How Lewis Acids Catalyze Diels–Alder Reactions
Pascal Vermeeren, Trevor A. Hamlin, Israel Fernández, F. Matthias Bickelhaupt
Abstract
Abstract 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 <SnCl 4 <TiCl 4 <ZnCl 2 <BF 3 <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.