Fourier transform ion cyclotron resonance mass spectrometry and theoretical studies of gas phase SN2 nucleophilic substitution reactions at sp3-carbon atoms

Abstract

Gas phase S_N2 intramolecular displacements are reported in which neutral nucleophiles displace neutral leaving groups within cationic substrates. In the intramolecular S_N2 reaction of N-(2-piperidinoethyl)-2,4,6-triphenylpyridinium cation (11), the cationic product 12 was detected directly. Intramolecular reactions of N-(ω-aminoalkyl)-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]acridinium salts 13a–c, N-(5-hydroxypentyl)-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]acridinium salt 14 and N-(ω-aminoalkyl)-2,4,6-triphenylpyridinium salts 20a,b afforded the corresponding protonated acridinium cation (15) or pyridinium cation (21) and, presumably, a neutral heterocycle. This interpretation is supported by isotopic labeling control experiments. No evidence has been obtained for any intermolecular gas phase S_N2 reaction with a pyridine as leaving group; theoretical calculations suggest an explanation for these experimental results.