Diazo compounds Tautomerism

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... 

With 9-diazofluorene, [3 + 2]cycloaddition products of type 111 were obtained in pentane or hexane, while N2 loss and subsequent cyclotrimerization occurred in CH2CI2, CCLt, and benzene (178). With acceptor-substituted diazo compounds of the type RCH=N2 [R = C02Et (179), COMe (180,181), and PO(OEt)2 (182)], the initially formed A -phosphapyrazolines rapidly tautomerized and bicyclic A -phosphapyrazolines such as 112 and 113 were formed. 

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 reaction of azides with ethoxyacetylene (98) has been studied by Italian authors. With aliphatic and aromatic azides, the expected 1-substituted 5-ethoxy-l,2,3-triazoles (99) are obtained.310-311 Sulfonyl azides, on the other hand, react with ethoxyacetylene at room temperature to yield no triazoles but the valence tautomeric diazo compounds (100).312-313... 

A new synthetic approach to heterophospholes based on the [3+2] cycloaddition reactions of phosphaalkynes has been developed by Regitz and co-workers <1998S1305>. In specific examples, a mixture of tautomeric 1,2,4-diazaphospholes 56 was prepared by cycloaddition of phosphaalkyne 55 and diazo compounds, while 1,2,4-oxazaphospholes 57 were prepared similarly from phosphaalkyne 55 and appropriate nitrile oxides (Scheme 1) <1998S1305>. 

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. 

By thermolysis or photolysis of appropriate diazo compounds, various substituted cy-anocarbenes can be generated. The addition of these carbenes to benzene gave 7-substituted bicyclo[4.1,0]hepta-2,4-diene-7-carbonitrile/cyclohepta-l, 3,5-triene-7-earbonitrile 9/10, valence-tautomeric isomers. ... 

Some insight into the nature of these catalyzed cyclizations is provided by the synthesis of benzobicyclo[3.1.0]hexene 17. Intramolecular cycloaddition under base catalysis (from the tosylhydrazone 14) or by heating compound 18 gave the [3.3.0] bicyclic product 16 but, in the boron trifluoride catalyzed reaction, the [3.2.1] bicyclic product 13 was formed. In the base-catalyzed reaction the intermediate 4,5-dihydro-3//-pyrazole 16 suffered tautomerization (to give 19). The diazo compound 13 cannot tautomerize, and gave a quantitative yield of benzobicyclohexene 17 when heated. These boron trifluoride catalyzed addition reactions do not involve the free diazo compound and in this particular case the cyclic A -tosyl intermediate 12 could be isolated from the reaction mixture. ... 

Flash thermolysis of 3-aryl[l,2,3]triazolo[l,5-a]pyridines (277) under mild conditions (380-500°C, 10-3 torr) affords carbazoles (279, 280) in nearly quantitative yields.232 The triazoles exist as the valence tautomeric diazo compounds 278 in the gas phase.189 The substitution patterns in the products (279 and 280) demonstrate that the reactions take place exclusively by a carbene-nitrene rearrangement in which the pyridylcarbenes insert into the 2,3-bond in pyridine (Scheme 52). 

This corresponds to the neutralization reaction just given except that X is used in place of the OH. The general principle of intermediate compound formation, though not mentioned every time, must be understood to apply to all these reactions. If nitrous acid plays the part of BX and an alcohol that of HA, then the ready formation of nitrite esters is not surprising when the two interact. From this point of view the nitrite esters are the aquo-esters of nitrous acid, and diazo compounds are the ammono-esters of nitrous acid, ON.OR and ON.NHR (not considering tautomeric rearrangements within the molecule). 

This does not react as a diazo-compound unless a mineral acid is added to the solution. Bamberger called it an isodiazo -compound, which isomerises to a diazo-compound. By the action of hot concentrated potassium hydroxide solution on Griess s diazobenzene potassium (see p. 771), Schraube and Schmidt obtained an isomeric compound giving the diazo-reaction with acid. Von Pechmann and L. Frobenius found that the silver salt of /)-nitrodiazo-benzene with methyl iodide does not give a nitrosamine but the methyl ether CeH4(N02)-N N-0CH3, so that the nitrosamine formula appeared to represent a tautomeric form of diazobenzene. 

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 ... 

Weak base such as cesium carbonate has been utilized in this reaction to generate diazo compounds in situ from tosylhydrazones through the Bamford-Stevens reaction. The reaction is initiated by palladium-promoted decarboxylation of propargylic carbonate to form propargylpalladium complex A, which then tautomerizes to afford allenylpalladium intermediate B. Subsequently, the common carbene formation-migratory insertion-p-hydride elimination occurs to afford various vinylallenes (Fig. 30). 

Synthesis. Almost without exception, azo dyes ate made by diazotization of a primary aromatic amine followed by coupling of the resultant diazonium salt with an electron-rich nucleophile. The diazotization reaction is carried out by treating the primary aromatic amine with nitrous acid, normally generated in situ with hydrochloric acid and sodium nitrite. The nitrous acid nitrosates the amine to generate the N-nitroso compound, which tautomerizes to the diazo hydroxide. 

In 1882 Griess discovered that in aqueous acidic solution 4-diazobenzenesulfonate and 4-methylaniline react quantitatively to yield 4-toluenediazonium ion and 4-ami-nobenzenesulfonic acid. This phenomenon is called diazo migration or diazo exchange. We now know that it is a consequence of the tautomerism of the initially formed l-(4 -methylphenyl)-3-(4 -sulfophenyl)-triazene, as discussed above. Griess also found another reaction that could not be explained at his time, but which is based on the tautomerism of intermediate triazenes occasionally, the reaction of an arenediazonium salt with a primary aromatic amine in weakly acidic solution yields a mixture of two isomeric aminoazo compounds (Scheme 13-21). 

Bis-acceptor-substituted diazomethanes are most conveniently prepared by diazo group transfer to CH acidic compounds either with sulfonyl azides under basic conditions [949,950] or with l-alkyl-2-azidopyridinium salts [951] under neutral or acidic conditions [952-954]. Diazo group transfer with both types of reagents usually proceeds in high yield with malonic acid derivatives, 3-keto esters and amides, 1,3-diketones, or p, y-unsaturated carbonyl compounds [955,956]. Cyano-, sulfonyl, or nitrodiazomethanes, which can be unstable or sensitive to bases, can often only be prepared with 2-azidopyridinium salts, which accomplish diazo group transfer under neutral or slightly acidic reaction conditions. Other problematic substrates include amides of the type Z-CHj-CONHR and P-imino esters or the tautomeric 3-amino-2-propenoic esters, which upon diazo group transfer cyclize to 1,2,3-triazoles [957-959]. 

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