Mechanism of the Addition Reaction of Alkyl Azides to [60]Fullerene and the Subsequent N₂ Extrusion to Form Monoimino-[60]fullerenes

The 1,3-dipolar cycloaddition of methyl azide to C60 and the subsequent nitrogen elimination from the formed triazoline intermediate to yield the aziridine adduct have been studied using semiempirical and density functional methods. The results obtained show that the addition of methyl azide to C60 takes place in the ring junction between two six-membered rings leading to a closed [6,6]-trizoline intermediate with an energy barrier of about 20 kcal mol-1 and an exothermicity of ca. 2 kcal mol-1 at the B3LYP/6-31G**//AM1 level of theory. The subsequent thermal loss of N2 takes place through a stepwise mechanism in which the cleavage of the N-N single bond precedes the breaking of the N-C bond, with a total activation energy of approximately 45 kcal mol-1. The N2 loss occurs simultaneously with the formation of the new N-C bond. During the process, the steric effects of the leaving N2 molecule prevent the addition of the nitrene substituent to the [6,6]-ring junction attacked initially and force the addition to an adjacent [5,6]-ring junction.