Benzyl cations resonance interactions

Nakata et al, 1996, 1999). The agreement between the theoretical dihedral angles Ocaic and the empirical Oexpi of the twisted benzylic cations confirms that the observed decrease in the r value should be ascribed to a loss of resonance interaction caused by deviation from coplanarity of the carbocation centre and the benzene 7r-system. 

The intermediate for a-attack is stabilised - it is a benzylic cation - but it cannot derive assistance from the nitrogen without disrupting the benzenoid resonance (a resonance contributor, which makes a limited contribution, is shown in parenthesis). The more stable intermediate from P-attack, has charge located adjacent to nitrogen and able to derive the very considerable stabilisation attendant upon interaction with the nitrogen lone pair of electrons. 

The correlation with suggests that the equilibrium population of the benzylic carbocation (23) is enhanced by a substituent X that can stabilize the carbocation through a resonance interaction. The correlation with o-y suggests that substituent Y has a much weaker effect. This result is not compatible with an alternative mechanism involving rate-limiting formation of a bridged cationic intermediate such as 24, since in that case the X and Y substituents would be expected to have a similar effect on cation stability. ... 

Replacing an a-alkyl snbstituent by an a-aryl group is expected to stabilize the cationic center by the p-Jt resonance that characterizes the benzyl carbocations. In order to analyze such interaction in detail, the cumyl cation was crystallized with hexafluoroantimonate by Laube et al. (Fig. 13) A simple analysis of cumyl cation suggests the potential contributions of aromatic delocalization (Scheme 7.3), which should be manifested in the X-ray structure in terms of a shortened cationic carbon—aromatic carbon bond distance (C Cat). Similarly, one should also consider the potential role of o-CH hyperconjugation, primarily observable in terms of shortened CH3 distances. Notably, it was found experimentally that the Cai distance is indeed shortened to a value of 1.41 A, which is between those of typical sp -sp single bonds (1.51 A) and sp -sp double bonds (1.32 A). In the meantime, a C -CH3 distance of 1.49 A is longer than that observed in the tert-butyl cation 1 (1.44 A), and very close to the normal value for an sp -sp single bond. 

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