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 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.
Highlights
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
It is well known that Diels–Alder reactions are greatly accelerated by Lewis acids (LAs) via complexation to the dienophile.[5]
According to frontier molecular orbital (FMO) theory and a plethora of mechanistic studies on these reactions,[6] it is nowadays widely accepted that the donor–acceptor interaction established between the dienophile and the LA catalyst results in a significant stabilization of the LUMO of the dienophile, which is translated into a smaller HOMOdiene– LUMOdienophile energy gap and, as a consequence, to a lower reaction barrier as compared to the uncatalyzed reaction.[7]
Summary
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. 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 p-electron systems of the diene and dienophile.
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