THERMAL AND MICROWAVE-ASSISTED SYNTHESIS OF DIELS-ALDER ADDUCTS OF [60]FULLERENE WITH 2,3-PYRAZINOQUINODIMETHANES - CHARACTERIZATION AND ELECTROCHEMICAL PROPERTIES
Um. Fernandezpaniagua et al., THERMAL AND MICROWAVE-ASSISTED SYNTHESIS OF DIELS-ALDER ADDUCTS OF [60]FULLERENE WITH 2,3-PYRAZINOQUINODIMETHANES - CHARACTERIZATION AND ELECTROCHEMICAL PROPERTIES, Journal of organic chemistry, 62(11), 1997, pp. 3705-3710
[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.