The role of ion/neutral complexes in the fragmentation of N-benzyl-(alkylpyridinium) ions

N-Benzylpyridinium ions bearing an alkyl group at the pyridine nucleus were studied as potential precursors of gaseous ion/neutral complexes. The occurrence of I/N complexes [C6H5CH2+ ··· alkylpyridine] was probed by the reactivity of the potential benzylic hydride donor sites present in the ortho-, meta- and para-alkyl groups (R = methyl, ethyl, isopropyl and benzyl). Collision-induced dissociation of the ions, carried out in an electrical ion cage mass spectrometer, revealed that hydride transfer strongly depends both on the energy requirements of the hydride transfer but also on the position of the hydride donor. Hydride transfer, giving rise to the loss of toluene, was found to occur exclusively with those N-benzylpyridinium ions which bear an isopropyl or a benzyl substituent in the ortho position of the pyridine ring, thus reflecting the intermediacy of I/N complexes. All of the putative hydride donor alkyl groups were found to be non-reactive in the meta and para positions, as were methyl and ethyl groups even in the ortho positions. Density functional calculations (B3LYP/6-311+G/3d,2p)//(B3LYP/6-31+G(d)) on the hydride-transfer and simple-cleavage channels were carried out to help rationalizing these observations. The results suggest that the intra-complex hydride abstraction from the 3- and 4-isopropyl- and from the 3- and 4-benzylpyridine neutrals, although being thermodynamically favorable, is suppressed by substantial intra-complex rotational (or reorientation) barriers.

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► N-benzylpyridinium ions were found to fragment via ion/neutral complexes.
► Alkyl substituents at the pyridine nucleus were used as hydride-donor probes.
► Intra-complex H− transfer occurs with ortho-isopropyl and -benzyl groups only.
► Reorientation within the I/N complexes is a crucial factor.
► The C7H7+ ion leaving the N atom can undergo intra-complex electrophilic attack.