Transition-metal-catalyzed divergent functionalization of [60]fullerene with propargylic esters

We have demonstrated that transition-metal-catalyzed divergent reactions between [60]fullerene (C₆₀) and propargylic esters allow easy access to formal [2 + 2] and [4 + 2] cycloadducts in reasonable yields, and that the production ratios depend on the metal catalyst used. The molecular structures of the cycloadducts were characterized by means of spectroscopic analyses and theoretical calculations. A plausible reaction mechanism is proposed that shows that two competing routes are likely to be involved after the initial transition-metal-activated cyclization of the carbonyl group and the C-C triple bond in the 6-endo-dig mode. Further rearrangement into allenol esters followed by formal [2 + 2] cycloaddition with C₆₀ gives the [2 + 2] cycloadducts, whereas rearrangement into 1,3-dienyl esters followed by [4 + 2] cycloaddition with C₆₀ gives the [4 + 2] cycloadducts.