Electrophilic addition to enamines

The activation of carbonyl compounds via enamine intermediates is an important part of organocatalysis. List and coworkers analyzed crystal structures of several proline enaminones and compared them with density functional theory-calculated structures of the ground and transition states.  

Among the many interesting observations, we will only discuss how the extent of electron density removal from the tt-bond affects the negative hyperconjugation. The observed changes in N-C distances 

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