Organocatalyzed Diels-Alder Reactions: Unexplored Hydrogen Bond Donor Catalysts

We have quantum chemically investigated the catalytic effect of hydrogen bonding organocatalysts, (H₂N)₂C=X (X=O, S, Se, NH, PH, AsH, CH₂, SiH₂ GeH₂), such as urea, on the classic Diels-Alder reaction. All studied hydrogen bond donor catalysts enhance the Diels-Alder reaction between acrolein and 1,3-butadiene to a similar extent. Our activation strain and Kohn-Sham molecular orbital analyses show that these organocatalysts lower the reaction barrier by polarizing the π-orbitals away from the reactive carbon atoms of acrolein, reducing the Pauli repulsion between the reactants. Interestingly, this catalytic mechanism is not limited to >C=X moieties with relatively electronegative X (e. g., O, S, NH) but extends to situations like >C=CH₂ and even >C=SiH₂.