Carbocations, benzylic heteroatoms

Just as N for a nucleophile can be determined from a plot of log k against E for a series of electrophiles, in principle, the value of E for an electrophile can be determined from the intercept (at E + N = 0) of a plot of log (k/s) versus N for a series of nucleophiles (or indeed, if need be, from the measurement for a single nucleophile). In this way E values have been determined for many electrophiles other than benzhydryl cations, including metal-coordinated cations,186 BF3-coordinated aldehydes,274 tropylium ions, and many benzylic-and heteroatom-substituted carbocations. In the low reactivity range... 

Resonance effects. Conjugation with a double bond increases the stability of a carbocation. Thus, allylic and benzylic cations are more stable than their saturated counterparts. (For example, see Problem 1.4.c.) Heteroatoms with unshared electron pairs, e.g., oxygen, nitrogen, or halogen, can also provide resonance stabilization for cationic centers, as in the following examples ... 

The GDC reaction of a benzylic C-H bond without an adjacent heteroatom is more challenging. Sinee benzylic carbocations can be generated under oxidative reaction conditions, asymmetric allqrlation is considered to be possible if the carbocations are trapped with chiral enamines or chiral metal enolates. 

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