Behaviour of arylalkylamines toward trimethyl borate as a gas-phase reagent

Abstract

Gas-phase reactions of a number of arylalkylamines of general structure R 1R 2C 6H 3CHR 3(CH 2) n NR 4R 5 , where R 1 = H, F, NO 2, OH or OCH 3, R 2 = H or OH, R 3 = H or OH, R 4 and R 5 = H or CH 3 with the dimethoxyborinium ion, (CH 3O) 2B +, m/z 73, obtained by electron ionization of trimethyl borate have been studied. Mass-analysed ion kinetic energy (MIKE) spectra of the [M + 73] + adducts, generated in a chemical ionisation source, together with MIKE spectra of some of their decomposition products, have been taken and interpreted. The interpretation is discussed in terms of structural features present in the neutral molecules, such as aliphatic chain length, methyl substitution at the amino group, presence of electron-donating or electron-withdrawing substituents in the aromatic ring and, finally, presence of a benzylic hydroxy group. The analysis of the results shows that the fragmentations of the adduct ions [M + 73] + are much more structurally sensitive than those for the [M + 45] + adducts previously observed with dimethyl ether, with the loss of methanol generating the most abundant ion only for three of the amines studied. The amino group is an important, but not always the dominant site of initial reaction, the exceptions being: (1) the aromatic ring, if substituted by activating substituents, and (2) a benzylic hydroxy group, if present in the structure. The 2-(4-nitrophenyl)-ethylamine constitutes the only exception to the general behaviour just described, because in that case the site of reaction of the electrophilic dimethoxyborinium ion is the oxygen atom of the nitro group.