Prediction of the experimental regioselectivity of C60 fullerene bis-adducts

Regiochemical control of multiple additions to the fullerene core of C-60 is attractive and feasible but not fully understood. Specifically, the second nucleophilic addition of bis-adducts produces unpredicted products. Here, we sought to elucidate the extent to which the first addition influenced, electronically and structurally, the regioselectivity of the second nucleophilic attack on C-60 fullerene by exploring three combinations of N-(diphenyl-methylene)glycinate and malonate addends connected through 1,3-benzenedimethanol. Molecular orbitals of C-60 and the N-(diphenyl-methylene)glycinate and malonate mono-adducts were determined at the B3LYP/6-31G**/B3LYP/3-21G* level of theory. We observed that employing only the lowest unoccupied molecular orbital (LUMO) coefficients was not sufficient to understand the regioselectivity of the tethered second addend. Also, structural perturbations due to the presence of the first addend were largely local. These findings suggested that the selectivity should be explained by the combination of the orbital coefficient and the thermodynamic factors. Thus, the structure and the stability of 21 bis-adducts of C-60 fullerene, containing bis-methano, bis-dihydropyrrole, or methanodihydropyrrole, were optimized at the PCM/B3LYP/6-31G*//PCM/B3LYP/3-21G*//B3LYP/3-21G* level of theory. In all cases, the relative energies and Boltzmann populations of the bis-adducts were in agreement with the main regioisomers obtained experimentally. This study showed that the regioselectivity of the second addition is governed by electronic contributions (LUMO coefficients) and by the strong effect of the strain produced by the linker and the addends (three-membered and/or five-membered rings). This insight should be taken into consideration when designing the synthesis of functionalized fullerenes. Copyright (C) 2012 John Wiley & Sons, Ltd.