Cyclohexadienyl radicals dimerization

This conclusion is supported by results of detailed study on the decay of hydroxyhexa-dienylperoxyl radicals, formed by addition of OH to benzene, followed by addition of dioxygen molecule. It was found that in the high dose rate of pulse radiolysis, hydro-quinone is the major product whereas catechol was not observed, indicating that only the 1,3-isomer loses HO2" and hence does not lead to dihydroxybenzene. The observation that the yield of 02 is 60% of the yield of the cyclohexadienyl radicals indicates that when dioxygen molecules react with the cyclohexadienyl radical, the radical is 60% trapped in the mesomeric form of 5b, whereas the results from the final products of dimerization in /-radiolysis show that 60% react in the form 5a. 

Neither the relative number of benzylic hydrogens nor the base strength accounts for the slow oxidation rate of the methylnaphthalenes. Formation of radicals in the presence of aromatic hydrocarbons can lead to radical attack on the aromatic ring. Addition of phenyl or methyl radical to the ring gives a cyclohexadienyl radical that may disproportionate or dimerize, or undergo hydrogen abstraction by another radical (3, 9,13). 

The necessity for another radical to complete the substitution sequence opens the way for complications. The cyclohexadienyl radical 40 may well react in some other way. It may dimerize, undergo self-disproportionation, or couple. Furthermore, since the cyclohexadienyl radical has three positions at which it can react, isomeric products are possible in each case. 

The discovery of muonic radicals in 1978 by Roduner opened up a broad field of interesting experiments. Aromatic substitution is normally studied by the measurement of the yields of stable products. This includes not only the site of addition of the reactant, but also the splitting off of the atom that is substituted. With the MuSR technique only the first step is studied exclusively. Another development concerns the measurements of the reactions of muonic radicals dimerization, ring opening, and ring closure. The observations of different reaction rates of the three isomeric muonic cyclohexadienyl radicals from anisole with quinones also opens a new field of investigation. 

The reaction of a nucleophilic alkyl radical R with benzene affords the a-complex 1, a fairly stable cyclohexadienyl radical, which under oxidizing conditions leads to cation 2 (Scheme 1). Depending on the stability of the attacking radical, the formation of 1 is a reversible process. Deprotonation eventually affords the homolytic aromatic substitution product 3. If the reaction is performed under non-oxidizing conditions, cyclohexadienyl radical 1 can dimerize (—> 4), disproportionate to form cyclohexadiene 5 and the arene 3, or further react by other pathways [3]. 

Stable longlived cyclohexadienyl radical 66 can either dimerize to give 69 or disproportionate to form cyclohexadiene 70 and arene 68. Moreover, H-abstraction from cyclohexadienyl radical 66 directly leads to arene 68. 

Many side reactions compete efficiently with the HAS process. Under non-oxidizing conditions, the cyclohexadienyl radical 1 is rather long lived and can dimerize to 5 (radical-radical coupling) or disproportionate to cyclohexadiene 6 (H-atom abstraction) and substitution product 3 (Scheme 9.2). Long-lived radical 1 may also couple with radicals derived from the radical initiator [8]. These reactions can be considered as termination steps [9]. In addition, radical R can be reduced by hydrogen atom abstraction from the solvent to yield R-H, before the attack on the benzene core preventing the HAS reaction. 

The formation of multilayers is presented in Figure 3.42 [126]. Reaction [R15] involves the transfer of 2e and the formation of two aryl radicals 1, where one of them attaches to the surface 2. These electrons can be obtained from an electrode, a reducing substrate or a reducing reagent in solution. At this point, there are two possible reaction pathways. In route A, the first grafted aryl group is attacked by a radical [R17] leading to a dimeric cyclohexadienyl radical 3 that can be reoxidized... 

A similar case of concurrence of one-electron transfer and nucleophilic addition is observed in the thermal ion-pair annihilation of CpMo(CO)3 anion with (dienyl)Fe(CO)3+ cations [84, 179]. Thus, spontaneous electron transfer (A et) occurs upon mixing of ( / -cyclohexadienyl)Fe(CO)3 with CpMo(CO)3 in acetonitrile to afford the transient 19-electron iron radical and the 17-electron molybdenum radical which both rapidly dimerize (Eq. 51). 

---