Theoretical study on the reaction mechanism of bis-addition of methyl azide to C60

The selectivity of attacking sites and the reaction mechanisms of the bis-addition of methyl azide with its corresponding azafulleroid (C60NCH3) have been investigated using AM1 semi-empirical and density functional methods. The whole reaction processes can be divided into two stages. The first stage is the 1,3-dipolar cycloaddition (1,3-DC) reaction of methyl azide with C60NCH3 giving rise to a triazoline intermediate and the second is the N2 elimination. Based on the charge distributions, four patterns of the addition sites have been discussed. In view of the energy barriers, two kinds of 6-6 double bonds, which are in the most and the second vicinities of the -NCH3 addend group of the C60NCH3, are the two most possible attack sites in the reaction of 1,3-DC. The analyses of the π-orbital axis vector (POAV) and the deformation and interaction energies indicate that it is the favorable interaction energy rather than the strain release that dominates the two preferential attacking patterns. The subsequent thermal elimination of N2 takes place via two steps in which the breaking of N-N single bond precedes the cleavage of the C-N bonds of the unsubstituted N atom. The N2 elimination occurs simultaneously with the formation of the new C-N bonds (corresponding to the substituted N atom), giving rise to two isomers of the bisadducts. One is a double azafulleroid with two N atoms bonding to two consecutive 5-6 junctions of the same pentagon, and the other with two N atoms bonding to two alternate 5-6 junctions of the same pentagon. The analysis of the energy results shows that although the former reaction is preferred to some extent, both of the two reactions can take place and both of the two bisadducts are in principle obtainable.