The Bürgi‐Dunitz (BD) angle plays a pivotal role in organic chemistry to rationalize the nucleophilic addition to carbonyl groups. Yet, the origin of the obtuse trajectory of the nucleophile remains incompletely understood. Herein, we quantify the importance of the underlying physical factors quantum chemically. The obtuse BD angle appears to originate from the concerted action of a reduced Pauli repulsion between the nucleophile HOMO and carbonyl π bond, a more stabilizing HOMO‐π*‐LUMO(C=O) interaction, as well as a more favorable electrostatic attraction.
The physical factors governing the catalysis in Lewis
acid-promoted
carbonyl-ene reactions have been explored in detail quantum chemically.
It is found that the binding of a Lewis acid to the carbonyl group
directly involved in the transformation greatly accelerates the reaction
by decreasing the corresponding activation barrier up to 25 kcal/mol.
The Lewis acid makes the process much more asynchronous and the corresponding
transition state less in-plane aromatic. The remarkable acceleration
induced by the catalyst is ascribed, by means of the activation strain
model and the energy decomposition analysis methods, mainly to a significant
reduction of the Pauli repulsion between the key occupied π-molecular
orbitals of the reactants and not to the widely accepted stabilization
of the LUMO of the enophile.
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