13C NMR Chemical Shift Correlations in Application of ``Tool of Increasing Electron Demand'' to Stable Long-Lived Carbocations: Comprehensive Evaluation

The reliability of13C NMR chemical shift correlations in the application of the ``tool of increasing electron demand'' to stable long-lived carbocationic systems is demonstrated by a comprehensive analysis of 22 stable aryl-substituted carbocationic systems. The observation of slopes of less than unity in such chemical shift correlations for several cationic systems has been attributed to additional charge delocalizing mechanisms present in the system (such as homoallylic, cyclopropyl, and π conjugations). The onset of nonclassical σ -delocalization in 2-aryl-2-norbornyl cations with electron withdrawing-substituents previously observed was further verified by using σC+substituent constants. Difficulties in relating the CαNMR shifts in different carbocationic systems are also discussed.