Wallach rearrangement of azoxypyridines and azoxypyridine N-oxides -: Charge distributions and dramatic reactivity differences

Extension of our studies of the generic Wallach rearrangement (of azoxybenzene to 4-hydroxyazobenzene) to the heteroaromatic series (azoxypyridines and axoxypyridine N-oxides) has revealed some dramatic reactivity differences, particularly for the alpha and beta compounds. We have studied the 3-isomers and the 4-isomers in each series, each with alpha and beta forms, eight compounds in all, in the 100 wt% sulfuric acid region of acidity. In those cases in which a product could be observed, the alpha and beta isomers both give the same one, the corresponding 4'-hydroxyazo compounds. All the compounds react much more slowly than does azoxybenzene itself, presumably because of the extra positive charge present in the substrates, but the beta isomers have half-lives of seconds and the alpha isomers half-lives of hundreds of hours in the 100 wt% H2SO4 acidity region. The alpha compounds have measurable pK(BH)+ values, but the beta compounds do not, exhibiting only a medium effect in the acidity region in which the alpha compounds protonate. This means that for the beta compounds, the protonated intermediates must be much less stable and the postulated reaction intermediates must be much more stable than for the alpha compounds. To clarify this, we have obtained Mulliken charge distributions for the various species concerned, calculating the charge carried by each half of the molecule, larger charge separations being taken to indicate lesser stability. As far as we can establish, this is the first time that this technique has been used to indicate the stabilities of carbocationic species.