Non-radical process

Another danger, shared with many of the classical probes for free-radical mechanisms, is that minor radical involvement, perhaps in some competing side reaction, will be revealed, whilst the dominant non-radical process will go unnoticed. 

Heterocyclic substrates in SET processes have been widely studied, including the reactions of diliydroiiicotiiiamide,116 pyridine, and quinoline117 and also phenoxazine and phenotiiiazines.118 Phenothiazine has also been shown by ESR analysis to undergo an electron-transfer reaction witii its radical cation with an appreciable 15N/14N isotope effect.119 The reaction of phenazine di-A -oxidc radical cations with hydrocarbons shows evidence of non-radical processes.120... 

Although the BL reaction contains no organic compounds, the list of reactions to be considered is lengthy and complicated. For example, O2 has to be considered as aquated (i.e., dissolved) reaction intermediate, and as gaseous product. Nevertheless, the principles of the FKN mechanism have been applied to the BL reaction with some success23, though a number of questions still remain about certain details of the mechanism24. The mechanism contains a non-radical process,... 

Lipid peroxidation is a free radical-mediated, chain reaction resulting in the oxidative deterioration of polyunsaturated fatty acids (PUFAs) defined for this purpose as fatty acids that contain more than two double covalent carbon-carbon bonds. Singlet oxygen can produce lipid hydroperoxides in unsaturated lipids by non-radical processes (Pryor and Castle, 1984), but the reaction usually requires a radical mechanism (Porter, 1984). Polyunsaturated fatty acids are particularly susceptible to attack by free radicals. Lipid peroxidation is a complex process, and three distinct phases are recognized (a) initiation, (b) propagation and (c) termination (see Fig. 2.10). 

Examples of non-platinum metal hydrogenation catalysts include (arene)-chromiumtricarbonyls which will hydrogenate dienes, alkynes, and so on, while ReH7(PCy3)2 will selectively hydrogenate acenaphthalene. Lanthanides and early transition metals are discussed later. Those catalysts operate via non-radical processes, but a few systems are known to involve radical reactions. The complex [CoH(CN)5]3 is a water-soluble catalyst that is selective for the hydrogenation of ajS-unsaturated compounds. 

Monomer complexes play an important role even in non-radical processes. In coordination polymerizations, the interactions of monomers with catalysts are evidently of greatest importance without them this type of addition would not be possible. The formation of unstable complexes between the electrophilic initiator and nucleophilic monomer is also necessary in cationic polymerizations. The idea that under certain conditions the formation of stable complexes between initiator and monomer may prevent polymerization [171] is now frequently accepted [172-174]. 

The evidence suggests that 5-33 and 5-34 are formed by different types of mechanisms structure 5-33 by a radical process and structure 5-34 by a non-radical process. Formation of 5-33 is suppressed completely by addition of a free radical inhibitor, but is stimulated by light. These observations strongly support a radical pathway to 5-33. Other factors also make ionic Sfj pathways unlikely. An Sfj2 reaction is ruled out because 5-33 is formed by substitution at a tertiary carbon. An S l ionization also is unlikely because formation of a carbocation next to the partially positive... 

Finally it is to be noted that allyl alcohol does not completely suppress ethane formation, which indicates that ethane may also be formed in a non-radical process of some sort. 

A similar beneficial effect of 1,1-diphenylethylene on the yields of arylation products was later observed in the synthesis of benzonitrile derivatives by reaction of diaryliodonium salts with potassium cyanide and in the reaction of diaryliodonium salts with the sodium salt of nitroalkanes. 7 In the latter case, the reaction was therefore considered to result from intermediate inner-sphere radicals. Some years later. Barton et al. showed that 1,1-diphenylethylene acts as an efficient inhibitor of the radical chain process in the reaction of enolates with diaryliodonium salts. They concluded that the arylation products arose from a non-radical process. 

Dimers and trimers are produced by radiolysis of n-pentane in the solid phase. The yields of the various dimers have been measured. These yields suggest a much higher production of 1-pentyl radicals than does the ESR study of these transformations. Occurrence of non-radical processes in dimer production would account for this discrepancy. Trimer production strongly suggests the occurrence of ion-molecule reactions. These reactions would also yield dimers. Dimers would thus be formed by ionic and by radical processes. 

Bu3SnH reacts spontaneously at ambient temperature with acid chlorides in a non-radical process, whereas (TMS)3SiH does not. Therefore, acid chlorides can be used under free-radical conditions only with the silane. 

This is the Norrish II reaction, occurring via a six-membered ring intermediate, and is a non-radical process. 

Alkane activation by a non-radical process is an unusual reaction which was recently achieved by several transition-metal complexes (as reagent or catalyst). Although there are presently, no synthetic applications of such a reaction it remains a potentially important field. Surprisingly, organolutetium (or organoytterbium) complexes are also able to interact with alkanes under very smooth conditions (67) (eq. 44 ). 

There is no effective epoxidation of cholest-4-ene-3-one, which has a carbonyl conjugated with the olefin bond, but ketalization of the conjugated carbonyl shifts the double bond to the 5,6-position and epoxidation occurs as described above ". The non-conjugated cholest-5-ene-3-one yields a mixture of epimeric 6-hydroxy-4-ene-3-ones, where the C=C bond has been shifted, and a 4-ene-3,5-dione this reaction was insensitive to the addition of a radical inhibitor, indicating a non-radical process. Ru(TMP)CO also catalyzes equally well this same reaction, but the true catalyst was again the trans-dioxo species formed from the carbonyl via reaction with a 6-hydroperoxy-4-ene-3-one (cf. Fig. 5), formed by radical-initiated, incipient autoxidation of the cholest-5-ene-3-one. 

Diphenylethylperoxy radicals undergo self-reaction to give almost equal yields of benzaldehyde, benzyl alcohol, 1,2-diphenylethanol, and benzoin. Benzaldehyde is produced by 6-scission of 1,2-diphenylethoxyls while 1,2-diphenylethanol and benzoin are formed by a non-radical process. 

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