Dienophiles local electrophilicity

Since most of the dienophiles considered are substituted ethylenes, we can take ethylene as a reference to discuss the variations in local reactivity induced by chemical substitution. Ethylene (29) presents a local electrophilicity value tok = 0.37eV at the equivalent carbon atoms Cl and C2. Note that acetylene (32), having equivalent Fukui functions for both electrophilic and nucleophilic attacks, presents a lower electrophilicity pattern as compared with that of ethylene (tok = 0.27 eV at the equivalent carbon centres of structure 29). 

In Chart 7 we included a short series of benzylidenemalononitriles as well as a-nitrostilbenes.116 Note that the most common functionalities associated with the chemistry of the nucleophilic addition to the C=C bond, namely aldehydes, ketones, esters, anhydrides and nitriles, are EW substituents present in dienophiles and dipolarophiles used in NED DA and 1,3 dipolar cycloadditions, and they are included in Table l.121 In Table 14, we summarize the global properties and local electrophilicity values for the series of benzylidenemalononitriles and a-nitrostilbenes. Both series present ethylene derivatives with large electrophilicity values, to > 2.3 eV, they being classified as strong electrophiles within the theoretical electrophilicity scale.39... 

The issue of regiochemistry can be addressed by identifying sites of local electrophilicity and local nucleophilicity. This was done by calculation of a local electrophilicity index The index of nucleophilicity can be taken as / , the local Fukui function for electrophilic attack. The regiochemistry is then predicted by matching the highest local electrophilicity in the electrophilic component with the largest f for the nucleophilic component. Table 10.10 gives some values of representative dienes and dienophiles. 

Table 10.10. Local Electrophilicity and Fukui Functions for Some Dienes and Dienophiles ... 

According to the global electrophilicity index co shown, the dienes will act as nucleophiles and the dienophiles as electrophiles. To study the regioselectivity we used the local... 

Normally the polarity of the normal electron demand DA process has been studied by means of global electrophilicity index difference between reactants and the regioselectivity of the normal electron demand DA reaction using the local electrophilidty index for dienophiles (electrophiles in the reaction) and the local nudeophilicity index for dienes (nucleophiles in the reaction). 

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