Radicals, and hydrogen abstraction

Figure 8 An overall mechanism for the dehydration of propane-1,2-diol by DDH. The Co-C bond of adenosylcobalamin is homolytically cleaved in a preliminary, highly unfavorable equilibrium to the 5 -deoxyadenosyl radical and cob(ll)alamin. Hydrogen atom abstraction from the substrate, isomerization of the substrate radical, and hydrogen abstraction from...

Two classes are known based on free radicals and hydrogen abstraction techniques, respectively. Free residual types are receptive to UV light by absorbing radiation energy such that free radicals result and these produce a chain polymerization reaction and eventually a solid polymer matrix. An example of the photoinitiation reaction sequence, which follows a Norrish I-type cleavage, is given in Fig. 12.10. Another photoinitiator would be benzoin butyl ether shown in Fig. 12.11. 

Some details of the chain-initiation step have been elucidated. With an oxygen radical-initiator such as the /-butoxyl radical, both double bond addition and hydrogen abstraction are observed. Hydrogen abstraction is observed at the ester alkyl group of methyl acrylate. Double bond addition occurs in both a head-to-head and a head-to-tail manner (80). 

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

Complete dissociation of the primary product (160) would result in the loss of the formyl fragment and in the stabilization of the remaining oxydiene radical by hydrogen abstraction from any suitable donor. As a competitive primary photoprocess bridging between the C-5 jS-carbon and the... 

The a,( -unsaturated aldehyde 452 is generated from the unstable spiro-oxetane 451, and hydrogen abstraction from the aldehydic C-H bond by 3449 gave a triplet radical pair 453 and 454. Intersystem crossing and radical recombination followed by intramolecular nucleophilic attack of the hydroxyl group toward the ketene functionality furnish the diastereomeric products 54 and 55 (Scheme 102) <20000L2583>. 

Ethylene-propylene and silicone rubbers are crosslinked by compounding with a peroxide such as dicumyl peroxide or di-t-butyl peroxide and then heating the mixture. Peroxide cross-linking involves the formation of polymer radicals via hydrogen abstraction by the peroxy radicals formed from the decomposition of the peroxide. Crosslinks are formed by coupling of the polymer radicals... 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

Continued addition of 02 beyond one-half the stoichiometric value with the hydrocarbons present encourages a net destruction of the hydrocarbon radicals. For the temperature range 1200-1300 K, production of the hydrocarbon radicals via hydrogen abstraction by 02 is rapid, even assuming an activation energy of 520kJ/mol, and more than adequate to provide sufficient radicals for NO reduction in the stay time range of 125 ms. 

It would be interesting to test with other Rh(III) complexes, whether the direct oxidation of the base (by photo-electron transfer) could also be a primary step responsible for photocleavages. Indeed, as outlined before in Sect. 5, radiation studies have shown that the radical cation of the base can produce the sugar radical, itself leading to strand scission [122]. Moreover base release, as observed with the Rh(III) complexes, can also take place from the radical cation of the base [137]. Direct base oxidation and hydrogen abstraction from the sugar could be two competitive pathways leading to strand scission and/or base release. 

The persistence of alkoxamidyls (164) probably facilitates rebound capture of acyloxyl radicals in competition with their decarboxylation and hydrogen abstraction reactions. 

Comparison of the behavior of free (mono) radicals with that of electronically excited sensitizers has led to the assumption that the excited sensitizer, Sensrad, is best described as a phototropic-isomeric diradical. Thus, photodimerization and photodehydrogenation, exhibited by certain sensitizers in the absence of oxygen, reflect radical-combination and hydrogen-abstraction reactions. Furthermore, fluorescein (a photosensitizer) becomes paramagnetic when excited... 

It has been mentioned that phenol is formed via Path A, by diffusion of radicals from the solvent cage and hydrogen abstraction from the solvent. This process is undoubtedly favored (and the yield of phenol is increased) when the phenoxy radical 65 already loses its counterpart in the solvent cage, i.e., when it loses the acyl radical 68 as a consequence of its decarbonylation. From the hitherto reported results it can be assumed that decarbonylation is significant and proceeds very readily under two conditions. It occurs (1) if the acyl radical formed possesses excess energy ( hot radical) due to excitation of high energy, e.g., by y-radiolysis,41,46 and (2) if the alkyl or aryl radicals formed by the decarbonylation of the acyl radical are exceptionally stable.61... 

The bromine radical abstracts an allylic hydrogen atom of the cyclohexene, and forms a resonance stabilized allylic radical and hydrogen bromide. 

Figure 6. Relationship between yields of low molar mass nitrogen-containing products resulting from elimination of a section of the main chain and hydrogen abstraction radical from the irradiation of poly(amino acid)s at 25° C. (A) tyr, (A) phe, ( ) gly, (O) ala, ) val.

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