Rearrangements benzidine-like rearrangement

The benzidine-like rearrangement of 2-hydrazinothiazoles has been observed by the Beyer group (523, 546). This rearrangement may be very fast (539). but does not occur with 2.2 -hydrazothiazole (300). where the... 

The unsubstituted 5,5 -isomer (76) is not readily available by the Hantzsch method. It was first prepared (Scheme 29) by diazotization of a diamino derivative (53HCA354) obtained by a benzidine-like rearrangement (51CB518), and has since been prepared by cross-coupling of the chloro- and trimethylstannyl-thiazoles (87S185). The 4,4 -bis(ethoxycar-bonyl) derivative has been prepared from thiooxalic esters by reaction... 

Benzidine-like rearrangements are reported on acid treatment of suitable imidazoles , pyridine and thiazole hydrazocompounds , but not ferrocene derivatives. Related, probably intramolecular, rearrangements of/>-quinamines and 0,iV-diarylhydroxylamines , (reactions 106 and 107) have been arbitrarily assigned w-bond mechanisms, whereas reactions (108) and (109) are probably inter-molecular, although the observed formation of cross-over products counts for litde . ... 

The mechanism of this elegant, surprising, and widely applicable synthesis of indole derivatives was only explained recently (R. Robinson). It must be assumed that the keto-phenylhydrazones, in tautomeric hydrazo-form, undergo a species of benzidine rearrangement which, like the latter, can often occur even in dilute acid solution, e.g. with the phenylhydrazone of pyruvic acid. 

Benzidine Dyes.— The importance of hydrazo compounds in connection with dyes is not on their own account for, as has been stated, they are colorless compounds but because they are easily oxidized to azo compounds which are dye compounds and because of the above rearrangement into compounds like benzidine which yield dyes known as benzidine dyes (p. 787). 

Quinamines having no substituents at the para-position of the aniline ring gave 4-aminodiphenyl ethers 77 as the principal products in almost quantitative yields. The behavior of quinamines 76 and their geometric features as well as the kinetic characteristics of the reaction make this transformation very similar to the well-known benzidine rearrangement (Section III.B.l). Like the latter, the mechanism of the quinamine rearrangement involves a transition state that resembles a sandwich of two rings (jr-complex)129. 

The arylhydrazine derivatives can also participate in other than benzidine isomeriza-tions. Thus, a novel rearrangement of fused /V-arylamino-substituted 1,4-dihydropyridines 137 resulted in the formation of quinindoline derivatives 1392 30,231 (equation 53). A proposed mechanism of this reaction assumes a [3,3]-sigmatropic shift like a Cope rearrangement through unstable intermediate 138. 

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