Diazo tars

The reason is soon discovered on making a serious attempt to investigate such a system on the one hand, numerous polymeric products (diazo tars) that are difficult to identify are formed at pH 6-11, and on the other hand these preparative and kinetic experiments are not readily reproducible. The material of the reaction vessel, light, and the atmosphere influence the product formation and the rate and order of the reaction to an extent rarely encountered in organic chemistry. 

On the basis of the nucleophilicity parameters B, NBs, and fi (see Table 8-2) one expects less of the homolytic product in water than in methanol. This is, however, not the case. It has been known for many decades that a very complex mixture of products is formed in the decomposition of diazonium ions, including polymeric products, the so-called diazo tars. In alcohols this is quite different. The number of products exceeds three or four only in exceptional cases, diazo tars are hardly formed. For dediazoniation in weakly alkaline aqueous solutions, there has, to the best of our knowledge, been only one detailed study (Besse et al., 1981) on the products of decomposition of 4-chlorobenzenediazonium fluoroborate in aqueous HCOf/ CO]- buffers at pH 9.00-10.30. Depending on reaction conditions, up to ten compounds of low molecular mass were identified besides the diazo tar. 

In this context two observations reported by Rondestvedt (1960, p. 214) should be mentioned (i) Meerwein reactions proceed faster in the presence of small amounts of nitrite ion. Meerwein reactions in which N2 evolution ceased before completion of the reaction can be reinitiated by addition of some NaN02. (ii) Optimal acidity for Meerwein reactions is usually between pH 3 and 4, but lower (pH — 1) with very active diazonium compounds such as the 4-nitrobenzenediazonium ion or the diphenyl-4,4 -bisdiazonium ion. At higher acidities more chloro-de-diazoniation products are formed (Sandmeyer reaction) and in less acidic solutions (pH 6) more diazo tars are formed. 

The Pd°-catalyzed arylations using arenediazonium tetrafluoroborates are limited to those diazonium salts that can be manipulated at room temperature. The reaction can, if necessary, be performed at temperatures up to 50 °C by using a mixture of an arylamine and tert-butyl nitrite in chloroacetic acid or in a mixture of chloroacetic and acetic acid (Kikukawa et al., 1981a). Styrene reacted with fourteen arylamines in the presence of 5 mol-% Pd(dba)2 to give the corresponding substituted stilbenes in yields of 46-97%. It is important for good yields to carry out these reactions in an acidic system. Without acid the yield was low (11%), and diazo tars were also formed. 

With regard to the mechanism of these Pd°-catalyzed reactions, little is known in addition to what is shown in Scheme 10-62. In our opinion, the much higher yields with diazonium tetrafluoroborates compared with the chlorides and bromides, and the low yields and diazo tar formation in the one-pot method using arylamines and tert-butyl nitrites (Kikukawa et al., 1981 a) indicate a heterolytic mechanism for reactions under optimal conditions. The arylpalladium compound is probably a tetra-fluoroborate salt of the cation Ar-Pd+, which dissociates into Ar+ +Pd° before or after addition to the alkene. An aryldiazenido complex of Pd(PPh3)3 (10.25) was obtained together with its dediazoniation product, the corresponding arylpalladium complex 10.26, in the reaction of Scheme 10-64 by Yamashita et al. (1980). Aryldiazenido complexes with compounds of transition metals other than Pd are discussed in the context of metal complexes with diazo compounds (Zollinger, 1995, Sec. 10.1). 

For the dyestuff and pigment industry, a better knowledge of dediazoniation under such conditions would be useful because we estimate the loss in yields of industrially produced azo compounds due to competitive (unwanted) dediazoniations to be at least 10% of the production. These 10% are mainly diazo tars which have been investigated systematically in only three papers since the 1950s124. 

Diazo-4,5-dicyanoimidazole in acetic acid decomposed to the corresponding imidazole, but in hot water or aqueous acetic acid it gave quantitative evolution of nitrogen and intractable tars (79JOC1717). 4-Diazoi-midazole-5-carboxamide treated with 30% sulfuric acid at 95°C in the presence of copper bronze did not give the hydroxy compound, but cy-clized to 2-azahypoxanthine, the same compound obtained by thermolysis and photolysis in acid [81JCS(P1)1433]. 

The development of modern organic pigments started with the synthesis of dyestuffs for the textile industry. The period up to 1900 was characterized by the discovery and development of many dyes derived from coal-tar intermediates. Rapid advances in color chemistry were initiated after the discovery of diazo compounds and azo derivatives (shown to be largely hydrazone derivatives). The wide color potential of this class of pigments and their relative ease of preparation led to the development of azo colors, which represent the largest fraction of manufactured organic pigments. 

Tar formation is observed in the reduction of most diazo compounds and is not peculiar to the use of alcohol as the reducing agent. In some instances only slight losses are thus incurred in otherB, appreciable amounts of diazonium salt are reslnified. 

Conversion of stabilized diazo compounds to hydrocarbons does not depend upon the reducing action of ethyl alcohol, for the reaction takes place smoothly in acetone, ether, nitrobenzene, benzene, chloroform, apd carbon tetrachloride. No acetaldehyde is produced when ethanol is the solvent. In the absence of metals, however, when the stabilized salts of diazotized a-naphthylamine are treated with ethyl alcohol, acetaldehyde, naphthalene, a-ethoxynaphthalene, and tar are obtained. 

Commercial Preparation.—The most important method for preparing phenol on a commercial scale is the potash fusion of benzene sul-phonate (p. 520), though it may also be prepared by the diazo synthesis (p. 597). Its chief natural source is coal tar, from which it is obtained in the fractions of coal tar distillation, boiling below 210°, i.e., in the light and middle oils (p. 497). The process of isolating it has been described (p. 498), the purest product being in the form of the hydrate, m.p. 16°. The yield from coal tar is 0.4 to 0.5 per cent, it being one of the five most important coal tar distillation products. 

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