Carbocation stereoelectronic stabilization

An immediate presumption that the more stable allyl ion will be formed overlooks the stereoelectronic aspects of the reaction. Protonation at the center carbon without rotation of one of the terminal methylene groups leads to a primary carbocation which is not stabilized by resonance, because the the adjacent it bond is orthogonal to the empty p orbital. 

As the oxidative carbon-carbon bond breakage of alcohols, leading to a stable carbocation, depends not only on the stability of the resulting carbocation but also on very exacting stereoelectronic factors, many cases are known in which alcohols are successfully oxidized to ketones, regardless of apparently easy oxidative carbon-carbon bond breakages. In fact, in synthetic experimental practice, it is recommended not to fail in trying a Jones oxidation because of fear of such side reactions. 

Substituent effects on stability of a carbocation can be estimated from the following isodesmic reactions (Figure 6.43, performed at HF/6-31G level). Again, stabilization energies have a clearly manifested stereoelectronic component as illustrated by differences in stabilization energies imposed by substituents in the eclipsed and perpendicular conformations. 

Cooperativity/competUion among acceptors (armed/disarmed carbohydrates) Cleavage of the C-X bond at the anomeric position proceeds via evolution of the anomeric n o C-X interaction that culminates in the oxocarbenium ion. This carbocation is so strongly stabilized by n j- p interaction that this interaction is commonly described as a dative it-bond. Because oxacarbenium ions are strongly stabilized, their formation is relatively facile. In cross-coupling glycosylalion reactions, however, it is beneficial to differentiate the reactivity of two saccharides for the selectivity reasons. The competition and cooperativity between stereoelectronic effects can be used to fine-tune the reactivity of anomeric systems. 

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