Effect of substituents and protonation on the mechanism of 1,3‐dipolar retro‐cycloaddition reaction of pyrrolidino[60]‐ and [70]fullerenes

Abstract

The mass spectra of new substituted pyrrolidino[60]- and [70]fullerenes have been obtained using electrospray ionization conditions in the positive and negative mode of detection with two different mass spectrometers, a quadrupole ion trap and a Fourier transform ion cyclotron resonance. Radical anions M(●-) and deprotonated molecules [M-H](-) are formed under negative electrospray ionization mass spectrometry conditions, and the collision-induced dissociations of both ionic species have been studied. Either negative odd-electron ions or negative even-electron ions undergo a retro-cycloaddition process forming the corresponding fullerene product ions C(60)(●-) and C(70)(●-). The generation of fullerene radical anions from deprotonated molecules is a new exception of the "even-electron rule." In contrast, the protonated molecules [M + H](+) obtained from the positive mode of detection do not undergo this cycloreversion reaction, and the MS(n) experiment reveals a variety of eliminations of neutral molecules involving different hydrogen shifts and multiple bond cleavages that lead eventually to substituted methanofullerene fragment ions. The observed fragmentations can be correlated with the electronic character of the substituents attached to the heterocyclic moiety. The results obtained from the thermal reactions of these compounds, carried out under different pH conditions, correlate well with those obtained in gas phase. The different behaviors between protonated and unprotonated molecules and ions can be explained assuming that the retro-cycloaddition reaction takes place only when the nitrogen atom of the pyrrolidine ring (the basic center of the molecule) is unprotonated both in gas and condensed phase. The protonation of the NH group inhibits the cycloreversion process, and therefore different fragmentations take place. The detailed mechanisms of the formation and evolution of the intermediate fragments are described.

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

“…The corresponding protonated molecule [M+H] + ( 1b ) was observed at m/z 1013 and the mass spectrum obtained by CID reveals a main fragment 7 at m/z 926 (75%). The formation of this ion can be explained by assuming a loss of a methyliminoacetate molecule (C 3 H 5 NO 2 , 87 u) (Scheme ), similarly to the eliminations reported to explain the fragmentations of protonated fulleropyrrolidines . The elimination of the pyrrolidine ring does not take place and the loss of the imino derivative molecule produces the formation of 7 , while the isoxazoline ring remains attached to the C 60 core.…”

“…Despite these data, the reduced basicity seems to be enough to permit the protonation of the nitrogen atom resulting in the formation of the corresponding [M+H] + ion. We have previously reported that the protonated molecules [M+H] + of pyrrolidinofullerenes do not undergo the retro‐cycloaddition reaction under CID conditions . In this case, the protonation seems to avoid the loss of the corresponding azomethine ylide molecule.…”

“…In this case, the protonation seems to avoid the loss of the corresponding azomethine ylide molecule. Instead, different fragmentations take place at the pyrrolidine ring depending on the nature of the substituents attached to the heterocyclic moiety . Similarly, the protonation of isoxazolinofullerenes takes place at the nitrogen atom .…”

“…We have previously reported that the protonated molecules [M+H] + of pyrrolidinofullerenes do not undergo the retro-cycloaddition reaction under CID conditions. [25] In this case, the protonation seems to avoid the loss of the corresponding azomethine ylide molecule. Instead, different fragmentations take place at the pyrrolidine ring depending on the nature of the substituents attached to the heterocyclic moiety.…”

“…[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. [26] Values of proton affinity also indicate a reduced gas-phase basicity for this heterocyclic amine.…”