Oxonium ions superelectrophiles

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. 

There have been several distonic superelectrophiles described in the literature that are oxonium-centered dications. For example, a series of diols were solvated in FSOsH-SbFs-SC and the dicationic species were persistent at —80°C.77 Upon warming to 25°C, the bis-oxonium ions undergo rearrangement to the more stable carboxonium monocations (eq 77). Barring a concerted mechanism, the transformations are thought to involve the carbo-oxonium dications (i.e., 225) and concomitant hydride shifts. Interestingly, 2,5-hexanediol ionizes in superacid, and with warming the cyclic oxonium ion (229) is formed (eq 78). 

Several gitonic superelectrophiles have been reported having closely oriented oxonium and carboxonium ion centers, some of which may be considered 1,3-dications. A series of hydroxy-substituted carboxylic acids were studied in FSOsH-SbFs in solution and the oxonium-carbonium dications could be directly observed at low temperature.57 In the case of lactic acid, dication 147 is a persistent ion at — 80°C, but at temperatures above — 60°C, formation of the diprotonated lactide (148) is observed (eq 48). 

The electrophilicity index also accounts for the electrophilic activation/deactivation effects promoted by EW and electron-releasing substituents even beyond the case of cycloaddition processes. These effects are assessed as responses at the active site of the molecules. The empirical Hammett-like relationships found between the global and local electrophilicity indexes and the reaction rate coefficients correctly account for the substrate selectivity in Friedel-Crafts reactions, the reactivity of carbenium ions, the hydrolysis of esters, the reactivity at the carbon-carbon double bonds in conjugated Michael additions, the philicity pattern of carbenes and the superelectrophilicity of nitronium, oxonium and carboxonium ions. This last application is a very promising area of application. The enhanced electrophilicity pattern in these series results from... 

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