Thermolysis hydrogen abstraction reaction

The initiating radicals (Inif) can also be generated from peroxides, azo compounds (Init-Init), etc., via thermolysis or photolysis (Eq. 9.8). A hydrogen abstraction reaction between radical Inif and a given H-donor (R-H) affords Init-H and a new radical (R ) that can initiate the radical chain reaction (Eq. 9.9) ... 

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. 

A number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

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. 

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. 

The decomposition of 2-azido-4,6-dimethylpyrimidine (186) (in equilibrium with the tetrazolo compound 185) in the presence of copper acetylacetonate in cyclohexane at 140° gave the hydrogen abstraction product (187) (46%) and C—H insertion product (188) (8-5%) The same products were obtained in essentially the same yields by thermolysis of 186 at 185° °, (see section III.B.5). It was felt that the reaction proceeded via a copper-nitrene complex. The copper-catalysed decomposition of 186 and also 2-azidopyridine (in... 

As shown in Scheme 1.55, chain branching by thermolysis or photolysis, reaction (4), of polymer hydroperoxides (POOH) results in the formation of very reactive polymer alkoxy radicals (PO-) and hydroxyl radicals ( OH). The highly mobile hydroxyl and polymer alkoxy radical can abstract hydrogen atoms from the same or a nearby polymer chain by reactions (5) and (6), respectively (Zweifel, 1998). These polymer oxy radicals can react further to result in p-scission, by reaction (13), or the formation of in-chain ketones by reaction (14), as shown in Scheme 1.57, or can be involved in termination reactions. 

Finally, the ferrocenyl complexes were decomposed photochemically and thermally. Thermolyses can be performed in this case because the decomposition temperature of the azide (96) (80 °C) is much lower than those employed for other diazirines and azides. The results obtained from photolyses and thermolyses do not differ significantly. Here again, a-CD causes the most drastic changes because of the complete encapsulation of the guest in a 1 2 complex (Scheme 10.27). In accord with the other reactions performed in a-CD, the main reaction pathway is hydrogen abstraction from the host. Upon thermolysis ferrocenyl amine (112) is obtained in a yield up to 60%. More remailcably, ferrocenyl nitrene (111) seems to react in very low yields with a-CD. However, the structure of the reaction product 113 has not been fully established yet and is quite unexpected because 113 is the result of a glucopyranose-furanose conversion. In contrast, the products obtained by thermolysis of ferrocenyl nitrene in the soUd state, namely... 

The diketone 64 was also readily prepared from 59 as outlined in Scheme 20.15. Condensation between 64 and 2 equiv. of 51b gave 65 in excellent yield. Thermolysis of 65 in 1,4-CHD at 75 °C also promoted the Myers-Saito cyclization reaction to generate the biradical 66. The aryl radical center in 66 was then captured by the allenic moiety to form 67, having two stabilized triarylmethyl radical centers. Subsequent hydrogen-atom abstractions from 1,4-CHD then furnished 68. 

The thermolysis of 2-methoxyphenol in the presence of cumene as a radical scavenger occurs via two possible pathways. A homolytic cleavage of the methoxyl 0-C bond leads to methane and 1,2-dihydroxybenzene whereas an induced route starting with abstraction of the phenolic hydrogen by cumyl radicals leads, after a cascade of reactions, to phenol, 2-hydroxybenzaldehyde, and 2-hydroxybenzyl alcohol. ... 

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