Thermal and microwave-assisted synthesis of Diels-Alder adducts of [60]fullerene with 2,3-pyrazinoquinodimethanes: Characterization and electrochemical properties

[4+2] Cycloaddition reactions of six-membered heterocyclic o-quinodimethanes, generated ''in situ'' from pyrazine derivatives, to [60]fullerene, either under thermal or microwave irradiation are described. Other microwave assisted cycloadditions involving o-quinodimethanes derived from thiophene were also performed. A comparative study of the activation energy for the boat-to-boat conformational inversion has been carried out by dynamic NMR experiments, the Delta G(double dagger) values being highly dependent on the nature of the covalently attached heterocyclic systems. Theoretical calculations predict a more planar cyclohexene ring for the five member containing cycloadducts. The cycloaddition process is controlled by the HOMO of the heterocyclic o-quinodimethanes showing a LUMO(C-60)-HOMO(diene) energy differences typical for favoured cycloadditions. The redox properties of the novel organofullerenes have been determined by cyclic voltammetry in solution, showing a cathodically shifted first reduction potential values, related to [60]fullerene. Compound 15c bearing two cyano groups exhibited an opposite trend which was accounted for by the lower LUMO energy determined by semiempirical calculations.