Reaction of C60F18 with diethyl bromomalonate: diversion of the Bingel reaction and formation of the first 18π annulenic fullerene

Abstract

Reaction of C60F18 with diethyl bromomalonate in the presence of DBU results in the nucleophilic replacement of either one, two, or three of the most accessible fluorine atoms by CBr(CO2Et)2 moieties, in preference to formation of a cyclopropanated derivative (the normal Bingel reaction). Substitution that takes place δ to the departing fluorine, is the first proven example of SN2′ substitution in a fullerene, and appears to be sterically driven. The ratio of mono-/poly-substitution products can be controlled by varying the rate of addition of the DBU and the molar ratio between C60F18 and the other reagents. The tri-substituted product is an [18]annulene, has an intense emerald-green colour ascribable to the electron delocalisation in the (equatorial) annulene belt (bond length variation 0.018 A), and has C3v symmetry. This is the first example on an annulenic fullerene (moreover of an all-trans annulene or trannulene). The extent of substitution in each compound is identified from the fluorinated fragments (C60F15, C60F16, and C60F17, respectively, for tri-, di-, and mono-substitution) in the EI mass spectra, and by their 1H and 19F NMR spectra. The structure of the tri-substituted [18]annulene was confirmed by single crystal X-ray diffraction. Normal Bingel cycloaddition also takes place between C60F18 and diethyl malonate–DBU in CBr4, to give C60F18C(CO2Et)2 and C60F16C(CO2Et)2 in relatively low yields. Calculations indicate a critical size of substituent required to produce δ-substitution, rather than ipso-substitution of the departing fluorine.