Theoretical Study of the Regioselectivity of Successive 1,3-Butadiene Diels−Alder Cycloadditions to C60

The possibility of successive Diels−Alder cycloadditions of 1,3-butadiene to C 60 has been studied theoretically by means of the AM1 semiempirical method. The nine unique possible reaction pathways leading to the experimentally observed T h -symmetric hexakisadduct have been described, and the most thermodynamically favored has been analyzed in more detail. The enthalpy barrier for the cycloaddition changes from 16.2 kcal/mol in the formation of the monoadduct to 17.8 kcal/mol for the hexakisadduct, increasing slightly with successive attacks. However, once the hexakisadduct is reached, addition of a new 1,3-butadiene has an enthalpy barrier as high as 35.5 kcal/mol, in agreement with the fact that experimentally the heptakisadduct has not yet been observed.