1.5- Hexadienyl radical

Radical cyclization to trans-hydrindanes. Although 1,5-hexadienyl radicals generally cyclize to a five-membered ring, the radical formed from the vinyl bromide 1 [Bu3SnH-B(C2H5)3] cyclizes to a trans-hydrindane 2 selectively, possibly via radicals a and b. The presence of an angular methyl group does not prevent a similar... 

The initiation reaction is assumed to be the decomposition of diallyl into two allyl radicals. Lossing et al. (16), Ruzicka and Bryce (17) and Akers and Throssell (18) also suggested the same initiation reaction. An allyl radical generated by reaction 1 abstracts hydrogen from the parent molecule to produce propylene as in reaction 2. Existence of the 1,5-hexadienyl radical (II) is supported by Ruzicka and Bryce (17) and James and Troughton ( ). 

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

Radical attack on methylbenzene (toluene, 60) results in preferential hydrogen abstraction by Cl leading to overall substitution in the CH3 group, rather than addition to the nucleus. This reflects the greater stability of the first formed (delocalised) benzyl radical, PhCH2 (61), rather than the hexadienyl radical (62), in which the aromatic stabilisation of the starting material has been lost ... 

ESR studies and product determination they concluded that the main primary process for radiolysis of both isomers is the dissociation of allylic C—H bonds. The formed hydrogen atoms may add to double bonds or abstract other hydrogen atoms (mainly allylic ones). The ESR spectrum of the radiolysis product at 77 K showed the presence of the cyclo-hexadienyl radical in the case of 1,4-cyclohexadiene, whereas the main intermediate from... 

Study of isomer distribution in substitution of benzene rings already carrying one substituent presents some potential pitfalls. Inspection of product ratios for ortho, meta, and para substitution, as in investigation of electrophilic substitution (Section 7.4, p. 392), might be expected to give misleading results because of the side reactions that occur in radical substitution. The isomeric substituted cyclo-hexadienyl radicals first formed by radical attack partition between the simple substitution route and other pathways (Equation 9.102). In order for the... 

Bansal and Henglein (1974) have suggested the use of the polaro-gram of the radical as a criterion for the possibility of electron transfer. When the radical shows a steep polarographic wave with an anodic part immediately followed by a cathodic part, this radical can be both reduced and oxidized at the same potential, and consequently can be assumed to disproportionate by electron transfer. Such polarograms have been observed for several hydroxycyclo-hexadienyl radicals. 

The experimental methodology in radiation-induced oxidation of benzene systems involved the measurement of rate constants and the transient absorption spectra by pulse radiolysis and the determination of yields of hydroxylated products on oxidation of the hydroxycyclo-hexadienyl radicals under steady-state conditions. The two commonly used oxidants — K3Fe(CN) "and IrCl " — convert quantitatively the OH adducts to the corresponding phenolic products. Thus, the pulse radiolysis technique in combination with product analysis using analytical techniques such as UV-VIS spectroscopy, HPLC, GC-MS, etc. under steady state conditions has provided valuable information in the understanding of the oxidation reaction mechanism of aromatics in... 

Recently, Al-Sheikhly et al studied the radiation-induced destruction of benzene and dienes. Their pulse radiolysis work gave a rate constant k = 7.8 x 10 dm mol s to form the hydroxycyclo-hexadienyl radical, absorbing at 315 nm with a molar extinction coefficient of 4.2 x 10 dm mol cm (Fig. 1). The OH adduct... 

A triplet with a splitting constant of 50 Gauss has also been observed in the solution radiolysis of benzene and definitively assigned to a cyclo-hexadienyl radical [113]. 

This dramatic difference of behavior has been interpreted in terms of transition state 81). The benzene series has not enough electron-deficient character to determine a transition state similar to a charge-transfer complex, so that the reaction rates would be affected more by the stability of the intermediate cyclo-hexadienyl radicals (23, 24) than by polar effects... 

