Cyclohexenyl radical

Pulse radiolysis of N20-saturated aqueous solution of 1,4-cyclohexadiene leads to formation of three radicals, two by addition of either OH or H atoms to give the cyclohexenyl radicals 3 and 4 (equation 12 and 13) and one by abstraction of H atoms (equation 14). The last one, the cyclohexadienyl radical, can exist in two mesomeric forms (5a and 5b). Fessenden and Schuler16 found that the spin density of the cyclohexadienyl radical was highest at the central atom, i.e. form 5a is the predominant one. 

The first step of the reaction may be the formation of cyclohexyl radicals and cyclohexenyl radicals from cyclohexene by silent discharge. These free radicals might be formed by a cyclohexenyl radical and a hydrogen atom s being generated by electron impact, and the hydrogen atom may add to cyclohexene to give a cyclohexyl radical. 

The cyclohexenyl radical is resonance-stabilized. It reacts further via the following two propagation steps ... 

However the subsequent reactions of the cyclohexenyl radical have to be considered. The most probable sequences are summarized in Fig. 20... 

Autoxidation, the oxidation of organic compounds by air, normally occurs via a radical chain mechanism. For example, cyclohexene undergoes allylic CH abstraction by an initiator, and the resulting cyclohexenyl radical reacts with O2 to give the corresponding hydroperoxy radical that abstracts an H from cyclohexene. In this case the final product is the allylic hydroperoxide. Conversion of ethers to the hydroperoxides is another familiar example. The conversion of cumene to phenol and acetone is a commercial application of the reaction (equation 9). 

Indirect evidence that intermediate radicals are involved stems from the formation of two regioisomers when an unsymmetrical olefin is employed, as indicated by the NMR spectrum of 13e. More direct evidence is obtained upon conducting the reaction in the presence of the radical scavenger diphenylpicrylhydrazyl [194]. In the case of cyclohexene, formation of the corresponding addition product is indicated by MS analysis [142]. The cyclohexenyl radical was postulated also in the electrochemical oxidation of cyclohexene in methanolic solution [195]. 

As mentioned above, some arylalkenes, such as stilbene, form complexes with Ag+. Such complexes are also formed between Ag+, as added silver perchlorate, and simpler alkenes. Typical of this is the interaction with 1-methylenecyclohexane when a crystalline complex is formed. Irradiation of this complex in the polycrystalline state or in solution in methanol affords isomerization to 1-methylcyclohexene by a 1,3-hydrogen migration path. Further irradiation brings about the stereospecific formation of the exo,trans,exo-dimer (24) of 1-methylcyclohexene. Less specific photodimerization is also reported for the irradiation of the /S-pinene (25)/silver perchlorate complex69 The mechanism was thought to involve a silver/cyclohexenyl radical similar in type to that observed in the y-radiolysis of silver/cycloalkene complexes70,71. 

The activation energies for the rearrangements are 5-7 kcal/mol higher than the BDE of the C1-C8 bond, so biradicals are probably involved, but a conformation-ally random species will not account for the relative rates. It was suggested that the conformation of the initially generated biradical leads to the preference for closure and that the conformation of the cyclohexenyl radical is particularly important. 

The allylic 2-cyclohexenyl radical has its unpaired electron delocalized over two secondary carbon atoms, so it is even more stable than the unsubstituted allyl radical. The second propagation step may occur at either of the radical carbons, but in this symmetrical case, either position gives 3-bromocyclohexene as the product. Less-symmetrical compounds often give mixtures of products resulting from an allylic shift In the product, the double bond can appear at either of the positions it occupies in the resonance forms of the allylic radical. An allylic shift in a radical reaction is similar to the 1,4-addition of an electrophilic reagent such as HBr to a diene (Section 15-5). 

The initiation steps in this reaction are formation of succinimide (NHS), followed by formation of bromine atoms (Fig. 11.57). The bromine radical abstracts the allylic hydrogen to produce the cyclohexenyl radical, which in turn reacts with... 

FIGURE 11.58 The bromine radical abstracts a hydrogen to form the cyclohexenyl radical. Independently, a molecule of bromine is formed from the NBS.The Bt2 then reacts with the cyclohexenyl radical to give the product and a new bromine atom. 

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