Eliminations radical

SQnued olefin is spontaneously formed by an Spj l reaction of the aruon derived from gem-nitroLmidazolylethane with fim-chloronitropropane fEq 7 107 Similar sequend il S l reaction followed by radical elimination is observed in the reaction of 1-methyl-2-trichlo-romethyl-5-nitroLmida2ole with the anion of 2-nitropropane fsee Eq 7 108, or di-... 

Stereoselective preparation of CEi-allyl alcohols via radical elimination from ruin -y-phenylthio-fi-nkro alcohols has been reported. The requisiteruin -fi-nitro sulfides are prepared by protonadon of nitronates at low temperanire Isee Chapter 4, and subsequent treatment v/ith Bu-vSnH induces and eliminadon to givelE -alkenes selecdvely IseeEq. 7.112. Unfortunately, it is difficult to get the pure syu-fi-nitro sulfides. Treatment of a rruxnire of syu- and ruin -fi-nitrosulfides v/ith Bu- SnH results in formadon of a rruxnire of (Ey and lZ -alkenes. 

These results are in accordance with the findings of Boothe and coworkers26, who found that the reactions of four diastereomeric 2-bromo-3-phenylsulfinylbutanes with tributyl-tin radicals generate /3-phenylsulfinyl sec-butyl radicals (8) which eliminate PhSO radicals to form the 2-butenes in a stereoselective manner. The stereoselectivities observed in this free radical elimination must result from the fact that the rate constant for elimination is greater than that for rotation about the C—C bond. Furthermore, a neighboring phenyl group on the radical center seems to stabilize the radical enough so that the internal rotation can compete with the -elimination reaction. It is also noteworthy that the small... 

Free-radical mechanisms are mostly found in pyrolyses of polyhalides and of primary monohalides,though they also have been postulated in pyrolysis of certain carboxylic esters/ Much less is known about these mechanisms and we shall not consider them further. Free-radical eliminations in solution are also known but are rare. ... 

Rearrangements must be involved also in radical eliminations in which the open shell species X is not present as a structural unity of M+ but has to be created prior to the eventual dissociation step. Examples of such processes are the loss of HC = O from ionized methylvinylsulfide 6- 7i) or the elimination of both OH and HC = O from the cation radical of 8S,6> (the arrows in (2) indicate the atoms of 8 which are involved in the formation of the radicals OH and HCO. Both radicals are obviously not present as structural functions in the precursor. For HCO loss from 6 no labelling work has been reported). 

Essentially different from the reactions described in (2) are radical eliminations in which the radical X is already-present in the ionized molecule Rt — X)+ but where the actual dissociation of the R, — X bond is preceded or accompanied by an isomerization of the charge carrying part R. This situation (3) may occur in cases, in which the direct cleavage 11- 12 is energetically less likely than the two-step (or generally multi-step) pathway 11-+13-+14. 

The multistep radical elimination may involve the generation of discrete intermediates, which for instance could be formed by a cyclization process7) such as 15- 16- 17e). Alternatively, there may be no intermediate involved in the elimination sequence, but the actual transition states 19, 22 are substantially lower in energy due to the anchimeric assistance of suitable functional groups9,10) (4). 

Hydrogen Radical Elimination from Ionized Ethylene... 

Schwarz, H. Some Newer Aspects of Mass Spectrometric Ortho Effects. 73, 231-263 (1978). Schwarz, H. Radical Eliminations From Gaseous Cation Radicals Via Multistep Pathways — The Concept of Hidden Hydrogen Rearrangements, 97, 1-31 (1981). 

The nitro groups in Eqs. 7.88-7.90 are readily replaced by hydrogen with tin hydride under radical conditions as discussed already. However, the nitro groups in the a-nitrosulfides or (3-nitrosulfides are not replaced by hydrogen on treatment with tin hydride but the reaction affords desulfonated products (Eq. 7.51) and alkenes (Eq. 7.97) such radical elimination reactions are discussed in Section 7.3.1. (see Eqs. 7.91 and 7.92).138... 

Ono and coworkers have extended the radical elimination of v/c-dinitro compounds to P-nitro sulfones151 and P-nitro sulfides.138,152 As P-nitro sulfides are readily prepared by the Michael addition of thiols to nitroalkenes, radical elimination of P-nitrosulfides provides a useful method for olefin synthesis. For example, cyclohexanone is converted into allyl alcohol by the reaction shown in Eq. 7.110. Treatment of cyclohexanone with a mixture of nitromethane, PhSH, 35%-HCHO, TMG (0.1 equiv) in acetonitrile gives ahydroxymethylated-P-nitro sulfide in 68% yield, which is converted into the corresponding allyl alcohol in 86% yield by the reaction with Bu3SnH.138 Nitro-aldol and the Michael addition reactions take place sequentially to give the required P-nitro sulfides in one pot. 

Stereoselective preparation of ( )-allyl alcohols via radical elimination from anti-j-phenylthio-P-nitro alcohols has been reported.154 The requisite anti-P-nitro sulfides are prepared by protonation of nitronates at low temperature (see Chapter 4), and subsequent treatment with Bu3SnH induces anti elimination to give (E)-alkenes selectively (see Eq. 7.112). Unfortunately, it is difficult to get the pure yyw-P-nitro sulfides. Treatment of a mixture of syn- and anti-P-nitrosulfides with Bu3SnH results in formation of a mixture of (E)- and (Z)-alkenes. 

Ono and coworkers have devised a new acetylene equivalent for the Diels-Alder reaction namely, l-phenylsulfony-2-nitroethylene is a very reactive dienophile, and the radical elimination from the adduct gives the Diels-Alder adduct of acetylene, as exemplified in Eq. 7.113. Other acetylene equivalents are summarized in a review.156... 

Due to the toxicity of tin reagents, a new radical elimination without using Bu3SnH is highly desirable. B arton has reported that nitro olefins are converted into olefins via radical elimination of P-nitro trithiocarbonates (Eq. 7.116).160 The Michael addition of trithiocarbonate to nitroalke-nes is carried out in CS2 to avoid the addition of EtSH. 

Alkoxyl radicals can result from the isomerization of peroxyl radicals of oxidized PP (see above 13.1.6). If alkoxyl radicals cause polymer destruction, then, as they are produced from alkyl radicals, their accumulation and quasistationary concentration must decrease with increasing p02. However, despite varying p02, vs = const, in the oxidized PE and PP and, therefore, alkoxyl radicals essentially do not contribute to the oxidative destruction of polymers. At moderate temperatures, alkoxyl radicals eliminate hydrogen atoms from PH more rapidly than they undergo degradation. 

Using basic pH leads to higher plateau rate constants, indicating that the ratedetermining step is reaction 18. Reaction 17 must be at least as fast as the rate of 02 addition in the highest 02 concentration used, kn 8 x 105 s 1, which is the limit of the instrument measurement. The G of benzene in pulse radiolysis was found to be equal to that of the nitroform anion (1.6 x 10-7 molJ-1) as can be expected from reactions 17-19. Since the yield of the cyclohexadienylperoxyl radical is 2.9 x 10-7 mol. 1 1 it means that only a fraction (ca 60%) of the cyclohexadienylperoxyl radicals eliminates HO2. The H02 elimination occurs by H-transfer of the allylic hydrogen to the oxygen... 

For example, the 0,C-trisaccharide 153 was obtained by a Henry reaction of nitro disaccharide 151 and sugar aldehyde 150 followed by the dehydration of (3-nitro alcohol 152 and reduction of the resulting nitroolefin, to give nitro sugar 153. Finally, a radical elimination of the nitro group afforded the target 154 (Scheme 47).105... 

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