The major OH radical reaction pathway is OH radical addition to the aromatic ring to yield hydroxycyclohexadienyl or alkyl-hydroxycyclo-hexadienyl radicals [reaction (63b)]. The reported rate constants for the reactions of the hydroxycyclohexadienyl-type radicals are in reasonable agreement, and the most recent data (Knispel et al., 1990 Zetzsch et al., 1990 Goumri et al., 1990, 1992) show that the hydroxycyclohexadienyl and methylhydroxycyclohexadienyl radicals both react rapidly with N02 with similar room-temperature rate constants of 3 X 10 11 cm3 molecule-1 s-1. The corresponding reactions with 02 have much lower reported room temperature rate constants, of 1.8 X 10-16 cm3 molecule-1 s-1 for the hydroxycyclohexadienyl radical and 5 X 10 16 cm3 molecule-1 s-1 for... 

Trradiation of aqueous solutions of aromatic solutes is believed to lead initially to adding H atoms and OH radicals to the ring to form cyclo-hexadienyl radicals, which then react further to form the observed products (17, 21, 34). The hydrated electron can also react to form transient negatively-charged entities (8, 29, 40) which may either be protonated by reaction with the solvent molecules (8, 40, 41) or else dissociate to form a stable anion and a substituted phenyl radical (8). In some systems (12, 41) the lifetime of the electron adduct is sufficient for its detection by pulse radiolysis techniques, but in many cases only the protonated species is seen. The rates of reaction of H atoms, OH radicals and hydrated electrons with aromatic solutes vary widely, but some success... 

Feldman and coworkers have generated radicals from vinylcyclopropanes in the presence of thiyl radicals and studied their reaction with alkynes to give vinylcyclopentenes. A general [3+2] annulation strategy considers vinylcyclopropanes 39 as the three atom components and alkynes 40 as the two atom counterparts (Scheme 14). The vinylcyclopentene products have been obtained as mixtures of syn- and a /-substituted stereoisomers the authors considered that product stereochemistry is set during the cyclization of the 5-hexadienyl radical 41 and derives from the two conformations (chair- or boat-like) in the transition state. 

For aromatic compounds, again there is some, but not much, discrimination based on electron density of the ring. The electron-rich compound anisole (methoxy-benzene), for example, has a rate constant for reaction with HO- only 2.6 times larger than that of nitrobenzene, a much more electron-poor aromatic compound (Zepp et al., 1987b). Addition to the aromatic ring (to form the intermediate cyclo-hexadienyl radicals 21) often predominates by a factor of approximately 10 over... 

Mu-substituted hexadienyl radicals are easily observed by high-field tS Rotation and their reaction with benzoquinone has been studied. The method is principally the same as in the Mu relaxation kinetics but, since Mu-substituted tmlicals are measured, the disappearance rate of MuO H is determined from the line broadening of the Fourier transformation power spectrum. The obtained value is kjj = 2.8 x 10 M- s- . It is important to note tlmt the reaction rate constant of Mu-substituted radicals may not differ much from that of the H-counterparts. The isotope effect should be small since the mass of Mu- and H-radicals are not much different, and since the reaction center is arated from the site of Mu- and H-addition. 

Fig. 12a, b. Resonance of benzoquinone solution in benzene, a The spin evolution at just-resonance of the Mu-substituted radicals of benzene (Mu-substituted hexadienyl radicals) in neat benzene (upper spectrum). When benzoquinone is added (lower two spectra, 2 mM and 8 mM), the spin evolution is damped showing that Mu-substituted hexadienyl radicals are reacting with benzoquinone. The reaction rate constant is derived from this damfung. b The Mu-substituted hexadienyl radicals resonance in neat benzene presented by the resonance lineshape (open circles). When 8 mM benzoquinone is added (black circles), the resonance signal becomes weak and the linewidth is broadened. The reaction rate constant is derived from the broadtening of the linewidth, too... 

