Semidines

ZININ Benzidine (semidine) Rearrangement Acid catatyzed rearrangement of hydrazobenzenes to benzidines and sermdines... 

If one of the nuclei of hydrazobenzene is already substituted in the y5(znr-position, the reaction maygi e rise to diphcnylamine derivatives, which are known as ortho- or para-semidines t Jacobson),... 

The rearranged dicationic species 4, which has been shown to be an intermediate, leads to the stable benzidine 2 upon deprotonation. It has been demonstrated by crossover experiments that the rearrangement does not proceed via a dissoci-ation/recombination process. From the reaction of hydrazobenzene the benzidine is obtained as the major product (up to 70% yield), together with products from side reactions—2,4 -diaminobiphenyl 5 (up to 30% yield) and small amounts of 2,2 -diaminobiphenyl 6 as well as o- and /j-semidine 7 and 8 ... 

Later it was shown24 that ortho and para semidines (X and XI) could also be formed in this reaction together with oxidation and disproportionation products... 

Diaminobiphenyl is formed by a completely different mechanism, though the details are not known. There is rate-determining breaking of the N—N bond, but the C—C bond is not formed during this step. The formation of the o-semidine also takes place by a nonconcerted pathway. Under certain conditions, benzidine rearrangements have been found to go through radical cations. 

Other evidence, for instance the observation of a semidine as one of the reaction products,256 led to the realization that the reaction is a benzidine disproportionation, such as those observed when benzidines with two p-substituents are subjected to benzidine rearrangement conditions 260,261 this also very conveniently explains the products formed. The mechanism is given in Scheme 13.258... 

This form of the isomerisation is known as the semidine transformation (P. Jacobson). 

The next benzidine-type rearrangement studied by Shine and coworkers was the p-semidine rearrangement of 4-methoxyhydrazobenzene26. In this reaction, 4-methoxy hydrazobenzene rearranges in acidic medium to form p-semidine (4) and an o-semidine (5) see equation 12. 

Rhee and Shine39 used an impressive combination of nitrogen and carbon kinetic isotope effects to demonstrate that a quinonoidal-type intermediate is formed in the rate-determining step of the acid-catalyzed disproportionation reaction of 4,4 -dichlorohydrazobenzene (equation 26). When the reaction was carried out at 0°C in 60% aqueous dioxane that was 0.5 M in perchloric acid and 0.5 M in lithium perchlorate, extensive product analyses indicated that the major pathway was the disproportionation reaction. In fact, the disproportionation reaction accounted for approximately 72% of the product (compounds 6 and 7) while approximately 13% went to the ortho-semidine (8) and approximately 15% was consumed in the para-semidine (9) rearrangement. 

TABLE 4. The nitrogen, carbon-13 and carbon-14 kinetic isotope effects found for the acid-catalyzed formation of the disproportionation product, the ortho-semidine and the para-semidine at 0°C in 60% aqueous dioxane that was 0.5 M in perchloric acid and 0.5 M in lithium perchlorate... 

An interesting ortho semidine-type rearrangement of a l,2-diazetidin-3-one has been reported. However, it is probably not general for other derivatives that are unable to stabilize positive charge (Scheme 20) (63JCS674), and a different course has been reported for l,2,4-triphenyl-l,2-diazetidin-3-one (67AG(E)630). 

Further complications of the reduction of aromatic nitro compounds are the possibility of complete reduction to aromatic amines (which may condense with nitroso compounds to give the desired azo compounds), reduction of azo compounds to the corresponding hydrazo compounds, followed by a benzidine (or semidine) rearrangement. It is clear, therefore, that the level of reducing agent used and other reaction conditions are quite critical. 

This reaction can be extended to almost all non -para substituted hydrazo compounds if one or both para-positions are substituted, either ortAo-benzidine or diphenylamine derivatives known as ortho- or para-semidines are formed. 

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