Diels-Alder transition state charge transfer

Compare electrostatic potential maps for the following Diels-Alder transition states cyclopentadiene+ethene, cyclopentadiene+acrylonitrile and cyclopentadiene+ tetracyanoethylene, with those of reactants cyclopentadiene, ethene, acrylonitrile and tetracyanoethylene. Are electrons transferred from diene to dienophile in the transition states (relative to reactants) or vice versa For which reaction is the transfer the greatest The least Quantify your conclusion by measuring the total charge on the diene and dienophile components in the three transition states. 

The first step 202 is now a pericyclic reaction. It looks like a cycloaddition, though it involves a hydrogen transfer as well. It is in fact a carbonyl (or oxo- ) ene reaction. It is like a Diels-Alder cycloaddition in which a C-H bond has replaced one of the double bonds in the diene and a C=0 group is the dienophile. Many Prins reactions are probably carbonyl ene reactions. In his excellent review30 in Comprehensive Organic Synthesis, B. J. Snider says The (carbonyl) ene reaction and the Prins reaction are not mechanistically distinct . Though this step is pericyclic, it is very polar and the transition state 203 no doubt contains partial charges. It is therefore stabilised and the reaction accelerated by protic acids 205 and Lewis acids 207. 

Although the mechanism of the Diels-Alder reaction is still controversial, it is practically useful to envision a relationship between its transition state (t.s.) and a charge transfer complex. S) In its simplest version, the two enantiomeric t.s. s 6a and 6b become diastereomeric t.s. s if a chiral "solvator" is present Preferential solvation of one of the two enantiotopic... 

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