Why Do Catalytic Quantities of Lewis Acid Generally Yield More Product than 1.1 Equiv in the Intramolecular Diels−Alder Reaction with a Furan Diene? 2.1 AM1 Calculations and Mathematical Simulation of the Equilibria

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Abstract
The results presented here provide additional support for our hypothesis based on the relative basicity of the reaction controlling functional group to rationalize experimental observations on intramolecular Diels-Alder reactions with a furan diene (IMDAF) regarding the quantity (0.1 or 1.1 equiv) of Lewis acid required to facilitate the reaction most effectively. Heats of formation, DeltaH(f), and heats of reaction, DeltaH(R), have been obtained using AM1 calculations for 26 IMDAF reactions. These DeltaH(R) are generally exothermic (indicating that these IMDAF reactions are favorable) and can be qualitatively correlated with experimental yields of adduct, thereby providing a means of predicting the feasibility of the IMDAF promoted by 0.1 equiv of Lewis acid. The equilibria involved in the Lewis acid-promoted IMDAF reaction have been qualitatively interpreted using reaction coordinate diagrams and quantitatively investigated by generating a mathematical simulation of the reaction scheme. This demonstrates that the experimental behavior of the IMDAF reaction is well represented by the relative basicity hypothesis and that the LA concentration-dependent behavior should also be observed for other Lewis acid-promoted organic reactions.