Diazo compounds Constitution

Diazo compounds constitute the most common class of carbene precursors. The stability of diazo compounds is very much influenced by the substituents present. Simple diazoalkanes such as diazomethane tend to be rather unstable and because... 

Intramolecular [3+2] cycloaddition reactions of unsaturated diazocarbonyl compounds as well as transition-metal-catalyzed intramolecular carbene-type reactions of diazo compounds constitute an important strategy in contemporary synthesis of alicyclic and heterocyclic systems [1]. In a program directed towards the synthesis of silaheterocycles according to this concept, we have used various silicon-functionalized (silyl)diazoacetates [2] as starting materials. In this communication, we report on the synthesis of silaheterocycles from a-(alkynyloxy)silyl-a-diazoacetates [3], which can be prepared easily by successive reaction of a silyl bis(triflate) with an alkyl diazoacetate and a propargyl alcohol [4],... 

Kekule (1866, vol. II, p. 716) realized that the diazo compounds were structurally analogous to their most important derivatives, the azo compounds, and he therefore described their constitution with the formula C6H5-N = N-X. The fact that the diazo compounds are much less stable than the azo species was thought to detract from Kelule s formulation. Blomstrand (1869, 1875) proposed the structure C6H5 — N — X and, in analogy to the ammonium salts, the name azoammonium . 

Nor can there be any question of real tautomerism in the case of phenol. In its chemical properties phenol resembles the aliphatic enols in all respects. We need only recall the agreement in the acid character, the production of colour with ferric chloride, and the reactions with halogens, nitrous acid, and aromatic diazo-compounds (coupling), caused by the activity of the double bond and proceeding in the same way in phenols and aliphatic enols. The enol nature of phenol provides valuable support for the conception of the constitution of benzene as expressed in the Kekule-Thiele formula, since it is an expression of the tendency of the ring to maintain the aromatic state of lowest energy. In this connexion the hypothetical keto-form of phenol (A)—not yet obtained—would be of interest in comparison with... 

There are several reports in the literature dealing with the bimolecular [3 + 2] cycloaddition reactions of alkynyl-substituted diazo compounds. Propargyl diazoacetate 212, when stored for 2 weeks at 0 °C, was transformed into an oligomer to which the constitution 213 was assigned (273) (Scheme 8.50). The alkynyl-diazoketone 214 requires a much higher temperature and is transformed into pyrazole 215, which probably arises from intermolecular cycloaddition, pyrazole tautomerization, and carbenic N/H insertion (274). The inter-intramolecular... 

The ease with which diazo compounds are formed in these reactions should constitute reasonable warning that the nitrosation of all amines is fraught with considerable hazard. Not only can the unstable diazo compounds be formed, but also the intermediate /V-nitrosoamines themselves may be explosive in nature. Therefore each particular case must be carefully examined with all caution. As a matter of fact, even though reactions may have been run successfully at one time or another, this does not necessarily imply that the reaction is indeed safe (see safety note in Redemann et al. [18a]). 

OXYAZO COMPOUNDS. Compounds of the type RN=NC6H4OH, containing both the azo group — N=N—, and a hydroxyl group —OH, both attached to carbon atoms in the same ring. These compounds are commonly produced by ihe action of diazo compounds upon phenols in alkaline solution. They constitute a class of dyes. See Dyes. 

Although the thermal decomposition of diazo compounds has received an extraordinary amount of attention, nevertheless, most of the studies were run in the presence of a catalyst (generally, copper metal or a copper salt), and therefore the catalyzed decomposition of diazo compounds by transition metal derivatives constitutes one of the most intensely investigated fields of organic chemistry. The use of such catalysts has resulted in a considerable improvement of the efficiency and selectivity of many carbene reactions (see Section 9). 

The full discussion of these diazo compounds with the explanation of their constitution will come later but, as they are involved in the preparation of some of the azo compounds, it is necessary to introduce the subject at this time. They are formed by the action of nitrous acid... 

The details of this reaction and the one following will be considered later in duscussing the constitution of diazo compounds. The reaction always takes place in an acid solution, i.e., with the salt of the amine, so that the product obtained is not the diazo benzene as written above, but the salt of it. 

