Radical fragmentation reactions

Aliphatic Ethers The abundance of the molecular ion from aliphatic ethers is usually low but higher than that of the corresponding alcohols. These compounds exhibit radical site- and charge site-initiated fragmentation reactions. Radical site-initiated reactions prodnce resonance-stabilized oxyninm ions (e.g., of m/z 45,59,73,. ..). As exemplified in the reaction... 

SECTION 12.7. REARRANGEMENT AND FRAGMENTATION REACTIONS OF FREE RADICALS... 

Rearrangement and Fragmentation Reactions of Free Radicals 12.7.1. Rearrangement Reactions... 

Earlier sections have already provided several examples of radical fragmentation reactions, although this terminology was not explicitly used. The facile decarboxylation of acyloxy radicals is an example. 

Another common fragmentation reaction is the cleavage of an alkoxy radical to an alkyl radical and a carbonyl compound ... 

A stereoelectronic requirement in radical addition to carbon-carbon double bonds first became apparent from studies on radical cyelization and the reverse (Fragmentation) reactions.54 56 It provides a rationalization for the preferential formation of the less thermodynamically stable exo-product (i.e. head addition) from the cyelization of to-alkcny] radicals (16 - Scheme 1.5). s 57 G4... 

This comparison suggests that of these two similar reactions, only alkene additions are likely to be a part of an efficient radical chain sequence. Radical additions to carbon-carbon double bonds can be further enhanced by radical stabilizing groups. Addition to a carbonyl group, in contrast, is endothermic. In fact, the reverse fragmentation reaction is commonly observed (see Section 10.3.6) A comparison can also be made between abstraction of hydrogen from carbon as opposed to oxygen. 

Among the most useful radical fragmentation reactions from a synthetic point of view are decarboxylations and fragmentations of alkoxyl radicals. The use of (V-hydroxy-2-thiopyridine esters for decarboxylation is quite general. Several procedures and reagents are available for preparation of the esters,353 and the reaction conditions are compatible with many functional groups.354 f-Butyl mercaptan and thiophenol can serve as hydrogen atom donors. 

Schuster and Brizzolara<61> have provided a very nice study of the photochemistry of 10-hydroxymethyl-A1 9-2-octalone (87). Schuster and Patel<13) previously used radical fragmentation reactions as probes for the nature of the intermediates in the cyclohexadienone rearrangement. This compound (87) was designed so that it could undergo a radical fragmentation reaction in competition with the usual type A rearrangement if the intermediate involved has radical character (n -> n triplet). Photolysis produced (88)—(92) ... 

The total fluorescence intensity saturated around a few hundreds of mJ/cm2 which corresponds to the irradiation condition where the new plasma-like emission was observed. Above this value fluorescence intensity decreased, which is accompanied with the recovery of the relative intensity of excimer emissions. This means that a quite efficient deactivation channel of excitation intensity opens in this energy range, and the contribution of Si -Si annihilation is depressed. This suggests that fragmentation reactions to diatomic radicals are not induced by the annihilation process. Multi-photon absorption processes via the Si states and chemical intermediates should be involved, although no direct experimental result has as yet been obtained. 

The last few years have seen numerous applications of spin trapping to biological systems, and in these the trapping of hydroxyl radicals has assumed some importance. This work has been confined almost exclusively to nitrone scavengers 4 the fact that the hydroxyl adduct [6] of DMPO is much more persistent than that [7] of the commonly used nitrone, benzylidene-t-butylamine-N-oxide ( phenyl t-butyl nitrone ,3 or PBN) [3], may be due to a fragmentation reaction, with subsequent oxidation of the cr-hydroxybenzyl radical, as shown. 

Due to the fact that the removal of a bonding electron from the HOMO of the substrate RH leads to a radical cation with enhanced reactivity with respect to fragmentation reactions, the pathway often employed in radical cation chemistry results in the separation of charge and spin by dissociative processes, such as deprotonation, desilylation or cleavage of a stable cationic leaving group... 

The reaction of amino acids with HOCl was smdied using EPR spin trapping and UV-vis spectroscopy. Some nitrogen-centred radicals, which then undergo a variety of abstraction, rearrangement, and fragmentation reactions, were detected (Scheme 27). 

Dnring an electron transfer, the acceptor places its LUMO at the electron disposal and the donor releases an electron that is located on its HOMO. These orbitals are frontier orbitals. In the corresponding ion-radicals, the distribution of an unpaired electron proceeds, naturally, under frontier-orbital control. This definitely reflects in the ion-radical reactivity and not always by a self-obvions manner. Let ns concisely trace peculiarities of ion-radical fragmentation reactions that are very important in organic synthesis. 

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