Directing effects and activation

A Summary of Substituent Effects in Aromatic Substitution A summary of the activating and directing effects of substituents in electrophilic aromatic substitution is shown in Table 16.2. 

C. The Sulfone and Sulfoxide Functionality in Three-Membered Ring Systems Activating and Directive Effects... 

Substituents on an aromatic ring may show activating and directing effects in electrophilic substitution. The same situation is encountered in nucleophilic photosubstitution. 

X-Activation and directive effects of the nitro- and methoxy groups may play a role, too. 

In Section II we begin by surveying the factors that lead to activation and direction. In Section III these concepts are then applied to the various systems in which silicon exerts a stabilizing or destabilizing effect. Where possible we have tried to give quantitative information on the size of the effect. In the final sections we survey reactions which exemplify the activating and directing effects of silicon. 

In this section we concentrate mainly on mechanistic aspects of reactions involving ft-silylcarbocations. The structure and properties of /J-silylcarbocations are discussed in detail in Chapter 12 on Silicon-substituted Carbocations in this volume. The strong stabilization of /J-silylcarbocations is of particular importance in relation to activating and directing effects in organic syntheses using silicon compounds, and is still an important area for mechanistic, theoretical and synthetic studies. 

Activating and directive effects of silicon TABLE 3. Stabilization of vinyl cations by /S-trimethylsilyl groups... 

IV. ACTIVATING AND DIRECTIVE EFFECTS OF SILICON IN ELECTROPHILIC AROMATIC SUBSTITUTION... 

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