The Pauli Repulsion-Lowering Concept in Catalysis

Abstract

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Unveiling the Ionic Diels–Alder Reactions within the Molecular Electron Density Theory

“…The present MEDT study establishes that the high GEDT taking place at the TSs of the I-DA reactions resulting from the superelectrophilic character of the iminium cations is responsible for the features of these types of DA reactions. This contrasts with the steric (Pauli) repulsions origin proposed recently by Bickelhaup et al [29,45] based on an ASM analysis, which is markedly different from the MEDT.…”

Switch From Pauli‐Lowering to LUMO‐Lowering Catalysis in Brønsted Acid‐Catalyzed Aza‐Diels‐Alder Reactions

“…The electronic energy (Δ E ) is decomposed into two terms: the strain energy (Δ E strain ) that results from the distortion of the individual reactants and the interaction energy (Δ E int ) between the deformed reactants along the reaction coordinate [17] . In this study, all energy terms were projected onto the length of the shorter one of the two forming C⋅⋅⋅C bonds, which undergoes a well‐defined change during the reaction and has proven to provide reliable results for Diels‐Alder reactions [14–16,23] . In the following, we compare the reactivity of 1 and 1* (Figure 2) and also find that the same general conclusions hold for the other systems, that is, 2 / 2* and 3 / 3* (Figures S6–S7).…”

“…In the previous section, we have already established that the “Pauli‐lowering catalysis”‐controlled reactions have the more asynchronous TS (Figure 1 ) that minimizes the overlaps of the closed‐shell orbital interactions (Figure 4 ). [ 14 , 15 ] In this case, the superelectrophiles 3** and 3*** display such stabilized LUMO(s) that the “LUMO‐lowering catalysis” becomes operative (Figure 6 and 7 ). Accordingly, these “LUMO‐lowering catalysis”‐controlled reactions of 3** and 3*** become more synchronous (Figure 5 ), to maximize the stabilizing orbital and electrostatic interactions.…”

“…As described above, the acid‐catalyzed aza‐Diels‐Alder reaction of various 2‐aza‐dienes are employed in organic synthesis, but the “LUMO‐lowering catalysis” mechanism [5] has solely been attributed to the enhanced donor–acceptor interactions caused by the stabilized LUMOs of cationic 2‐aza‐dienes upon protonation [6,12] . Our previous studies of Lewis acid‐catalyzed Diels‐Alder reactions revealed that Lewis acids activate dienophiles by reducing the Pauli repulsion between the reactants and not due to the previously expected enhanced donor–acceptor interactions [14,15] . In the present study, we aimed to uncover the actual mechanism of Brønsted acid‐catalyzed inverse electron demand aza‐Diels‐Alder reactions of 2‐aza‐dienes using density functional theory (DFT) calculations at BP86/TZ2P as implemented in ADF [14,16] .…”

Unveiling the Ionic Diels-Alder Reactions within the Molecular Electron Density Theory

“…On the other hand, the high GEDT taking place in the cationic TSs allows explaining the low endo selectivity and low solvent effects found in I-DA reactions. The present MEDT study makes it possible establishing that the high GEDT taking place at the TSs of the I-DA reactions resulting of the superelectrophilic character of the iminium cations, and not steric (Pauli) repulsions such as Bickelhaup et al have been recently proposed [29,46], are responsible for the features of these type of DA reactions.…”