Ring destruction reaction

Though the fragmentation is one of the basic reactions of radical ions, this destructive reaction pathway seems to be synthetically useless at first sight. Nevertheless, the electron-transfer-induced bond cleavage can be specific and, for this reason, synthetically useful (e.g., for ring enlargement reactions). 

Some oxygen functional groups do not react with water molecules and behave thus as neutral. Their fundamental structures are shown in Fig. 22. Some carbonyl groups can still react with sodium ethoxide. The ether or xanthene structure is fully inactive in aqueous media. Both neutral structures are believed to occur frequently due to their structural simplicity and the fact that in any oxidation reaction these structures have to occur as intermediates in ring destruction sequences. In quantitative studies where the oxygen content is compared to the sum of all neutralization reactions a significant excess of chemically unreactive oxygen is stated [149, 146] which is accounted for by these neutral functions. 

The reaction of AI with hydrazines follows a complex pathway. Thus 5-amino-isoxazoles react with hydrazine to furnish a mixture of pyrazolone 83, 4-amino-pyrazol-5-one 84, and l-aminopyrazol-5-one 85 (72JHC1219). Bipyrazole 86 and isoxazole ring destruction product 87 were obtained from the reactions of 5-AI with phenylhydrazine (53JPJ387) and semicarbazide (54JPJ138), respectively. This issue seems to require reinvestigation however, these reactions have a complex nature and... 

Ring destruction with the formation of olefins is shown in reaction (6.38) ... 

Triazine (1) reacts with various nucleophiles under ring cleavage. Reactions with ammonia, hydroxylamines and amines follow a similar scheme.18 The initial step is a nucleophilic attack of the nitrogen nucleophile to an electron-deficient triazine carbon. The reaction is assumed to proceed via ring opening and further addition of the nucleophile as shown below. The result, in every case is the complete destruction of the triazine ring with formation of three moles of ammonia and three moles of formimidamide derivatives 2.18... 

So now that we have all the reagents out of the way let s see how the reaction proceeds. There s the clear- yellow "safrole" sitting in the bottom if the flask and the clear saturated KOH solution is dumped in. The solution is heated to reflux etc. and yes, some brown byproducts and destruction artifacts will appear. Especially if the safrole is not pure. These byproducts should be expected to some extent because concentrated basic (OH) solutions can be as nasty as concentrated acidic solutions. One is mindful that KOH is less intrusive towards the delicate methylenedioxy ring structure of the safrole/isosafrole molecule. 

It was not their reactivity but their chemical inertness that was the true surprise when diazirines were discovered in 1960. Thus they are in marked contrast to the known linear diazo compounds which are characterized by the multiplicity of their reactions. For example, cycloadditions were never observed with the diazirines. Especially surprising is the inertness of diazirines towards electrophiles. Strong oxidants used in their synthesis like dichromate, bromine, chlorine or hypochlorite are without action on diazirines. Diazirine formation may even proceed by oxidative dealkylation of a diaziridine nitrogen in (186) without destruction of the diazirine ring (75ZOR2221). The diazirine ring is inert towards ozone simple diazirines are decomposed only by more than 80% sulfuric acid (B-67MI50800). 

Destruction of the aromatic ring is the mam reaction in the oxidation of tetrafluoro-o phenyleiiediamine with lead tetraacetate by products are tetrafluorobenzotnazole and tetrafluorochinoxalme denvatives [92] (equation 85) Polyfluonnated benzylideneanilines are oxidized by peroxyacids to different products dependmg on reaction contitions at room temperature the benzylidene carbon is oxidized with the formation of peroxy bonds [93 94] (equation 86), whereas in refluxing agent, the azomethme bond is cleaved [93] (equation 86) Pentafluorobenzylidencanilme is oxidized by peroxyacetic acid in dichlo-romethane at room temperature to perfluorobenzoic acid in a 77% yield [93]... 

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

Observation of the Norrish Type II reaction presents some difficulty in that generation of the biradical intermediate 12 requires a six-membered transition state and this is in conflict with the linear guest arrangement normally expected in the channel. However, as noted earlier, accommodation of planar six-membered rings in urea inclusion complexes has been observed 38. It appears that in this case the necessary six-membered transition state can be produced in the channel without destruction of the crystal structure. 

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