Neighboring group contribution

The local contribution, [Pg.521]

FT 6. The observed shielding constant is the sum of a local contribution, a neighboring group contribution, and a solvent contribution. 

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

The exo/endo rate ratio in the unsaturated series compares closely to that observed with the saturated tosylates homoconjugative interaction is therefore not apparent. Also, the products were those expected for solvolysis without participation.373 The dihydrotriquinacenyl tosylates solvolyze more slowly due die inductive contributions of the double bonds. The geometry of the p7T orbitals in the exo isomer are seen to have a very poor alignment for neighboring group participation. 

The solvolytic reactions of a-metallocenylcarbinyl acetates, which presumably involve a-metallocenyl carbonium ions, provide further examples of reactions in which metal orbital electrons may be involved. Although the evidence for neighboring group participation by the metal atom is fairly compelling, the precise mode by which the metal atom may contribute to charge stabilization is still a matter of some dispute 6, 15),... 

Neighboring group participation is also another important factor for predicting the reactivity of secondary hydroxyl groups, particularly at the C-2 position. Under basic conditions, the C-2 hydroxyl tends to be more acidic than the C-3 hydroxyl and this may be advantageously exploited in certain cases such as partial benzylation under phase-transfer catalysis. The latter reaction conditions also contribute to the relatively good selectivity for substitution at a primary hydroxyl group in preference to a secondary one at either C-3 or C-4. 

The two classes of carbocations are not mutually exclusive but represent the limits of a spectrum of carbocations with various types and degrees of charge delocalization. The interaction of neighboring groups with the vacant p-orbital of a carbenium ion center can contribute to the ions stability. This may involve the interaction of unshared electron pairs (n-donors), bent... 

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