1H and 13C NMR spectroscopic investigation of long-lived ortho- and meta-substituted di-1-adamantylbenzyl cations: highly deshielded crowded benzylic cation centers

A 1H and 13C NMR spectroscopic investigation of the o-methyl, m-methyl and m-tert-butyl derivatives of 1,1-di-1-adamantylbenzyl cations is presented. The results showed that o-tolyldi-1-adamantylmethyl has the most deshielded carbocationic centers for a tertiary benzylic cation in solution. The steric effect virtually excludes p-pi resonance overlap with the adjacent empty p-orbital. However, the lack of resonance stabilization is compensated by the hyperconjugative contributions from the two adamantyl groups.