Wheland complexes electronic effects

In the first step of the actual Ar-SE reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a sigma or Wheland complex. Sigma complexes are described by at least three carbenium ion resonance forms (Figure 5.1). There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a pi electron-donating (+M) effect (see Section 5.1.3). This resonance form is an all-octet formula. 

Which Wheland complexes are the most stable This is determined to a small extent by steric effects and to a considerably greater extent by electronic effects As a car-bocation, a substituted Wheland complex is considerably more stable than an unsubstituted one only when it carries one or more donor substituents, and unsubstituted Wheland complexes E—C6Hj are still considerably more stable than Wheland complexes that contain one or more acceptor substituents. Therefore, donor-substituted benzenes are attacked by electrophiles more rapidly than benzene, and acceptor-substituted benzenes are attacked more slowly. 

Electrophilic Substitution Reactivity Much of the electrophilic reactivity of aromatics is described in great detail in a comprehensive recent book of Taylor [10]. We shall focus attention on the electrophilic substitution reactivity of annelated benzenes and try to interpret the orientational ability of fused small rings. For this purpose we consider here Wheland transition states of the electrophilic substitution reactions. It is also convenient to take the proton as a model of the electrophilic reagent. In order to delineate rehybridization and 7r-electron localization effects, let us consider a series of angularly deformed benzenes (Fig. 21), where two vicinal CH bonds bent toward each other mimick a fused small ring. Angles c of 110° and 94° simulate five and four membered... 

The spectroscopic manifestation of symmetry effects in cyclic hyperconjugation was reported in electron spin resonance (ESR) spectra. " Symmetric LUMOs show cooperative effects to hyperconjugative interactions, while interactions with antisymmetric LUMOs are cancelled by symmetry (Figure 8.28). Such effects can be extended to spin-paired molecules, Wheland intermediates of electrophilic aromatic substitution, metal cation/arene complexes, and n -cyclopentadieneylmetal compounds. ... 

The Hammett equation usually predicts substituent effects with reasonable accuracy, provided the correct choice is made between o and o. a values are based on a system in which there is little change in the degree of conjugation between the substituent and the remainder of the molecule on moving from one side of the transition state to the other. Electrophilic aromatic substitutions pass through a positively charged complex, the Wheland intermediate (I), in which the electronic distribution is different from that of either reactants or products. A generalised sequence is shown in Equation (10). The standard reaction on... 

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