Oxonium concerted mechanism

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). 

The 3+2 cycloreversion of the transient bicyclo[m.3.0]alkan-3-on-2-yl-l-oxonium ylide that was genenrated by the rhodium-catalyzed intramolecular reaction of a tetrahydrofuran substituted diazoketone was found to be stereospecific as illustrated below. The result supports a concerted mechanism <04JOCI331>. 

The regio- and stereo-chemistry of the nucleophilic attack of (5)-trani -hex-3-en-2-ol and (5)-trani-hex-4-en-3-ol on the corresponding O-protonated or -methylated derivatives have been examined in the gas phase at 40 °C and 720 Torr. Firm evidence of various kinds was obtained for the concerted 5ivr2 pathway accompanying the classical 5n2 mechanism. Competition between the two processes is essentially governed by the orienting properties of the oxonium intermediate towards the approaching nucleophile. Many other details were elucidated. 

The mechanism of rhodium-catalysed O—H insertion of diazomethane and diazoacetate with water has been computationally investigated. These DPT calculations have unmasked that rhodium oxonium ylide is largely easier to form than the free ylide. The results have further indicated the preference for an ionic stepwise pathway rather than a concerted one. 

Elimination reactions of primary alcohols occurs by an E2 mechanism in an acid-cataly2ed reaction. First, the acid protonates the oxygen of a primary alcohol to give a primary alkyl oxonium ion. Then, water is lost by an E2 mechanism because a primary carbocation is too unstable to form in an El process. This concerted step resembles the reaction of primary alkyl hahdes with a base. The proton of the alkyl oxonium ion is deprotonated by a Lewis base, which is water in the dehydration of alcohols. The electron pair in the C—H bond moves to form a carbon-carbon double bond, and the electron pair of the C—O bond is retained by the oxygen atom. The reaction with ethanol illustrates the process. In both the El process and the E2 dehydration reaction, the acid serves only as a catalyst. It is regenerated in the last step of the reaction. 

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