Diazoamino rearrangement

It was called the diazoamino rearrangement until 1979, when IUPAC proposed replacing the term diazoamino by triazene. 

The most significant difference from C-coupling is the facile reversibility of N-coupling. In acidic solutions the diazonium ion and the (protonated) amine are regenerated. This back reaction is important in the context of the diazoamino rearrangement discussed later in this Section. 

Evidence that the diazoamino rearrangement can be partly intramolecular was brought forward qualitatively by O ta et al. l,3-Bis(4-methylphenyl)triazene 144 rearranges in acidic ethanol, affording 2-amino-S,4 -dimethylazobenzene 145. In the presence of N,N-dimethylaniIine as a scavenger of diazonium ion, 4-dimethyl-... 

The early work on diazoamino rearrangements has been well summarized by Shine 3,244) Briefly, all the previous evidence supported an intermolecular, specifically acid-catalyzed process, the so-called Friswell-Green mechanism, postulated in 1884(1). Under certain conditions, particularly when not water, but the corresponding amine is used as solvent, a modification of this mechanism can occur. Thus Goldschmidt et al. found in 1924 that the decomposition of the protonated diazoamino compound to amine and diazonium ion can be catalyzed by the anion of the acid when the latter is weak, for example, nitrobenzoic acid. With mineral acids the anion is a weak nucleophile and no evidence was found for such a pathway, but it was postulated that here the amine itself can catalyze the fission of the protonated diazoamino compound. Neither of these processes has been observed in aqueous or partially aqueous solution, for example, in 95 % aqueous ethanol... 

Diazoamino Rearrangement. When a primary or secondary aromatic amine is used in the coupling process, attack of the diazonium ion usually occurs at the amino group rather than at the nucleus ... 

Benzilic acid rearrangement Benzoin reaction (condensation) Blanc chloromethylation reaction Bouveault-Blanc reduction Bucherer hydantoin synthesis Bucherer reaction Cannizzaro reaction Claisen aldoi condensation Claisen condensation Claisen-Schmidt reaction. Clemmensen reduction Darzens glycidic ester condensation Diazoamino-aminoazo rearrangement Dieckmann reaction Diels-Alder reaction Doebner reaction Erlenmeyer azlactone synthesis Fischer indole synthesis Fischer-Speior esterification Friedel-Crafts reaction... 

The mechanism of the diazoamino-aminoazo rearrangement is dependent upon —... 

The formation of diazoamino compounds, on coupling ArN2 with primary amines, does not constitute a total preparative bar to obtaining products coupled on the benzene nucleus for diazoamino compounds (33) may be rearranged to the corresponding amino-azo... 

The rearrangement has been shown under these conditions to be an infermolecular process, i.e. the diazonium cation becomes free, for the latter may be transferred to phenols, aromatic amines or other suitable species added to the solution. It is indeed found that the rearrangement proceeds most readily with an acid catalyst plus an excess of the amine that initially underwent coupling to yield the diazoamino compound (33). It may then be that this amine attacks the protonated diazoamino compound (39) directly with expulsion of PhNH2 and loss of a proton ... 

Dialkylanilines, pure, from commercial products, 572 Di-n-amyl ether, 313 Di-iso-amyl ether, 313 Di-n-amyl ketone, 340 Diazoaminobenzene, 622, 627 Diazoamino-aminoazo rearrangement, 622J, 626, 627 Diazo ketone, 903, 904, 905 Diazomethane, 946, 967, 968, 969-972 determination of, 972 methylation with, 973 precautions in the use of, 968, 969 ring enlargement with, 946J, 947 Diazonium fluoborates, 594, 610, 611, 612, 613... 

If diazoaminobenzene is dissolved in aniline, to which a small quantity of aniline hydrochloride has been added, and the mixture kept at about 40 °C for a short time, it is converted into p-aminoazobenzene (Expt 6.86). The mechanism of this diazoamino-aminoazo rearrangement is dependent on (a) the heterolytic cleavage of a protonated diazoaminobenzene molecule to yield the benzenediazonium ion and aniline, and (b) a recoupling reaction, under weakly acidic conditions, of the diazonium ion at the para position of aniline. 

Mechanisms of N-Coaplii Formation and Rearrangement of Diazoamino Compoonds (Triazenes)... 

Primary aromatic amines (e.g. aniline) and secondary aliphatic-aromatic amines (e.g. N-methylaniline) usually form diazoamino compounds in coupling reactions with ben nediazonium salts. If the nucleophilic reactivity of the aryl residue is raised by sutetituents or fused rinp, as in m-toluidine and I - and 2-naphthylamine, aminoazo formation takes place (C-coupling). Hov ver, the possibility has also been noted that in aminoazo formation, initial attack of the diazonium ion may still be at the amine N-atom, but the a -complex might rearrange too rapidly to allow its identification... 

Penton and Zollinger 233.238) reported recently that this could indeed be the case. The coupling reaction of m-toluidine and N,N-dimethylaniline with 4-methoxy-benzenediazonium tetrafluoroborate in dry acetonitrile showed a number of unusual characteristics, in particular an increase in kinetic deuterium isotope effect with temperature. Predominant C-coupling occurs ( 86% for m-toluidine). but on addition of a tert.-alkyl ammonium chloride the rate became much faster, and predominantly diazoamino compounds (with loss of a methyl group from N,N-dimethylaniline) were formed. Therefore, initial attack of the diazonium ion is probably at the amine N-atom and aminoazo formation occurs via rearrangement. 

The intramolecular rearrangement of the conjugate acid of the diazoamino compound into the a -complex without an additional molecule of amine corresponds to a thermal [1, 3] sigmatropic rearrangement. Such a mechanism can be ruled out, however, by the antarafacial pathway required from orbital symmetry considerations (Woodward-Hoffmann rules). 

The first experiment is a synthesis of 4-chlorotoluene. p-Toluidine is dissolved in the required amount of hydrochloric acid, two more equivalents of acid are added, and the mixture cooled in ice to produce a paste of the crystalline amine hydrochloride. When this salt is treated at 0-5°C with one equivalent of sodium nitrite, nitrous acid is liberated and reacts to produce the diazonium salt. The excess hydrochloric acid beyond the two equivalents required to form the amine hydrochloride and react with sodium nitrite maintains acidity sufficient to prevent formation of the diazoamino compound and rearrangement of the diazonium salt. 

This accounts for the precaution of having an excess of acid during diazotization otherwise the diazonium salt, as it forms, would combine with the unreacted base. The diazoamino compounds are not very stable. They can be made to rearrange to the more stable azo compounds. 

The well-known diazoamino-aminoazo rearrangement (158 —> 159) is an inter-molecular reaction occurring stepwise via acidolysis followed by azo coupling23,244,245 (equation 62). This mechanism was supported by isolation of 4-hydroxyazobenzene when the reaction was carried out in the presence of phenol23. 

The 3-diazoamino-5-methylisoxazole (88) readily rearranged in the presence of base to the 2-substituted 5-acetonyltetrazole (89) two different mechanisms were proposed for this reaction58,59. 

---