Constitution.—In the following discussion of the constitution and reactions of diazo compounds diazo benzene and its salts will be taken as examples. The reactions, however, are to be considered as typical of all diazo compounds. The physical properties of free diazo com-... 

What now is the constitution of these diazo compounds The facts thus far considered and which must be explained by an accepted constitutional formula are, (i) Their formation by the action of nitrous acid on a primary- amine, (2) Their conversion into azo, amino azo, hydroxy azo and hydrazine compounds and (3) the basic character of the hydroxy compound, diazo benzene, and the true salt character of the compounds formed with strong acids. 

Such a reaction agrees perfectly with the new formula which has been generally accepted as expressing the constitution of the free diazo base and the salts. The base thus takes the name benzene diazonium hydroxide and the salts, e.g., the chloride, benzene diazonium chloride signifying their ammonium character. The new formula does not, however, fit the reactions by which diazo. compounds are converted into azo or amino azo compounds nor the reduction of diazo benzene to phenyl hydrazine. Thus we have two formulas neither of which... 

Thus the acceptance of the Bloomstrand-Strecker-Erlenmeyer formula for the diazonium base and salts, and of the Hantzsch modification of the Kekule formula for the isomeric diazotates and the acid diazo hydroxide, together with the tautomeric transformations which occur, makes possible the explanation of all the facts which we have considered in the preceding discussion. These may be stated again briefly for the sake of emphasis and review, (i) The reaction of diazotization (2) the diazonium base and salts, (3) the diazotates and acid diazo hydroxide, (4) the isomerism of the diazotates, (5) the diazonium sulphites and diazo sulphonic acidsj (6) the relationship of diazo compounds to nitroso amines (7) to hydrazines (8) the coupling with amino and hydroxy compounds to form amino azo and hydroxy azo compounds, (9) the reactions of decomposition. Most of these last reactions we have not considered, but will do so presently and we shall find that they all may be likewise satisfactorily explained. The tautomeric constitution of the diazo compounds, therefore, meets every test and is generally accepted. 

At present three monosulphonic acids have been obtained from a-naphthol, but the constitution of only one of these is knowTi with any degree of certainty. This is the aa-acid prepared by Nevile and "V inther, and studied more closely by Witt. It is obtained by decomposition of the diazo-compound of naphthionic acid with water, and from its method of formation is constituted according to the formula ... 

The behaviour of the o-acid is totally different. The dyestuffs do not resemble those from Schaeffer s acid and /3-naphthol in the least, either as regards shade or solubility. Further, this acid may easily be converted into a dinitro-compound with nitric acid, and this is not the case with either y8-naphthol or Schaeffer s aeid. Another striking property of the a-acid is that its combination with diazo-compounds takes place with much more diflSculty than is experienced with Schaeffer s acid. Its constitution is therefore... 

According to their method of formation and constitution, these compounds may be divided into different classes. The first of these classes, the type of which is phenolbidiazobenzene discovered by Griess, contain the two azo-groups and the auxochrome amido-or hydroxyl groups in one benzene nucleus. These compounds are obtained by the action of diazo-compounds on oxy- or amido-azo-compounds. 

It has been held that the mother substance of the diazo-compounds, —free diazolenzene—possessed the constitution ... 

At this place, two examples may be mentioned which illustrate the theoretical as well as the practical value of the reaction by the oxidation of a mixture of aniline and p-toluidine, there is formed a complex dye, para-fuchsine, the constitution of which was unknown for a long time. This was first explained by E. and O. Fischer. They heated the diazo-compound of the leuco-base of the dye, paraleucaniline with... 

Concerning the constitution of the azo dyes, provided the components are known, the only question to solve is which hydrogen atom of the undiazotised component combines with the add radical of the diazo-compound (the add thus formed being eliminated). The question may be answered by investigating the reduction products of the azo dyes. By energetic reduction, best in add solution with stannous chloride, the double NrrN union is broken up, thus forming, with the addition of 4 atoms of hydrogen, two molecules of a primary amine, e.g. ... 

