Nitrenes hydrogen abstraction

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). 

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 first tantalum nitrene was obtained in 1959 by thermolysis of [Ta(NEt2)]5-288 This class of compounds is presently accessible by several routes, including hydrogen abstraction from the mono- or di-alkylamides, reaction of metallacarbenes with organic imines, oxidation of low valent species by organic azides, or reductive coupling of nitriles (Table 13). The tantalum derivatives are usually stabler than those of niobium. 

Nitrenes have a short lifetime (only several microseconds)86- 8 and undergo stabilization by the following reactions isomerization to imines, dimerization to azo compounds, hydrogen abstraction followed by ring closure to heterocyclic compounds, bimolecular insertion into C-H bonds to secondary amines, addition to solvent yielding ylids, and addition to unsaturated systems yielding heterocyclic compounds. Table 117-106 includes the reaction products and references for the different classes of nitrenes. 

As in the carbon series, this photolytic rearrangement involved a nitrene intermediate. Indeed, in the case of thio phospho-razides, beside the formation of dimers 3 and polymers, the product was characterized and arises from hydrogen abstraction from the solvent by the nitrene intermediate. 

Metal nitrene complexes were used in a number of C-H amination reactions (recent reviews [358, 359]). Copper ketiminate complexes react with azides to nitrene complexes, which were isolated [360]. (p-Ketiminate)copper(I) complex 262 (2.5 mol%) serves therefore as an efficient catalyst for the intermolecular C-H amination of alkylarenes, cycloalkanes, or benzaldehydes 260 using adamantyl azide 261 as the nitrogen source ig. 68) [361]. The corresponding adamantyl amines or amides 263 were isolated in 80-93% yield. Copper complex 262 forms initially a dinuclear bridged complex with 261. From this a copper nitrene complex is generated by elimination of nitrogen, which mediates the hydrogen abstraction from 260. 

Triplet nitrenes mostly react by hydrogen abstraction (inter- and intramolecularly). Cycloaddition reactions with Ti-bonds occur nonstereoselectively. 

Among these, a variety of oxygen atom transfer reactions have been described [la,b] and highly stereoselective reactions have been reported [2]. Although the formation of aziridines by the reaction of nitrenes with olefins is well known, the efficiency is moderate, because of the competition between hydrogen abstraction and insertion processes [3]. A typical example is shown (Eq. 2) [3d]. 

The typical reactions of nitrenes generated from aliphatic azide precursors are shown below and include isomerization to imines (a), intramolecular insertion into C—H bonds (b) has been questioned (see 1. c. n> p. 57) and intermolecular hydrogen abstraction (c). 

Smith 65> concluded the strong preference of hydrogen abstraction by the nitrene from the starting material rather than from alkane solvents from the results of the thermolysis of C6D5N3 in pentane 66h The two main products were aniline, which was found to contain 18% of N-D-aniline. 

If this mechanism is really operative, the "abstraction product" formed in benzene is not necessarily due to a triplet nitrene precursor. Recently a careful study of the thermolysis of methylazide in substituted benzenes demonstrated that the unsubstituted primary sulfonamide is a product of hydrogen abstraction by the nitrene 72). On the other hand there are remarkable differences in isomer ratios (o m p) of the ring-substituted anilides formed depending on the spin state of the reacting nitrene. The triplet was shown to attach the aromatic nucleus mainly in the o-position, as is expected from a highly electrophilic diradical. Dehydrogenations by carbonylnitrenes have been reported by several authors for a variety of systems. In the direct photolysis of ethylazidoformate 29 in cyclohexene, the amide 30 and the bicyclohexenyl 31 were isolated 35b Both products result from an abstraction reaction. 

A novel tricyclic mesoionic tetrazolium derivative of 1,3,5-triazocine 9 was the major product of photochemical conversion of 5-azido-l-mesityl-3-phenyltetrazolium tetrafluoroborate 32 (Scheme 4, Section 14.08.5.2.1 <2000JHC1129>). The proposed mechanism of the transformation included benzylic hydrogen abstraction by the triplet nitrene intermediate to produce biradical species, which captures the solvent acetonitrile to afford 9. 

The decrease in solubility upon exposure in this type of resist was first ascribed to the formation of a secondary amine generated from nitrene insertion into C-H bonds of the polymer (see Scheme 3.6) (88). However, gel permeation chromatographic analyses revealed that the molecular weight of poly(p-vinylphenol) increased upon irradiation in the presence of the azide. Hydrogen abstraction from the polymer by nitrene and subsequent polymer... 

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