Diazo coupling kinetics

Imidazole, 4-methyl-annular tautomerism, 5, 363 association, 5, 362 boiling point, 5, 362 bromination, 5, 398 deuteration, 5, 417 diazo coupling, 5, 403 hydrogen bonding, S, 350 hydroxymethylation, 5, 404 iodination, 5, 400 kinetics, 5, 401 mass spectra, 5, 358 melting point, 5, 362 methylation, 5, 364 sulfonation, 5, 397 synthesis, 5, 479-480, 482, 484, 489 Imidazole, 5-methyl-annular tautomerism, 5, 363 Imidazole, l-methyl-4-chloro-ethylation, 5, 386 Imidazole, l-methyl-5-chloro-ethylation, 5, 386 nitration, 5, 395... 

Because of the small concentration of the 2 1 complex the last term can be ignored. From the extreme rate values in the absence of zinc and with an excess of zinc, 2i and 22 are determined as 2.4 X 104 min.-1 and 1.57 min.-1 respectively. These values can be combined with the trend in the rate constants to give the stability constant of the reactive complex, presumably Zn(OR)(OAc), as 3 X 107. For the simple zinc complex in water the literature values of the stability constant for the 1 1 complex vary from 2.5 X 108 to 6.3 X 108. The diazo coupling reaction of the complex indicates the smaller effect of coordination vis a vis protonation since this reaction is very sensitive to such effects and does not proceed with phenols. Unfortunately the choice of cations for such a reaction is restricted since the cation should not interfere with the analytical methods used to obtain the kinetic data nor should it introduce additional reactions such as occur with transition metal cations which can catalyze the decomposition of the diazonium salt via a redox process. 

A great deal of evidence has accumulated to show that a metal ion in a chelate ring can alter the electron density in a molecule. One method of obtaining quantitative data on such changes in electron density is to compare the rates of electrophilic attack at a point in the unchelated and chelated molecules as, for example, in the iodination of 8-hydroxyquinoline chelates (Section II,A). Another such kinetic study has been carried out on the diazo coupling of 8-hydroxyquinoline-5-sulfonic acid and its zinc(II) chelate with diazotized sulfanilic acid (141)-... 

Since the 5-position is blocked by a sulfonic acid group, the position of electrophilic attack is the 7-position in both the free ligand and the zinc chelate. The kinetic data were obtained at pH 5.00 in an acetate-acetic acid buffer at temperatures between 5° and 15°C. The diazo coupling reaction is a second-order reaction, but a pseudo first-order rate was obtained, since a large excess of p-diazobenzenesulfonate was used. 

In fact, when either chloride ion or water is added to the reaction mixture, both the rate and the product composition change markedly. For example, the rate increases by fourteen times and the product composition changes from 9% of the N-coupled diazoamino compound to 79% of the diazoamino compound when the chloride ion concentration is increased from zero to 1.4 x 10" M. The amount of the N-coupled diazoamino product also increased at all three temperatures used in the study as the amount of amine (base) increased. On the basis of the base catalysis and the hydrogen-deuterium kinetic isotope effects and their studies of other diazo coupling reactions, the authors concluded that the N-coupling and the C-coupling reactions both proceed by an S 2 mechanism with the proton transfer from the cr-complex rate-determining. The results are best explained by the mechanism presented in Scheme 3. 

Dobas I, Sterba V and Vecera M, Kinetics and mechanism of diazo coupling. X. The coupling kinetics of para-substituted phenol,... 

H. Maire, Kinetics of the diazo coupling between 1-naphthol and diazotized sulfanilic add. Ind. Eng. Chem. Res., 1990,... 

The acid—base equiUbtia are fundamental to the kinetics of azo coupling and of practical significance for azo technology. Thus it is important that coupling reactions be carried out in a medium such that the acid—base equiUbtia of the diazo and coupling components favor as much as possible the diazonium ions and the phenolate ions or the free amine, respectively. 

We now know that Hammett s explanation is correct in all its aspects. This result is especially noteworthy because Hammett arrived at his conclusions not through extensive experimentation in his laboratory, but by the consistent application of the newer theories of organic chemistry to kinetic results already published by others. This is not the only example of such anticipation of views (now generally accepted) to be found in Hammett s book, and it is worth remembering that Hammett expressly postulates the diazonium ion as the reactive form of the diazo compound in coupling, in contrast to the then current opinion that the diazohydroxide was the effective species. 

Other applications of NHC-based rhodium catalysts include the cyclization of acetylene carboxylic acids, cyclopropanation of olefins with diazo compounds, or aryl-aryl cross-coupling combined with dynamic kinetic resolution with the help of a lipase or Beckmann rearrangement. Thus, the chemistry of NHC-Rh catalysts is rich and varied, and we expect new catalysts and patterns of reactivity in the years to come. These will provide new tools for synthetic organic chemistry. 

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