Radical isomerization

The same mixture of H and I was obtained starting with either of the geometrically isomeric radical precursors E or F. A possible explanation is based on the assumption of a common radical conformer G, stabilized in the geometry shown by electron delocalization involving the radicaloid p-orbital, the p-peroxy oxygen and Jt of the diene unit. The structure of the compounds H and I were determined by H NMR spectra and the conversion of H to diol J, a known intermediate for the synthesis of prostaglandins. 

In Scheme 1, the radical cations of the linear hexadienes and some cyclic isomers are contrasted. The heats of formation, AHr, as determined from the heats of formation of the species involved, as well as the heats of formation of the isomeric radical cations themselves clearly reveal the favourable stability of the cyclic isomers and/or fragment ions. Thus, instead of the linear pentadienyl cation (3), the cyclopenten-3-yl cation (2) is eventually formed during the loss of a methyl radical from ionized 1,3-hexadiene (1). Since 1,2-H+ shifts usually have low energy requirements (5-12 kcalmol-1), interconversion of the linear isomers, e.g., 4, and subsequent formation of the cyclic isomers, in particular of the ionized methylcyclopentenes 5 and 6, can take place easily on the level of the... 

The oxidation of 3,6-dehydrohomoadamantane (52) with NO+BF4, photo-excited tetracyanobenzene, and under anodic conditions has been found to involve a common radical cation intermediate. The study has shown that the activation of propellane cTc-c bonds with strong oxidizing electrophiles occurs by a sequence of single-electron transfer steps. These findings are supported by ab initio computations showing that the isomeric radical cations can equilibrate with low barriers and lead to a common product. ... 

Alkyl substituted cyclopropanols and cyclopropanone hemiacetals 115,116a) aiso undergo oxidative cleavage when exposed to air or peroxides the initial products are hydroperoxides such as 148. In the case of l-methoxy-2,2-dimethylcyclopropanol, the reaction can be followed by observing the emission peaks in the NMR spectrum, and these CIDNP effects have enabled identification of radical intermediates.1154) With di-f-butylperoxylate (TBPO), the isomeric radicals 143 and 144 are formed and these may undergo a diverse number of further reactions as indicated by the complex product mixture given in Table 20. 

The free energy difference between pairs of isomeric radical cations may be reduced because of the different oxidation potentials of the precursors (vide supra), or even reversed the second case is particularly interesting, as it may provide access to otherwise inaccessible products (vide infra)... 

The rapid chemical transformation of the initial radical anion precludes the use of ETC-substitution chemistry to make substituted derivatives, because of the short lifetime and hence inactivity of the isomerized radical anion [299] - towards the CO/PR3 replacement reaction.484... 

In common with other aromatic ketyls, the anion-radicals from the acetylpyridines show restricted rotation about the Ar—COMe bond, deriving from an increased bond order in the radicals, by comparison with their parent molecules.88 This manifests itself in the 4-acetylpyridine anion-radical in the nonequivalence of the pairs of ortho and meta protons (44), while 3-acetylpyridine gives rise to two geometrically isomeric radicals, 45 and 46. 

Whereas Scheme 13.6 focuses on the kinetics of stereochemical changes to R)-3h induced by its photolyses. Scheme 13.7 focuses on regiochemical considerations of product formation. Scheme 13.7 links in-cage formation of 2-BN and 4-BN to specific, regio-isomeric radical pairs, [radical pair]2B and [radical pair]4B, and Equation 13.16 can be derived from it. 

In this respect, theoretical studies have shown that in halobenzenes, halobenzo-nitriles, haloacetophenones and halonitrobenzenes, by considering the intramolecular electron transfer as the rate limiting step of the cleavage reaction, the energy difference between the two isomeric radical anions correlates with the ex-... 

The importance of such calculations not only lies with the prediction of the absolute values, but also in determining the relative site specific rate coefficients, and hence, the fractions of the different isomeric radicals which can be formed (Table 2.4). Experimental determinations of site specific rate coefficients are scarce. As seen in subsequent sections, the final products of combustion depend strongly on the nature of the radical intermediates. 

Irradiation of the nitrone nitroxide 160 in pentane gives the two isomeric radicals 162 and 163, the formation of which was postulated to occur through the intermediate nitroxide 161. °... 

These latter workers liave demonstrated, by electrochemical experiments, the different reactivities of 125 and its cis isomer.Thus, whereas 125 is formed essentially reversibly and lost in a slow dimerization reaction and a slow coupling with unreduced thioindigo, its cis isomer is rapidly consumed in isomerization, radical-radical, and radical-parent coupling reactions. Similar differential reactivities with carbon dioxide, acrylonitrile, and cin-namonitrile were also observed. For 6,6 -diethoxytliioindigo, it was found that the electroreduction of the cis and trans isomers occurred at different... 

The unreacted residual trans-monomer was found to consist of only fumarate after polymerization of the complex 6a by means of HPLC. In contrast, the residual 7a was detected to consist of cis- and trans-isomer during the polymerization. This means that the complexed ds-isomer 7a isomerized to trans configuration in the presence of radicals. Figure 13 shows the chromatograms of monomers obtained during the polymerization of complex 7a with redox initiator. It indicates that the peak of trans-monomer increased with increasing reaction time. Therefore, it was clear that the polymerization of cis-olefin monomers 7 in the presence of both CD and redox initiator takes place via an isomerization radical polymerization... 

More interest has been shown in the past year in the laser-induced processes involving organic molecules. One such study is the laser irradiation (193.3 nm) of the ketones (1-4). This study has shown that the Norrish Type I process is dominant resulting in a-fission and the formation of alkyl and acyl radicals. The ultimate products formed are alkanes and carbon monoxide. Norrish Type I reactivity is also observed in more complex molecules such as the carbohydrate derivatives (5) and (6). Irradiation of these in solution again brings about a-cleavage to give the isomeric radicals ]) ... 

A weak ortho effect The isomeric radical cations of ortfto-cresol also eliminate water but to lesser extents. 

If propyl radicals are generated into the gas phase by an active catalyst, the comparison of residence time with lifetimes given Fig. 12 can say whether or not such reactions can proceed while the gas mixture resides in the reactor. Also, if only these reactions are controlling the olefin distribution, the (C3 = )-to-(C2 =) ratio would be as shown in Fig. 13. The relative amount of propylene formed from both isomeric radicals is increasing with temperature. In fact, some additional amounts of propylene that can form in homogeneous reactions should be even higher due to several additional processes... 

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