Regioselectivity of 1,3-Butadiene Diels-Alder Cycloaddition to C59XH (X=N, B)

The regioselectivity of Diels-Alder cycloaddition of 1,3-butadiene to C59XH (X=N, B) has been studied theoretically by means of the semiempirical AM1 and DFT (B3LYP/6-31G*) methods. The mechanisms of the cycloaddition on some selected 6.6 bonds of C59XH (X=N, B) have been analyzed. For C59NH, the activation energies become lower with the addition site increasingly farther from the N atom; however, they are all higher than that of the reaction of 1,3-butadiene with C60. In contrast to C59NH, for the cycloaddition to C59BH, the activation energies corresponding to 2,12/r- and 2,12/f-transition states, in which the addition sites are the nearest ones to the B atom, are the lowest ones, and are lower than that of the reaction of 1,3-butadiene with C60 by over 18 kJ·mol−1, and the products corresponding to these two transition states are the most stable ones. The different electronic natures of N and B atoms results in different effects on the Diels-Alder reactions of 1,3-butadiene with C59NH and C59BH; the former makes the reactivity of C59NH reduced and the latter makes the reactivity of C59BH enhanced, relative to that of C60.