Does Cr(CO) 3 Really behave as Catalyst in the Diels‐Alder Reaction of Styrene with Cyclopentadiene? A Molecular Electron Density Theory Study

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The role of the coordination of the Cr(CO) 3 complex to the aromatic ring of styrene in the Diels‐Alder (DA) reaction towards cyclopentadiene (Cp) has been studied within the Molecular Electron Density Theory (MEDT) at the ω B97X−D/6‐311G(d,p) level. While Electron Localization Function (ELF) topological analysis of the reagents indicates that coordination of Cr(CO) 3 to the aromatic ring of styrene does not cause any remarkable change in its electronic structure, it increases its global electrophilicity but does not efficiently activate the C−C double bond. Thus, the DA reaction of styrene with Cp, which has a high activation energy, 15.9 kcal ⋅ mol −1 , is decreased by only 2.1 kcal ⋅ mol −1 with the formation of the corresponding Cr(CO) 3 complex. This DA reaction is endo and anti stereoselective. Analysis of the geometrical parameters and the electron density distribution at the endo TSs confirms that coordination of the Cr(CO) 3 complex to the aromatic ring of styrene does not produce any appreciable change at the corresponding TSs. These DA reactions, which have low polar character, take place through a one‐step mechanism via an asynchronous endo TS, in which the formation of the C4−C5 simple bond is more advanced than the C1−C6 one. The present MEDT study shows that the Cr(CO) 3 complex has no effective catalytic activity in the non‐polar DA reactions of styrene with Cp.