Synthesis and antioxidant activity of [60]fullerene-BHT conjugates

Fullerene derivatives incorporating one or two 3,5-di-tert-butyl-4hydroxyphenyl groups were synthesized by 1,3-dipolar cycloaddition of azomethine ylides to C-60. The O-H bond dissociation enthalpies (BDEs) of these compounds were estimated by studying, by means of EPR spectroscopy, the equilibration of each of these phenols and 2,6-di-tert-butyl-4-methyl-phenol (BHT) with the corresponding phenoxyl radicals. The antioxidant activity of the investigated phenols was also determined by measuring the rate constants for their reaction with peroxyl radicals in controlled autoxidation experiments and compared to that recorded under identical experimental settings for [60]fullerene itself and unlinked BHT. The results indicate that linking of the BHT structure to C-60 does not substantially alter the thermochemistry and kinetics of its reaction with peroxyl radicals, but such adducts may behave as interesting bimodal radical scavengers. The inherent rate constant for trapping of peroxyl radicals by C-60 per se (k(inh) = 3.1 +/- 1.1 x 10(2) M-1 s(-1)) indicates that, contrary to previous reports, [60]fullerene is an extremely weak chain-breaking antioxidant.