The ionized methylene transfer from the distonic radical cation (+)CH 2-O-CH 2 to heterocyclic compounds. A pentaquadrupole mass spectrometric study

Abstract

Ion-molecule reactions of the mass-selected distonic radical cation (+)CH2-O-CH 2 (·) (1) with several heterocyclic compounds have been investigated by multiple stage mass spectro- metric 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 CH 2 (+·) 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 δ-cleavage cyclization process induced by the free-radical character of the N-methylene group that yields bicyclic pyridinium cations. On the other hand, extensive CH 2 (+·) 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 CH 2 (+·) to tetrahydrofuran, whereas a new α-distonic sulfur ion is formed in reactions with tetrahydrothiophene. Unstable N-methylene distonic ions, likely formed by transfer of CH 2 (+·) to the nitrogen of piperidine and pyrrolidine, undergo rapid fragmentation by loss of the α-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.