Sigmatropic rearrangements benzidine rearrangement

As a result of these heavy-atom KIE experiments the principal features of the benzidine rearrangements have now been firmly established. The two main products arise from two parallel reactions one of which is concerted and the other is not. Other concerted processes have been identified and all of the concerted processes can be readily classified in the terminology of sigmatropic rearrangements within the general class of percyclic reactions. 

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. 

Sigmatropic rearrangements of quaternary ammonium salts 8-38 Rearrangement of benzidines... 

There are a few sigmatropic rearrangements with more than six electrons, such as the 10-electron doubly vinylogous Stevens rearrangement 6.86—>6.87 of the unsaturated ammonium salt 6.85, and the 10-electron benzidine rearrangement 6.88 —> 6.89 of the unsaturated ammonium salt 6.85. 

The benzidine rearrangement is an important example of the [5,5]-sigmatropic rearrangement (Scheme 8.4). 

The benzidine rearrangement was employed recently for the preparation of unique bridged diaminodiphenyls 130 and 131227,228. An unprecedented [9,9]-sigmatropic shift was discovered in the acid-catalyzed benzidine rearrangement of bis[4-(2-furyl)phenyl]... 

Analogously to the benzidine rearrangement, hydrazo-biaryls do undergo the reaction furnishing the tetraaryls in good yields. For example, bis[4-(2-furyl)phenyl]diazane (514) by [9,9]-sigmatropic shift gave tetraaryl 515 in 75% yield [89], Scheme 46. 

The Woodward—Hoffmann rules predict that [5,5] sigmatropic shifts would proceed suprafacially through a ten-membered transition state. A common example of [5,5] sigmatropic shift is benzidine rearrangement. This reaction is an acid-catalyzed rearrangement of hydrazobenzenes (in general, the NJ f-diarylhydrazines) to 4,4-diaminobiphenyls (benzidines) (Scheme 3.27). 

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. 

SCHEME 3.28 [5,5] Sigmatropic shift in benzidine-type rearrangement. 

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... 

A review of the reactions and cationoid skeletal transformations in the series of polyfluoroarenes has been reported. The monoprotonated mechanism of the benzidine acid-catalysed rearrangement of hydrazobenzene has been compared with the diproto-nated mechanism monoprotonation results in concerted closed-shell sigmatropic shift, whereas diprotonation leads to a stepwise radical cation recoupling. ... 

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