Nitrenes abstraction

Alkyl nitrenes have been isolated by trapping in matrices at 4K, while aryl nitrenes that are less reactive can be trapped at 77 K. Occasionally, a nitrene has been trapped by its reaction with CO to yield an isocyanate (Scheme 6.2). In general, aryl nitrenes abstract hydrogen from hydrocarbons more reluctantly than do triplet carbenes. For example, phenylnitrene abstracts H from toluene with a rate constant 10 times lower than triplet diarylcarbenes. 

Fig. 6. Coupling of polymer chains via (a) photoinduced hydrogen abstraction free-radical reactions and (b) nitrene insertion/addition reactions.

Early attempts to induce the thermal ring expansion of arenes with (arylsulfonyl)nitrenes were disappointing in that primary sulfonamides, formed by hydrogen abstraction by the singlet nitrene, and A-phenylsulfonamides, were the major products.157 l-(Arylsulfonyl)-l//-azepines were produced in low yields and were isolated only as their [4 + 2] eycloadducts with ethene-... 

More recent work 8> shows that the S—N bond can be cleaved by hydroperoxides and that aromatic sulphonyl azides only undergo free radical thermal decomposition if a source of radicals is provided. Some light on the nature of the radical transfer agent has recently been shed by the observation 14> that dodecyl azides are formed (2.3%) in the thermolysis of mesitylene-2-sulphonyl azide (3) at 150 °C in w-dodecane under nitrogen. It seems likely that a dodecyl radical is produced by hydrogen abstraction by the triplet nitrene (5) [mesitylene-2-sulphonamide was also formed (1.1%)] which then attacks undecomposed sulphonyl azide... 

Evidence for the formation of alkyl and aryl radicals in some cases following loss of SO2 (Scheme 1) has been obtained. Thus, a small amount of M-pentane was formed in the decomposition of M-pentanesulphonyl azide in mineral oil ). Thermolysis of diphenyl sulphone-2-sulphonyl azide (8) in dodecane at 150 °C gave diphenyl sulphone 9 (27%) and diphenyl sulphone-2-sulphonamide 10 (9%) which arise by hydrogen abstraction by the aryl radical and sulphonyl nitrene, respectively. When this thermolysis was carried out in Freon E-4 at 150 °C, the products were diphenylene sulphone 77 (1.3%) (Pshorr-type cyclization product of the aryl radical) and 10 (1.5%) together with tars 16h Ferro-... 

The success of this reaction was ascribed to the solubility of the chlorozinc intermediate, whereas other chloramine-T derivatives (e.g. the sodium salt) are insoluble. An alternative non-nitrene pathway was not eliminated from consideration. On the other hand, no aromatic substitution or addition, characteristic of a free sulphonyl nitrene (see below), took place on treatment of jV,lV-dichloromethanesulphonamides with zinc powder in benzene in the cold or on heating. The only product isolated was that of hydrogen-abstraction, methanesulphonamide 42>, which appears to be more characteristic of the behaviour of a sulphonyl nitrene-metal complex 36,37). Photolysis of iV.iV-dichloromethanesulphonamide, or dichloramine-B, or dichloramine-T in benzene solution led to the formation of some unsubstituted sulphonamide and some chlorobenzene but no product of addition of a nitrene to benzene 19>. 

The chemical reactions of sulphonyl nitrenes include hydrogen abstraction, insertion into aliphatic C—H bonds, aromatic substitution , addition to olefinic double bonds, trapping reactions with suitable nucleophiles, and Wolff-type rearrangement. Hydrogen-abstraction from saturated carbon atoms is usually considered to be a reaction typical of triplet... 

Arylsulphonyl nitrenes usually give better yields of hydrogen-abstraction products from aliphatic hydrocarbons. -Toluenesulphonyl azide gave a 5% yield of -toluenesulphonamide on thermolysis in cyclohexane... 

On the other hand, thermolysis of ferrocenylsulpkonyl azide (14) in aliphatic solvents may lead to the predominant formation of the amide (16) 17>. A 48.4% yield of (16) was obtained from the thermolysis in cyclohexane while an 85.45% yield of 16 was formed in cyclohexene. Photolysis of 14 in these solvents led to lower yields of sulphonamide 32.2% in cyclohexane, 28.2% in cyclohexene. This suggests again that a metal-nitrene complex is an intermediate in the thermolysis of 14 since hydrogen-abstraction appears to be an important made of reaction for such sulphonyl nitrene-metal complexes. Thus, benzenesulphonamide was the main product (37%) in the copper-catalyzed decomposition of the azide in cyclohexane, and the yield was not decreased (in fact, it increased to 49%) in the presence of hydroquinone 34>. On the other hand, no toluene-sulphonamide was reported from the reaction of dichloramine-T and zinc in cyclohexane. 

This indicates that the thermally or photochemically decomposed azide (Figure 4) inhibits the dissolution of the styrene resin into the alkaline developer. The inhibition may be due to the increase of the molecular weight of the styrene resin in the presence of the decomposed azide. Hydrogen abstraction from the polymer by nitrene of the decomposed azide and subsequent polymer radical recombination result in a increase in the molecular weight of the polymer (17). 

Although mononitrenes do not act as crosslinking agents themselves, they can generate unpaired electrons in the polymer chains by the abstraction of hydrogen molecules from the polymer. As the result, the polymer chains are crosslinked by the recombinations of these unpaired electrons. Crosslinking by mononitrene in such a way, depends on the activity of the nitrene and may be not necessarily less efficient in some combination of monoazide with the polymer than that by dinitrene as is seen in the case for... 

Mechanisms [i] and [II] may occur through the recombination of unpaired electrons which are formed by the hydrogen abstraction of nitrenes, and the insertion reaction of nitrenes to C-H bonds, respectively. However, it has not been revealed in this study which C-H bond is attacked by the nitrene. 

The deuterium kinetic isotope effect for intramolecular CH insertion of the nitrene (87), generated by photolysis of the corresponding azide, is 14.7 0.3 at 20 °C and is consistent with the H-abstraction-recombination mechanism from the triplet state. The temperature dependence of the kinetic isotope effect suggests that quanmm mechanical tunnelling is important in this process. 

Preparation of the prototype drug departs from the phenylenediamine strategy used in all of the previous examples. Condensation of thiazolo nitrile (53-2) with aniline catalyzed by aluminum chloride affords the amidine addition product (53-3). This is then converted to its reactive A -chloro derivative (53-4) by reaction with sodium hypochlorite. Treatment of that intermediate with a base such as potassium hydroxide leads directly to the cyclization product and thus the benzimidazole thiabendazole (53-6) [56]. The reaction can be rationalized by invoking as the first step the abstraction of chloride to leave behind a nitrene species such as (53-5) this would then readily insert in the CH bond at the ortho position. 

Solvolysis of MC15 by ammonia was studied as early as 1924, but the products were not conclusively characterized.260,261 Halo monoalkylamides262 and dialkyl amides were prepared. The monoalkylamides undergo facile cr-H abstraction reactions to nitrenes, especially when labile substitutents are present (Section 34.2.3.5) dialkylamides (Table 12) have been extensively studied by Bradley.263-265... 

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