Improved Purification of C₆₀ and Formation of σ‐ and π‐Homoaromatic methano‐bridged fullerenes by reaction with alkyl diazoacetates

Abstract

Abstract A rapid and inexpensive method for the large‐scale purification of C 60 is the simple filtration of the toluenesoluble extract of commercial fullerene soot through a short plug of charcoal/silica gel with toluene as the eluent. Reactions of C 60 with ethyl and tert ‐butyl diazoacetates in refluxing toluene lead to the formation of the (alkoxycarbonyl)methylene‐bridged isomers 1a – 3a and 1b – 3b , respectively, which can be equilibrated, upon further heating, into the single compounds 1a and 1b , respectively. Isomers 1a / b possess the methano bridge at the 6–6 ring junction, whereas structures 2a / b and 3a / b are bridged at the 6–5 junction. A dramatic influence of local and π‐ring current anisotropic effects of the fullerene sphere on the NMR chemical shifts of the methine protons in the bridge is observed: the chemical shifts of the protons located over a pentagon ring in 2a / b and over a hexagon ring in 3a / b differ by Δδ = 3.47 and 3.45 ppm, respectively. The analysis of the 13 C‐NMR chemical shifts of the bridgehead C‐atoms and the 1 J (C,H) coupling constants for the methano‐bridge atoms reveals conclusively that the 6‐5‐ring‐bridged structures 2a / 2b and 3a / 3b are π‐homoaromatic (‘open’ transannular bond) and the 6‐6‐ring‐bridged structures 1a / b are π‐homoaromatic (‘closed’ transannular bond). The electronic absorption spectra show that π‐homoconjugation in 2a / b and 3a / b represents a much smaller electronic perturbation of the original C 60 chromophore than σ‐homoconjugation in 1a / b . The results of this study demonstrate an impressive linkage between the chemistry of methano‐bridged annulenes and methano‐bridged fullerenes.