Benzidine rearrangement thermal

Scheme 13 may look unfavorable on the face of it, but in fact the second two reactions are thermally allowed 10- and 14-electron electrocyclic reactions, respectively. The aromatic character of the transition states for these reactions is the major reason why the benzidine rearrangement is so fast in the first place.261 The second bimolecular reaction is faster than the first rearrangement (bi-molecular kinetics were not observed) it is downhill energetically because the reaction products are all aromatic, and formation of three molecules from two overcomes the entropy factor involved in orienting the two species for reaction. 

The benzidine rearrangements can also be brought about thermally, but very few mechanistic studies have been carried out. One set of heavy-atom KIE measurements has been made in the reaction of 2,2 -hydrazonaphthalene (18)21. Substantial nitrogen (1.0611 for the [15N, 15N ]) and carbon (1.0182 for the [1,1 -13C2]) KIE values were obtained showing that, just as for the acid catalysed reaction, this is a [3,3]-sigmatropic rearrangement, this time presumably of the non-protonated reactant. 

TABLE 2. The nitrogen and carbon-13 kinetic isotope effects for the acid-catalyzed and for the thermal benzidine rearrangement of 2,2 -hydrazonaphthalene in 70% aqueous dioxane at 0°C and in 95% ethanol at 80 °C, respectively... 

Thermal dimerization of 2,3,5,6-tetrachloro-p-phenylenediimine 351 A remarkable benzidine rearrangement 352 Condensation of aromatic perchloroamines 353 Perchlorinated organic radicals and related intermediates 354 Perchlorobenzyl radicals 354 Perchlorodiphenylmethyl radicals 355 Perchlorotriphenylmethyi radicals 361 Perchloro-9-phenylfluorenyl radicals and related species 384 Inert perchloro-aza-radicals 389 Perchloro-p-quinodimethanes, perchloro-/ -xylylenes and related compounds 390... 

In a reaction analogous to that of stilbene, protonated azobenzene iso-merizes at room temperature under irradiation with ( )e, z = 0.27 and = 0.25. Subsequently, the dehydro-photocyclization starts from the photo-stationary E/Z mixture to give protonated benzo[c]cinnoline with a yield of (j) = 0.02. This is parallel to the stilbene-to-phenanthrene reaction, and it proceeds not in concentrated sulfuric acid where the thermal Z -> E reisomerization is fast, but best in ca. 66% acid. Protonated benzo[c]cinnoline and hydrazobenzene are formed, and the latter undergoes the benzidine rearrangement. The complexation with metal ions in azobenzene crown ethers under participation of the n-electrons of the azo group leads to an increase of the E Z quantum yield from 0.25 of the metal free compound to 0.4 to 0.6 in the Ba and Ca "" complexes. The Z E yield decreases from 0.18 to 0.13 and 0.05. If the crown is larger, the values increase to 0.35 and (pz-iE = 0.45. " ... 

Sigmatropic shift is thermally allowed suprafacial process of Site and 2oe. The thermal rearrangements of aryl ethers 154 to 155, l,2-(l,3-butadienyl) cyclohexyl enolates 156 to 157 and hydrazobenzene 158 to p-benzidine 159 are illustrative... 

---