Dynamic solvation effects on the endo/exo selectivity of the Diels-Alder reaction

The endo/exo stereoselectivity of the Diels-Alder cycloaddition between cyclopentadiene and 3-crotonoyl-2-oxazolidinone catalysed by Et₂AlCl is temperature- and solvent-dependent. Eyring plots of ln(endo/exo) versus 1/T show different inversion temperatures (T_inv) depending on the reaction solvent. Using toluene-d₈ we found a break temperature (T_NMR) in a variable temperature ¹³C NMR experiment of the starting oxazolidinone, and this T_NMR corresponded to the T_inv. In several cases the differential activation entropy manages the observed stereoselectivity.