the two prospective glycosyl donors. 

As far as the polymerisation of heterocyclic monomers is concerned, the situation is qualitatively similar, but quantitatively different. As a model for the active species in oxonium polymerisations, Jones and Plesch [10] took Et30+PF6 and found its K in methylene dichloride at 0 °C to be 8.3 x 10"6 M however, in the presence of an excess of diethyl ether it was approximately doubled, to about 1.7 x 10 5 M. This effect was shown to be due to solvation of the cation by the ether. Therefore, in a polymerising solution of a cyclic ether or formal in methylene dichloride or similar solvents, in which the oxonium ion is solvated by monomer, the ion-pair dissociation equilibrium takes the form... 

Although this work is still incomplete, it shows that some tertiary oxonium ions are formed in the reaction, but that by far the greater part of the active species are secondary oxonium ions. The origin of the tertiary oxonium ions, which yield the involatile phenyl ether by reaction with C6H5CT, is not at all clear at present. Some may be formed from an impurity in the monomer and others may arise from a slow side-reaction. 

Or one can say that even if the equilibrium constant for the above reaction is negligibly small, the unsolvated oxycarbenium ion (XII) is a part of a relatively unimportant canonical form of the tert.-oxonium ion, which implies a certain probability that the oxycarbenium ion may react as such by detaching itself from the ether and finding some other basic site. The important point, however, is that under the normal conditions under which DCA are polymerised the attempts to distinguish experimentally between oxycarbenium ions and tert.-oxonium ions as the active species appear at present to be quite pointless. 

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