DIELS-ALDER CYCLOADDUCTS OF [60]FULLERENE WITH PYRIMIDINE O-QUINODIMETHANES

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.