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

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 1J(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.