Radical ring-cleavage

In a stereochemical study, it was shown that the free radical ring cleavage of all-m-1,2,3-trimethylcyclopropane (10) by bromine proceeded with inversion of configuration at one center and by nonstereospecific reaction at the other center. Equal amounts of S)-meso-2,4-di-bromo-3-methylpentane and tf/-2,4-dibromo-3-methylpentane were obtained by this reaction. The reaction of the corresponding trans derivative was much slower than that of the all-cis compound. 

Free Radical Ring Cleavage of c/s-l,2,3-Trimethylcyclopropane by Bromine. 

The Mass Spectra of O-Isopropylidene Ketals 9 (Figure 6), 10, (Figure 7), and 11 (Figure 8). Three fragmentations characteristic of O-isopropylidene ketals are loss of a methyl radical from the ketal ring, cleavage of bonds adjacent to the ketal ring, and loss of acetone (12). 

Ketones are oxidatively cleaved by Cr(VI) or Mn(VII) reagents. The reaction is sometimes of utility in the synthesis of difunctional molecules by ring cleavage. The mechanism for both reagents is believed to involve an enol intermediate.206 A study involving both kinetic data and quantitative product studies has permitted a fairly complete description of the Cr(VI) oxidation of benzyl phenyl ketone.207 The products include both oxidative-cleavage products and benzil, 7, which results from oxidation a to the carbonyl. In addition, the dimeric product 8, which is suggestive of radical intermediates, is formed under some conditions. 

Ring cleavage of the vinyloxaspirohexane 208 to the alkoxy radical 211 followed by radical recyclization leads to ring expansion to the cyclic ketones 209 and 210 [114]. (Scheme 82)... 

The bottom part of Scheme 18.5 is intended to show another fate for the phenyl radicals produced other than those already discussed in Schemes 18.1 and 2. Simple hydrogen abstraction yields a phenyl radical as discussed above in conjunction with Scheme 18.1. Reaction with other chains yields a branch point as also discussed above. However, if after hydrogen abstraction by the phenyl radical the cleavage reaction that produced the phenyl radical originally were to be applied to the remaining ester group on the ring, then one could account for the report of benzene [25] as well. 

Chemical/Physical. Anticipated products from the reaction of chlorobenzene with ozone or OH radicals in the atmosphere are chlorophenols and ring cleavage compounds (Atkinson et al., 1985 Cupitt, 1980). 

Chemical/Physical. Anticipated products from the reaction of 1,2-dichlorobenzene with ozone or OH radicals in the atmosphere are chlorinated phenols, ring cleavage products, and nitro compounds (Cupitt, 1980). Based on an assumed base-mediated 1% disappearance after 16 d at 85 °C and pH 9.70 (pH 11.26 at 25 °C), the hydrolysis half-life was estimated to be >900 yr (Ellington et al, 1988). 

Photolytic. Photooxidation products reported include 2,2 -dihydroxy-4,4 -dimethylbiphenyl, 2-hydroxy-3,4 -dimethylbiphenyl ether, and 4-methylcatechol (Smith et al., 1978). Anticipated products from the reaction of 4-methylphenol with ozone or OH radicals in the atmosphere are hydroxynitrotoluene and ring cleavage compounds (Cupitt, 1980). Absorbs UV light at a maximum wavelength of 278 nm (Dohnal and Fenclov4, 1995). 

Anticipated products from the reaction of phenol with ozone or OH radicals in the atmosphere are dihydroxybenzenes, nitrophenols, and ring cleavage products (Cupitt, 1980). Reported rate constants for the reaction of phenol and OH radicals in the atmosphere 2.8 x 10 " cmVmolecule-sec at room temperature (Atkinson, 1985) and with NO3 in the atmosphere 2.1 x lO" cmVmolecule-sec at 296 K (Atkinson et al., 1984). 

As seen from Scheme 7.2, the epoxy-ring cleavage and nickel oxidation proceed simultaneously. The nickel-oxygen bond is formed. This results in the formation of the carbon-nickel biradical in which Ph-CH fragment can rotate freely. The cleavage of the (NiO)-C bond leads to the formation of a mixture of styrenes. At early reaction stages (30 min), cis and trans olefins are formed in 50 50 ratio. After a prolonged contact (30 h), when all possible transformations should be completed, the trans isomer becomes the main product and cis trans ratio becomes 5 95. Such enrichment of the mixture with the trans isomer follows from the formation of the di-P-(trimethylsilyl)styrene anion-radical and its isomerization. The styrene formed interacts with an excess of the nickel complex. 

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