Hydrazobenzene disproportionation

Disproportionation (equation 13) is one of the side reactions that can occur in benzidine rearrangements. Shine and coworkers measured the nitrogen and carbon kinetic isotope effects for the disproportionation reaction of 4,4 -diiodohydrazobenzene, which only yielded disproportionation products, at 25 °C in 70% aqueous dioxane that was 0.376 M in perchloric acid29. The reaction was first order in hydrazobenzene and it has been assumed that an intermediate was involved in the disproportionation reaction. This intermediate must be one of a radical ion30 (equations 14 and 15), a jr-complex31 (equation 16) or a quinonoid structure32 (equation 17). 

The reaction was second order in acid and first order in substrate, so both rearrangements and the disproportionation reaction proceed via the doubly-protonated hydrazobenzene intermediate formed in a rapid pre-equilibrium step. The nitrogen and carbon-13 kinetic isotope effects were measured to learn whether the slow step of each reaction was concerted or stepwise. The nitrogen and carbon-13 kinetic isotope effects were measured using whole-molecule isotope ratio mass spectrometry of the trifluoroacetyl derivatives of the amine products and by isotope ratio mass spectrometry on the nitrogen and carbon dioxide gases produced from the products. The carbon-12/carbon-14 isotope... 

Hydrazyl radicals derived from 1,2 diaryl- and 1,2-dialkyldiazenes have been postulated [186] and identified in homogeneous solution by ESR [187]. The observation that the product 15a, obtained from l-/er/-butyl-2-phenyldiazene, consists of only one isomer is in agreement with the expectation that the hydrazyl radical should be more stable when the unpaired electron is localized rather on the PhN than on the /BuN group. The oxidative IFET is assumed to proceed as described for the pho-todehydrodimerization. Heterocoupling of the hydrazyl and allyl radicals affords the allylhydrazine. Thus, formation of the addition product is a le lh+ process, whereas the by-products are formed via a 2e 2h+ process, irrespective whether the hydrazobenzene derivative is formed by subsequent reduction or disproportionation of the hydrazyl radical (Eqs. 34 and 35) ... 

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