From this equation it is evident that by reduction, the amine which was diazotised—in the above case sulphanilic add—may be obtained again on the one hand, on the other an amido-group is introduced into the second component. If the constitution of this second product can be determined, then the constitution of the azo dyes is also determined. It may be stated as a general rule that, when a diazo-compound combines with an amine or phenol, the hydrogen atom in the para position to the amido- or hydroxyl-group is always substituted. In accordance with this, in the above case, p-amidodimethyl aniline ought to be obtained on reduction. If the para position is already occupied, then, the o-hydrogen atom Mites With the acid radical-... 

Aryldiazoacetates constitute an effective class of donor/acceptor diazo compounds in the realm of intermolecular carbenoid transformations [79], Because of their electronic profile, they were expected to display increased chemoselectivity for the desired C-H insertion event, while suppressing unwanted side reactions such as carbene dimerization. Additionally, the regioselectivity was expected to be enhanced in more complex systems due to the attenuated reactivity of the carbenoid. 

Carbenes constitute a further class of neutral reactive species that provide a route to alkylation of the prosthetic heme group. The oxidation of sydnones by cytochrome P-450, which yields an alkyl diazonium product, results in inactivation of the enzyme (Ortiz de Montellano and Grab, 1986 Grab et al., 1988). This loss of activity is associated with the formation of (V-alkyl heme adducts best rationalized by addition of a catalytically generated carbene or carbene equivalent to the heme group (Fig. 32). Model studies have shown that diazo compounds do react with reduced iron porphyrins to give N-alkylated products (Komives et al., 1988). The details of the enzymic N-alkylation reactions remain to be defined, but evidence exists for two distinct reaction pathways. The first is... 

By warming ofthe diazo-compound with traces of rhodium acetate in toluene at 80°C, the carbenoid inserts smoothly into the neighbouring N-H bond. This reaction step is ofparticular importance, since it constitutes a new methodology for the construction ofthe carbapenem skeleton. Although investigations on comparable bicycUc -ketoesters showed that these exist almost entirely in their keto-form, reaction with diphenyl chlorophosphonate and Himig s Base leads to the enol phosphate. 

The diazo-compounds form the starting point of numerous organic compounds, including the azo-dyes, the first representative of which (amino-azobenzene hydrochloride, aniline yellow ) was prepared and shown to be a dye by Mene. What was called aniline yellow is the oxalate of aminoazo-benzene (CeH5 N N-CeH4 NH2), introduced as a dye by Simpson, Maule, and Nicholson in 1863 its constitution was established by C. A. Martius and Griess, and the isomeric change of diazoaminobenzene into aminoazobenzene was explained by Kekule. ... 

This was the state of diazo-chemistry when Hantzsch published an elaborate criticism which produced a profound impression and opened a violent controversy with Bamberger. Hantzsch asserted that there was no valid reason for the nitrosamine formula for the isodiazo-compounds, which on hydrolysis give normal diazo-compounds. The different behaviour of sodium and silver salts on alkylation is due to tautomerism. Normal and isodiazo-compounds have identical constitutions, and hence the difference must be due to stereoisomerism, which arises from the fact that the three valencies of nitrogen do not lie in a plane, an assumption which explained the isomerism of the oximes. The only new experimental evidence Hantzsch gave was the preparation of a very unstable isomer of Fischer s potassium diazobenzenesulphonate (see p. 823), assumed to have the same structure, and hence the two salts were stereo-isomers, Fischer s salt being the syn- and the new (iso-) salt the anti-ioim ... 

The aliphatic and aromatic diazo-compounds (diazo-nium derivatives) are differently constituted, hence the... 

Christian Wilhelm Blomstrand (1826-1897), Swedish chemist and mineralogist, was professor at Lund University 1862-1895 (3). He gained an intemationsd reputation for his inorganic experimental research and for his accurate analyses of minerals. He was also renowned for his views on the constitution of diazo compounds and metal complexes (coordination compounds), published in German periodicals during the 1870s but already developed in his book Die Chemie der Jetztzeit (Contemporary Chemistry), the 417-page treatise on the theoretical aspects of chemistry, from which Werner took his motto. 

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