Reactivity and endo–exo Selectivity in Diels–Alder Reaction of o-Quinodimethanes. An Experimental and DFT Computational Study

endo–exo Selectivity in Diels–Alder cycloadditions of several o-quinodimethanes (1–4) with acrylonitrile, 2-(5H)-furanone and N-methylmaleimide has been investigated in acetonitrile solution. Transition structures of the cycloaddition of the parent o-QDM (1), (E,E)-1,8-dimethyl-o-QDM (2), isoindene (3) and 2,3-dihydronaphthalene (4) with acrylonitrile and maleimide were located at both HF/6-31G∗ and B3LYP/6-31G∗ methods. Theoretical data reproduce fairly well both experimental absolute reaction rates and diastereoisomer ratios. The high endo selectivity has been rationalized mainly as a result of solvation effects (acetonitrile, PCM model) and reactant deformations. The latter is due to steric interactions.