Probing the Regioselectivity with Encapsulated H2 : Diels-Alder Reaction of an Open-Cage C60 Derivative with Anthracene.

The regioselectivity in the Diels-Alder reaction of an unsymmetrical open-cage C60 derivative with anthracene was studied. By using an encapsulated H2 molecule as a magnetic probe, the product population was successfully evaluated in detail, indicating the formation of approximately ten compounds as major components. The nucleus-independent chemical shift (NICS) calculations showed a close resemblance to the observed 1 H NMR spectrum, which allowed for a facile characterization of the products. Theoretical studies on the formation of all 29 possible anthracene adducts were also performed. The results indicated that the regioselectivity is strongly governed by steric factors, values of the frontier orbital coefficients, and thermodynamic stabilities. Single-crystal X-ray analysis of the dominant compound revealed the supramolecular architecture between the anthracene moiety and the π-sphere of a neighboring molecule.