Carbocations oxygen substituent effects

Carbocations, as we learned in Chapter 4 of Part A, can readily rearrange to more stable isomers. To be useful in synthesis, such reactions must be controlled and predictable. This goal can be achieved on the basis of substituent effects and stereoelectronic factors. Among the most important rearrangements in synthesis are those directed by oxygen substituents, which can provide predictable outcomes on the basis of electronic and stereoelectronic factors. 

The substitution of an a-oxygen by an a-sulfur or an a-selenium results in a decrease in ks/kp for partitioning of oxocarbenium ions between addition of solvent and deprotonation. The largest effect is observed for the a-selenium substitution. These changes are probably the result of several different effects on carbocation reactivity, each of which contributes to the observed substituent effect on kjkp. 

Effects of oxygen substitutents in an aromatic ring upon an exocyclic rather than endocyclic carbocation charge center have also been measured. The possibility of comparing HO, MeO, and O substituent effects for the benzylic cations is provided by recent studies of quinone methides, including the unsubstituted / -quinone methide 23, which may be considered as a resonance-stabilized benzylic cation with a /xoxyanion substituent. 

Table 4 Effects of oxygen substituents on stabilities of cyclic benzylic carbocations...

In principle, the effects of oxygen substituents are consistent with CF2 being a relatively stable carbene despite the corresponding carbocation being quite unstable. This is understandable if the dipolar structure produced by resonance interaction in the carbene (52) is compensated by an inductive back donation of electrons in the cr-bonds. In the cation (53), back donation accentuates rather than compensates charge separation arising from resonance... 

The reactivity trends observed for both the nucleophilic addition and cycloaddition of alkene radical cations parallel trends observed for the reactions of carbocations with nucleophiles and alkenes. However, the observed variations in reactivity towards oxygen - and substituent effects on the competition between addition of methanol and the neutral monomer for diphenylethene radical cations indicate that variations in both spin and charge density are important in determining the overall reactivity patterns. It is clear that further experimental and theoretical studies are required to provide a detailed model for understanding and ultimately predicting the reactivity of radical cations. 

The alkoxy substituent allows a delocalization of the positive charge. If the substituent were not present (e.g., in ethylene), the positive charge would be localized on the single a-carbon atom. The presence of the alkoxy group leads to stabilization of the carbocation by delocalization of the positive charge over two atoms—the carbon and the oxygen. Similar delocalization effects occur with phenyl, vinyl, and alkyl substituents, for example, for styrene polymerization ... 

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