B. Gonzalez et al., DIELS-ALDER CYCLOADDUCTS OF [60]FULLERENE WITH PYRIMIDINE O-QUINODIMETHANES, Journal of organic chemistry, 63(20), 1998, pp. 6807-6813
Novel organofullerenes bearing a pyrimidine nucleus covalently attache
d to the C-60 cage have been prepared by [4 + 2] cycloaddition reactio
ns of C-60 and pyrimidine o-quinodimethanes generated ''in situ'' from
the readily available cyclobutapyrimidines which are prepared in a on
e-pot procedure from cyclobutanone and alkyl or aryl nitriles. The rea
ction mechanism involves formation of a nitrilium cation with particip
ation of two molecules of the respective nitrile. A side-product (16)
formed from two cyclobutanone molecules is obtained together with the
target cyclobutapyrimidines 4a-d. Compounds 4a-d are appropriate precu
rsors for the generation of substituted pyrimidine o-quinodimethanes 5
a-d which are efficiently trapped by the C-60 molecule in a cycloaddit
ion reaction which according to theoretical calculations is controlled
by the HOMO of the diene. H-1 NMR spectra indicate the presence of a
dynamic process attributed to the boat-to-boat interconversion of the
cyclohexene ring. The activation free energy has been measured by dyna
mic NMR experiments showing values Delta G(double dagger) approximate
to 16-17 kcal/mol for both methylene groups, depending upon the substi
tuents on the pyrimidine unit. Theoretical calculations at the semiemp
irical PM3 level confirm the presence of a boat conformation for the c
yclohexene ring which undergoes a rapid flipping motion resulting in a
n average C-s symmetry as it is observed in the H-1 NMR spectra. The c
yclic voltammetry measurements show the presence of reduction waves ca
thodically shifted, related to C-60, due to the saturation of a double
bond of the C-60 cage. A weak electronic interaction is observed betw
een the pyrimidine moiety and the C-60 core.