Benzidine rearrangements. 18. Mechanism of the acid-catalyzed disproportionation of 4,4'-diiodohydrazobenzene. Application of heavy-atom kinetic isotope effects

The Acid-Catalysed Benzidine Rearrangements May Proceed via Cation Radicals Formed by Electron Transfer to a Proton from a Hydrazyl Nitrogen

“…The other nonconcerted rearrangements are the second-order proton [1,3] sigmatropic shift of 4,4'-dichlorohydrazobenzene [17], and also probably the first-order proton [1,3] sigmatropic shift of 4-methoxyhydrazobenzene [12] to yield the corresponding o-semidine products. The authors claim that the results rule out the possibility of the rate-determining formation of a π-complex or radical/radical ion intermediate in the disproportionation of 4,4'-diiodohydrazobenzene [16]. After the above 1980s work of Shine et al, interest in the mechanistic aspects of the rearrangements waned.…”

“…In the last step, the aniline radical abstracts a hydrogen from the hydrazyl radical to yield the second aniline and azo molecules. The 4,4'-diiodohydrazobenzene yielded about 100% disproportionation products whereas the 2,2'-isomer yielded 100% benzidine product [2,16].…”

“…Benzidine formation can be classified as an allowed [5,5] sigmatropic rearrangement while diphenyline formation is a [3,5] sigmatropic rearrangement which also follows the requirements of orbital symmetry and cannot be suprafacially concerted. The other concerted reactions are the first-order proton [1,5] sigmatropic rearrangement of 4-methoxyhydrazobenzene to p-semidine [12]; the first-order proton [5,5] sigmatropic rearrangement of 2, 2'-dimethoxyhydrazobenzene to 3,3'-dimethoxybenzidine [13]; both the first-order and second-order proton [3,3] sigmatropic rearrangements of 2,2'-hydrazonaphthalene [14] and N-2-naphthyl-N'-phenyl-hydrazine [15] to yield the o,o'-biaryl-linked diamines, 2,2'-diamino-1,1'-binaphthyl and 1-(o-aminophenyl)-2-naphthylamine, respectively; the second-order proton disproportionation reaction of 4, 4'-diiodohydrazobenzene [16]; the second-order proton rearrangement of 4,4'-dichlorohydrazobenzene to o-semidine may be a concerted [1,5] sigmatropic shift, but is not characterised as such and is, therefore, suggested to be non-concerted [17]. The other nonconcerted rearrangements are the second-order proton [1,3] sigmatropic shift of 4,4'-dichlorohydrazobenzene [17], and also probably the first-order proton [1,3] sigmatropic shift of 4-methoxyhydrazobenzene [12] to yield the corresponding o-semidine products.…”

Rearrangements of Hydrazo, Azoxy and Azo Compounds: Kinetic, Product and Isotope Studies

“…One recent fascinating example of semidine formation is the exclusive formation of osemidine 6 when hydrazobenzene 1 is treated with a rhodium catalyst in dichloromethane solution; the o-semidine bonds to rhodium as a bidentate ligand, and presumably the geometrical requirements for this ensure that it is the preferred rearrangement product in this case 27 The reaction of 4, 4 -diiodohydrazobenzene (21) was used to investigate the acidcatalyzed disproportionation which accompanies many benzidine rearrangements 19 19 . Thus N−N bond breaking and C−C bond forming are both taking place at the transition state, and the rearrangement mechanism is probably exactly the same as for hydrazobenzene itself.…”

Acid-catalysed disproportionation and benzidine rearrangement of phenylhydrazinopyridines: reaction pathways, kinetics and mechanism 1

“…These processes-oxidation and reduction-are reminiscent of the disproportionation which accompanies the classical benzidine rearrangement of some hydrazoarenes. [7][8][9] Evidence was presented that the hydroxylation of a phenylazopyridine involves formation of a phenylhydrazinopyridine as a transient reaction intermediate, which subsequently undergoes fast disproportionation to the observed oxidized and reduced products. 6 In the present work we have synthesized the surmised intermediate, 4-[NЈ-(4-hydroxyphenylhydrazino)]pyridine 1, as well as 4-(NЈ-phenylhydrazino)pyridine 2, and subjected them to the acid media used in the former study 6 in order to determine the products and evaluate the kinetics.…”