Radicals cyclohexadienyl radical

The majority of the reaction proceeds via the addition channel (b) to give a methyl hydroxycyclohexadienyl radical. Cyclohexadienyl radicals are resonance stabilized and are relatively unreactive. Typical alkyl radicals add O2 rapidly with rate constants of the order of 10 12, in contrast the reaction of cyclohexadienyl and methyl cyclohexadienyl radicals with O2 proceed with rate constants of (2 - 5) x 10 16 cm3 molecule 1 s 1 [67,68]. It can be calculated that in one atmosphere of air the cyclohexadienyl radicals have a lifetime of... 

Benzene anion radical Methanol Cyclohexadienyl radical Methoxide ion... 

Step 3 The cyclohexadienyl radical produced m step 2 is converted to an anion by electron transfer from sodium H H... 

Cyclohexadienyl radical Sodium Cyclohexadienyl anion Sodium ion... 

The more facile migration of aryl and other unsaturated groups involves bridged intermediates formed by an addition process. In the case of aryl migration, the intermediate is a cyclohexadienyl radical ... 

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

Benzene may react by addition as shown in Scheme 6.12 (this pathway is also open to other aromatic solvents). The cyclohexadienyl radical is a poor initiating species and may terminate a second chain by hydrogen atom transfer. According to this process, benzene is a retarder rather than a transfer agent. 

The 17-value of the pentachloroacetonyl radical is greater than that of the cyclohexadienyl radical and is positive. Pentachloroacetone... 

The number of reported reactions in which the radical derived from the decomposition of AIBN plays a role in the termination process has increased considerably. Often these reactions are not radical chain reactions, since the initiator is used in stoichiometric amounts. A few examples of rearomatization of cyclohexadienyl radicals by disproportionation have been reported herein. Below are some other examples, where the phenyl selenide 61 reacts with (TMSfsSiH (3 equiv), AIBN (1.2 equiv) in refluxing benzene for 24 h to give the coupling product of radicals 63 and 64 in good yields (Scheme 9).i24,i25 these cases,... 

Homolytic aromatic substitution often requires high temperatures, high concentrations of initiator, long reaction times and typically occurs in moderate yields.Such reactions are often conducted under reducing conditions with (TMSlsSiH, even though the reactions are not reductions and often finish with oxidative rearomatization. Reaction (68) shows an example where a solution containing silane (2 equiv) and AIBN (2 equiv) is slowly added (8h) in heated pyridine containing 2-bromopyridine (1 equiv) The synthesis of 2,3 -bipyridine 75 presumably occurs via the formation of cyclohexadienyl radicals 74 and its rearomatization by disproportionation with the alkyl radical from AIBN. ... 

Lyons and coworkers studied the ESR spectra of bakelite polysulfone [—CgH4— O—CgH4—SO2—CgH4—O—CgH4—C(CH3)2—] y-irradiated at 77 K and found features characteristic of at least four radicals, the cyclohexadienyl radical, formed from addition to the aromatic ring, methylene groups (— CH2) formed from H abstraction from the methyl group, phenoxy radicals and peroxy radicals. 

Loss of a hydrogen atom from the delocalised cyclohexadienyl radical intermediate (106) to yield the substituted end-product (107) does not proceed spontaneously, however, but requires intervention by a... 

When benzene adsorbed on silica gel was irradiated with 7 rays, a rather complex symmetric spectrum was observed. The spectrum consisted of a triplet of about 48 G splitting with each component being further split into a quartet of lines. The latter splitting was about 10.6 G. This spectrum is attributed to the cyclohexadienyl radical (I) which is formed by the... 

A reaction analogous to the formation of the cyclohexadienyl radical apparently occurs when mesitylene is adsorbed in a 13 X molecular sieve and then irradiated with high-energy electrons (64)- The resulting spectrum shown in Fig. 22 agrees well with the theoretical spectrum predicted for the radical (II) where the numbers indicate the splitting from the... 

The addition of dioxygen to the cyclohexadienyl radical is also reversible [86]. The following thermodynamic parameters were estimated for such equilibrium A//=21kJmol 1 and AA= 84 J mol-1 K-1. 

H" atom can both abstract hydrogen atoms and add to the double bonds. However it was found that the predominant reaction is the addition to the double bond. From the absorption of the cyclohexadienyl radical (formed by H abstraction) in acidic solution containing t-butanol (to scavenge the OH radicals) it was concluded14 that only 22% and 7% of the H atoms abstract hydrogen from 1,4- and 1,3-cyclohexadiene, respectively. 

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 cyclohexadienyl radicals decay by second-order kinetics, as proven by the absorption decay, with almost diffusion-controlled rate (2k = 2.8 x 109 M 1 s 1). The cyclohexyl radicals 3 and 4 decay both in pseudo-first-order bimolecular reaction with the 1,4-cyclohexadiene to give the cyclohexadienyl radical 5 and cyclohexene (or its hydroxy derivative) (equation 15) and in a second order bimolecular reaction of two radicals. The cyclohexene (or its hydroxy derivative) can be formed also in a reaction of radical 3 or... 

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. 

As mentioned above, the electrochemical oxidation of a diene yields 1,2- and 1,4-addition products when the reaction is carried out in the presence of a nucleophile such as methanol or acetic acid. When the oxidation is carried out in the absence of the nucleophile it usually yields a polymeric compound as the major product. The formation of a small amount of the Diels-Alder adduct is, however, observed when the reaction is carried out in CH2CI2 with graphite anode. One of the proposed reaction pathways is shown in equation 68, though it is not clear whether the cyclohexadienyl radical serves as a diene (as shown in equation 6) or a dienophile in the Diels-Alder reaction. 

In the case of the [4+ 2]-cycloadditions, the diradical analogous to 172 should contain an allyl radical subunit in the side-chain having the Z-configuration. There the closure of the six-membered ring occurs also employing the central carbon atom of the pentadienyl radical system. A quantum-chemical study reproduced the preference of the step 172 —y 163 over that from 172 to 173 [47]. This may have its origin in the higher spin density at C3 of the cyclohexadienyl radical as compared with Cl and C5 [108]. 

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