Radical Ring breaking

Phenyl radical reactions with 02, O, or H02 seem to be the most likely candidates for the first steps in the aromatic ring-breaking sequence [54, 61]. A surprising metathesis reaction that is driven by the resonance stability of the phenoxy product has been suggested from flow-reactor studies [54] as a key step in the oxidation of the phenyl radical ... 

The thermal rearrangement of spiropentanes to methylenecyclobutanes proceeds via two successive bond cleavages. First, a peripheral bond breaks to give a 1,3-diradical 28, and then a radical bond breaks to give a 1,4-diradical 29, until ring closure yields the product(s). 

The potential ring-opening of cyclopropane radical cations, breaking the weakened bond of type A radical cations (21 +), has been a subject of both interest and controversy. The ESR spectra of cyclopropane radical cation and its methyl-substituted derivatives decayed at temperatures near 100 K. They were replaced by secondary spectra, in which the protons at one cyclopropane center do not interact with the electron spin. This coupling pattern was interpreted as evidence for a ring-opened trimethylene species (22 +) in which one terminal carbon has rotated into an orthogonal orientation [105, 106, 140]. 

Figure 10 Model systems used for various ring-breaking sugar radicals radicals observed experimentally (I and II), model ring-breaking radical (M), C5 centered radical proposed experimentally (IV) and the model ring-breaking radical with a phosphate group (V).

So how are these products formed At such a high temperature, cr-bonds break and the weas -bonds in the molecule are the C-C and C-O bonds in the four-membered ring next to the ben ring. Breaking these bonds releases strain and allows one of the radical products to be secondar delocalized. 

PHOTODEGRADATION -bonds and rings breaking by radicals formation... 

A further example of the importance of this type of stereoelectronic effect is seen in the reactions of /-butoxy radicals with spiro[2,n]alkanes (22) where it is found that hydrogens from the position a- to the cyclopropyl ring arc specifically abstracted. This can be attributed to the favorable overlap of the breaking C-H bond with the cyclopropyl cr bonds.120131 No such specificity is seen with bicyclo[n, 1,0]alkanes (23) where geometric constraints prevent overlap. 

However, it may also reflect the greater ease with which the larger ring systems can accommodate lhe slereoelectronic requirements for fi-scission (Section 2.3.4). n Substituents (e.g. CH j, Ph) which lend stabilization to the new radical center, or increase strain in the breaking bond, also favor ring-opening (Tabic 4.5),... 

It would seem that all bonds must break in one of the two ways previously noted. But there is a third type of mechanism in which electrons (usually six, but sometimes some other number) move in a closed ring. There are no intermediates, ions, or free radicals, and it is impossible to say whether the electrons are paired or unpaired. Reactions with this type of mechanism are called pericyclic. ... 

Typically, the reaction mechanism proceeds as follows [6], By photoreaction, two chlorine radicals are formed. These radicals react with the alkyl aromatic to yield a corresponding benzyl radical. This radical, in turn, breaks off the chlorine moiety to yield a new chlorine radical and is substituted by the other chlorine, giving the final product. Too many chlorine radicals lead to recombination or undesired secondary reactions. Furthermore, metallic impurities in micro reactors can act as Lewis catalysts, promoting ring substitution. Friedel-Crafts catalyst such as FeClj may induce the formation of resin-Uke products. 

Now the second step. Make C7-N2. Break 01-N2, C5-C7. Heating the tricyclic compound causes thermolysis of the weak 01-N2 bond. The cyclopropyloxy radical quickly ring-opens to put the radical center at C7 then radical-radical recombination between C7 and N2 gives the product. 

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