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N-Benzylpyridinium ions bearing a tertiary alkyl or a ben-
zyl group at the ortho-position of the pyridine nucleus eliminate
toluene by hydride transfer from the ␣-C–H donor site of that
ortho substituent. Our ESI/CID measurements indicate that this
elimination pathway can compete with the simple C–N bond dis-
+
sociation releasing the C7H7 ions because of local C–H bond
activation at the hydride donor site. However, the very same bond
activation does not assist the elimination of toluene from the iso-
meric meta- and para-alkyl-substituted N-benzylpyridinium ions.
Therefore, it appears that the constituents of the ion/neutral com-
+
plexes [C6H5CH2 ··· alkylpyridine] generated from the reactive
ortho-isomers can adopt an intra-complex orientation enabling
the hydride transfer, whereas a suitable reorientation cannot be
adopted in the I/N complexes formed during the fragmentation
of the meta- and para-isomers [49]. The intermediacy of I/N com-
plexes is also supported by the loss of benzene originating from
the N-benzyl group of the N,2-dibenzylpyridinium ions 4a. Fur-
ther studies should shed more light onto the surprising ortho
specificity of the toluene elimination from the isomeric N-benzyl-
(alkylpyridinium) ions.
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