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

Abstract

AbstractThe 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 ωB97X−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.