Artículos de revistas
THE IONIZED METHYLENE TRANSFER FROM THE DISTONIC RADICAL-CATION (CH2)-C-+-O-CH2-CENTER-DOT TO HETEROCYCLIC-COMPOUNDS - A PENTAQUADRUPOLE MASS-SPECTROMETRIC STUDY
Journal Of The American Society For Mass Spectrometry. Elsevier Science Publ Co Inc, v. 6, n. 7, n. 554, n. 563, 1995.
Ion-molecule reactions of the mass-selected distonic radical cation (CH2)-C-+-O-CH2 (1) with several heterocyclic compounds have been investigated by multiple stage mass spectrometric experiments performed in a pentaquadrupole mass spectrometer. Reactions with pyridine, 2-, 3-, and 4-ethyl, 2-methoxy, and 2-n-propyl pyridine occur mainly by transfer of CH2+ to the nitrogen, which yields distonic N-methylene-pyridinium radical cations. The MS(3) spectra of these products display very characteristic collision-induced dissociation chemistry, which is greatly affected by the position of the substituent in the pyridine ring. Ortho isomers undergo a delta-cleavage cyclization process induced by the free-radical character of the N-methylene group that yields bicyclic pyridinium cations. On the other hand, extensive CH2+ transfer followed by rapid hydrogen atom loss, that is, a net CH+ transfer, occurs not to the heteroatoms, but to the aromatic ring of furan, thiophene, pyrrole, and N-methyl pyrrole. The reaction proceeds through five- to six-membered ring expansion, which yields the pyrilium, thiapyrilium, N-protonated, and N-methylated pyridine cations, respectively, as indicated by MS(3) scans. Ion 1 fails to transfer CH2+ to tetrahydrofuran, whereas a new alpha-distonic sulfur ion is formed in reactions with tetrahydrothiophene. Unstable N-methylene distonic ions, likely formed by transfer of CH2+ to the nitrogen of piperidine and pyrrolidine, undergo rapid fragmentation by loss of the alpha-NH hydrogen to yield closed-shell immonium cations. The most thermodynamically favorable products are formed in these reactions, as estimated by ab initio calculations at the MP2/6-31G(d,p)//6-31G(d,p) + ZPE level of theory.67554563