Laser-Induced Azomethine Ylide Formation and Its Covalent Entrapment by Fulleropyrrolidine Derivatives During MALDI Analysis

Abstract

Two novel monofunctionalized fulleropyrrolidine derivatives (Prato adducts) were prepared and characterized by matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI). MALDI experiments conducted in the positive-ion mode on pure and mixed samples of both monofunctionalized fullerene derivatives revealed the efficient formation of bisadducts (in the case of the pure samples) and mixed bisadducts (in the case of a mixed sample). Bisadducts were not observed in the ESI experiments and thus not present in the sample. A mechanism for the MALDI formation of these bisadduct ions is proposed in which an azomethine ylide fragment is formed in situ from the monofunctionalized fulleropyrrolidine species upon laser irradiation. This fragment, which can survive as an intact moiety in the gas phase in the special environment provided by the MALDI experiment, is then able to attach to a fulleropyrrolidine monoadduct which acts as a dipolarophile, thus leading to the formation of a bisadduct fullerene derivative. The unprecedented re-attachment of the azomethine ylide implies that the establishment of the ligand attainment of Prato adducts based on MALDI analysis alone can lead to wrong assignments.

Competitive retro‐cycloaddition reactions in heterocyclic fullerene bis‐adducts ions: selective removal of the heterocyclic moieties

“…The formation of fullerene ions was accounted for by the elimination of the corresponding 1,3‐ and 1,2‐dipole fragments through a retro‐cycloaddition reaction. The in situ generation of these ylides has been demonstrated by trapping this reactive intermediate under MS conditions with a fullerene monoadduct which acts as dipolarophile, leading to the formation of fullerene bis‐adducts …”

“…The in situ generation of these ylides has been demonstrated by trapping this reactive intermediate under MS conditions with a fullerene monoadduct which acts as dipolarophile, leading to the formation of fullerene bis-adducts. [24] Under positive ESI conditions, pyrrolidinofullerenes provide protonated molecules [M+H] + . [25] It is well known that pyrrolidine is a weak base and theoretical calculations and experimental data reveal that the fullerene moiety reduces dramatically the basicity of the nitrogen atom of the pyrrolidine ring.…”

Evidence of charge-remote fragmentation in protonated [60]fulleropyrrolidine multi-adducts

“…[13][14][15] Earlier mass spectrometry-based studies have concentrated on the 1 characterization of the retro-cycloaddition reaction evaluating the conditions leading to the 2 complete loss of the ligand. [16][17][18][19] In an earlier MALDI investigation, we showed that the 3 released ylide ligand can efficiently re-attach to an intact adduct, [20] thereby potentially 4 falsifying efforts aimed at the elucidation of the true amount of ligand attainment in the starting 5 material. [21] Following this earlier investigation, we studied several other fulleropyrrolidines 6 and with the Prato adduct discussed in this note we discovered a system in which neutral and The fulleropyrrolidine adducts were prepared as described previously.…”

“…[21] Following this earlier investigation, we studied several other fulleropyrrolidines 6 and with the Prato adduct discussed in this note we discovered a system in which neutral and The fulleropyrrolidine adducts were prepared as described previously. [20] Unreacted C60 was 13 removed from the raw product which was then used without further purification, as the multi-14 ligated adducts were the target of this study. A stock solution of the raw product in toluene (1 15 g/L) was prepared and further diluted with dichloromethane and methanol in a 1:1 ratio 16 resulting in a solution with a monoadduct concentration of roughly 5 x 10 -5 mol/L.…”

“…The authors attributed this behaviour to an assumed CRF of the bis-adduct 19 involving dissociation of the neutral ligand rather than the protonated as in the mono-adduct. 20 The dissociations of the mono-adduct would be less efficient as they involved the protonated 21 ligand. Unfortunately, no further evidence was provided to support this hypothesis.…”