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 I2<SnCl4<TiCl4<ZnCl2<BF3<AlCl3. 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 [HOMOdiene–LUMOdienophile] interaction and constitute a novel physical mechanism for this indispensable textbook organic reaction.
Bibliographical noteGerman Edition: DOI: 10.1002/ange.201914582
- Activation strain model
- density functional calculations
- Diels–Alder reactions
- Lewis acid catalysis
- Pauli repulsion