Isomerization of radicals

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Atom migrations in molecules represent another class of isomerization reactions. Examples for the isomerizations of radical and stable species include... 

The whole kinetic mechanism is essentially characterized by initiation, /1-scission, H-abstraction, internal isomerization of radicals, substitutive addition of radicals onto unsaturated molecules and radical recombination reactions. 

VIII. Isomerization of radical ions of arylethenes Acknowledgments References... 

However, the evidence is very strong (30) that in such reaction systems those phosphoranyl radicals observed by ESR have five-membered dioxa ring systems attached to phosphorus apical-equatorial as in 15 (15) and its mirror form 25. This applies to six-membered ring 1,3-diaza species too (39) The apparent failure of the isomerization of radical 15 to 16 (24) (a thermoneutral process which keeps the odd electron equatorial) to compete with loss of R (R = t-Bu), indicates that the cyclic phosphoranyl radicals are not simple analogs of pentacovalent phosphorus intermediates, at least where permutational properties are concerned. Thus by contrast to the apparent high AG of >11 kcal/mol assigned to process Mi of J.5 (15 24), that for the... 

Monomer. Ethylene (which according to lUPAC rules should be called ethene and the corresponding polymer, from source-based rules, polyethene) is a gas Tb = - 0A°C) obtained from the thermal cracking (free radical process) of oil products. The initial reaction is a homolytic rupture of the covalent bonds of hydrocarbons that generate primary free radicals but the subsequent reactions are extremely varied (H abstractions, additions, decompositions, and isomerizations of radicals, etc.) and lead to a complex mixture that must be fractionated. Ethylene can also be produced either by dehydration of ethanol... 

In a number of cases quantum-chemical calculations have confirmed the experimentally determined distribution of unpaired electron density in a radical ion [94]. In addition the hyperfine structure of the spectrum can serve as an experimental basis for verification and confirmation of the validity of complex quantum-chemical calculations applied to the structure of organic molecules and radicals. The hyperfine structure can also serve to identify a given type of radical, and to offer information on the coplanarity and isomerism of radical anions it can also be used to determine the rates of transi-... 

FIGURE 4 20 The bond dis sociation energies of methy lene and methyl C—H bonds in propane reveal difference in stabilities between two isomeric free radicals The secondary radical is more stable than the primary... 

Additional evidence indirectly supports the reversibiUty of reaction 2. The addition of oxygen to a highly conjugated radical is readily reversible even at 40°C according to a study of the isomerization of methyl linolenate hydroperoxides in the presence of 02 (95). 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

The transfer of a hydrogen atom resulting in isomerization of the radical... 

The preparation of the less stable isomer (53b) of the oxazolone 53a involves a rather tedious procedure. It has been reported that 53a is rapidly isomerized to 53b in 48% hydrobromic acid saturated with gaseous HBr. In this way four azlactones have been converted into their isomers.It has been established, moreover, that the isomerization is radical-initiated and does not involve a carbonium ion intermediate. The isomerization can be reversed by pyridine. ... 

Both CSs and CSs were also successfully generated by the fragmentation of ionized 4,5-dioxo-2-thioxo-l,3-dithione (65) and 2-thioxo-l,3-dithiole (66) (90JA3750). Tire three sulfur atoms in the anion and cation radicals were chemically equivalent, suggesting that they take the D h (or C2u) form (67 or 68). On the other hand, under similar conditions, 3-thioxo-1,2-dithiole (69) yielded two isomeric cation radicals the (or 2 ) form and the carbon disulfide 5-sulfide form (70). Ab initio calculations on three electronic states of CS3 at the 6-31G -l-ZPVE level indicated that the C21, form (68) was more stable than the carbon disulfide 5-sulfide form (70) in the neutral (both singlet and triplet states) and the anion radical states, but 68 was less stable than 70 in the radical cation state. 

Arylfuroxans 23 (R = Ar) underwent isomerization to 4-arylfuroxans 21 during electrolysis via formation of a cation radical (86BAU1543, 86IZV1691). The thermal isomerization of 4-arylfuroxans 21 afforded 3-arylfuroxans 23 (79BAU2118, 79IZV2295, 83G811). 

Most radicals located on double bonds (e.g. 4, 5) or aromatic systems (e.g. 6) are a-radicals. The free spin is located in an orbital orthogonal to the it-bond system and it is not delocalized. The orbital of the vinyl radical (4) containing the free spin can be cis- or trans- with respect to substituents on the double bond. The barrier for isomerization of vinyl radicals can be significant with respect to the rate of reaction. 

Rate constants for ring-opening of dioxolan-2-yl radicals have been measured by Barclay et a/.241 as 103-104 s 1 at 75 °C (Scheme 4.29). There is also evidence that ring-opening is reversible.24 244 Thus, isomerization of the initially formed product to one more thermodynamically favored is possible if propagation is slow. 

Baechler and coworkers204, have also studied the kinetics of the thermal isomerization of allylic sulfoxides and suggested a dissociative free radical mechanism. This process, depicted in equation 58, would account for the positive activation entropy, dramatic rate acceleration upon substitution at the a-allylic position, and relative insensitivity to changes in solvent polarity. Such a homolytic dissociative recombination process is also compatible with a similar study by Kwart and Benko204b employing heavy-atom kinetic isotope effects. 

There is clear evidence that the addition of a sulfonyl radical to an olefin is a reversible reaction. This has been demonstrated for the case of but-2-ene, where cis-trans isomerization of the olefin accompanies addition63,64. Furthermore, 18a and 18b radicals generated by the reaction of Bu3Sn with erythro- and tfireo-2-bromo-3-(phenylsulfonyl)butane, respectively, eliminate benzenesulfonyl radicals to some extent, even at — 67 °C, to form 2-butenes in a non-stereospecific manner, providing also indication that the rotation about the central C—C bond is faster than the /1-cleavage process65. 

In the thermal reaction of aliphatic and aromatic sulfonyl chlorides with acetylenes no adduct has been observed82. However, the light-catalyzed additions of sulfonyl iodides to acetylenes83 as well as the thermal addition of sulfonyl bromides to phenylacetylene84 to form 1 1 adducts have been shown to be stereoselective and to occur in good to excellent yields. The fact that the addition occurs in a trans manner forced the authors83,84 to suggest that chain transfer by the sulfonyl halide (k ) is much faster than isomerization of the intermediate vinyl radical (k2) (see Scheme 5). 

For many radicals, step 1 (C=C+Y- -C—C—Y) is reversible. In such cases, free radicals can cause cis — trans isomerization of a double bond by the pathway. ... 

The reductive elimination of a variety of )3-substituted sulfones for the preparation of di-and tri-substituted olefins (e.g. 75 to 76) and the use of allyl sulfones as synthetic equivalents of the allyl dianion CH=CH—CHj , has prompted considerable interest in the [1,3]rearrangements of allylic sulfones ". Kocienski has thus reported that while epoxidation of allylic sulfone 74 with MCPBA in CH2CI2 at room temperature afforded the expected product 75, epoxidation in the presence of two equivalents of NaHCOj afforded the isomeric j ,y-epoxysulfone 77. Similar results were obtained with other a-mono- or di-substituted sulfones. On the other hand, the reaction of y-substituted allylic sulfones results in the isomerization of the double bond, only. The following addition-elimination free radical chain mechanism has been suggested (equations 45, 46). In a closely related and simultaneously published investigation, Whitham and coworkers reported the 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulfones on treatment with either benzoyl peroxide in CCI4 under reflux or with... 

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