Diels-Alder cycloadducts of [60]fullerene with pyrimidine o-quinodimethanes

Novel organofullerenes bearing a pyrimidine nucleus covalently attached to the C-60 cage have been prepared by [4 + 2] cycloaddition reactions of C-60 and pyrimidine o-quinodimethanes generated "in situ" from the readily available cyclobutapyrimidines which are prepared in a one-pot procedure from cyclobutanone and alkyl or aryl nitriles. The reaction mechanism involves formation of a nitrilium cation with participation 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 precursors for the generation of substituted pyrimidine o-quinodimethanes 5a-d which are efficiently trapped by the C-60 molecule in a cycloaddition 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 dynamic NMR experiments showing values Delta G(double dagger) approximate to 16-17 kcal/mol for both methylene groups, depending upon the substituents on the pyrimidine unit. Theoretical calculations at the semiempirical PM3 level confirm the presence of a boat conformation for the cyclohexene ring which undergoes a rapid flipping motion resulting in an average C-s symmetry as it is observed in the H-1 NMR spectra. The cyclic voltammetry measurements show the presence of reduction waves cathodically shifted, related to C-60, due to the saturation of a double bond of the C-60 cage. A weak electronic interaction is observed between the pyrimidine moiety and the C-60 core.