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-quinodime thanes, generated ''in situ'' from pyrazine derivatives, to [60]fuller ene, either under thermal or microwave irradiation are described. Othe r microwave assisted cycloadditions involving o-quinodimethanes derive d from thiophene were also performed. A comparative study of the activ ation energy for the boat-to-boat conformational inversion has been ca rried out by dynamic NMR experiments, the Delta G(double dagger) value s being highly dependent on the nature of the covalently attached hete rocyclic systems. Theoretical calculations predict a more planar cyclo hexene ring for the five member containing cycloadducts. The cycloaddi tion process is controlled by the HOMO of the heterocyclic o-quinodime thanes showing a LUMO(C-60)-HOMO(diene) energy differences typical for favoured cycloadditions. The redox properties of the novel organofull erenes 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 oppos ite trend which was accounted for by the lower LUMO energy determined by semiempirical calculations.