Natural rubber

Catalysis

Biology

Photochemistry

Density functional theory

Reaction mechanism

Alder

Diene

Lewis acids and bases

Molecule

Botany

Diels–Alder reaction

Isoprene

Pauli exclusion principle

Polymer

Medicinal chemistry

Physics

Organic chemistry

HOMO/LUMO

Copolymer

Condensed matter physics

Computational chemistry

Methyl acrylate

Molecular orbital

Chemistry

Angewandte Chemie

The Lewis acid(LA)‐catalyzed Diels–Alder reaction between isoprene and methyl acrylate was investigated quantum chemically using a combined density functional theory and coupled‐cluster theory approach. Computed activation energies systematically decrease as the strength of the LA increases along the series I 2 &lt;SnCl 4 &lt;TiCl 4 &lt;ZnCl 2 &lt;BF 3 &lt;AlCl 3 . Emerging from our activation strain and Kohn–Sham molecular orbital bonding analysis was an unprecedented finding, namely that the LAs accelerate the Diels–Alder reaction by a diminished Pauli repulsion between the π‐electron systems of the diene and dienophile. Our results oppose the widely accepted view that LAs catalyze the Diels–Alder reaction by enhancing the donor–acceptor [HOMO diene –LUMO dienophile ] interaction and constitute a novel physical mechanism for this indispensable textbook organic reaction.

How Lewis Acids Catalyze Diels–Alder Reactions