C NMR chemical shift correlations in application of "tool of increasing electron demand" to stable long-lived carbocations: Comprehensive evaluation.

The reliability of (13)C 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 pi conjugations). The onset of nonclassical sigma-delocalization in 2-aryl-2-norbornyl cations with electron withdrawing-substituents previously observed was further verified by using sigma(C+) substituent constants. Difficulties in relating the C(alpha)NMR shifts in different carbocationic systems are also discussed.