Contribution of Quantum Tunneling to Intramolecular Diels-Alder Reactions

Intramolecular Diels-Alder (IMDA) reactions have been widely employed in the synthesis of complex organic compounds, and they comprise key steps in the enzyme-catalyzed biosynthesis of natural products. Tris(cyclohexeno)[12]annulene undergoes electrocyclization followed by Diels-Alder reaction to form a cage compound. Castro and Karney computed mechanisms for that and the analogous C12H12 system. Our work attempts to address whether formation of the cage product by IMDA reaction is partly due to heavy-atom tunneling––i.e. passage through the barrier rather than over the barrier––as a result of the reacting carbons needing to move only short distances. We report preliminary computational results (B3LYP/6-31G* level of theory) on two related IMDA reactions, including transition state structures, barrier heights, distances moved by the carbons, and relative barrier widths––a key parameter related to tunneling probability. We also present tunneling transmission coefficients computed using Bell’s formula in order to estimate the percentage of the rate due to tunneling at selected temperatures.