Theoretical investigation on two different mechanisms of fulleropyrrolidine formation

Abstract

Fulleropyrrolidine synthesis by photo-addition of glycine methyl ester (GME) to [60] fullerene has been recently realized and experimentally studied. Two possible hypotheses were suggested for its formation pathway, but there was no consensus about the most favorable one. Thus, in order to find the most probable mechanism, we performed a detailed theoretical investigation of the reaction between GME and [60] fullerene studying both mechanisms suggested experimentally. The first hypothesis involves two additions of two GME radicals in two steps to C60 followed by a NH3 departure, whereas the second one involves azomethine ylide formation in a first step and followed by a cycloaddition to [60] fullerene. All the transition states and the intermediates in the reaction steps for both mechanisms were determined. The energetic profiles of both mechanisms were drawn and compared. Several levels of theory were used for the purpose, with the aim to investigate which low-cost level is sufficient to settle and which mechanism is probably involved. For the purpose, semiempirical (AM1), DFT on geometries optimized at AM1 level, and finally DFT on geometries optimized at DFT level were considered. At DFT level, GGA (PBE), hybrid (PBE0) and meta-GGA (M06-2X) were used, with a 6-31+G(d) basis set. We proved that the release of NH3 and the ring formation step in the first mechanism require a higher energy barrier compared to the second mechanism reaction steps like tautomerization and H2O departure. Thus, we can conclude that the second mechanism involving in a first step the azomethine ylide formation is more favorable than the first mechanism. The interest in using in a first step a semiempirical determination of reaction paths is highlighted, and the choice of the exchange–correlation functional is discussed.