Isomerization chain mechanism

Graphs VIII and IX correspond respectively to the process of a —8 butylene isomerization (chain mechanism), and vinylchloride... 

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... 

It has been suggested [18] that the greater tendency for long-chain hydrocarbons to knock as compared to smaller and branched chain molecules may be a result of this internal, isomerization branching mechanism. 

Remarkable enhancements of the unimolecular c-t isomerization of c-S with p-MeO and oxidation of S with -MeO are explained by charge-spin separation in such S Unimolecular c-t isomerization of such c-S proceeds with a chain mechanism, while regioselective oxidation occurs in such S because of the spin localization. Cycloreversion of t,c,t-TPCB occurs to give a a-St 2, while the photochemical cycloreversion of TPCB and t,t,t-TPCB gives Tr-St 2 and t-St /t-St pair, respectively. Radical cations of phosphorus compounds (9 and 10 form intramolecular rr-dimer between two Nps from which Np 2 forms. Formation of intermolecular a-dimer of aromatic acetylene (11 - and 12 -) and intramolecular dimer of 13 and diarylmethanoT was observed, and the n = 3 rule is not effective for intramolecular dimer -. 

The 9,10-dicyanoanthracene sensitized irradiation of c/i-stilbene results in nearly quantitative isomerization (>98%) to the trans isomer with quantum yields greater than unity. Therefore, the isomerization was formulated as a free radical cation chain mechanism with two key features (1) rearrangement of the c/i-stilbene radical cation and (2) electron transfer from the unreacted cis-olefin to the rearranged (trans-) radical cation. 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

In connection with the oxirane ring opening, the isomerization of oxirane 6 to ketone 9 in an MeCN—LiClO —(Pt) and a CH Clj—Et NClO —(Pt) system has been reported and was explained on the basis of an electron-transfer chain mechanism via 7 and 8 However, 6 is actually converted to 9 under the preelectrolysis conditions so that the transformation may be explained in terms of an EGA catalyzed reaction... 

It is now well established that the cation radicals of unsaturated and strained hydrocarbons undergo a variety of isomerization (e.g., Scheme 18) and cycloaddition reactions with much faster rates than those of the corresponding neutral molecules [162-165]. A cation radical chain mechanism analogous to Scheme 17 was reported for one-way photoisomerization of cis-stilbene (c-S) to truws-stilbene (f-S) via photoinduced electron transfer, as shown in Scheme 18 [166], Once c-S + is formed, it is known to isomerize to t-S + [167,168]. The free energy change of electron transfer... 

An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

The general features of the isomerization are compatible with a free radical cation chain mechanism, featuring electron transfer from unreacted olefin to rearranged radical cation. This chain mechanism was firmly established in several other isomerizations by the observation of quantum yields greater than unity. Thus, the dicyanoanthracene sensitized irradiation of m-stilbene results in nearly quantitative isomerization (> 98%) to the trans-isomer. In this system, the quantum yield increases with increased ds-stilbene concentration, solvent polarity, salt concentration, as well as decreasing light intensity [159]. 

With large hydrocarbon molecules an important isomerization reaction will occur. Benson [I7b] has noted that with six or more carbon atoms, this reaction becomes a dominant feature in the chain mechanism. Since most practical fuels contain large paraffinic molecules, one can generalize the new competitive mechanisms as... 

G-values are reported for the electron induced isomerization which is interpreted in terms of a radical-chain mechanism which involves an addition elimination sequence in which both the added and eliminated moieties are RSO - radicals. 

Just as the equilibrium conformational properties of macromolecules, the theory of which has been developed in well-known classical works by Kuhn, Flory, Volken-stein and others the kinetic properties of polymer chains can be determined by two main mechanisms of intramolecular mobility. First, it is the discrete rotational isomeric (rotameric) mechanism of mobility caused by the jump of small-chain segments (kinetic units) from certain energically stable allowed conformers into others is4-i6S) gg ond it is the continuous mechanism of motion deter-... 

In 1989 Curran and co-workers reported on a photocatalytically induced free-radical cyclization leading to various cyclic, bi-, or polycyclic carbocycles (fused and spiro) via isomerization of unsaturated iodides (alkenes, alkynes) [63]. This corresponds to the nonreductive variant of the tin hydride method. Under sunlight irradiation and in the presence of 10 mol% hexabutylditin, a-iodo esters, ketones, and malonates are efficiently transformed via an iodide atom transfer chain mechanism (eq. (4)). 

Isomerization of hexamethyl Dewar benzene to hexamethylbenzene proceeds photochemically, some nitriles being used as sensitizers. In the photoisomerization in the presence of 1-eyanonaphthalene, an emission from exciplex has been observed 45). Furthermore, fluorescence of nitriles like telephthalonitrile is extinguished by hexamethyl Dewar benzene which isomerizes to the benzene 46). This fact shows that the singlet excited state of nitriles participates in the formation of the exciplex. The photosensitized isomerization of hexamethyl Dewar benzene in the presence of fumaronitrile is affected by the polarity of the solvent used. In a non-polar solvent the quantum yield is low while it is higher in a polar solvent. This fact suggests that this isomerization follows a radical-ion chain mechanism (44)47) ... 

A curious isomerization was observed on action of hydrazines on 131. In competition with the reduction, aziridine 132 is formed, probably by a radical-chain mechanism. 

Spectroscopic studies indicate homolytic cleavage of the enzyme bound coenzyme to a deoxyadenosyl radical (ACH2 ) and Bi2r as a common characteristic of these enzymatic reactions. Isomerization, described above, involves an apparent intramolecular 1,2-shift of a hydrogen and an electronegative group (X = OH, NH2, Table 1, entry 4 to 6) or a carbon skeleton (Table 1, entry 7 to 10). However, in the nucleotide reductase system, coenzyme B12 has a unique role of radical initiator in a radical chain mechanism rather... 

Effect on oxidation reactions. From a study of the oxidation of the isomeric octanes, Pope, Dykstra, and Edgar42 showed that the reaction proceeded by a chain mechanism and that the decomposition of the peroxide first formed between a hydrocarbon and an oxygen molecule resulted in the release of an aldehyde in a high state of activation which underwent further oxidation through a continuation of the chain. The proposed reactions for the oxidation of n-octane are as follows ... 

Whitten and co-workers [230] have amply investigated stilbene isomerization induced by excited porphyrins. Electron-deficient nitrostilbenes quench excited Zn or Mg etioporphyrins through exciplex formation. Light absorbed by the porphyrin affects isomerization of 4-nitro- or 4,4 -dinitro-stilbene with low quantum yields ((%trans)sens = 99.5), possibly via a stilbene exchange chain mechanism [230]. Azastilbenes coordinate to me-talloporphyrins in both the ground and the excited state (but do not quench it), and isomerize efficiently upon selective excitation of the porphyrin [231]. Complexed 1,4 -NPE gives d , = 6.6, = 0.2, and (%trans)sens = 96,... 

The mechanism by which propane isomerizes can also be explained by means of a chain mechanism, in a similar way as was done for butanes. 

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