The baseline shift shows how the Mu-substituted hexadienyl radicals are converted to diamagnetic muons by reaction 33, and start to evolve as a member of the resonant component. The following equation describes this situation, where the first term is the evolution of the diamagnetic muon already present at t = 0 (with the asymmetry the second term is that produced by reaction 33 with the rate 7 33 = kjj [Quinone] at time t = t and starts to evolve thereafter, and the third term is the asymmetry of Mu-hexadienyl radicals which are not on-resonant and are decreasing due to the reaction. 

Fig. 14. pS Repolarization for H O, and CSj. The steepness of the rise of polarization contain information about the species involved in the loss of polarization at low fields. Mu and Mu-substituted hexadienyl radials are responsible for the loss of polarization in Hj and respectively. Tire repolarization curve of CS suggests that a species with a hyperfine constant between those of Mu and Mu-substituted hexadienyl radicals fa tire precursor of the lost polarization... 

The results for the rate coefficients and the branching ratio for the initial addition/abstraction reactions of toluene with OH are in agreement with previous measurements [7]. For benzene and toluene the values for the rate coefficients of the reactions of X CHD with O2 tend to be lower than previously reported by Zetzsch et al. [7]. However, it has to be recognised that the data of the Cl atom initiated oxidation of benzene is based on the investigation of chlorine and hydroxyl containing hexadienyl radicals. A separation of the O2 reactions of the different radicals was not possible. 

The enthalpy of activation for the turnover-limiting hydrogen-atom transfer step depends upon the affinity of tire anthracene substrate for free radicals. The 9,10-dihydroanthracene is formed as the product because the radical is most stable in these positions, and the mixture of stereoisomers is formed because the process does not involve migratory insertion. This mechanism also explains why simple benzene derivatives are imreactive. The 1,4-hexadienyl radical lies too far uphill of benzene and the cobalt hydride to be a product from a reaction of a catalytic cycle that occurs under mild conditions. 

While radical species can hardly abstract H atom from benzene rings, some of them, in particular HO radical, are easily trapped by aromatic molecules. Radical hydroxylation most often follows Fenton-type chemistry that involves generation of HO in the course of homolytic decomposition of HjOj induced by an iron(II) salt and addition of HO to an aromatic nuclear to give hydroxycyclo-hexadienyl radical I. This radical may dimerize, be oxidized to phenols (Cu(II) is one of the most effective oxidants for this), or undergo an acid-catalyzed collapse to radical cation [15, 28]. Scheme 14.3 shows the classical mechanism suggested by Walling [28]. 

Grebenkin, S.Y., Krasnoperov, L.N. Kinetics and thermochemistry of the hydroxycyclo-hexadienyl radical reaction with O2 C6H60H-l-02 C6H6 (OH) OO. J. Phys. Chem. 108, 1953-1963 (2004)... 

Two possible mechanisms are proposed. Primarily the enol radical cation is formed. It either undergoes deprotonation because of its intrinsic acidity, producing an a-carbonyl radical, which is oxidized in a further one-electron oxidation step to an a-carbonyl cation. Cyclization leads to an intermediate cyclo-hexadienyl cation. On the other hand, cyclization of the enol radical cation can be faster than deprotonation, producing a distonic radical cation, which, after proton loss and second one-electron oxidation, leads to the same cyclo-hexadienyl cation intermediate as in the first reaction pathway. After a 1,2-methyl shift and further deprotonation, the benzofuran is obtained. Since the oxidation potentials of the enols are about 0.3-0.5 V higher than those of the corresponding a-carbonyl radicals, the author prefers the first reaction pathway via a-carbonyl cations [112]. Under the same reaction conditions, the oxidation of 2-mesityl-2-phenylethenol 74 does not lead to benzofuran but to oxazole 75 in yields of up to 85 %. The oxazole 75 is generated by nucleophilic attack of acetonitrile on the a-carbonyl cation or the proceeding enol radical cation. 

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