Diels−Alder Cycloadducts of [60]Fullerene with Pyrimidine o-Quinodimethanes

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Abstract
Novel organofullerenes bearing a pyrimidine nucleus covalently attached to the C60 cage have been prepared by [4 + 2] cycloaddition reactions of C60 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 C60 molecule in a cycloaddition reaction which according to theoretical calculations is controlled by the HOMO of the diene. 1H 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 ΔG⧧ ≈ 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 Cs symmetry as it is observed in the 1H NMR spectra. The cyclic voltammetry measurements show the presence of reduction waves cathodically shifted, related to C60, due to the saturation of a double bond of the C60 cage. A weak electronic interaction is observed between the pyrimidine moiety and the C60 core.